Designing an Electrical Installation Beginner Guide

Designing an electrical installation Beginner Guide

Designing an electrical installation (Beginner Guide)

SUMMARY 1. Consumers list……………………………………………...…………….5 1.1. Definition of voltage ranges………………………..…………………………5 1.2. Installed power loads - characteristics………………………...………….…6 1.2.1. Induction motors………………………….………………………….…....6 1.2.2. Resistive-type heating appliances and incandescent lamps (conventional or halogen)…………………………………………...……8

2. Power balance……..……………………………………………………12 2.1. Power loading of an installation……………………………………….……12 2.1.1. Installed power (kW)…………………………………….………………12 2.1.2. Installed apparent power (kVA)……………………………….…...……13 2.1.3. Estimation of actual maximum kVA demand………………….……..…13 de mand………………….……..…13 2.1.4. Example Example of application applicat ion of factors k u and k s………………….………..…16 2.1.5. Diversity factor……………………………………………….……….…17 2.2. Choice of transformer rating…………………………………….….………17 2.3. Example………………………………………...……………………….……18

3. Single line diagram….………………..……………………………...…19 4. Study of each electrical section…………… sec tion………………………..…… …………..…………….…21 ……….…21 4.1. Determination of the rated current of the protective devices……..…...…21 4.2. Determination of the sections of cables……………………………….……22 4.2.1. Determination of conductor size for unburied circuits…….…………….23 4.2.2. Determination of conductor size for buried circuits…………….…..…...27 4.2.3. Sizing the neutral conductor……………………………………….…….31 4.2.3.1. Influence Influence of the earthing ear thing system…………………….……...……31 4.2.3.2. Influence Influence of o f harmonic currents…………………….…………..…31 4.2.3.3. Protection of the neutral conductor…………….…...……………33 4.2.3.4. Breaking of the neutral conductor………..……...………….……34 4.2.3.5. Isolating of the neutral conductor………………………………...35 conducto r………………………………...35 4.2.4. Sizing the protective earthing conductor (PE)……………………..……35 4.2.4.1. Connection………………………………………………..………35 4.2.4.2. Type Type of materials...………………………………………….……36 4.2.4.3. Conductor sizing…………………………………………….……37 4.2.5. Calculation of L Lmax. for a TN-earthed system, using the conventional method……………………………………………….……39 4.2.6. Rules fore marine electrical cables according Bureau Veritas………..…40 4.3. Determination of the voltage drop……………………………………….…46 4.3.1. Maximum voltage drop limit………………………….…………...….…46

Romanian Electro Trade, Engineering & Consulting

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SUMMARY 1. Consumers list……………………………………………...…………….5 1.1. Definition of voltage ranges………………………..…………………………5 1.2. Installed power loads - characteristics………………………...………….…6 1.2.1. Induction motors………………………….………………………….…....6 1.2.2. Resistive-type heating appliances and incandescent lamps (conventional or halogen)…………………………………………...……8

2. Power balance……..……………………………………………………12 2.1. Power loading of an installation……………………………………….……12 2.1.1. Installed power (kW)…………………………………….………………12 2.1.2. Installed apparent power (kVA)……………………………….…...……13 2.1.3. Estimation of actual maximum kVA demand………………….……..…13 de mand………………….……..…13 2.1.4. Example Example of application applicat ion of factors k u and k s………………….………..…16 2.1.5. Diversity factor……………………………………………….……….…17 2.2. Choice of transformer rating…………………………………….….………17 2.3. Example………………………………………...……………………….……18

3. Single line diagram….………………..……………………………...…19 4. Study of each electrical section…………… sec tion………………………..…… …………..…………….…21 ……….…21 4.1. Determination of the rated current of the protective devices……..…...…21 4.2. Determination of the sections of cables……………………………….……22 4.2.1. Determination of conductor size for unburied circuits…….…………….23 4.2.2. Determination of conductor size for buried circuits…………….…..…...27 4.2.3. Sizing the neutral conductor……………………………………….…….31 4.2.3.1. Influence Influence of the earthing ear thing system…………………….……...……31 4.2.3.2. Influence Influence of o f harmonic currents…………………….…………..…31 4.2.3.3. Protection of the neutral conductor…………….…...……………33 4.2.3.4. Breaking of the neutral conductor………..……...………….……34 4.2.3.5. Isolating of the neutral conductor………………………………...35 conducto r………………………………...35 4.2.4. Sizing the protective earthing conductor (PE)……………………..……35 4.2.4.1. Connection………………………………………………..………35 4.2.4.2. Type Type of materials...………………………………………….……36 4.2.4.3. Conductor sizing…………………………………………….……37 4.2.5. Calculation of L Lmax. for a TN-earthed system, using the conventional method……………………………………………….……39 4.2.6. Rules fore marine electrical cables according Bureau Veritas………..…40 4.3. Determination of the voltage drop……………………………………….…46 4.3.1. Maximum voltage drop limit………………………….…………...….…46


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Designing an electrical installation (Beginner Guide) 4.3.2. Calculation of voltage drop in stead y load conditions……….……….…47 4.3.3. Examples……………….…………………………...………...…………49 4.4. Determination of the short-circuit currents…………………...………...…51 4.4.1. Calculation of maximum short-circuit currents in electrical ship mains according GERMANISCHER LLOYD “SCC363.EXE”……….………51 4.4.1.1. Principles Principles of o f the calculation………………………..…….………52 calculat ion………………………..…….………52 4.4.1.2. Structure of the Ship Mains to be calculated…………………..…54 4.4.1.3. Asymmetric Asymmetric short s hort circuit…………………………………………55 4.4.1.4. Remarks on input data and components…………………….……55 co mponents…………………….……55 4.4.1.5. Simplified calculation……………………………………….……59 4.4.1.6. Selection of switch switch gear…………………………………….……60 gear… ………………………………….……60 4.4.4.7. The documentation…………………………………………….…60 4.4.2. Short-circuit Short-circuit current cur rent calculation according acco rding BUREAU VERITAS………61 4.4.2.1. Main methods………………………………………………….…61 4.4.2.2. Theoretical considerations……………………………………..…62 4.4.2.3. Formulas…………………………………………………….……67 4.4.2.4. Selection of protective devices………………………………...…69 4.5. Worked example of cable calculation………………….………………...…71 4.6. Choice of the protective devices…………………………...……………..…74 4.6.1. The basic functions of LV switchgear…………….…...…………...……74 switc hgear…………….…...…………...……74 4.6.2. Elementary switching devices…………………….…...…………...……72 4.6.2.1. Disconnector (or isolator)……………………………………...…78 4.6.2.2. Load-breaking switch………………………………………….…78 4.6.2.3. Bistable switch (télérupteur)………………………………..……79 4.6.2.4. Contactor…………………………………………………………80 4.6.2.5. Fuses………………………………………………………...……84 4.6.2.5. Circuit breaker……………………………………………………87 4.6.3. Combined switchgear elements…………………………………….……93 4.6.3.1. Switch and fuse co mbinations…………………… mbinations……………………………………94 ………………94 4.6.3.2. Fuse - disconnector + discontactor, Fuse - switch-disconnector + discontactor…………………….…95 4.6.3.3. Circuit-breaker + contactor circuit-breaker + discontactor………96 4.6.4. Selection of a circuit breaker………………………………………….…96 4.6.4.1. Choice Choice of a circuit breaker……………………………………..…96 4.6.4.2. The selection of main and principal circuit-breakers………..……99 4.6.5. Protection of circuits according acco rding GL……………………………………102 4.6.6. Protection of circuits according Bureau Veritas………….…………….102 4.7. Selectivity of the protections………………………………...……..………103 4.7.1. Cascading………………………………………………………………103 4.7.2. Discriminative tripping (selectivity)……………………...……………103 (selectivity)……………………...……………103

5. Electrical machines….… machines….…..………………… ..……………………………...…… …………...……..………109 ..………109 5.1. Induction motors…...…………………………………………………….…109 5.1.1. The basic functions of the motor-starter s……….………………...……109 5.1.2. The motor start solutions…………………………………………..…...110 5.1.2.1. D.O.L. solutions………………………………………...………110 5.1.2.2. Star-delta solution………………………………………….……114 5.1.2.3. Star-double Star-double star solution (Dahlander connection)………………116 5.1.3. Variable speed drives for asynchronous motors (Altivar 38)……..……116


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Designing an electrical installation (Beginner Guide) 5.1.3.1. Applications…………………………………………………..…116 5.1.3.2. Functions………………………………………………..………117 5.1.3.3. Options...………………………………………………..………117 5.1.3.4. Characteristics……………………………………………..……118 5.1.3.5. Special uses………………………………………………..……118 5.1.3.6. Connection diagrams..……………………………………..……120 5.2. Connection diagrams for synchronous generators…...…….……………126 5.3. Connection diagrams for DC- Motors……………………..…………...…127 5.4. Protection of motors according Bureau Veritas……………..……….…..128 5.5. Protection of generators…………………………………………...……….129 5.6. Transformers……………………….………………………………………132 5.6.1. Basic principals……………………………………………..…...……..132 5.6.2. Circuit symbols……………..……………………………………….….133 5.6.3. Transformers types…………………..………………………………....133 5.6.4. Vectors-groups of transformers……..………………………………….135 5.6.5. Important equations…………………..………………………….....…..138 5.6.6. Protection of transformers according Bureau Veritas………..…….…..139

6. Technical information……………………….……………………..…140 6.1. Degrees of protection provided by enclosures……….…………………...140 6.2. Degrees of protection against mechanical impact…………………...…...142 6.3. Minimum required degrees of protection on ships (Bureau Veritas)…...143

Lexicon……………………………………………………………………145 Bibliography………………………………………………………...……150


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CHAPTER

1 1. Consumers list The first step for the designing of an electrical installation, whatever will be, involve the knowledge en detail of whole consumers which utilize her. In general the minimum of information required per consumer are: the type end the value of the power supply (alternative three-phase electrical voltage, D.C. voltage etc.); the electrical power of the consumer; the rated current of the consumer; the number of consumers of the same type. §

§ § §

In ships the principal consumers, supplied directly fro m the main switchboard, are: lighting equipment, power equipment, heating equipment, control & signaling, warming equipment, fire pumps, bilge pumps, radio equipment, steering gear, lateral thruster, sprinkler equipment, variable pitch propeller systems, CO2 system, auxiliary services for main engine, panels for ship and engine control.

§ § § § § § § § § § § § § § §

1.1. Definition of voltage ranges IEC voltage standards and recommendations

The nominal voltage of existing 220/380 V and 240/415 V systems shall evolve toward the recommended value of 230/400 V. The transition period should be as short as possible and should not exceed the year 2008. During this period, as a first step, the electricity supply authorities of countries having 220/380 V systems should bring the voltage within the range 230/400 V +6 %, -10 % and those of countries having 240/415 V systems should bring the voltage within the range 230/400 V +10 %, -6 %. At the end of this transition period, the tolerance of 230/400 V ± 10 % should have been achieved; after Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) this the reduction of this range will be considered. All the above considerations apply also to the present 380/660 V value with respect to t he recommended value 400/690 V.

Fig. 1.1. Standard voltages between 100 V and 1000 V

1.2. Installed power loads - characteristics The examination of actual values of apparent-power required by each load enables the establishment of: a declared power demand which determines the source for the supply of energy; the rating of the HV/LV transformer, where applicable (allowing for expected increases load); levels of load current at each distribution board. § §

§

1.2.1. Induction motors Current demand The full-load current I a supplied to the motor is given by the following formulae: P n ⋅ 1,000 3-phase motor: I a = ; 3 ⋅ U ⋅η ⋅ cos ϕ §

1-phase motor: I a

=

P n ⋅ 1,000

where U ⋅η ⋅ cos ϕ I a - current demand (in amps); P n - nominal power (in kW of active power); U - voltage between phases for 3-phase motors and voltage between the terminals for single-phase motors (in volts). A single-phase motor may be connected phase-to-neutral or phase-to-phase; η - per-unit efficiency, i.e. (output kW)/( input kW); cos ϕ - power factor, i.e. (kW input)/( kVA input). §

Subtransient current and protection setting Subtransient current peak value can be very high; typical value is about 12 to 15 times the RMS rated value I nm. Sometimes this value can reach 25 times I nm. Merlin Gerin circuit-breakers, Telemecanique contactors and thermal relays are designed to withstand motor starts with very high subtransient current (subtransient peak value can be up to 19 RMS rated value I nm). If unexpected tripping of the overcurrent protection occurs during starting, this means the starting current exceeds the normal limits. As a result, some maximum switchgears


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Designing an electrical installation (Beginner Guide) withstands can be reach, life time can be reduce and even some devices can be destroyed. In order to avoid such a situation, oversizing of the switchgear must be considered. Merlin Gerin and Telemecanique switchgears are designed to ensure the protection of motor starters against short circuits. According to the risk, tables show the combination of circuit breaker, contactor and thermal relay to obtain type 1 or type 2 coordination. Motor starting current Although high efficiency motors can be find on the market, in practice their starting currents are roughly the same as so me of standard motors. The use of start-delta starter, static soft start unit or speed drive converter allows to reduce the value of the starting current (Example: 4 ⋅I a instead of 7.5 ⋅I a). Compensation of reactive-power (kvar) supplied to induction motors It is generally advantageous for technical and financial reasons to reduce the current supplied to induction motors. This can be achieved by using capacitors without affecting the power output of the motors. The application of this principle to the operation of induction motors is generally referred to as “power-factor improvement” or “power-factor correction”. The apparent power (kVA) supplied to an induction motor can be significantly reduced by the use of shunt-connected capacitors. Reduction of input kVA means a corresponding reduction of input current (since the voltage re mains constant).

Fi g. 1.2. Rated operational power and currents


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Designing an electrical installation (Beginner Guide) Compensation of reactive-power is particularly advised for motors that operate for long periods at reduced power. As noted above cosϕ = (kW input)/(kVA input) so that a kVA input reduction in kVA input will increase (i.e. improve) the value of cosϕ. The current supplied to the motor, after power-factor correction, is given by: I a = cos ϕ / cos ϕ ' where cosϕ is the power factor before compensation and cosϕ’ is the power factor after compensation, I a being the original current. It should be noted that speed drive converter provides reactive energy compensation. F igur e 1.2. below shows, in function of motor rated power, standard motor current values for several voltage supplies.

1.2.2. Resistive-type heating appliances and incandescent lamps (conventional or halogen) The current demand of a heating appliance or an incandescent lamp is easily obtained .). from the nominal power P n quoted by the manufacturer (i.e. cosϕ = 1) (see F ig. 1.3

F ig. 1.3. Current demands of resistive heating and incandescent

lighting (conventional or halogen) appliances

The currents are given by: P n 3-phase case: I a = 3 ⋅ U P 1-phase case: I a = n U where U is the voltage between the terminals of the equipment. For an incandescent lamp, the use of halogen gas allows a more concentrated light source. The light output is increased and the lifetime of the lamp is doubled. §

§


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Designing an electrical installation (Beginner Guide) Note: At the instant of switching on, the cold filament gives rise to a very brief but

intense peak of current. Fluorescent lamps and related equipment The power P n (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current is given by: + P n P I a = ballast U ⋅ cos ϕ If no power-loss value is indicated for the ballast, a figure of 25% of P n may be used. Standard tubular fluorescent lamps The power P n (watts) indicated on the tube of a fluorescent lamp does not include the power dissipated in the ballast. The current taken by the complete circuit is given by: + P n P I a = ballast U ⋅ cos ϕ where U is the voltage applied to the lamp, complete with its related equipment. With (unless otherwise indicated): cos ϕ = 0.6 with no power factor (PF) correction ( capacitor); cos ϕ = 0.86 with PF correction (single or twin tubes); cos ϕ = 0.96 for electronic ballast. § § §

Fi g. 1.4. Current demands and power consumption

of commonly-dimensioned fluorescent lighting tubes (at 230 V-50 Hz)

Fi g. 1.5. Current demands and power consumption of compact

fluorescent lamps (at 230 V - 50 Hz)


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Designing an electrical installation (Beginner Guide) If no power-loss value is indicated for the ballast, a figure of 25% of P n may be used. F igur e 1.4. gives these values for different arrangements of ballast. Compact fluorescent lamps Compact fluorescent lamps have the same characteristics of economy and long life as classical tubes. They are commonly used in public places which are permanently illuminated (for example: corridors, hallways, bars, etc.) and can be mounted in situations otherwise illuminated by incandescent lamps (see F ig. 1.5.). Discharge lamps The power in watts indicated on the tube of a discharge lamp does not include the power dissipated in the ballast. F igur e 1.6. gives the current taken by a complete unit, including all associated ancillary equipment. These lamps depend on the luminous electrical discharge through a gas or vapour of a metallic compound, which is contained in a hermetically-sealed transparent envelope at a pre-determined pressure. These lamps have a long start-up time, during which the current I a is greater than the nominal current I n. Power and current demands are given for different types of lamp (typical average values which may differ slightly from one manufacturer to another).

Fi g. 1.6. Current demands of discharge lamps


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Designing an electrical installation (Beginner Guide) Note: §

§

These lamps are sensitive to voltage dips. They extinguish if the voltage falls to less than 50% of their nominal voltage, and will not re-ignite before cooling for approximately 4 minutes. Sodium vapour low-pressure lamps have a light-output efficiency which is superior to that of all other sources. However, use of these lamps is restricted by the fact the tallow-orange colour emitted makes colour recognition practically impossible.


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CHAPTER

2 2. Power balance The power balance consist in the determination, for every consumer, of the followings parameters: electrical power installed, load factor, time factor end the power required by the consumer in the normal work regime. The settlement of this balance is very serious for the dimension end the correct choice of the electrical lines, of the electrical cables, of the transformers end the main generator. In general the electrical power installed must to be smaller than the electrical power of the generator.

2.1. Power loading of an installation In order to design an installation, the actual maximum load demand likely to be imposed on the power-supply system must be assessed. To base the design simply on the arithmetic sum of all the loads existing in the installation would be extravagantly uneconomical, and bad engineering practice. The aim of this chapter is to show how some factors taking into account the diversity (nonsimultaneous operation of all appliances of a given group) and utilization (e.g. an electric motor is not generally operated at its full-load capability, etc.) of all existing and projected loads can be assessed. The values given are based on experience and on records taken from actual installations. In addition to providing basic installation-design data on individual circuits, the results will provide a global value for the installation, from which the requirements of a supply system (distribution network, HV/LV transformer, or generating set) can be specified.

2.1.1. Installed power (kW) The installed power is the sum of the nominal powers of all power-consuming devices in the installation. But this is not the power to be actually supplied in practice. Most electrical appliances and equipments are marked to indicate their nominal power rating ( P n). This is the case for electric motors, where the power rating refers to the output power at its driving shaft. The input power consumption will evidently be greater. Fluorescent and discharge lamps associated with stabilizing ballasts are other cases in which the nominal power indicated on the lamp is less than the power consumed by the lamp and its ballast. The power demand (kW) is necessary to choose the rated power of a generating set or battery, and where the requirements of a prime mover have to be considered. For a power


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Designing an electrical installation (Beginner Guide) supply from a LV supply network, or through a HV/LV transformer, the significant quantity is the apparent power in kVA.

2.1.2. Installed apparent power (kVA) The installed apparent power is commonly assumed to be the arithmetical sum of the kVA of individual loads. The maximum estimated kVA to be supplied however is not equal to the total installed kVA. The apparent-power demand of a load (which might be a single appliance) is obtained from its nominal power rating (corrected if necessary, as noted above for motors, etc.) and the application of the following coefficients: η = the per-unit efficiency = output kW / input kW cosϕ  = the power factor = kW / kVA The apparent-power kVA demand of the load: P a = P n /( η⋅ cosϕ ) From this value, the full-load current I a taken by the load will be: I a = (P a⋅103 )/V for single phase-to-neutral connected load I a

= ( P a ⋅ 10 3 ) /(

3 ⋅ V )

for three-phase balanced load where: V - phase-to-neutral voltage (volts); U - phase-to phase voltage (volts). It may be noted that, strictly speaking, the total kVA of apparent power is not the arithmetical sum of the calculated kVA ratings of individual loads (unless all loads are at the same power factor). It is common practice however, to make a simple arithmetical summation, the result of which will give a kVA value that exceeds the true value by an acceptable “design margin”. When some or all of the load characteristics are not known, the estimation of the values of installed apparent power may be used to give a very approximate estimate of VA demands.

2.1.3. Estimation of actual maximum kVA demand All individual loads are not necessarily operating at full rated nominal power nor necessarily at the same time. Factors k u and k s allow the determination of the maximum power and apparent-power demands actually required to dimension the installation. Factor of maximum utilization (k ) u - l oad f actor In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (k u) in the estimation of realistic values. This factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load. In an industrial installation this factor may be estimated on an average at 0.75 for motors. For incandescent-lighting loads, the factor always equals 1.


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Designing an electrical installation (Beginner Guide) For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the sockets concerned. Factor of simultaneity (k ) s - ti me f actor It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use of a simultaneity factor (k s). The factor k s is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited. For this reason, it is not possible to give precise values for general application. Example: Factor of simultaneity for an apartment block Some typical values for this case are given in F igur e 2.1. , and are applicable to domestic consumers supplied at 230/400 V (3-phase 4-wires). In the case of consumers using electrical heat-storage units for space heating, a factor of 0.8 is recommended, regardless of the number of consumers. §

Fi g. 2.1. Simultaneity factors in an apartment block

Example (see fi g. 2.2 .): Five storeys apartment building with 25 consumers, each having 6 kVA of installed load. The total installed load for the building is: 36 + 24 + 30 + 36 + 24 = 150 kVA. The apparent-power supply required for t he building is: 150·0.46 = 69 kVA From fi gure 2.2. , it is possible to determine the magnitude of currents in different sections of the common main feeder supplying all floors. For vertical rising mains fed at ground level, the cross-sectional area of the conductors can evidently be progressively reduced from the lower floors towards the upper floors. These changes of conductor size are conventionally spaced by at least 3-floor intervals. In the example, the current entering the rising main at ground level is:

150 ⋅ 0.46 ⋅10 3

= 100 A 400 ⋅ 3 the current entering the third floor is: (36 + 42) ⋅ 0.63 ⋅ 10 3 = 55 A 400 ⋅ 3 Romanian Electro Trade, Engineering & Consulting

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Fi g. 2.2. Application of the factor of simultaneity (k S )

to an apartment block of 5 storeys

Example: Factor of simultaneity for distribution boards Figure 2.3. shows hypothetical values of k S for a distribution board supplying a number of circuits for which there is no indication of the manner in which the total load divides between them. If the circuits are mainly for lighting loads, it is prudent to adopt k S values close to unity. §

Fi g. 2.3. Factor of simultaneity for distribution boards


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Designing an electrical installation (Beginner Guide) Example: Factor of simultaneity according to circuit function K s factors which may be used for circuits supplying commonly-occurring loads, are shown in fi gure 2.4. §

Fi g. 2.4. Factor of simultaneity according to circuit function

2.1.4 Example of application of factors k u and k s An example in the estimation of actual maximum kVA demands at all levels of an installation, from each load position to the point of supply (see F ig. 2.5.).

Fi g. 2.5. An example in estimating the maximum predicted loading of an installation

(the factor values used are fo r demonstration purposes only)


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Designing an electrical installation (Beginner Guide) In this example, the total installed apparent power is 126.6 kVA, which corresponds to an actual (estimated) maximum value at the LV terminals of the HV/LV transformer of 65 kVA only. Note: in order to select cable sizes for the distribution circuits of an installation, the current I (in amps) through a circuit is determined from the equation: kVA ⋅ 10 3 I = U ⋅ 3 where kVA is the actual maximum 3-phase apparent-power value shown on the diagram for the circuit concerned, and U is the phase-to-phase voltage (in volts).

2.1.5. Diversity factor The term diversity factor, as defined in IEC standards, is identical to the factor of simultaneity (k s) used in this guide. In some English-speaking countries however (at the time of writing) diversity factor is the inverse of k s i.e. it is always ≥1.

2.2. Choice of transformer rating When an installation is to be supplied directly from a HV/LV transformer and the maximum apparent-power loading of the installation has been determined, a suitable rating for the transformer can be decided, taking into account of the following .): considerations (see F ig. 2.6 the possibility of improving the power factor of the installation; anticipated extensions to the installation; installation constraints (temperature...) standard transformer ratings. § § §

Fi g. 2.6. Standard apparent powers for HV/LV

transformers and related nominal currents

The nominal full-load current I n on the LV side of a 3-phase transformer is given by: P a ⋅ 10 3 I n = U ⋅ 3 Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) where: P a - kVA rating of the transformer U - phase-to-phase voltage at no-load in volts (237 V or 410 V) I n is in amperes. For a single-phase transformer: P a ⋅ 10 3 I n = V where: V - voltage between LV terminals at no-load (in volts); I n = kVA·1.4 - simplified equation for 400 V (3-phase load). § § §

§ §

2.3. Example Consumer

anchor winch crane starting air compressor 1 starting air compressor 2 cargo hold ventilation stb. engine room ventilation stb. stearing gear 1 stearing gear 2 hfo separator 1 hfo separator 2 hfo heater 1 hfo heater 2 seawater cooling pump fuel pump 1 fuel pump 2 fuel transf. pump compressor aircond. plant lighting electric stove container ( 50 pcs, 10KW each) all others

Σ Σ Σ

Electrical power Installed KW 16.5 3.0 7.5

Load factor %

Time factor %

Power required KW

0 0 80

0 0 20

0 0 1.2

7.5

0

0

0

3.6

80

100

2.9

4.0

85

100

3.4

7.5 7.5 4.0 4.0 36.0 24.0 18.5

60 0 65 65 50 50 80

100 0 100 100 100 100 100

4.5 0 2.6 2.6 18.0 12.0 14.8

1.1 1.1 4.0 31.0

85 0 80 65

100 0 10 100

0.9 0 0.3 20.2

22.0 11.7 500

40 50 80

100 40 70

8.8 2.3 280

166

99.5 880.5

without container with container required electrical power at maneuvering area without container and bow thruster


194 474 209.3

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CHAPTER

3 3. Single line diagram A very important step in the designing of an electric installation is the realization of the one line diagram.

Fi g. 3.1. Single line diagram example

The one line diagram is the drawing of the simplified general electrical diagram. For example, for a three-phase electrical network the system wires are represented through a single line which shall be marked by three little parallel lines. In the case of a three-phase system with four wires, with a neutral conductor, in the one line diagram shall appear a Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) single line marked by four little parallel lines; same times the 4th little line, the neutral line, present on the end a litt le circle. In general in the one line diagram the consumer are groupated and bounded the in blocks and sections depending on the type and the value o f the electrical supply and on the consumers types.


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CHAPTER

4 4. Study of each electrical section The study of each electrical installation must be maked methodically while respecting the following most important stages: determination of the rated current, I n, of the protective devices, determination of the sections of cables, determination of the tension drop, determination of the short-circuit currents, choice of the protective devices, selectivity of the protections, verification of the protection of people. § § § § § § §

4.1. Determination of the rated current of the protective devices The determination of the rated current, I n, of each protected device is based on the maximum load current, I B.

Fi g. 4.1. Calculation of maximum load current I B


21

Designing an electrical installation (Beginner Guide) At the final circuits level, this current corresponds to the rated kVA of the load. In the case of motor-starting, or other loads which take an initially-high current, particularly where frequent starting is concerned (e.g. lift motors, resistance-type spot welding, and so on) the cumulative thermal effects of the overcurrents must be taken into account. Both cables and thermal type relays are affected. At all upstream circuit levels this current corresponds to the kVA to be supplied, which takes account of the factors of simultaneity (diversity) and utilization, k s and k u respectively, as shown in fi gure 4.1. The full-load currents calculation for the most used loads is described in the first chapter of this document at the paragraph 1.2. In conclusion the rated current I n of the protective devices must be equal to or greater than the maximum load current I B (f ig. 4.2.). I n ≥ I B

Fi g. 4.2. Determination of the rated current I n of protective devices

4.2. Determination of the sections of cables The first step is to determine the size of the phase conductors. In this clause the following cases are considered: unburied conductors, buried conductors. The tables in this clause permit the determination of the size of phase conductors for a circuit of given current magnitude. The procedure is as follows: determine an appropriate code-letter reference which takes into account: - the type of circuit (single-phase; three-phase, etc.); - the kind of installation. determine the factor K of the circuit considered, which covers the following influences: - installation method; - circuit grouping; - ambient temperature. § §

§

§

IMPORTANT NOTES: This procedure is a combination of IEC 60364-5-52 requirements and Schneider Electric recommendations (f ig. 4.3.). This procedure is also presented as a principle for the determination of the sections of cables. For different manufacturers of cables the principle rests the §

§


22


§

same with the specifics variations depending of the application type, of the environment temperature and the necessaries protections. For example for the electrical marine applications is utilized only copper cables because of the superior electric properties of the copper and because the crosssectional area of the aluminiun cables is bigger than the cross-sectional area of copper cables. The marine cables are also to be protected against the corrosion because the marine environment is very wet and very corrosive.

Fi g. 4.3. Logigram for the determination of minimum conductor size for a circuit

4.2.1. Determination of conductor size for unburied circuits Determination of the code-letter reference The size of a phase conductor is given in tables which relate: the code letter symbolizing the method of installation, and the factor of influence K . These tables distinguish unburied circuits from buried circuits. § §


23

Designing an electrical installation (Beginner Guide) The letter of reference (A to G) depends on the type of conductor used and its method of installation. The possible methods of installation are numerous, but the most common of them have been grouped according to four classes of similar environmental conditions, as shown below in fi gure 4.4 .

F i g. 4.4. G12 Code-letter reference, depending on type of conductor and method of installation

Determination of the factor K For circuits which are not buried, factor K characteristizes the conditions of installation, and is given by: K = K 1·K 2·K 3. The three component factors depending on different features of the installation. The values of these factors are given in f igu res 4.5. to 4.7. below. Correction factor K 1 (see F ig. 4.5.) Factor K 1 is a measure of the influence of the method of installation.

Fi g. 4.5. Factor K 1 according to method of circuit installation


24

Designing an electrical installation (Beginner Guide) Correction factor K 2 (see F ig. 4.6.) Factor K 2 is a measure of the mutual influence of two circuits side-by-side in close proximity. Two circuits are considered to be in close proximity when L, the distance between two cables, is less than double the diameter of the larger of the two cables.

F ig. 4.6. Correction factor K 2 for a group of conductors in a single layer

When cables are installed in more than one layer a further factor, by which K 2 must be multiplied, will have the following values: 2 layers: 0.80 3 layers: 0.73 4 or 5 layers: 0.70 Note: IEC 60364-5-52 recommends using a correction factor when cables are installed in more than one layer but no values are given. § § §

Correction factor K 3 (see F ig. 4.7.) Factor K 3 is a measure of the influence of the temperature, according to the type of insulation.

0

Fi g. 4.7. Correction factor K 3 for ambient temperature other than 30 C

Example A 3-phase 3-core XLPE cable is laid on a perforated cable-tray in close proximity to three other circuits, consisting of:


25

Designing an electrical installation (Beginner Guide) a 3-phase 3-core cable (circuit no. 1); three single-core cables (circuit no. 2); six single-core cables (circuit no. 3). Circuit no. 2 and no. 3 are 3-phase circuits, the latter comprising 2 cables per phase. There are, therefore, effectively 5 3-phase circuits to be considered, as shown in 0 fi gure 4.8. The ambient temperature is 40 C. The code letter indicated in fi gure 4.4. is E . § § §

K 1 = 1, given by fi gure 4.5. K 2 = 0.75, given by fi gure 4.6. K 3 = 0.91, given by fi gure 4.7. K = K 1·K 2·K 3 = 1·0.75·0.91 = 0.68

Fi g. 4.8. Example in the determination of factors K 1 , K 2 and K 3

Determination of the minimum cross-sectional area of a conductor The current I z when divided by K gives a fictitious current I’ z . Values of I’ z are given ., together with corresponding cable sizes for different types of insulation and in fi gure 4.9 core material (copper or aluminium). Example The example shown in fi gure 4.8. for determining the value of K , will also be used to illustrate the way in which the minimum cross-sectional-area of conductors may be found, by using fi gure 4.9. The XLPE cable to be installed will carry 23 amps per phase. Previous examples show that: the appropriate code letter is E the factor K = 0.68 § §

Determination of the cross-sectional areas A standard value of I n nearest to, but higher than 23 A is required. Two solutions are possible, one based on protection by a circuit breaker and the second on protection by fuses. Circuit breaker: - I n = 25 A - permissible: current I z = 25 A - fictitious current: I’ z = 25 / 0.68 = 36.8 A - cross-sectional-area of conductors is found as follows: 1. In the column XLPE3 corresponding to code letter E the value of 42 A (the nearest value greater than 36.8 A) is shown to require a copper conductor c.s.a. of 4 mm 2. 2 2. For an aluminium conductor the corresponding values are 42 A and 6 mm . Fuses: - I n = 25 A - permissible current I z = K 3·I n = 1.21·25 = 30.3 A §

§


26

Designing an electrical installation (Beginner Guide) - the fictitious current I’ z = 30.3 / 0.68 = 40.6 A - the cross-sectional-areas, of copper or aluminium conductors are (in this case) found to be the same as those noted above for a circuit-breaker protected circuit.

Fi g. 4.9. Case of an unburied circuit: determination of the minimum cable size derived from the code

letter; conductor material; insulation material and the fi ctitious current I’ z

4.2.2. Determination of conductor size for buried circuits In the case of buried circuits the determination of minimum conductor sizes, necessitates the establishement of a factor K . A code letter corresponding to a method of installation is not necessary.


27

Designing an electrical installation (Beginner Guide) Determination of factor K Factor K summarizes the global influence of different conditions of installation, and is obtained by multiplying together correction factors K 4, K 5, K 6 and K 7. The values of these several factors are given in f i gur es 4.10. to 4.13 . Correction factor K 4 Factor K 4 is a measure of the influence of the method of installation.

F ig. 4.10. Correction factor K 4 related to the method of installation

Correction factor K 5 Factor K 5 is a measure of the mutual influence of circuits placed side-by-side in close proximity. Cables are in close proximity when the distance L separating them is less than double the diameter of the larger of the two cables concerned. When cables are laid in several layers, multiply K 5 by 0.8 for 2 layers, 0.73 for 3 layers, 0.7 for 4 layers or 5 layers.

Fi g. 4.11. Correction factor K 5 for the grouping of several circuits in one layer

Correction factor K 6 This factor takes into account the nature and condition of the soil in which a cable (or cables) is (are) buried; notably its thermal conductivity.

Fi g. 4.12. Correction factor K 6 for the nature of the soil

Correction factor K 7 This factor takes into account the influence of soil temperature if it differs from 20 0C. Example (see fi gure 4.15.) A single-phase 230 V circuit is included with four other loaded circuits in a buried 0 conduit. The soil temperature is 20 C. The conductors are PVC insulated and supply a 5 kW lighting load. The circuit is protected by a circuit breaker. K 4 = 0.8, from fi gure 4.10. K 5 = 0.6 , from fi gure 4.11.


28

Designing an electrical installation (Beginner Guide) K 6 = 1.0, from fi gure 4.12. K 7 = 1.0, from fi gure 4.13. K = K 4 ·K 5 ·K 6 ·K 7 = 0.48 Determination of the smallest cross-sectional-area of a conductor, for buried circuits Knowing I z and K , the corresponding cross-sectional-areas are given in fi gure 4.14.

0

Fi g. 4.13. Correction factor K 7 for soil temperatures different than 20 C

Example This is a continuation of the previous example, for which the factors K 4, K 5, K 6 and K 7 were determined, and the factor K was found to be 0.48.

Full load current : I b

=

5,000 230

= 22 A

F ig. 4.15. Example for the determination of K 4 , K 5 , K 6 and K 7

Selection of protection: a circuit-breaker rated at 25 A would be appropriate. Maximum permanent current permitted (i.e. the circuit-breaker rating I n): I z = 25 A


29

Designing an electrical installation (Beginner Guide) Fictitious current : I ' Z =

I Z K

=

25 0.48

= 52.1 A

C.s.a. of circuit conductors: In the column PVC, 2 conductors, a current of 54 A 2 corresponds to a 10 mm copper conductor. In the case where the circuit conductors are in aluminium, the same fictitious current (52 A) would require the choice of 16 mm2 corresponding to a fictitious current value (for aluminium) of 62 A.

Fi g. 4.14. Case of a buried circuit: minimum c.s.a. in t erms of type of conductor; type of

insulation; and value of fictitious current I’ z (I’ z = I z / K)


30


4.2.3. Sizing the neutral conductor The c.s.a. and the protection of the neutral conductor, apart from its current-carrying requirement, depend on several factors, namely: the type of earthing system, TT, TN, etc. the harmonic currents, the method of protection against indirect contact hazards according to the methods described below. The color of the neutral conductor is statutorily blue. PEN conductor, when insulated, shall be marked by one of the following methods: green-and-yellow throughout their length with, in addition, light blue markings at the terminations, or light blue throughout their length with, in addition, green-and-yellow markings at the terminations. § § §

§

§

4.2.3.1. Influence of the type of earthing system TT, TN-S and IT schemes Single-phase circuits or those of c.s.a. ≤ 16 mm2 (copper) 25 mm2 (aluminium): the c.s.a. of the neutral conductor must be equal to that of the phases. Three-phase circuits of c.s.a. > 16 mm2 copper or 25 mm2 aluminium: the c.s.a. of the neutral may be chosen to be equal to that of the phase conductors, or smaller, on condition that: - the current likely to flow through the neutral in normal conditions is less than the permitted value I z . The influence of the 3rd and multiples of the 3rd harmonic must be given particular consideration or - the neutral conductor is protected against short-circuit. §

§

TN-C scheme The same conditions apply in theory as those mentioned above, but in practice, the neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor. IT scheme In general, it is not recommended to distribute the neutral conductor, i.e. a 3-phase 3-wire scheme is preferred. When a 3-phase 4-wire installation is necessary, however, the conditions described above for TT and TN-S schemes are app licable. 4.2.3.2. Influence of harmonic currents Effects of order 3 and multiple of 3 harmonics Harmonics are generated by the non-linear loads of the installation (computers, ballast lighting, rectifiers, power electronic choppers) and can produce high currents in the neutral. In particular order 3 or multiple of 3 harmonics of the three phases have a tendency to cumulate in the neutral as: fundamental currents are out-of-phase by 2π/3 so that their sum is zero on the other hand, order 3 harmonics of the three phases are always positioned in the same manner with respect to their own fundamental, and are in phase with each (see F ig. 4.16. ). § §


31


Fi g. 4.16. Order 3 harmonics are in phase and cu mulate in the neutral

F igur e 4.17. shows the load rate of the neutral conductor in funct ion of the percentage

of harmonic 3. In practice, this maximum load rate cannot exceed 3 .

Fi g. 4.17. Load rate of the neutral conductor vs the percentage of harmonic 3

Reduction factors for harmonic currents in four-core and five-core cables with four cores carrying current

The basic calculation of a cable concerns only cables with three loaded conductors i.e there is no current in the neutral conductor. Because of the third harmonic current, there is a current in the neutral. As a result, this neutral current creates an hot environment for the 3 phase conductors and for this reason, a reduction factor for phase conductors is necessary (see F ig. 4.18.). Reduction factors, applied to the current-carrying capacity of a cable with three loaded conductors, give the current-carrying capacity of a cable with four loaded conductors where the current in the fourth conductor is due to harmonics. The reduction factors also take the heating effect of the harmonic current in the phase conductors into account. Where the neutral current is expected to be higher than the phase current then the cable size should be selected on the basis of the neutral current. §


32

Designing an electrical installation (Beginner Guide) §

§

Where the cable size selection is based on a neutral current which is not significantly higher than the phase current it is necessary to reduce the tabulated current carrying capacity for three loaded conductors. If the neutral current is more than 135% of the phase current and the cable size is selected on the basis of the neutral current then the three phase conductors will not be fully loaded. The reduction in heat generated by the phase conductors offsets the heat generated by the neutral conductor to the extent that it is not necessary to apply any reduction factor to the current carrying capacity for three loaded conductors.

Fi g. 4.18. Reduction factors for harmonic currents in four-core and five-core cables

(according to IEC 60364)

Examples Consider a three-phase circuit with a design load of 37 A to be installed using four., a 6 core PVC insulated cable clipped to a wall, installation method C. From fi gure 4.14 2 mm cable with copper conductors has a current-carrying capacity of 40 A and hence is suitable if harmonics are not present in the circuit. If 20 % third harmonic is present, then a reduction factor of 0.86 is applied and thedesign load becomes: 37/0.86 = 43 A. For this load a 10 mm 2 cable is necessary. If 40 % third harmonic is present, the cable size selection is based on the neutral current which is: 37·0.4·3 = 44.4 A and a reduction factor of 0.86 is applied, leading to a design load of: 44.4/0.86 = 51.6 A. For this load a 10 mm 2 cable is suitable. If 50 % third harmonic is present, the cable size is again selected on the basis of the neutral current, which is: 37·0.5·3 = 55.5 A .In this case the rating factor is 1 2 and a 16 mm cable is required. §

§

§

4.2.3.3. Protection of the neutral conductor (see F i g. 4.19.) Protection against overload If the neutral conductor is correctly sized (including harmonics), no specific protection of the neutral conductor is required because it is protected by the phase protection. However, in practice, if the c.s.a. of the neutral conductor is lower than the phase c.s.a, a neutral overload protection must be installed. Protection against short circuit If the c.s.a. of the neutral conductor is lower than the c.s.a. of the phase conductor, the neutral conductor must be protected against short-circuit.


33

Designing an electrical installation (Beginner Guide) If the c.s.a. of the neutral conductor is equal or greater than the c.s.a. of the phase conductor, no specific protection of the neutral conductor is required because it is protected by the phase protection.

Fi g. 4.19. The various situations in which the neutral conductor may appear

4.2.3.4. Breaking of the neutral conductor (see F ig. 4.19 .)

The need to break or not the neutral conductor is related to the protection against indirect contact In TN-C scheme The neutral conductor must not be open-circuited under any circumstances since it constitutes a PE as well as a neutral conductor.


34

Designing an electrical installation (Beginner Guide) In TT, TN-S and IT schemes In the event of a fault, the circuit breaker will open all poles, including the neutral pole, i.e. the circuit breaker is omnipolar. The action can only be achieved with fuses in an indirect way, in which the operation of one or more fuses provokes a mechanical trip-out of all poles of an associated seriesconnected load-break switch. 4.2.3.5. Isolation of the neutral conductor (see F ig. 4.19.)

It is considered to be the good practice that every circuit be provided with the means for its isolation.

4.2.4. Sizing the protective earthing conductor (PE) Protective (PE) conductors provide the bonding connection between all exposed and extraneous conductive parts of an installation, to create the main equipotential bonding system. These conductors conduct fault current due to insulation failure (between a phase conductor and an exposed conductive part) to the earthed neutral of the source. P.E. conductors are connected to the main earthing terminal of the installation. PE conductors must be: insulated and coloured yellow and green (stripes), be protected against mechanical and chemical damage. § §

4.2.4.1. Connection

PE conductors must: not include any means of breaking the continuity of the circuit (such as a switch, removable links, etc.), connect exposed conductive parts individually to the main PE conductor, i.e. in parallel, not in series, have an individual terminal on common earthing bars in distribution boards.

§

§

§

TT scheme The PE conductor need not necessarily be installed in close proximity to the live conductors of the corresponding circuit, since high values of earth-fault current are not needed to operate the RCD-type of protection used in TT installations.

Fi g. 4.20. Direct connection of the PEN conductor to the earth

terminal of an appliance


35

Designing an electrical installation (Beginner Guide) IT and TN schemes The PE or PEN conductor, as previously noted, must be installed as close by as possible to the corresponding live conductors of the circuit and no ferro-magnetic material must be interposed between them. A PEN conductor must always be connected directly to the earth terminal of an appliance, with a looped connection from the earth terminal to the neutral terminal of the appliance (see F ig. 4.20.).

TN-C scheme (the neutral and PE conductor are one and the same, referred to as a PEN conductor) The protective function of a PEN conductor has priority, so that all rules governing PE conductors apply strictly to PEN conductors. TN-C to TN-S transition The PE conductor for the installlation is connected to the PEN terminal or bar (see F ig. 4.21.) generally at the origin of the installation. Downstream of the point of separation, no PE conductor can be connected to the neutral conductor. §

§

F ig. 4.21. The TN-C-S scheme

4.2.4.2. Types of materials

Materials of the kinds mentioned below in fi gure 4.22. can be used for PE conductors, provided that the conditions mentioned in the last column are satisfied.

F ig. 4.22. Choice of protective conductors (PE)

(1) In schemes TN and IT, fault clearance is generally effected by overcurrent devices (fuses or circuit breakers) so that the impedance of the fault-current loop must be sufficiently low to assure positive protective device operation. The surest means of achieving a low loop impedance is to use a supplementary core in the same cable as the circuit conductors (or taking the same route as the circuit conductors). This stratagem minimizes the inductive reactance and therefore the impedance of the loop. (2) The PEN conductor is a neutral conductor that is also used as a protective earth conductor. This means that a current may be flowing through it at any time (in the absence of an earth fault). For this reason an insulated conductor is recommended for PEN operation. (3) The manufacturer provides the necessary values of R and X components of the impedances (phase/PE, phase/PEN) to include in the calculation of the earth-fault loop impedance.


36

Designing an electrical installation (Beginner Guide) (4) Possible, but not recomended, since the impedance of the earth-fault loop cannot be known at the design stage. Measurements on the completed installation are the only practical means of assuring adequate protection for persons. (5) It must allow the connection of other PE conductors. Note : these elements must carry an indivual green/yellow striped visual indication, 15 to 100 mm long (or the letters PE at less than 15 cm from each extremity). (6) These elements must be demountable only if other means have been provided to ensure uninterrupted continuity of protection. (7) With the agreement of the appropriate water authorities. (8) In the prefabricated pre-wired trunking and similar elements, the metallic housing may be used as a PEN conductor, in parallel with the corresponding bar, or other PE conductor in the housing. (9) Forbidden in some countries only-universally allowed to be used for supplementary equipotential conductors.

4.2.4.3. Conductor sizing Adiabatic method(which corresponds with that described in IEC 60724) This method, while being economical and assuring protection of the conductor against overheating, leads to small c.s.a.’s compared to those of the corresponding circuit phase conductors. The result is sometimes incompatible with the necessity in IT and TN schemes to minimize the impedance of the circuit earth-fault loop, to ensure positive operation by instantaneous overcurrent tripping devices. This method is used in practice, therefore, for TT installations, and for dimensioning an earthing conductor. §

For any size of the phase conductor: - for a period of 5 seconds or less, the relationship I 2 ⋅ t = k 2 ⋅ S 2 characterizes the time in 2 seconds during which a conductor of c.s.a. S (in mm ) can be allowed to carry a current I amps, before its temperature reaches a level which would damage the surrounding insulation. I ⋅ t = k ⋅ S 2

2

2

⇒ S PE =

I ⋅ t

(1)

k The c.s.a. of earthing conductor between the installation earth electrode and the main earth terminal: when protected against mechanical damage: §

S PE =

I ⋅ t

k without mechanical protection, but protected against corrosion by impermeable 2 cable sheath. Minimum 16 mm for copper or galvanized steel. without either of the above protections; min. of 25 mm 2 for bare copper and 50 2 mm for bare galvanized steel. (1) When the PE conductor is separated from the circuit phase conductors, the following minimum values must be respected: - 2.5 mm2 if the PE is mechanically protected, - 4 mm2 if the PE is not mechanically protected. §

§

Simplified method This method is based on PE conductor sizes being related to those of the corresponding circuit phase conductors, assuming that the same conductor material is used in each case. §


37


≤ 16 mm 2 ⇒ S PE = S ph < S ph ≤ 35 mm 2 ⇒ S PE = 16 mm 2

S ph 16 mm 2

S ph

> 35 mm 2 ⇒ S PE =

S ph 2

Note : when, in a TT scheme, the installation earth electrode is beyond the zone of

influence of the source earthing electrode, the c.s.a. of the PE conductor can be limited to 2 2 25 mm (for copper) or 35 mm (for aluminium). The neutral cannot be used as a PEN conductor unless its c.s.a. is equal to or larger than 10 mm2 (copper) or 16 mm2 (aluminium). Moreover, a PEN conductor is not allowed in a flexible cable. Since a PEN conductor functions also as a neutral conductor, its c.s.a. cannot, in any case, be less than that necessary for the neutral. This c.s.a. cannot be less than that of the phase conductors unless: the kVA rating of single-phase loads is less than 10% of the total kVA load, and I max likely to pass through the neutral in normal circumstances, is less than the current permitted for the cable size selected. Furthermore, protection of the neutral conductor must be assured by the protect ive devices provided for phase-conductor. § §

Values of factor k to be used in the formulae These values are identical in several national standards, and the temperature rise ranges, together with factor k values and the upper temperature limits for the different classes of insulation, correspond with those published in IEC 60724 (1984). The data presented in figure 4.23. are those most commonly needed for LV installation design.

Fi g. 4.23. k factor values for LV PE conductors, commonly used in national

standards and complying with IEC 60724


38


4.2.5. Calculation of L max. for a TN-earthed system, using the conventional method The maximum length of a circuit in a TN-earthed installation is given by the formula: Lmax

=

0.8 ⋅ U 0 ⋅ S ph ρ ⋅ (1 + m) ⋅ I a

where: Lmax - maximum length in metres, U 0 - phase volts, 230 V for a 230/400 V system, ρ - resistivity at normal working temperature in ohm-mm2/metre (22.5·10-3 for -3 copper; 36·10 for aluminium), I a - trip current setting for the instantaneous operation of a circuit breaker, or I a - the current which assures operation of the protective fuse concerned, in the specified time,

Fi g. 4.24. Calculation of L max. for a TN-earthed system, using

the conventional method

m = S ph / S PE S ph - cross-sectional area of the phase conductors of the circuit concerned in mm2, S PE - cross-sectional area of the protective conductor concerned in mm2. Tables

The tables give maximum circuit lengths, beyond which the ohmic resistance of the conductors will limit the magnitude of the short-circuit current to a level below that required to trip the circuit-breaker (or to blow the fuse) protecting the circuit, with sufficient rapidity to ensure safety against indirect contact. Correction factor m F igur e 4.25. indicates the correction factor to apply to the values given in figures 4.26., according to the ratio Sph/SPE, the type of circuit, and t he conductor materials. The tables take into account:


39

Designing an electrical installation (Beginner Guide) the type of protection: circuit breakers or fuses, operating-current settings, cross-sectional area of phase conductors and protective conductors, type of system earthing, type of circuit breaker (i.e. B, C or D). Equivalent tables for protection by Compact and Multi 9 circuit breakers (Merlin Gerin) are included in the relevant catalogues. § § § § §

Fi g. 4.25. Correction factor to apply to the lengths given for

TN systems (may be used for 230/400 V systems)

F ig. 4.26. Maximum circuit lengths (in metres) for different sizes of copper conductor and

instantaneous-tripping-current settings for general-purpose circuit breakers in 230/240 V TN system with m = 1

4.2.6. Rules for marine electrical cables according Bureau Veritas General 1. All electrical cables and wiring external to equipment shall be at least of a flameretardant type, in accordance with IEC Publication 60332-1. 2. When cables are laid in bunches, cable types are to be chosen in appliance with IEC Publication 60332-3 Category A, or over means are to be provided such as not to impair their original flame-retarding properties. 3. Where necessary for specific applications such as radio frequency or digital communications systems, which require the use of particular types of cables, the Society may permit the use of cables with do not comply with the provisions of 1 and 2.


40

Designing an electrical installation (Beginner Guide) 4. Cables which are required to have fire-resisting characteristics are to comply with the requirements stipulated in IEC Publications 60331. Choice of insulation 1. The maximum rated operating temperature of the insulating material is to be at least 100C higher than the maximum ambient temperature liable to occur or to be produced in the space where the cable is installed. 2. The maximum rated conductor temperature for normal and short-circuit operation, for the type of insulating compounds normally used for shipboard cables, is not to exceed the values stated in Tab 1 . Special consideration will be given to other insulating materials. 3. PVC insulated cables are not to be used either in refrigerated spaces, or on decks exposed to the weather of ships classed for unrestricted service.

Maximum rated conductor temperature Tabl e 1: Type of insulating compound

a)Thermoplastic: - based upon polyvinyl chloride or copolymer of vinyl chloride and vinyl acetate b) Elastomeric or thermosetting: - based upon ethylene-propylene rubber or similar (EPM or EPDM) - based upon high modulus or hardgrade ethylene propylene rubber - based upon cross-linked polyethylene - based upon rubber silicon - based upon ethylene-propylene rubber or similar (EPM or EPDM) halogen free - based upon high modulus or hardgrade halogen free ethylene propylene rubber - based upon cross-linked polyethylene halogen free - based upon rubber silicon halogen free - based upon cross-linked polyolefin material for halogen free cable (1)

Abbreviated designation

Maximum rated conductor 0 temperature ( C) Normal Short-circuit operation

PVC/A

60

180

EPR

85

250

HEPR

85

250

XLPE S 95 HF EPR

85 95 85

250 250 250

HF HEPR

85

250

HF XLPE HF S 95 HF 85

85 95 85

250 350 250

(1) Used on sheathed cable only

Choice of protective covering 1. The conductor insulating materials are to be enclosed in an impervious sheath of material appropriate to the expected ambient conditions where cables are installed in the following locations: - on decks exposed to the weather, - in damp or wet spaces (e.g. in bathrooms), - in refrigerated spaces, - in machinery spaces and, in general, - where condensation water or harmful vapour may be present.


41

Designing an electrical installation (Beginner Guide) 2. Where cables are provided with armour or metallic braid (e.g. for cables installed in hazardous areas), an overall impervious sheath means to protect the metallic elements against corrosion is to be provided. 3. An impervious sheath is not required for single-core cables installed in tubes or ducts inside accommodation spaces, in circuits w ith maximum system voltage 250 V. 4. In choosing different types of protective coverings, due considerations is to be given to the mechanical action to which cable may be subjected during installation and in service. If the mechanical strength of the protective covering is considered insufficient, the cables are to be mechanically protected (e.g. by an armour or by installation inside pipes or conduits). 5. Single-core cables for a.c. circuits with rated current exceeding 20 A are to be either non-armoured or armoured with non-magnetic material. Cables in refrigerated spaces 1. Cables installed in refrigerated spaces are to have a watertight or impervious sheat and are to be protected against mechanical damage. If an armour is applied on the sheath, the armour is to be protected against corrosion by a further moisture-resisting covering. Cables in circuits fore fire alarm, fire detection and fire-extinguishing 1. In general, in circuits intended for fire alarm and detection, emergency fireextinguishing service, fire telecommunication (e.g. communication between the navigating bridge and the main fire control station), remote stopping and similar control circuits for safety purposes, cables are to be of a fire-resistant type unless: - the systems are of self-monitoring type or failing to safety, - the systems are duplicated. 2. Cables for services that are required to maintain operation of equipment during a fire (e.g. cables for the general emergency alarm, the public address system when it is the only system to provide the general emergency alarm, the fireextinguishing medium alarm and their power supplies) are to be of a fire-resistant type. 3. Cables connecting fire pumps to the emergency switchboard shall be of fireresistant type where they pass through fire risk areas. Cables fore submerged bilge pumps 1. Cables and their connections to such pumps are to be capable of operating under a head of water equal to their distance below the bulkhead deck. The cable is to be impervious-sheathed and armoured, is to be installed in continuous lengths from above the bulkhead to the motor terminals and is to enter the air bell from the bottom. Internal wiring of switchboard and other enclosures for equipment 1. For installations in switchboards and other enclosures for equipment, single-core cables may be used without further protection (sheat h). Other types of flame-retardant switchboards wiring may be accepted.


42


Tabl e 2: Current carrying capacity, in A,

Table : Current carrying capacity, in A,

in continuous service for cables based on maximum conductor operating temperature of 600C (ambient temperature 450C)

in continuous service for cables based on maximum conductor operating temperature of 0 0 75 C (ambient temperature 45 C)

Nominal section 2 mm 1 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240 300

Number of conductors 1 2 3 or 4 8 7 6 12 10 8 17 14 12 22 19 15 29 25 20 40 34 28 54 46 38 71 60 50 87 74 61 105 89 74 135 115 95 165 140 116 190 162 133 220 187 154 250 213 175 290 247 203 335 285 235





in continuous service for cables based on maximum conductor operating temperature of 800C (ambient temperature 450C)

in continuous service for cables based on maximum conductor operating temperature of 0 0 85 C (ambient temperature 45 C)





Current carrying capacity of cables 1. The current carrying capacity for continuous service of cables given in Tab 2 to Tab 6 is based on the maximum permissible service temperature of the conductor also 0 indicated therein and on an ambient temperature of 45 C.


43

Designing an electrical installation (Beginner Guide) 2. The current carrying capacity is applicable, with rough approximation, to all types of protective covering (e.g. both armoured and non-armoured cables). 3. Values other then those shown in Tab 2 to Tab 6 may be accepted provided they are determined on the basis of calculation methods or experimental values approved by the Society. 0 4. When the actual ambient temperature obviously differs from 45 C, the correction factors shown in Tab 7 may be applied to the current carrying capacity in Tab 2 to Tab 6 . 5. Where more than six cables are bunched together in such a wa y that is an absence of free air circulating around them, and the cables can be expected to be under full load simultaneously, a correction factor of 0.85 is to be applied.


in continuous service for cables based on maximum conductor operating temperature of 0 0 95 C (ambient temperature 45 C) Nominal section 2 mm 1 1.5 2.5 4 6 10 16 25 35 50 70 95 120 150 185 240 300


7. For supply cables to single services for intermittent loads (e.g. cargo winches or machinery space cranes), the current carrying capacity obtained from Tab 2 to Tab 6 may be increased by applying the correction factors given in Tab 9 . The correction factors are calculated with rough approximation for periods of 10 minutes, of witch 4 minutes with a constant load and 6 minutes without load. Minimum nominal cross-sectional area of conductors 1. In general the minimum allowable conductor cross-sectional areas are those given in tables above. 2. The nominal cross-sectional area of the neutral conductor in three-phase distribution systems is to be equal to at least 50% of the cross-sectional areas of the phases, unless the latter is less than or equal to 16 mm2. In such case the cross-sectional of the neutral conductor is to be equal to that of the phase.


44

Designing an electrical installation (Beginner Guide) Table7: Correction factors for various ambient air temperatures

Maximum conductor 0 temperature C 60 75 80 85 95

35 C

0

40 C

0

1.29 1.15 1.13 1.12 1.10

1.15 1.08 1.07 1.06 1.05

Correction factors for ambient air temperature of: 0 0 0 0 0 0 0 0 45 C 50 C 55 C 60 C 65 C 70 C 75 C 80 C

1.00 1.00 1.00 1.00 1.00

0.82 0.91 0.93 0.94 0.95

0.82 0.85 0.87 0.89

0.71 0.76 0.79 0.84

0.58 0.65 0.71 0.77

0.53 0.61 0.71

0.50 0.63

0.55

0

85 C

0.45

Choice of cables 1. Rated voltage of any cable is to be not lower than the nominal voltage of the circuit which it is used. 2. The nominal cross-sectional area of each cable is to be sufficient to satisfy the following conditions with reference to the maximum anticipated ambient temperature: the current carrying capacity is to be not less than the highest continuous load carried by the cable, the voltage drop in the circuit, by full load on this circuit, is not to exceed the specified limits, the cross-sectional area calculated on the basis of the above is to be such that the temperature increases which may be caused by overcurrents or starting transients do not damage the insulation. §

§

§

Tabl e 8: Corrections factors for short-time loads

½ - hour service Sum of nominal cross-sectional areas of all 2 conductors in mm

Cable with metallic sheath and armoured cables

Cable with non-metallic sheath and nonarmoured cables

up to 20 21 - 41 41 - 65 66 – 95 96-135 136-180 181-235 236-285 286-350

up to 75 76-125 126-180 181-250 251-320 321-400 401-500 501-600 -

1 – hour service Sum of nominal cross-sectional areas Correction factor of all conductors in 2 mm Cable Cable with with metallic nonsheath metallic and sheath armoured and noncables armoured cables up to 80 up to 230 1.06 81-170 231-400 1.10 171-250 401-600 1.15 251-430 601-800 1.20 431-600 1.25 6001-800 1.30 1.35 1.40 1.45

3. The highest continuous load carried by a cable is to be calculated on the basis of the power requirements and of diversity factor of the loads and machines supplied through that cable.


45

Designing an electrical installation (Beginner Guide) 4. When conductors are carrying the maximum nominal service current, the voltage drop from the main or emergency switchboard busbars to any point in the installation is not to exceed 6% of the nominal voltage. For battery circuits with supply voltage less than 55 V, this value may be increased to 10%. For circuits of navigation lights, the voltage drop is not to exceed 5% of the rated voltage under normal conditions. Tabl e 9: Correction factors for intermittent service

Sum of nominal cross-sectional areas of all 2 conductors in mm Cable with metallic Cable with metallic sheath and armoured sheath and non-armoured cables cables S≤5 5
Correction factor

1.10 1.15 1.20 1.25 1.30 1.35 1.40 1.45 1.50

4.3. Determination of voltage drop The impedance of circuit conductors is low but not negligible. When carrying load current there is a fall in voltage between the origin of the c ircuit and the load terminals. The correct operation of an item of load (a motor; lighting circuit; etc.) depends on the voltage at its terminals being maintained at a value close to its rated value. It is necessary therefore to dimension the circuit conductors such, that at full load current, the load terminal voltage is maintained within the limits required for correct performance. This section deals with methods of determining voltage drops, in order t o check that: they conform to the particular standards and regulat ions in force; they can be tolerated by the load; they satisfy the essential operational requirements. § § §

4.3.1. Maximum voltage drop limit Maximum allowable voltage-drop limits vary from one country to another. Typical values for low-voltage installations are given below in fi gure 4.27.

Fi g. 4.27. Maximum voltage-drop between the service-connection

point and the point of utilization


46

Designing an electrical installation (Beginner Guide) These voltage-drop limits refer to normal steady-state operating conditions and do not apply at times of motor starting; simultaneous switching (by chance) of several loads, et c. When voltage drops exceed the values shown in fi gure 4.27. larger cables (wires) must be used to correct the condition.

F ig. 4.28. Maximum voltage drop

The value of 8%, while permitted, can lead to problems for motor loads; for example: in general, satisfactory motor performance requires a voltage within ± 5% of its rated nominal value in steady-state operation, starting current of a motor can be 5 to 7 times its full-load value (or even higher). If 8% voltage drop occurs at full-load current, then a drop of 40% or more will occur during start-up. In such conditions the motor will either: - still (i.e. remain stationary due to insufficient torque to overcome the load torque) with consequent over-heating and eventual trip-out, - or accelerate very slowly, so that the heavy current loading (with possibly undesirable low-voltage effects on other equipment) will continue beyond the normal start-up period, - finally an 8% voltage drop represents a continuous ( E 2 /R watts) of power loss, which, for continuous loads will be a significant waste of (metered) energy. For these reasons it is recommended that the maximum value of 8% in steady operating conditions should not be reached on circuits which are sensitive to undervoltage problems (see F ig. 4.28.). §

§

4.3.2. Calculation of voltage drop in steady load conditions Use of formulae F igur e 4.29. below gives formulae commonly used to calculate voltage drop in a given circuit per kilometer of length. If: I B - the full load current in amps; L - length of the cable in kilometers; § §


47


R - resistance of the cable conductor conductor in Ω/km; 2 22.5 Ω ⋅ mm / km for copper R = 2 S (c s . .a. in mm ) R

§ §

§ §

=

36

Ω ⋅ mm 2 / km

. .a. in mm 2 ) S (c s

for aluminium

X X - inductive reactance of a conductor in Ω/km; ϕ - phase angle angle between voltage and current in the circuit considered, generally: - lighting: cos ϕ = 1 - motor power: - at start-up: start- up: cos ϕ = 0.35 - in normal service: cos ϕ = 0.8 U n - phase-to-phase voltage; V n - phase-to-neutral voltage.

Fi g. 4.29. 4.29. Voltage-drop formulae

Fi g. 4.30. 4.30. Phase-to-phase voltage drop Δ drop ΔU U for a circuit, in volts per ampere per km


48

Designing an electrical installation (Beginner Guide) 2

X is negligible for conductors of c.s.a. Note: R is negligible above a c.s.a. of 500 mm ; X is 2

less than 50 mm . In the absence of any other information, take X as X as being equal to 0.08 Ω/km. For prefabricated pre-wired ducts and bustrunking, resistance and inductive reactance values are given by the manufacturer. Simplified table Calculations may be avoided by using fi gure 4.30. , which gives, with an adequate 4.30. , approximation, the phase-to-phase voltage drop per km of cable per ampere, in terms of: kinds of circuit use: motor circuits with cosϕ close to 0.8, or lighting with a cos ϕ in the neighborhood of unity, type of cable; single-phase or 3-phase. Voltage drop in a cable is then t hen given by: K ·I B·L K K - is given by the table, I B - is the full-load current in amps, L - is the length of cable in km. The column motor power “cos ϕ = 0.35” of Figure 4 may be used to compute the voltage drop occurring during the start-up period of a motor (see example no.1). §

§

4.3.3. Examples Example 1 A three-phase 35 mm2 copper cable 50 meters long supplies supplies a 400 V motor taking: 100 A at a cos ϕ = 0.8 on normal permanent load, 500 A (5·I ( 5·I n) at a cos ϕ = 0.35 during start-up. The voltage drop at the origin of the motor cable in normal circumstances (i.e. with the distribution board of Figure 5 distributing a total of 1000 A) is 10 V phase-to-phase. What is the volt drop at the t he motor terminals: In normal service? During start-up? § §

§ §

F ig. 4.31. 4.31. Example 1


49

Designing an electrical installation (Beginner Guide) Solution: Voltage drop in normal service conditions: §

∆U % = 100 ∆U U n

F igur e 4.30. 4.30. shows 1V/A/km so that:

∆U = 1·100·0.05 = 5V for the cable 15 ∆U total = 10 + 5 = 15V = ⋅ 100 = 3.75% 400 This value is less than that aut horized (8%) and is satisfactory. §

Voltage drop during motor start-up: ΔU cable = 0.52·500·0.05 = 13 V

Owing to the additional current taken by the motor when starting, the volt drop at the distribution board will exceed 10 Volts. Supposing that the infeed to the distribution board during motor starting is 900 + 500 = 1400 A then the volt-drop at the distribution board will increase approximately pro rata, i.e. 10 ⋅ 1,400

= 14V 1,000 ∆U = 14 V for the distribution board ∆U = 13 V for the motor cable 27 ∆U total = 13 + 14 = 27 V = ⋅ 100 = 6.75% 400 a value which is satisfactory satisfact ory during motor starting. Example 2 A 3-phase 3-phase 4-wire copper line line of 70 mm2 c.s.a. and a length of 50 m passes a current of 150 A. The line supplies, among other loads, 3 single-phase lighting circuits, each of 2.5 mm2 c.s.a. copper 20 m long, and each passing 20 A. It is assumed that the currents in the 70 mm2 line are balanced and that the three lighting circuits are all connected to it at the same point. What is the voltage drop at the end of the t he lighting circuits? Solution: Voltage drop in the 4-wire line: §

∆U % = 100

∆U U n

F igur e 4.30. 4.30. shows 0.55 V/A/km

ΔU line = 0.55·150·0.05 = 4.125 V phase-to-phase

which: 4 ⋅ 125 3

= 2.38 V

phase to neutral. Voltage drop in any one of the lighting single-phase circuits: ΔU for a single-phase circuit = 18·20·0.02 = 7.2 V The total volt-drop is therefore: §


50


7.2 + 2.38 = 9.6 V =

9.6V 230V

⋅ 100 = 4.2%

This value is satisfactory, being less than the maximum permitted voltage dro p of 6%.

Fi g. 4.32. Example 2

4.4. Determination of the short circuit currents 4.4.1. Calculation of maximum short-circuit currents in electrical ship mains according GERMANISCHER LLOYD “SCC363.EXE” The program is capable of calculating short-circuit currents according to IEC Publication 61363-1 in electrical installations of ships and offshore units. The electrical mains must have a standard, unmeshed configuration with one main distribution board. All generators, electric consumers and distribution boards are connected to the said main distribution. The calculation program may be used on electr ical system as specified below: low or medium voltage level, three-phase alternating current, 50Hz or 60Hz, different voltage levels downstream of the main distribution, up to 20 main generators, connected directly to t he main distribution, up to 20 single motors, connected to the main distribution with or without transformer, 1 equivalent motor (substituting the small motors), § § § § § §

§


51


§

up to 50 distribution boards, connected to the main distribution with or without transformer, up to 5 subdistribution boards, connected to each distribution board, with or without transformer (i.e. 250 is the max. number of subdistribution boards).

Important remarks: §

§

§

Please note that the results of this program have to be checked by another, different computer program, a simplified calculation or another suited method, in order to avoid failures in the selection of electrical switch gear. It is important to observe that only the maximum values of the short-circuit currents will be calculated by this program. The attenuation of short-circuit currents by electric arcs or other physical phenomena are not part of this calculation. The calculation of the peak value of the short-circuit current depends, among other factors, on the time constants, which are part of the input data. If the time constants of generators especially for the dc-component and the subtransient component are small, a low peak value of the short-circuit current will be calculated. For a rough estimation of short-circuits at the main distribution, please observe that the following relation between I k ” and I p is realistic for most applications: I p

=

2 ⋅ k ⋅ I k "

k = 1.5 … 1.8 I p - peak value of the short-circuit current; I k " - subtransient short-circuits current. 4.4.1.1. Principles of the calculation

In case of short-circuit the active components of a ship mains, which are the generators and the motors, will feed electric currents into the faulty point. Having the first and highest maximum some milliseconds (= T/2) after the beginning of the short-circuit condition the current will decrease until reaching a constant level after some hundred milliseconds. (T is the length of a period of the mains frequency.) The short-circuit current of a typical synchronous generator, for marine applications, is to be divided into the following different components: dc current, subtransient current, transient current and steady state current. The short-circuit current of an asynchronous motor is comprised of an ac component and a dc component decaying typically faster that the current of a generator. Asynchronous motors do not supply a steady short circuit current. The decreasing of the short circuit-current of both generators and motors may be expressed mathematically by means of exponential functions as carried out by this program, to calculate the upper envelope of the current marked by the maximal values. Non-active components of a ship mains are cables, transformers and motors, witch attenuate the short currents and change the time constants of motors and generators. The short-circuit calculations of the program described by this paper are performed in the following steps: 1. First, the program checks if the input data are complete and reasonable. If data are missing and there are not internal values the calculation will be disabled and F (failure) is placed in the main menu and also on the dat a input pages. If data are not reasonable (e.g. Romanian Electro Trade, Engineering & Consulting

52

Designing an electrical installation (Beginner Guide) voltage and frequency have been obviously interchanged e.g. 60V /440Hz) a warning will be written on the monitor. Despite of a warning the calculation may be started giving twice the calculation command from main menu level. A warning will be displayed in following cases: voltage at main bus U < 360 V or U > 460 V mains frequency f < 45 Hz or f > 65 Hz resistance of a generator r a > 30 % of the subtransient react ance xd ” subtransient time constant of a generator T d ” < 10 ms internal resistance Rtraf of a transformer is larger than the internal impedance Z traf . 2. After performing the check routine the program starts the calculation of the time dependent ac end dc current of generators, single motors and the equivalent motor. The ac and dc current are calculated for t = 0 and t = T/2. 3. Next, the equivalent generators are calculated at the points of common connection, i.e. at the main distribution as described in the following: 3.1. Calculation of the equivalent generator, comprising all active components, generators, single motors and equivalent motor. 3.2. Calculation of a number of equivalent generators in a program sequence (loop). These generators comprise all active components but neglecting one single motor in each sequence, beginning with motor no.1 and ending with motor no.20, if provided. The number, of this equivalent generators, to be calculated equals the number of single motors. 4. Finally the short circuit currents at the various fault points of the ship mains will be calculated as follows: 4.1. Calculation of the fault currents at main bus, generator breakers and breakers of single motors is based mainly on the results described by the above mentioned item 2 = currents of active components. The power factor in case of a short circuit at main bus is calculated using the data of the equivalent generator, which is comprised of all active components, as described by 3.1., see above. 4.2. Calculation of the time dependent short circuit currents at distributions and subdistributions using as active component the equivalent generator described by 3.1., see above. 4.3. Calculation of the time dependent short circuit currents at the terminals of the single motors using as active component the equivalent generators described by 3.2., see above. 5. After performing the calculation data are written into an ASCII-file and afterwards displayed on the monitor. The said file contains the following data: 5.1. All input data 5.2. The root mean square value of the ac component at the beginning of the short circuit I k ” at t = 0 or t = T/2. 5.3. The peak value Ip of the current calculated at t ime t = T/2. 5.4. The power factor The calculation carried out by this program is based on the following assumptions in compliance with the IEC Paper: The short circuit occurs at the same time between all phases. The calculation is carried out, neglecting the time dependent characteristic of the voltage regulators, which are part of the generators. These regulators start to increase the interval voltage and hence the currents of the generator about 100 ms after the drop of the voltage caused by a short circuit. The calculation is performed, assuming that t he continuous short circuit current is three times higher § § § § §

§ §


53


§

§ §

§

than the nominal current of a generator if there are no input data ( I kd /I n = 0). I kd is the steady state current and I n is the nominal current. Low ohmic short circuit is applied to all phases. Therefore, the impedance of the faulty point (i.e. the connection from phase to phase) is neglected by the calculation. System capacitances are neglected. Generators running in parallel have the same power factor (i.e. equal proportion between active and reactive load). Harmonic distortions of the currents are neglected.

4.4.1.2. Structure of the Ship Mains to be calculated

The program is capable of calculating short-circuits in a ship mains constructed as follows: Equivalent motor - sum of small motors; G - generator; FP - fault point; M - motor; TR - transformer;

Notes on the system structure: the installation of transformers is optional in any case; a max number of 5 subdistributions may be connected to each d istribution board. Description of the fault points: FP.1. short circuit at main distribution (worst case), FP.2. short circuit at the generator breaker (generator side), FP.3. short circuit at distribution board, § §

§ § §


54

Designing an electrical installation (Beginner Guide) § § §

FP.4. short circuit at subdistribution board, FP.5. short circuit at breaker of single motor (motor side), FP.6. short circuit at terminals of single motor.

4.4.1.3. Asymmetric short circuit 1. Unearthed system

A large number of electrical systems installed in vessels is operated with the neutral point insulated from the ship’s hull. In such system, the most critical condition concerning short-circuit is an insulation fault between all phases as calculated by this program. 2. Earthed systems

If the neutral point is connected directly to the ship’s hull, higher currents than calculated by this program may be produced if one or two phases are in contact with the ship’s hull. Asymmetric failures are not calculated by this program. If a separate detailed calculation will not be carried out it is recommended to multiply the symmetric fault currents of the generators and synchronous motors by a factor of 1.5. In case of an asymmetric short circuit asynchronous motors supply currents only if two phases are connected to the ship’s hull because the neutral points of these motors are operated unearthed in most applications. The asymmetric currents of motors may be derived from the symmetric currents using a factor of 0.866 (=square root of three divided by two). 4.4.1.4. Remarks on input data and components 1. Generators

Definitions: x d " [%] - subtransient reactance of a synchronous machine in the d-axis, x d ' [%] - transient reactance of a synchronous machine in the d-axis, x d [%] - reactance in the d-axis, r a [%] - stator resistance, t d " [ms] - subtransient time constant, t d ' [ms] - transient time constant, t dc [ms] - dc time constant. To convert percent values (e.g. r a of a generator) into figures with dimension and vice versa the following formula is to be used for: R a = r a ·U g 2 / (100· S g ), S g - nominal power of the generator, U g - resistance voltage of the generator, R a - resistance of the stator winding stated in ohm, r a - resistance of the stator winding stated in %. For the conversion of reactance the same formula is to be used in principle. Romanian Electro Trade, Engineering & Consulting

55

Designing an electrical installation (Beginner Guide) If the input value for the resistance of the stator winding is zero the program will calculate with the following data basing on experience: r a = 0.3 · x d " r a = 0.25 · x d "

S g <= 150 KVA 150 KVA < S g < 1000KVA

r a = 0.15 · x d "

S g >= 1000KVA

2. Asynchronous motors Single motors All motors rated above 100 KW or exceeding the generator power by 15 % are large motors and should be calculated individually acc. to the revised IEC Publication 363. Motors witch do not match above categories will be classified as small motors. Definitions: r s [%] - stator resistance, §

r r [%] - rotor resistance, x s [%] - stator reactance, x r [%] - rotor reactance. If the motor data except for the shaft power and the voltage are not available (i.e. input data = 0) the program will calculate the short circuit current by means of the following parameters taken from IEC Publication except for the efficiency and the power factor, witch are based on experience. Internal data: a) Impedance, reactance and resistances of large motors z m" = 16 % x m" = 15 % ( x m

= x r + x s )

r s = 3.4 % r r = 2.1 % b) Impedance, reactance and resistances of small motors z m" = 20 % x m" = 18.8 % ( x m

= x r + x s )

r s = 4.3 % r r = 2.7 % c) Time constants at 60 Hz for large and small motors T m" = 18.67 ms T dc = 11.73 ms d) Time constants at 50 Hz for large and small motors T m" = 22.4 ms T dc = 14.076 ms Romanian Electro Trade, Engineering & Consulting

56

Designing an electrical installation (Beginner Guide) e) Power factor and efficiency power factor = 0.8 efficiency = 0.9 Equivalent motor The equivalent motor will be calculated as a small motor despite of the KW power. For the definition of small and large motors reference is made to the IEC Publication 61363-1. The fault currents of the equivalent motor will be evaluat ed as follows: I ac = 5 ⋅ In - ac component of the short circuit current at t = 0 §

= 8 ⋅ I n - peak value of the short circuit current I dc = 2 ⋅ I ac - dc component of the short circuit current at t = 0 I p

I n - is the theoretical nominal current of the equivalent motor calculated by means of main bus voltage and a power factor and an efficiency as mentioned above. Transformers The complex internal impedance of a transformer is calculated by the program, using the following input data: a) short-circuit voltage ( z - component of the internal impedance) b) copper losses (r - component of the internal impedance) In short circuit condition a transformer is normally operated with reduced voltage, therefore losses of the iron core are not relevant. Calculation of the internal resistance and the impedance: §

Z traf

2 = u k ⋅ U traf /(100 ⋅ S traf )

Z traf - impedance of transformer, u k - short circuit voltage [%], U traf - nominal voltage of transformer on that side witch is opposite to the short circuit, S traf - nominal power of transformer, Rtraf

2 2 = P cu ⋅ U traf / S traf

Rtraf - internal resistance of a transformer, P cu - copper losses of a transformer. The reactance X is derived from Z and R by means of the following formula: X traf

=

( Z traf − Rtraf ) 2

2

If short circuit voltage and copper losses are not available, the following table may be used to obtain the missing data: a) 220 V transformers power [KVA] short-circuit voltage [%] copper losses [KW]

50 2.0 1.0

100 3.0 1.5


250 3.0 2.5

500 3.5 4.0

1000 5 7

57

Designing an electrical installation (Beginner Guide) b) 3 kVA transformer power [KVA] short-circuit voltage [%] copper losses [KW]

500 4.0 5.0

1000 6.0 9.0

2500 6.0 16.0

5000 6.0 25.0

The data of tables 1 and 2 above are based o n experience. Cables R and X values (resistances and reactances) of cables are calculated by means of a program routine based on the data specified by the table below. If this internal data shall be used, the input of the “specific r - and x - values”, which can be found on several pages, must be zero. §

CrossSection [mm x mm] 3x1.5 3x2.5 3x4 3x6 3x10 3x16 3x25 3x35 3x50 3x70 3x95 3x120

Resistance R [mΩ/m] 13.1 7.86 4.91 3.28 1.965 1.23 0.786 0.560 0.393 0.280 0.206 0.164

Resistance X at 50 Hz [mΩ/m] 0.126 0.117 0.107 0.100 0.098 0.091 0.082 0.082 0.075 0.075 0.075 0.072

Resistance X at 60 Hz [mΩ/m] 0.152 0.140 0.128 0.120 0.113 0.109 0.098 0.098 0.090 0.090 0.090 0.086

Table: Resistances and reactances related to length of marine cable made of copper

for nominal voltages up to 1000 V. The data are based on experience. If the “specific r -and x -values” are unequal to zero, the program will ignore the internal data and calculate resistances and impedances of cables by means of the specific data. Under “number of conductors” enter the number of parallel conductors per phase. Parameters After entering the submenu “parameters” the following may be specified: 1. Modification of time constants - the time constants which are derived from external or internal data will remain unchanged during the calculation. 2. Modification of time constants - the time constants which are derived from external or internal data will be modified by the resistances and the reactances of passive components which are cables and transformers. The equation used for the modification of time constants are extracted unchanged from the IEC Publication 61363-1. 3. The setting “modification of time constants ” should be standard due to fact that the dc time constants of generators will not be derived from the input data but calculated by means of reactances and resistances. If the mentioned setting is true a possible further element of uncertainty (i.e. the dc time constant) may be eliminated from the calculation. The short-circuit program is using the following equation from the calculation of the modified dc time constant of the generators acc. To the IEC Publication 61363-1: §


58

Designing an electrical installation (Beginner Guide) T mdc

= ( X d " + X cab ) / (2 ⋅ π ⋅ f ⋅ ( Ra + Racb ))

T mdc - dc time constant of a generator , modified; X d " - sub transient reactance of a generator; X cab - reactance of the cable between a generator and the main distribution ; f – frequency; R a - resistance of the stator winding of a generator; Rcab - resistance of the cable between the generator and the main distribution. The equation for the modification of the subtransient time constant as stated by the IEC Publication will change this constant, even if no cable is provided between generator and main distribution. 4. Include preload conditions of motors < false> - for the calculation of fault currents of motors the preload conditions (i.e. motor was running with nominal load prior to the short circuit) will be neglected. The internal voltage of the motor will be the nominal voltage at the terminals of this machine. 5. Include preload conditions of motors - the preload conditions of a motor will be taken into account. The internal voltage of this motor is now the voltage at the terminals minus the voltage drop caused by the current and by the internal impedance. The fault currents of a motor become smaller if preload conditions are not neglected. Enable back up file all input data are written into a file named “bak” on the following occasions: before leaving the short circuit calculation before deleting an input page before starting a print out before starting a calculation To recover data the bak-file has to be renamed using an appropriate command from system level. The modified file name must have the extension “.dat”. The status of the above mentioned parameters (true/false) is written into a file named “default” when leaving the submenu “parameters”. After each start on the short calculation these parameters will be set acc. to the content of the default-file. A new calculation will start with the setting of the previous one. § § § §

Detailed Results Instead of showing the complete results after performing a calculation the program may display detailed results enabling the operator to check the influence of parameters or to verify the results of a calculation. To enable a more detailed output enter the submenu “parameters” and specify under menu item “display detailed calculation” that part of the calculation which shall be displayed in detail. To produce hard-copies of the detailed results use the key combination + . If a detailed calculation is selected the complete results will not be displayed. 4.4.1.5. Simplified Calculation

For a simplified calculation of short-circuit currents at the main distribution supplied by generators and motors the following equations may be used. " I kg = I gn ·100 / x d " [%] Romanian Electro Trade, Engineering & Consulting

59

Designing an electrical installation (Beginner Guide) " I km = 6 · I mn " " " I ktot = I kgtot + I kmtot " I ptot = 2.3 · I ktot

I gn - nominal current of a single generator; " I kg - subtransient short-circuit current of a single generator; " I kgtot - subtransient short-circuit current of all generators which may operate in

I mn

parallel; - nominal current of a single motors;

" - subtransient short-circuit current of a single motor; I km " I kmtot - subtransient short-circuit current of all motors, which may be operated

" ktot

I

simultaneously; - subtransient short-circuit current of all generators and motors;

I ptot - peak value of the short-circuit current of all motors and generators. 4.4.1.6. Selection of switch gear

Beside the calculation of electrodynamics forces between busbars and heat dissipation in electric components a short-circuit evaluation is required for the selection of the protective devices which are fuses and circuit breakers. For the selection of breakers the following ratings must be available: rated short-circuit making capacity; rated short-circuit breaking capacity; rated operational voltage; power factor. The rated short-circuit breaking capacity of a circuit breaker must not be less than the calculated symmetric ac-component. Concerning the breaking operation delay times of the switch gear may be taken into account. The rated short-circuit making capacity of a circuit-breaker must not be lees than the calculated peak value of the short-circuit current. The rated operational voltage of the breaker must not be lees than the voltage at the point of installation of the breaker. If the above mentioned conditioned are not fulfilled the circuit-breaker manufacturer may be consulted as to state whether the may be modified. § § § §

4.4.1.7. The documentation

The documentation of a short-circuit calculation submitted for approval should comprise: 1. The principle lay-out of the electrical mains which has been calculated showing all: - active components, e.g. generators and motors; - cables; - transformers; - distributions and sub distributions; - fault points with identifications markings. The electrical mains may be displayed in the form of a single line diagram.


60

Designing an electrical installation (Beginner Guide) 2. Table of all input data required for the calculation, e.g. list of cables with cross section, lengths and number of parallel conductors per phase, data of motors, generators, transformers, etc. It should be clearly if data are based on assumption. 3. Results of the short-circuit calculation for the worst case condition, comprising the short-circuit current (peak current, symmetrical, ac- currents and power factors) of the active components, and the short-circuit currents at all relevant fault points, e.g.: - main bus, - breaker of generators, - breaker of single motors, - distributions, - subdistribution. 4. For practical reasons the document containing the calculated results should be grouped with regard to the fault points and should comprise a list stating the make and type of all installed breakers with their making capacity, breaking capacity and power factors, see example below showing the procedure for a sub distribution and two breakers. Place of Installation: subdistribution EXAMPLE Fault currents and power factors at the place of installation, calculated values I [kA] I [kA] p.f. p

25

ac

16

0.45

Data of breakers for 440 V operation makers data Type

Make

Break

p.f.

XYZ 63A UVW 125A

35 40

18 20

0.25 0.3

4.4.2. Short circuit current calculation according BUREAU VERITAS 4.4.2.1. Main methods

There are only 2 methods of calculation: Equivalent impedance method : by this method, the impedances on network are reduced to an equivalent impedance by considering the connection of impedances (series, parallel) in relation to the point of calculation. Contribution method : by this method, each equipment is considered separately in relation to the point of calculation and the total short circuit current on the point is the summation of the short circuit currents of elements connected to the point of calculation. Any other method is a variation, a combination or a particular case of the two methods. §

§

Real units method By this method, all operators are measured directly in ohm, V, A, VA. The main problem of method is that the impedances must be corrected to the voltage level of the point of calculation if TXs are present in the network; if there are many voltage levels (many TXs) in the network, the method may be quite elaborated.


61

Designing an electrical installation (Beginner Guide) The correction formula for Z, R and X, from one voltage level to ot her voltage level 2

is:

Z 2

 U  = Z 1 ×  2    U 1 

where: Z 1 is the value calculated for voltage level U 1

= U nom

of equipment,

Z 2 is the corrected value from U 1 to U 2 = voltage level of the point of calculation. Alternatively, the values can be calculated directly to the voltage level of the point of calculation; by this way, each Z , R and X will have one value for each point of calculation (for each voltage level). Proportional units method (pu) By this method, each equipment’s impedance is referred to an apparent base power Sb and to a base voltage Ub; by this way, the corrections of values from one voltage level to other voltage level are eliminated. The three (3) formulae of the method are: S Z [ pu ] = Z [ohm] × b2 [VA,V ] U S b I b = [VA, V , A] × 3 U b

I k

=

I b Z b

[ A, pu , A]

where: U [V] - is the nominal voltage of equipment, regardless of voltage level of the intended point of calculation; U b [V] - is the voltage level of the point of calculation; S b [VA] - is the base power and has an arbitrary value (e.g. 1kVA, 10MVA, 100MVA, 0.444VA etc), provided that is has the same value for calculation of all I b , Z , R and X in the network, I b [A·pu] - is the base current at the point of the calculation, Z b [pu] - is equivalent or calculation Z at the point of calculation, I k [A] - is short circuit current at the point of calculation. By using the above formulae, the calculation formulae of Z , R and X are changed in comparison to real units method. 4.4.2.2. Theoretical considerations

The following steps are to be performed for calculation of short-circuit currents: Step 1: Calculation of each equipment impedance Z i. Step 2: Development of impedances diagram, based on one-line diagram system. Step 3: Calculation of short-circuit impedances Z ki on the points of calculations. Ai Parallel connection:

Ac

∏ = ∑ A i

i

i


62

Designing an electrical installation (Beginner Guide) Ac

Serial connection:

= ∑ Ai i

Convert from ∆ to Y:

A1

=

A3

=

A2

=

A12 ⋅ A13

+ A13 + A23

A12

A13 ⋅ A23

+ A13 + A23

A12

A12 ⋅ A23 A12

+ A13 + A23

A12 2

1 A1

A2

A13

A23

A3

3

Convert from Y to ∆:

A12

= A1 + A2 +

A23

= A2 + A3 +

A13

= A1 + A3 +

A1 + A2 A3 A2 + A3 A1

A1 + A3 A2

Step 4: Calculation of short-circuit currents.

By short-circuit is meant the contact with a very small resistance between two or more conductors being under voltage; the circuit is closed by a small resistance, the current on this circuit resulting to be of a very high value (as I = U/R). Based on the above, the following can be deduced: the short-circuit is basically a normal circuit with a small resistance as consumer; as bigger the voltage is, as higher t he short-circuit current is. the calculation of the short-circuit current is simplified reduced to determining the resistance at the point of short-circuit. There are defined three short-circuit currents according to the type of short-circuit, respectively (RMS values): U k 3P short-circuit: I k (3) = 3 ⋅ Z k §

§

§


63


§

2P short-circuit: I k (2 )

=

§

1P short-circuit: I k (1)

=

U k 2 ⋅ Z k U k 2 ⋅ Z k + Z 0

where: Z k - is the equivalent short-circuit impedance, U k - is the line voltage at t he point of calculation, Z 0 - is the impedance of neutral conductor (= Z ∞ ); if the neutral is connected to earth by an impedance Z n, then Z 0 = Z ∞ + Z n. Three-phase short-circuit This fault involves all three phases. Short-circuit current I sc3 is equal to:

I sc 3

=

U

3

Z SC

where U (phase-to-phase voltage) corresponds to the transformer no-load voltage which is 3 to 5 % greater than the on-load voltage across the terminals. For example, in 390 V networks, the phase-to-phase voltage adopted is U = 410 and the phase-to-neutral voltage is U 3 = 237 V . Calculation of the short-circuit current therefore requires only calculation of Z sc, the impedance equal to all the impedances through which I sc flows from the generator to the location of the fault, i.e. the impedances of the power sources and the lines (see f i g. 4.33. ). This is, in fact, the "positive-sequence" impedance per phase:

Z sc

= (∑ R )2 + (∑ X )2

where: ∑ R - is the sum of series resistances, ∑ X - is the sum of series reactances. It is generally considered that three-phase faults provoke the highest fault currents. The fault current in an equivalent diagram of a polyphase system is limited by only the impedance of one phase at the phase-to-neutral voltage of the network. Calculation of I sc3 is therefore essential for selection of equipment (maximum current and electrodynamic withstand capability). Phase-to-phase short-circuit clear of earth This is a fault between two phases, supplied with a phase-to-phase voltage U . In this case, the short-circuit current ISC2 is less than that of a three-phase fault:

I sc 2

=

U 2 ⋅ Z sc

=

3 2

⋅ I sc3 ≈ 0.86 ⋅ I sc3


64


Fi g. 4.33. The various short-circuit currents (Z sc = Z k ; I sc = I k )

Phase-to-neutral short-circuit clear of earth This is a fault between one phase and the neutral, supplied with a phase-to-neutral

voltage V = U / 3 . The short-circuit current I SC1 is: I sc1

=

U

3

Z sc + Z Ln

In certain special cases of phase-to-neutral faults, the zero-sequence impedance of the source is less than Z sc (for example, at the terminals of a star-zigzag connected transformer or of a generator under subtransient conditions). In this case, the phase-to-neutral fault current may be greater than that of a three phase fault. Phase-to-earth fault (one or two phases) This type of fault brings the zero-sequence impedance Z (0) into play. Except when rotating machines are involved (reduced zero-sequence impedance), the short-circuit current I sc(0) is less than that of a three-phase fault. Calculation of I sc(0) may be necessary, depending on the neutral system (system earthing arrangement), in view of defining the setting thresholds for the zero-sequence (HV) or earth-fault (LV) protection devices.


65

Designing an electrical installation (Beginner Guide) Initial short circuit current The initial short-circuit current is the total short-circuit current i k ( iksym + idc ) at time

t = 0 sec. Sometime, the initial short-circuit current is understood as the RMS value of symmetrical short circuit current. Steady state short circuit current The steady state (permanent regime) short-circuit current is the total ik short-circuit

current after idc decayed to zero; it is actually the symmetrical short-circuit current iksym . Peak current i p

The peak short circuit current is the maximum instantaneous value of short circuit current at a point of calculation. This current occurs within the first ½ cycle of short circuit, but it is generally accepted to be calculated at ½ cycle = 0.01 sec on 50 Hz based networks and at 0.0083 sec on 60 Hz based networks ( ωt = π ⇒ 2π ft = π ⇒ t = 1 / 2 f ) .

i p

= ik =

 

2 × I k × sinα × e

−ωt

X R

 + sin(ωt − α ) 

Thermal short circuit current I kt

By thermal equivalent short-circuit current I kt in AC circuits is meant the effective value of an AC current that in 1 sec develops into a network element a heat which is equal to the heat developed by the ik on entire short circuit duration, and serves at verification of circuit breakers at thermal stability, having the formula: I ktAC = I k 0 × ( m + n)

t k t t

where: §

I k 0 - is the initial symmetrical short-circuit current,

§

m - is the factor of influence of idc , depending of K s and t sc ,

§

n - is the factor of influence of variat ion of ikaym , depending of t sc and I k 0 / I k ,

§

t k [sec] - is the duration of short-circuit that is equal with the opening time of circuit breaker (from catalog) plus the time for breaking the electric arc (= apr. 0.15 sec for U n ≤ 35 kV and 0.08 sec for U n ≥ 35 kV),

§

t t =1 sec is the corresponding time of I kt .

In DC circuits, the I k 0

= i s = I k = U / Rk , and the I kt is: I ktDC = I k × t k / t t

=

U Rk


×

t k / t t

66

Designing an electrical installation (Beginner Guide) 4.4.2.3. Formulas Motors a) proportional units method: sub transient reactance X” (%) and motor resistance (ohm). When missing, these data can be approximated; for project no 7015 X”=17%, K m =10. §

X m =

x" 100

×

S b S m

[ pu ]

;

Rm =

X m K m

[ pu ] ; K m =10 ; Z m = Rm2

+ X m2 [ pu ]

Other method is to calculate the Rm and X m in real units and to refer them to S b and U b ; the method is valid too for X me and Rme (see below): X m [pu] = X m [ohm] ×

S b 2 b

U

;

Rm [pu] = Rm [ohm] ×

S b 2 b

U

;

2

Z m = Rm

+ X m2 [ pu ]

b) real units method : resistance Rm [ohm], reactance X m [ohm], nominal current I n , power factor cosφ and starting factor K p . X m =

U × sin ϕ 3 × K p × I n

U × cos ϕ

[ohm] ; Rm =

3 × K p × I n

[ohm] ; Z m =

U 3 × K p × I n

[ohm]

In case of more motor connected to the point of calculation, it can be calculated the total equivalent resistance Rme and reactance X me of these motors by approximating cosφ , sinφ ; the total current I ne is the sum of nominal currents of motors; K pe the equivalent starting factor:

∑ I = ∑ I

pi

I ne = Σ I ni

;

K pe

i

;

Ame =

ni

U [× sin α ;× cos α ] 3 × K pe × I ne

i

Where Ame has the value Z, R, X, being the equivalent data of motors connected to the point of calculation. Transformer a) proportional units method: short circuit voltage u k (%) and short circuit power §

p k (kW). Z t =

u k 100

×

S b S t

[ pu ] ; Rt = p k [kW] ×

S b 2 t

S

[ pu ] ;

2

X t = Z t

− Rt 2 [ pu ]

b) real units method : short circuit voltage u k (%) and short circuit power p k (kW).


67


Z t =

u k 100

U t 2

×

S t

[ohm] ; Rt = p k ×

U t 2 2 t

S

[ohm] ;

X t = Z t 2 − Rt 2 [ohm]

Where U t is the voltage of the side (primary, secondary) on witch the item is calculated. Generally, U t = U 2 , resulting the item directly referred to secondary side, thus being avoided the referring of Z 1 to Z 2 . DG set a) proportional units method : subtransitorial reactance X” (%) and DG resistance (ohm). When missing, these data can be approximated. §

X DG =

x" 100

×

S b S t

[ pu ] ; R DG =

X DG K DG

[pu] ; K DG =20

b) real units method : subtransient reactance X” (%) and DG resistance (ohm). When missing, these data can be approximated. X” is appr. equal to 18%. X DG =

x" 100

×

2 U DG

S DG

[ohm] ; R DG =

X DG K DG

[ohm]

; K DG =20

Cables a) proportional units method: the resistance R0 (ohm/km), reactance X 0 (ohm/km), §

length of circuit l and number of cables per phase n. X c =

x0 1000 × n

× l ×

S b 2 b

U

[ pu ] ; Rc =

r 0 1000 × n

× l ×

S b 2 b

U

[ pu ] ; Z c = Rc2 + X c2 [ pu ]

b) real units method : the resistance R0 (ohm/km), reactance X 0 (ohm/km), length of circuit l and number of cables per phase n. X c =

x 0 1000 × n

× l [ohm] ; Rc =

r 0 1000 × n

× l [ohm] ; Z c =

Rc2

+ X c2 [ohm]

System/utility a) proportional units method: the short circuit power MVA available at the point of connection and the power MVA: §

X s

= 0.99 × Z s = 0.99 ×

S b S s

[ pu ] ; R s

= 0.15 × X s = 0.1485 ×

S b S s

b) real units method: the short circuit power MVA available at the point of connection and rated voltage at the point of connection: X s

= 0.99 × Z s = 0.99 ×

U 2 S s

[ohm] ; R s


= 0.15 × X s = 0.1485 ×

U 2 S s 68

Designing an electrical installation (Beginner Guide) Percentage: X = 99% ; R = 14.85% 4.4.2.4. Selection of protective device Nominal current I nPD :

I nDP ≥ I c

where I c is the current required by load; the is valid for all protection devices (PD). Circuit breakers Breaking capacity I cm : it indicates the maximum instantaneous current to witch CB

withstand without being damaged; I cm must be bigger that peak value of short circuit current: I cm

≥ i p .

Service short circuit breaking current I cs : the breaking capacity of CB is to be bigger than the RMS value of short circuit current calculated at the time when contacts start to open. The I cs indicates the maximum current (RMS value) than the CB can trip/break without being damaged or being affected - the CB is still operational after breaking the short circuit current. The I cs is generally indicated in % of I cu . I cs

≥ I k (t = t opening )

Ultimate short circuit current I cu : the breaking capacity of CB is to be bigger than the RMS value of short circuit current calculated at the time when contacts start to open. The I cu indicates the maximum current (RMS value) that the CB can trip/break and being damaged - the CB will not be operational after breaking the short current and to be replaced. I cu ≥ I k (t = topening ) Example: a CB having I cu =10 kA and I cs =50% · I cu =5 kA will trip short currents up

to 5 kA without being damaged and will trip short currents from 5 kA to 10 kA with coming out of operation (damaged). Generally, incoming CBs and essential consumers CBs have I cs = I cu , while outgoing CBs and non-essential CBs have: I cs = (25%, 50%, 75%) · I cu Short time withstands current I cw : this current is only defined for CBs of utilization category B.: a) utilization category A: the CBs of this category are not intended to be used in series (downstream of CB) with another protection devices; it is e.g. a motor protection CB (end of line) b) utilization category B: the CBs of this category are intended to be used in series (downstream of CB) with another protection devices; it is e.g. a line protection CB (beginning of line or intermediate in line CB). In order to assure the selectivity of protection, the CB is provided with an intentional time delay release and with I cw in order to withstand to short circuit current on the duration of intentional delay. I cw must be bigger than the RMS value of symmetrical short circuit current.


69

Designing an electrical installation (Beginner Guide) I cw ≥ I k ( symmetrical − RMS ) Thermal limit current I lt : it indicates the maximum thermal equivalent current to which CB withstand in respect to heat developed at short circuit; it must be bigger than the thermal equivalent short circuit current: I 1t ≥ I kt where I 1t is the thermal limit current for 1 sec of circuit breaker indicated on catalog. If the thermal limit current indicated for other time than 1 sec ( I lt ), then I 1t is determined by formula: I 1t = I lt × t . Fuses Breaking capacity I r must be bigger than RMS value or symmetrical short circuit current: I r ≥ I k 0 Current transformers Thermal stability:

I lt 2 ⋅ t lt ≥ I k 2 ⋅ t f Where: I lt [ A] - is the limit current at time t lt [sec] , indicated in catalog; §

§ § §

t f = t fp + t fa [sec]; t fp [sec] - is the period fictive time depending of t k and β = I k0 /I k ; t fa = 0.05· β 2, because t fa ≈ 0.05 sec, it can be ignored for t k > 1 sec.

Dynamic stability (breaking): 2 ⋅ I ld ≥ I s Where I ld is the dynamic limit current, indicated in cat alog and is the shock current. Separators Breaking capacity I cm: it indicates the maximum current (RMS value) that the separator can break under load conditions; it must be bigger than peak value of short circuit current. Cables Thermal stability:

S ≥ I k ⋅

t k C

Where: S [mm2] is the cable’s cross section and C is the constant depending of the type and material of cable: - C = 122 for PVC insulated cable of copper, maximum admissible temperature at short circuit is 1500C, - C = 104 for EPR insulated cable of copper, max. admissible temperature at short circuit is 1200C.


70


4.5. Worked example of cable calculation The installation is supplied through a 1,000 kVA transformer. The process requires a high degree of supply continuity and this is provided by the installation of a 500 kVA 400 V standby generator and the adoption of a 3-phase 3-wire IT system at the main general distribution board. The remainder of the installation is isolated by a 315 kVA 400/400 V transformer. The isolated network is a TT-eart hed 3-phase 4-wire system. Dimensioning circuit C1 The HV/LV 1,000 kVA transformer has a rated no-load voltage of 420 V. Circuit C 1 must be suitable for a current of 1,000 ⋅ 10 3 = 1,374 A I n = 3 ⋅ 420 per phase. Six single-core PVC-insulated copper cables in parallel will be used for each phase. These cables will be laid on cable trays corresponding with reference F . The “ K ” correction factors are as follows: K 1 = 1 (see fi gure 4.5.) K 2 = 1 (see fi gure 4.6.) 0 K 3 = 1 (temperature 30 C) If the circuit breaker is a Masterpact, one might choose:

I z = 1,374 A I n

=

I z

=

K 1 ⋅ K 2 ⋅ K 3

1,374 1

= 1,374 A

Each conductor will therefore carry 229 A. F igur e 4.9. indicates that the c.s.a. is 95 2 mm . The resistances and the inductive reactances for the six conductors in parallel are, for a length of 5 metres: ρ ⋅ l 18.5 ⋅ 5 = = 0.1623 mΩ R = S 95 ⋅ 6 R

=

0.13 ⋅ 5 6

= 0.1083 mΩ

Dimensioning circuit C6 Circuit C6 supplies a 315 kVA 3-phase 400/400 V isolating transformer.

Primary current

=

315 0.42 ⋅ 3

= 433 A

A single-core cable laid on a cable tray (without any other cable) in an ambient air temperature of 30 0C is proposed. The circuit breaker is regulated to 433 A: I z = 433 A Romanian Electro Trade, Engineering & Consulting

71


F ig. 4.34. Example of one-line diagram

The method of installation is characterized by the reference letter F , and the “ K ” correcting factors are: K 1 = 1 K 2 = 1 K 3 = 1 I z '

=

I z K

= 433 A


72

Designing an electrical installation (Beginner Guide) A c.s.a. of 240 mm2 is appropriate. The resistance and inductive reactance are respectively: ρ ⋅ l 18.5 ⋅ 5 = = 1.1568 mΩ R = S 240 R = 0.08 ⋅ 15 = 1.2 mΩ Calculation of short-circuit currents for the selection of circuit breakers Q 1 and Q 6 (see F ig. 4.34.)

Fi g. 4.35. Example of short-circuit current evaluation

The protective conductor Thermal requirements: fi gure 4.23. show that, when using the adiabatic method the c.s.a. for the protective earth (PE) conductor for circuit C 1 will be:

34,800 ⋅ 0.2 143

= 108 mm 2

A single 120 mm2 conductor dimensioned for other reasons mentioned later is therefore largely sufficient, provided that it also satisfies the requirements for indirectcontact protection (i.e. that its impedance is sufficiently low). For the circuit C 6 , the c.s.a. of its PE conductor should be: 29,300 ⋅ 0.2 143

= 92 mm 2

2

In this case a 95 mm conductor may be adequate if the indirect-contact protection conditions are also satisfied. Protection against indirect-contact hazards For circuit C 6 of fi gure 4.34., the maximum permitted length of the circuit is given by:

Lmax

=

0.8 ⋅ U 0 ⋅ S ph ρ ⋅ (1 + m) ⋅ I a

⇒ Lmax =

0.8 ⋅ 240 ⋅ 230 ⋅ 3 ⋅ 1,000

 240  ⋅ 630 ⋅ 11 2 ⋅ 22.5 ⋅ 1 +  95  

= 70 m

(The value in the denominator 630·11 = I m i.e. the current level at which the instantaneous short-circuit magnetic trip of the 630 A circuit breaker operates). The length of 15 metres is therefore fully protected by “instantaneous” overcurrent devices. Romanian Electro Trade, Engineering & Consulting

73

Designing an electrical installation (Beginner Guide) Voltage drop From fi gure 4.30. it can be seen that: For the cable C1 (6·95 mm2 per phase) §

∆U =

−1

0.42 VA Km

∆U % =

§

−1

⋅ 1,374 A ⋅ 0.008

3 100 400

= 1.54 V

⋅1.54 = 0.38%

For the circuit C6

∆U =

−

0.21 VA 1 Km

∆U % =

−1

3 100 400

⋅ 433 A ⋅ 0.015

= 1.36 V

⋅ 1.34 = 0.34%

At the circuit terminals of the LV/LV transformer the percentage volt-drop ΔU% = 0.72%

4.6.

Choice of the protective devices

4.6.1. The basic functions of LV switchgear National and international standards define the manner in which electric circuits of LV installations must be realized, and the capabilities and limitations of the various switching devices which are collectively referred to as switchgear. The main functions of switchgear are: electrical protection; electrical isolation of sections of an installation; local or remote switching. § § §

Fi g. 4.36. Basic functions of LV switchgear

Electrical protection at low voltage is (apart from fuses) normally incorporated in circuit-breakers, in the form of thermal-magnetic devices. In addition to those functions shown in fi gure 4.36. , other functions, namely: over-voltage protection; under-voltage protection are provided by specific devices (lightning and various other types of voltage-surge arrester; relays associated with: contactors, remotely § §


74

Designing an electrical installation (Beginner Guide) controlled circuit-breakers, and with combined circuit-breaker/isolators… and so on). Electrical protection

Electrical protection assures: protection of circuit elements against the thermal t hermal and mechanical stresses of shortcircuit currents; protection of persons in the event of insulation failure; protection of appliances and apparatus being supplied supplied (e.g. motors, etc.). The aim is to avoid or to limit the destructive or dangerous consequences of excessive (short-circuit) currents, or those due to overloading and insulation failure, and to separate the defective circuit from the rest of the installation. A distinction is made between the protection prot ection of: the elements of the installation (cables, wires, switchgear…); persons and animals; equipment and appliances supplied from the installat ion; the protection of circuits: - against overload ; a condition of excessive current being drawn from a healthy (unfaulted) installation; - against short-circuit currents due to complete failure of insulation between conductors of different phases or (in TN systems) between a phase and neutral (or PE) conductor. Protection in these cases is provided either by fuses or circuit-breaker, at the distribution board from which the final circuit (i.e. the circuit to which the load is connected) originates. The protection of persons: - against insulation failures. failures. According to the system of earthing eart hing for the installation (TN, TT or IT) the protection will be provided by fuses or circuit-breakers, residual current devices, and/or permanent monitoring of the insulation resistance of the installation to earth. The protection of electric motors: - against overheating , due, for example, to long term overloading; stalled rotor; single-phasing, etc. Thermal relays, specially designed to match the particular characteristics of motors are used. Such relays may, if required, also protect the motor-circuit cable against overload. Short-circuit protection is provided either by type aM fuses or by a circuit-breaker from which the thermal (overload) protective element has been removed, or otherwise made inoperative. §

§ §

§ § § §

§

§

Isolation

A state of isolation clearly indicated by an approved “fail-proof” indicator, or the visible separation of contacts, are both deemed to satisfy the national standards of many countries. The aim of isolation is to separate a circuit or apparatus, or an item of plant (such as a motor, etc.) from the remainder of a system which is energized, in order that personnel may carry out work on the isolated part in perfect safety.


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Designing an electrical installation (Beginner Guide) In principle, all circuits of an LV installation shall have means to be isolated. In practice, in order to maintain an optimum continuity of service, it is preferred t o provide a means of isolation at t he origin of each circuit. An isolating device must fulfil the following requirements: all poles of a circuit, including the neutral (except where the neutral is a PEN conductor) must be open; it must be provided with a means of locking open with a key (e.g. by means of a padlock) in order to avoid avoid an unauthorized reclosure by inadvertence; it must conform to a recognized national or international standard concerning clearance between contacts, creepage distances, over voltage withstand capability, etc. and also verification that the contacts of the isolating device are, in fact, open. The verification may be: - either visual, where the device is suitably designed to allow the contacts to be seen (some national standards impose this condition for an isolating device located at the origin of a LV installation supplied directly fro m a HV/LV transformer); - or mechanical, by means of an indicator solidly welded to the operating shaft of the device. In this case the construction of the device must be such that, in the eventuality that the contacts become welded together in the closed position, the indicator cannot possibly indicate that it is in the open position; - leakage currents; with the isolating device open, leakage currents between the open contacts of each phase must not exceed: - 0.5 mA for a new device; - 6.0 mA at the end of its useful life. - voltage-surge withstand capability, across open contacts. The isolating device, when open must withstand a 1.2/50 μs impulse, having a peak value of 6, 8 or 12 kV according to its service voltage, as shown in fi gur e 4.37. 4.37. The device must satisfy these conditions for altitudes up to 2,000 metres. Correction factors are given in IEC 60664-1 for altitudes greater than 2,000 metres. Consequently, if tests are carried out at sea level, the test values must be increased by 23% to take into account the effect of o f altitude. §

§

§

Fi g. 4.37. 4.37. Peak value of impulse voltage

according to normal service voltage of test specimen

The degrees III and IV are degrees of pollution defined in IEC 60664-1 Switchgear control

Switchgear-control functions allow system operating personnel to modify a loaded system at any moment, according to requirements. In broad terms “control” signifies any facility for safely modifying a load-carrying power system at all levels of an installation. The operation of switchgear is an important part of power-system control.


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Designing an electrical installation (Beginner Guide) Functional control This control relates to all switching operations in normal service conditions for energizing or de-energizing a part of a system or installation, or an individual piece of equipment, item of plant, etc. Switchgear intended for such duty must be installed at least : at the origin of any installation; installat ion; at the final load circuit or circuits (one switch may control several loads). Marking (of the circuits being controlled) must be clear and unambiguous. In order to provide the maximum flexibility and continuity of operation, particularly where the switching device also constitutes the protection (e.g. a circuit-breaker or switch-fuse) it is preferable to include a switch at each level of o f distribution, i.e. on each outgoing way o f all distribution and subdistribution boards. The manoeuvre may be: either manual (by means of an operating operat ing lever on the switch) or electric, by push-button on the switch or at a remote location (load-shedding and reconnection, for example). These switches operate instantaneously (i.e. with no deliberate delay), and those that provide protection are invariably omni-polar. omni-polar. The main circuit-breaker for the entire installation, as well as any circuit-breakers used for change-over (from one source to another) must be omni-polar units. § §

§ §

Emergency switching - emergency stop An emergency switching is intended to de-energize a live circuit which is, or could become, dangerous (electric shock or fire). An emergency stop is intended to arrest a movement which has become dangerous. In the two cases: the emergency control device or its means of operation (local or at remote location(s)) such as a large red mushroom-headed emergency-stop pushbutton must be recognizable and readily accessible, in proximity to any position at which danger could arise or be seen; a single action must result r esult in a complete switching-off of all live conductors; co nductors; a “break glass” emergency switching initiation device is authorized, but in unmanned installations the re-energizing of the circuit can only be achieved by means of a key held by an authorized person. It should be noted that in certain cases, an emergency system of braking, may require that the auxiliary supply to the braking-system circuits be maintained until final stoppage of the machinery. §

§ §

Switching-off for mechanical maintenance work This operation assures the stopping of a machine and its impossibility to be inadvertently restarted while mechanical maintenance work is being carried out on the driven machinery. The shutdown is generally carried out at the functional switching device, with the use of a suitable sa fety lock and warning notice at the switch sw itch mechanism.


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4.6.2. Elementary switching devices 4.6.2.1. Disconnector (or isolator) (see F i g. 4.38.)

This switch is a manually-operated, lockable, two-position device (open/closed) which provides safe isolation of a circuit when locked in the open position. A disconnector is not designed to make or to break current and no rated values for these functions are given in standards.

Fi g. 4.38. Symbol for a disconnector (or isolator)

A LV disconnector is essentially a deadsystem switching device to be operated with no voltage on either side of it, particularly when closing, because of the possibility of an unsuspected short-circuit on the downstream side. Interlocking with an upstream switch or circuit breaker is frequently used. It must, however, be capable of withstanding the passage of short-circuit currents and is assigned a rated short-time withstand capability; generally for 1 second, unless otherwise agreed between user and manufacturer. This capability is normally more than adequate for longer periods of (lower-valued) operational overcurrents, such as those of motor-starting. Standardized mechanical-endurance, overvoltage, and leakage-current tests, must also be satisfied. 4.6.2.2. Load-breaking switch (see F ig. 4.39.)

This control switch is generally operated manually (but is sometimes provided with electrical tripping for operator convenience) and is a non-automatic two-position device (open/closed).

Fi g. 4.39. Symbol for a load-breaking switch

It is used to close and open loaded circuits under normal unfaulted circuit conditions. It does not consequently, provide any protection for the circuit it controls. It is characterized by: - the frequency of switch operation (600 close/open cycles per hour maximum); - mechanical and electrical endurance (generally less than that of a contactor); - current making and breaking ratings for normal and infrequent s ituations. When closing a switch to energize a circuit there is always the possibility that an (unsuspected) short circuit exists on the circuit. For this reason, load-break switches are assigned a fault-current making rating, i.e. successful closure against the electrodynamics forces of short-circuits current is assured. Such switches are commonly referred to as Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) “fault-make load-break” switches. Upstream protective devices are relied upon to clear the short-circuit fault Category AC-23 includes occasional switching of individual motors. The switching of capacitors or of tungsten filament lamps shall be subject to agreement between manufacturer and user. The utilization categories referred to in fi gur e 4.40. do not apply to an equipment normally used to start, accelerate and/or stop individual motors.

Example: A 100 A load-break switch of category AC-23 (inductive load) must be able: - to make a current of 10⋅ I n (1,000 A) at a power factor of 0.35 lagging; - to break a current of 8⋅ I n (800 A) at a power factor of 0.45 lagging; - to withstand short duration short-circuit currents when close.

Fi g. 4.40. Utilization categories of LV AC switches

4.6.2.3. Bistable switch (télérupteur) (see F i g. 4.41.)

This device is extensively used in the control of lighting circuits where the depression of a pushbutton (at a remote control position) will open an already-closed switch or close an open switch in a bistable sequence. Typical applications are: - two-way switching on stairways of large buildings; - stage-lighting schemes; - factory illumination, etc. Auxiliary devices are available to provide: - remote indication of its state at any instant; - time-delay functions; - maintained-contact features.

F ig. 4.41. Symbol for a bistable remotelyoperated switch

(télérupteur)


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Designing an electrical installation (Beginner Guide) 4.6.2.4. Contactor (see F ig. 4.42 .)

The contactor is a solenoid-operated switching device which is generally held closed by (a reduced) current through the closing solenoid (although various mechanically latched types exist for specific duties). Contactors are designed to carry out numerous close/open cycles and are commonly controlled remotely by on-off pushbuttons. The large number of repetitive operating cycles is standardized by: the operating duration: 8 hours; uninterrupted; intermittent; temporary of 3, 10, 30, 60 and 90 minutes; utilization category: for example, a contactor of category AC3 can be used for the starting and stopping of a cage motor; the start-stop cycles (1 to 1,200 cyles per hour); mechanical endurance (number of off-load manoeuvres); electrical endurance (number of on-load manoeuvres); a rated current making and breaking performance according to the category of utilization concerned. §

§

§ § § §

Fi g. 4.42. Symbol for a contactor

Example: A 150 A contactor of category AC3 must have a minimum current-breaking capability of 8⋅ In (1,200 A) and a minimum current-making rating of 10⋅ In (1,500 A) at a power factor (lagging) of 0.35. Characteristics Altitude The rarefied atmosphere at high altitude reduces the dielectric strength of the air and hence the rated operational voltage of the contactor breaker. It also reduces the cooling effect of the air and hence the rated operational current of the contactor breaker (unless the temperature drops at the same time). No derating is necessary up to 3000 m. Derating factors to be applied above this altitude for main pole operational voltage and current (a.c. supply) are as follows: Altitude Rated operational voltage Rated operational current

3500 m 0.9 0.92

4000 m 0.8 0.9

4500 m 0.7 0.88

5000 m 0.6 0.86

Ambient air temperature The temperature of the air surrounding the device, measured near to the device. The operating characteristics are given: - with no restriction for temperatures between –5 and +55, - with no restriction, if necessary, for temperatures between –50 and +70.


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Designing an electrical installation (Beginner Guide) Rated operational current ( I e) This is defined taking into account the rated operational voltage, operating rate and duty, utilization category and air temperature around the device. Rated conventional thermal current ( I th) The current witch a closed contactor breaker can sustain for a minimum of 8 hours without its temperature rise exceeding the limits given in the standards. Short time rating The current witch a closed contactor breaker can sustain for a short time, after a period of no load, without dangerous overheating. Rated operational (U e) This is the voltage value witch, in conjunction with the rated operational current, determines the use of the contactor breaker or starter, and on witch the corresponding tests and the utilization category are based. For 3-phase circuits, it is expressed as the voltage between phases. Apart from exceptional cases such as rotor short-circuiting, the rated operational voltage U e is less than or equal to the rated insulation voltage U i. Rated control circuit voltage (U c) The rated value of the control circuit voltage, on witch the operating characteristics are based. For a.c. applications, the values are given for a near sinusoidal wave from (less than 5 % total harmonic distortion). Rated insulation voltage (U i) This is the voltage value used to define the insulation characteristics of a device and referred to in dielectric tests determining leakage paths and creepage distances. As the specifications are not identical for all standards, the rated values given for each of them are not necessarily the same. Rated impulse withstand voltage (U imp) This is the highest peak value of an impulse voltage, of prescribed from and polarity, witch the device is able to withstand without failure under specified test conditions, and to which isolation clearance values are referred. The rated impulse withstand voltage of a device must be equal to or higher than the values stated for the transient overvoltages appearing in the circuit in which the device is fitted. Rated operational power (expressed in kW) The rated power of the standard motor, which can be switched by the contactor breaker, at the stated operational voltage. Note: these definitions are based on extracts from standard IEC 947 Rated breaking capacity ( I q) This is the current value, which the contactor breaker can break in accordance with the breaking conditions specified in the IEC st andard. Rated making capacity This is the current value, which the contactor breaker can make in accordance with the making conditions specified in the IEC standard.


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Designing an electrical installation (Beginner Guide) On- load factor (m) This is the ratio between the time the current flows (t) and the duration of the cycle (T ): m = t / T

Cycle duration: duration of current flow + time at zero current. Pole impedance The impedance of one pole is the sum of the impedance of all the circuit components between the input terminals. The impedance comprises a resistive component ( R) and an inductive component ( X = L·ω). The total impedance therefore depends on the frequency and is normally given for 50 Hz. The average value is given for the pole at its rated operational current. Electrical durability This is the average number of on-load operating cycles, which the main pole contacts can perform without maintenance. The electrical durability depends on the utilization category, the rated operational voltage. Mechanical durability This is the average number of no-load operating cycles (i.e. with zero current flow through the main poles), which the contactor breaker can perform without mechanical failure.

The standard uti li zation categori es define the current values, which the contactor breaker must be able to make or break. These values depend on: the type of load being switched: squirrel cage or slip ring motor, resistors, the conditions under which making or breaking takes place: motor stalled, starting or running, reversing, plugging. § §

Utilization categories for a.c. applications (contactor breakers)

Category AC-1: applies to all types of a.c. load with a power factor equal to or greater than 0.95 (cos ϕ ≥0.95). Non-inductive or slightly inductive loads. Application examples: heating, distribution. Category AC-2: - applies to starting, plugging and inching of slip motors, - on closing, the contactor breaker makes the starting current, which is about 2.5 times the rated current of the motor, -on opening, it must break the starting current, at a voltage less than or equal to the mains supply voltage. Category AC-3: - applies to squirrel cage motors with breaking during normal running of the motor, - on closing the contactor breaker makes the starting current, which is about 5 to 7 times the rated current of the motor, Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) - on opening, it breaks the rated current drawn by the motor; at this point, the voltage at the contactor breaker terminals is about 20 % of the mains supply voltage. Breaking is light. Application examples: all standard squirrel cage motors (lifts, escalators, conveyor belts, bucket elevators, compressors, pumps, mixers, air conditioning units). Category AC-4: - covers applications with plug braking and inching of squirrel cage motors, - on closing, the contactor breaker makes a current peak which may be as high as 5 or 7 times the rated motor current, - on opening, it breaks this same current at a voltage which is higher, the lower the motor speed. This voltage can be the same as the mains voltage. Breaking is severe. Application examples: printing machines, wire drawing, hoisting equipment, metallurgy industry. Utilisation categories for a.c. applications (auxiliary contacts)

Category AC-14: applies to the switching of electromagnetic loads whose power drawn with the electromagnet closed is less than 72 VA. Application example: switching the operating coil of contactors and re lays. Category AC-15: This category applies to the switching of electromagnetic loads whose power drawn with the electromagnet closed is great er than 720VA. Application example: switching the operating coil of contactors. Category AC-41: applies to all types of a.c. device (load) with a power factor equal to or greater than 0.95 (cosϕ ≥0.95). Non-inductive or slightly inductive loads. Application example: heating, distribution Category AC-42: - applies to starting, plugging and inching of slip ring motors, - on closing, the contactor breaker makes the starting current, which is about 2.5 times the rated current of the motor, - on opening, it must break the starting current, at a voltage less than or equal to the mains supply voltage. Category AC-43: - applies to squirrel cage motors with breaking while motor running; inching or occasional reversing of limited duration are permissible if the number of operating cycle does not exceed 5 per minute, or 10 within a 10 minute period, - on closing the contactor breaker makes the starting current, which is about 5 to 7 times the rated motor current, - on opening, it breaks the rated current drawn by the motor; at this point, the voltage at the contactor breaker terminals is about 20 % of the mains supply voltage. Breaking is light. Application examples: all started squirrel cage motors (lifts, escalators, conveyor belts, bucket elevators, compressors, pumps, mixers, air conditioning units).


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Designing an electrical installation (Beginner Guide) Category AC-44: - covers applications with plug braking and inching of squirrel cage or slip ring motors, - on closing, the contactors breaker makes a current peak which may be as high as 5 or 7 times the rated motor current, - on opening, it breaks this same current at a voltage which is higher, the lower the motor speed. This voltage can be the same as the mains voltage. Breaking is severe. Application examples: printing machines, wire drawing machines, hoisting equipment, metallurgy industry, etc. Utilization categories for d.c. applications

Category DC-13 This category applies to the switching of electromagnetic loads for whthe time taken to reach 95% of the steady state current (T = 0.95) is equal to 6 times the power P drawn by the load (with P ≤ 5pW). Application example: switching the operating coil of contactor breakers. 4.6.2.5. Fuses (see F ig. 4.43.)

The first letter indicates the breaking range: “g” fuse-links (full-range breaking-capacity fuse-link); “a” fuse-links (partial-range breaking-capacity fuse-link). The second letter indicates the utilization category; this letter defines with accuracy the time-current characteristics, conventional times and currents, gates: For example: “gG” indicates fuse-links with a full-range breaking capacity for general application; “gM” indicates fuse-links with a full-range breaking capacity for the protection of motor circuits; “aM” indicates fuse-links with a partial range breaking capacity for the protection of motor circuits. Fuses exist with and without “fuse-blown” mechanical indicators. Fuses break a circuit by controlled melting of the fuse element when a current exceeds a given value for a corresponding period of time; the current/time relationship being presented in the form of a performance curve for each t ype of fuse. Standards define two classes of fuse: those intended for domestic installations, manufactured in the form of a cartridge for rated currents up to 100 A and designated type gG; those for industrial use, with cartridge types designated gG (general use); and gM and aM (for motor-circuits). § §

§

§

§

§

§

Fi g. 4.43. Symbol for fuses

The main differences between domestic and industrial fuses are the nominal voltage and current levels (which require much larger physical dimensions) and their fault current breaking capabilities. Type gG fuse-links are often used for the protection of motor Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) circuits, which is possible when their characteristics are capable of withstanding the motor-starting current without deterioration. A more recent development has been the adoption by the IEC of a fuse-type gM for motor protection, designed to cover starting, and short-circuit conditions. This type of fuse is more popular in some countries than in others, but at the present time the aM fuse in combination with a thermal overload relay is more-widely used. A gM fuse-link, which has a dual rating, is characterized by two current values. The first value I n denotes both the rated current of the fuse-link and the rated current of the fuseholder; the second value I ch denotes the time-current characteristic of the fuse-link. These two ratings are separated by a letter which defines the applications. For example: I n MI ch denotes a fuse intended to be used for protection of motor circuits and having the characteristic G. The first value I n corresponds to the maximum continuous current for the whole fuse and the second value I ch corresponds to the G characteristic of the fuse link. Important: Some national standards use a gI (industrial) type fuse, similar in all main essentails to type gG fuses. Type gI fuses should never be used, however, in domestic and similar insta llations. Fusing zones - conventional currents The conditions of fusing (melting) of a fuse are defined by standards, according to their class. Class gG fuses These fuses provide protection against overloads and short-circuits. Conventional non-fusing and fusing currents are standardized, as shown in fi gure 4.44. and in figure 4.45.

The conventional non-fusing current I nf is the value of current that the fusible element can carry for a specified time without melting. Example: A 32 A fuse carrying a current of 1.25·I n (i.e. 40 A) must not melt in less than one hour. The conventional fusing current I f (= I 2 in f ig. 4.44.) is the value of current which will cause melting of the fusible element before the expiration of the specified time. §

§

Fi g. 4.44. Zones of fusing and non-fusing for gG and gM fuses

Example: A 32 A fuse carrying a current of 1.6·I n (i.e. 52.1 A) must melt in one hour or less IEC 60269-1 standardized tests require that a fuse-operating characteristic lies


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Designing an electrical installation (Beginner Guide) between the two limiting curves (shown in fi gure 4.44.) for the particular fuse under test. This means that two fuses which satisfy the test can have significantly different operating times at low levels of overloading.

F ig. 4.45. Zones of fusing and non-fusing for LV types gG and gM class fuses

The two examples given above for a 32 A fuse, together with the foregoing notes on standard test requirements, explain why these fuses have a poor performance in the low overload range. It is therefore necessary to install a cable larger in ampacity than that normally required for a circuit, in order to avoid the consequences of possible long term overloading (60% overload for up to one hour in the worst case). By way of comparison, a circuit breaker of similar current rating: which passes 1.05·In must not trip in less than one hour; and when passing 1.25·In it must trip in one hour, or less (25% overload for up to one hour in the worst case).

§

§

§ §

§

Fi g. 4.46. Current limitation by a fuse

Class aM (motor) fuses These fuses afford protection against short-circuit currents only and must necessarily be associated with other switchgear (such as discontactors or circuit breakers) in order to ensure overload protection < 4·I n. They are not therefore autonomous. Since aM fuses are not intended to protect against low values of overload current, no levels of conventional non-fusing and fusing currents are fixed.


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Designing an electrical installation (Beginner Guide) Rated short-circuit breaking currents A characteristic of modern cartridge fuses is that, owing to the rapidity of fusion in the case of high short-circuit current levels, a current cut-off begins before the occurrence of the first major peak, so that the fault current never reaches its prospective peak value (see f ig. 4.46.). This limitation of current reduces significantly the thermal and dynamic stresses which would otherwise occur, thereby minimizing danger and damage at the fault position. The rated short-circuit breaking current of the fuse is therefore based on the RMS value of the AC component of the prospective fault curr ent. No short-circuit current-making rating is assigned to fuses. A gM type fuse is essentially a gG fuse, the fusible element of which corresponds to the current value I ch (ch = characteristic) which may be, for example, 63 A. This is the IEC testing value, so that its time/current characteristic is identical to that of a 63 A gG fuse. This value (63 A) is selected to withstand the high starting currents of a motor, the steadystate operating current ( I n) of which may be in the 10-20 A range. This means that a physically smaller fuse barrel and metallic parts can be used, since the heat dissipation required in normal service is related to the lower figures (10-20 A). A standard gM fuse, suitable for this situation would be designated 32M63 (i.e. I n M I ch). The first current rating I n concerns the steady-load thermal performance of the fuse-link, while the second current rating ( I ch) relates to its (short-time) starting-current performance. It is evident that, although suitable for short-circuit protection, overload protection for the motor is not provided by the fuse, and so a separate thermal-type relay is always necessary when using gM fuses. The only advantage offered by gM fuses, therefore, when compared with aM fuses, are reduced physical dimensions and slightly lower cost. 4.6.2.6. Circuit breaker

The fundamental characteristics of a circuit breaker are: its rated voltage U e; its rated current I n; its tripping-current-level adjustment ranges for overload protection ( I r or I rth) and for short-circuit protection ( I m); its short-circuit current breaking rating ( I cu for industrial CBs; I cn for domestic type CBs). Rated operational voltage (U e) This is the voltage at which the circuit breaker has been designed to operate, in normal (undisturbed) conditions. § § §

§

Rated current (I n ) This is the maximum value of current that a circuit breaker, fitted with a specified overcurrent tripping relay, can carry indefinitely at an ambient temperature stated by the manufacturer, without exceeding the specified temperature limits of the current-carrying parts. Example

A circuit-breaker rated at I n = 125 A for an ambient temperature of 40 0C will be equipped with a suitably calibrated overcurrent tripping relay (set at 125 A). The same circuit-breaker can be used at higher values of ambient temperature however, if suitably 0 “derated”. Thus, the circuit breaker in an ambient temperature of 50 C could carry only


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Designing an electrical installation (Beginner Guide) 117 A indefinitely, or again, only 109 A at 60 0C, while complying with the specified temperature limit. Derating a circuit-breaker is achieved therefore, by reducing the trip-current setting of its overload relay, and marking the CB accordingly. The use of an electronic-type of tripping unit, designed to withstand high temperatures, allows circuit-breakers (derated as described) to operate at 60 0C (or even at 70 0C) ambient. Note: I n for circuit breakers is equal to I u for switchgear generally, I u being rated uninterrupted current. Frame-size rating A circuit-breaker which can be fitted with overcurrent tripping units of different current level-setting ranges is assigned a rating which corresponds with that of the highest current-level-setting tripping unit that can be fitt ed. Example

A NS630N circuit-breaker can be equipped with 4 electronic trip units from 150 A to 630 A. The size of the circuit-breaker is 630 A. Overload relay trip-current setting (I r th or I r ) Apart from small circuit-breakers which are very easily replaced, industrial circuit-breakers are equipped with removable, i.e. exchangeable, overcurrent-trip relays. Moreover, in order to adapt a circuit-breaker to the requirements of the circuit it controls, and to avoid the need to install over-sized cables, the trip relays are generally adjustable. The trip-current setting Ir or Irth (both designations are in common use) is the current above which the circuit breaker will trip. It also represents the maximum current that the circuit-breaker can carr y without tripping. That value must be greater than the maximum load current I B, but less than the maximum current permitted in the circuit I z . The thermal-trip relays are generally adjustable from 0.7 to 1.0 times I n, but when electronic devices are used for this duty, the adjustment range is greater; typically 0.4 to 1 times I n. Example (see F ig. 4.47.) A NS630N circuit-breaker equipped with a 400 A STR23SE overcurrent trip relay, set at 0.9, will have a trip-current setting: I r = 400⋅0.9 = 360 A Note: For circuit breakers equipped with non-adjustable overcurrent-trip relays, I r = I n. Example: for C60N 20 A circuit-breaker, I r = I n = 20 A.

Fi g. 4.47. Example of a NS630N circuit breaker equipped with

a STR23SE trip unit adjusted to 0.9, to give I r = 360 A


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Fi g. 4.48. 4.48. Tripping-current ranges of overload

and short-circuit protective devices for LV circuit breakers

Short-circuit relay trip-current setting (I m ) Short-circuit tripping relays (instantaneous or slightly time-delayed) are intended to trip the circuit-breaker rapidly on the occurrence of high values of fault current. Their tripping threshold I m is either fixed by standards for domestic type CBs, Indicated by the manufacturer for industrial-type CBs according accord ing to related standards. For the latter circuit-breakers there exists a wide variety of tripping devices which allow a user to adapt the protective performance of the circuit breaker to the particular 4.48. , 4.49. and F ig. 4.50. 4.50.). requirements of a load (see F ig. 4.48. , F i g. 4.49. Isolating feature A circuit-breaker is suitable for isolating a circuit if it fulfills all the conditions prescribed for a disconnector (at its rated r ated voltage) in the relevant re levant standard. st andard. In such a case it is referred to as a circuit-breaker-disconnector and marked on its front face with the symbol:

Rated short-circuit breaking capacity ( I I cu I cn cu or I cn) The short-circuit current-breaking rating of a CB is the highest (prospective) value of current that the CB is capable of breaking without being damaged. The value of current quoted in the standards is the RMS value of the AC component of the fault current, i.e. the DC transient component (which is always present in the worst possible case of shortcircuit) is assumed to be zero for calculating the standardized value. This rated value ( I cu cu) for industrial CBs and ( I cn cn) for domestic-type CBs is normally given in kA RMS. I cu cu (rated ultimate short-circuit breaking capacity) and I cs cs (rated service short-circuit breaking capacity) are defined in IEC 60947-2 together with a table relating relat ing I I cs with I cu cs with I cu for different categories of utilization A (instantaneous tripping) and B (time-delayed tripping). Rated insulation voltage (U i) This is the value of voltage to which the dielectric tests voltage (generally greater than 2·U 2·U i) and creepage distances are referred.


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Designing an electrical installation (Beginner Guide) The maximum value of rated operational voltage must never exceed that of the rated insulation voltage, i.e. U e ≤ U Ui .

F ig. 4.49. 4.49. Performance Performance curve of a circuit-breaker thermal-magnetic

protective scheme

Fi g. 4.50. 4.50. Performance Performance curve of a circuit-breaker electronic protective scheme scheme

I r r - overload (thermal or short-deley) relay trip-current setting I m - short-circuit (magnetic or long-delay) relay t rip-current setting I - short-circuit instantaneous relay relay trip-current setting setting PdC - breaking capacity

Rated impulse-withstand voltage (U imp imp) This characteristic expresses, in kV peak (of a prescribed form and polarity) the value of voltage which the equipment is capable of withstanding without failure, under test conditions.


90

Designing an electrical installation (Beginner Guide) Generally, for industrial circuit breakers, U imp imp = 8 kV and for domestic types, U imp imp = 6 kV . Category (A or B) and rated short-time withstand current ( I I cw cw) There are two categories of LV industrial switchgear, A and B, according to IEC 60947-2:

Fi g. 4.51. 4.51. Category A circuit breaker §

§

those of category A, for which there is no deliberate delay in the operation of the “instantaneous” short-circuit magnetic tripping device (see Fig. 4.51.), are generally moulded-case type circuit-breakers, and those of category B for which, in order to discriminate with other circuit breakers on a time basis, it is possible to delay the tripping of the CB, where the faultcurrent level is lower than that of the short-time withstand current rating ( I ( I cw cw) of the CB (see F ig. 4.52. 4.52.). This is generally applied to large open-type circuit breakers and to certain heavy-duty moulded-case types. I cw cw is the maximum current that the B category CB can withstand, thermally and electrodynamically, without sustaining damage, for a period of time given by the manufacturer .

Fi g. 4.52. 4.52. Category B circuit breaker


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Designing an electrical installation (Beginner Guide) Rated making capacity ( I cm) I cm is the highest instantaneous value of current that the circuit breaker can establish at rated voltage in specified conditions. In AC systems this instantaneous peak value is related to I cu (i.e. to the rated breaking current) by the factor k , which depends on the power factor (cos ϕ) of the short-circuit current loop (as shown in F igur e 4.53.).

Fi g. 4.53. Relation between rated breaking capacity I cu and rated making capacity I cm at

different power-factor values of short-circuit current

Example: A Masterpact NW08H2 circuit-breaker has a rated breaking capacity I cu of

100 kA. The peak value of its rated making capacity I cm will be 100⋅2.2 = 220 kA. Rated service short-circuit breaking capacity ( I cs) The rated breaking capacity ( I cu) or ( I cn) is the maximum fault-current a circuit breaker can successfully interrupt without being damaged. The probability of such a current occurring is extremely low, and in normal circumstances the fault-currents are considerably less than the rated breaking capacity ( I cu) of the CB. On the other hand it is important that high currents (of low probability) be interrupted under good conditions, so that the CB is immediately available for reclosure, after the faulty circuit has been repaired. It is for these reasons that a new characteristic ( I cs) has been created, expressed as a percentage of I cu: 25, 50, 75, 100% for industrial circuit breakers. For domestic CBs, I cs = k ⋅I cn. In Europe it is the industrial practice to use a k factor of 100% so that I cs = I cu. Fault-current limitation The fault-current limitation capacity of a CB concerns its ability, more or less effective, in preventing the passage of the maximum prospective fault-current, permitting only a limited amount of current to flow, as shown in fi gure 4.54 .

Fi g. 4.54. Prospective and actual currents

The current-limitation performance is given by the CB manufacturer in the form of curves (see f i g. 4.55.).


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F ig. 4.55. Performance curves of a typical LV current-limiting circuit breaker

diagram (a) shows the limited peak value of current plotted against the RMS value of the AC component of the prospective fault current (“prospective” fault-current refers to the fault-current which would flow if the CB had no current-limiting capability) limitation of the current greatly reduces the thermal stresses (proportional I 2·t ) and ., again, versus the RMS this is shown by the curve of diagram (b) of fi gur e 4.55 value of the AC component of the prospective fault curr ent. LV circuit breakers for domestic and similar installations are classified in certain standards (notably European Standard EN 60 898). CBs belonging to a class (of current limiters) have standardized limiting I 2·t let-through characteristics defined by that class. In these cases, manufacturers do not normally provide characteristic performance curves. §

§

The advantages of current limitation The use of current-limiting CBs affords numerous advantages: better conservation of installation networks: current-limiting CBs strongly attenuate all harmful effects associated with short-circuit currents; reduction of thermal effects: conductors (and therefore insulation) heating is significantly reduced, so that the life of cables is correspondingly increased; reduction of mechanical effects: forces due to electromagnetic repulsion are lower, with less risk of deformation and possible rupture, excessive burning of contacts, etc. reduction of electromagnetic-interference effects: less influence on measuring instruments and associated circuits, telecommunication systems, etc. These circuit breakers therefore co ntribute towards an improved exploitation of: cables and wiring; prefabricated cable-trunking systems; switchgear, thereby reducing the ageing of the insta llation. §

§

§

§

§ § §

4.6.3. Combined switchgear elements Single units of switchgear do not, in general, fulfil all the requirements of the three basic functions: protection, control and isolation. Romanian Electro Trade, Engineering & Consulting

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Designing an electrical installation (Beginner Guide) Where the installation of a circuit-breaker is not appropriate (notably where the switching rate is high, over extended periods) combinations of units specifically designed for such a performance are employed. The most commonly-used combinations are described below. 4.6.3.1. Switch and fuse combinations

Two cases are distinguished: The type in which the operation of one (or more) fuse(s) causes the switch to open. This is achieved by the use of fuses fitted with striker pins, and a system of switch tripping springs and toggle mechanisms (see F i g. 4.56.)

§

F ig. 4.56. Symbol for an automatic tripping switch-fuse

The type in which a non-automatic switch is associated with a set of fuses in a common enclosure. In some countries, the terms “switch-fuse” and “fuse-switch” have specific meanings: A switch-fuse comprises a switch (generally 2 breaks per pole) on the upstream side of three fixed fuse-bases, into which the fuse carriers are inserted (see Fig. 4.57.)

§

§

Fi g. 4.57. Symbol for a non-automatic switch-fuse

A fuse-switch consists of three switch blades each constituting a double-break per phase. These blades are not continuous throughout their length, but each has a gap in the centre which is bridged by the fuse cartridge. Some designs have only a single break per phase, as shown in fi gure 4.57. and fi gure 4.58. §

Fi g. 4.58. Symbol for a non-automatic fuse-switch


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Designing an electrical installation (Beginner Guide) The current range for these devices is limited to 100 A maximum at 400 V 3-phase, while their principal use is in domestic and similar installations. To avoid confusion between the first group (i.e. automatic tripping) and the second group, the term “switchfuse” should be qualified by the adjectives “auto matic” or “non-automatic”. 4.6.3.2. Fuse - disconnector + discontactor, fuse - switch-disconnector + discontactor

As previously mentioned, a discontactor does not provide protection against shortcircuit faults. It is necessary, therefore, to add fuses (generally of type aM ) to perform this function. The combination is used mainly for motor-control circuits, where the disconnector or switch-disconnector allows safe o perations such as: the changing of fuse links (with the circuit isolated); work on the circuit downstream of the discontactor (risk of remote closure of the discontactor). § §

Fi g. 4.59. Symbol for a fuse-disconnector + discontactor

The fuse-disconnector must be interlocked with the discontactor such that no opening or closing manoeuvre of the fuse-disconnector is possible unless the discontactor is open (F igur e 4.59.), since the fusedisconnector has no load-switching capability. A fuse-switch-disconnector (evidently) requires no interlocking ( F igur e 4.60 . The switch must be of class AC22 or AC23 if the circuit supplies a motor.

Fi g. 4.60. Symbol for a fuse-switchdisconnector + discontactor


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Designing an electrical installation (Beginner Guide) 4.6.3.3. Circuit-breaker + contactor circuit-breaker + discontactor

These combinations are used in remotely controlled distribution systems in which the rate of switching is high, or for control and protection of a circuit supplying motors.

4.6.4. Selection of a circuit breaker 4.6.4.1. Choice of a circuit breaker

The choice of a CB is made in terms of: electrical characteristics of the installation for which the CB is destined; its eventual environment: ambient temperature, in a kiosk or switchboard enclosure, climatic conditions, etc. short-circuit current breaking and making requirements; operational specifications: discriminative tripping, requirements (or not) for remote control and indication and related auxiliary contacts, auxiliary tripping coils, connection; installation regulations; in particular: protection of persons; load characteristics, such as motors, fluorescent lighting, LV/LV transformers. The following notes relate to the choice LV circuit breaker for use in distribution systems. § §

§ §

§ §

Choice of rated current in terms of ambient temperature The rated current of a circuit breaker is defined for operation at a given ambient temperature, in general: 30 0C for domestic-type CBs; 40 0C for industrial-type CBs. Performance of these CBs in a different ambient temperature depends principally on the technology of their tripping units (see F ig. 4.61 .) § §

Fi g. 4.61. Ambient temperature

Uncompensated thermal-magnetic tripping units Circuit breakers with uncompensated thermal tripping elements have a trippingcurrent level that depends on the surrounding temperature. If the CB is installed in an enclosure, or in a hot location (boiler room, etc.), the current required to trip the CB on overload will be sensibly reduced. When the temperature in which the CB is located


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Designing an electrical installation (Beginner Guide) exceeds its reference temperature, it will therefore be “derated”. For this reason, CB manufacturers provide tables which indicate factors to apply at temperatures different to the CB reference temperature. It may be noted from typical examples of such tables (see F ig. 4.62.) that a lower temperature than the reference value produces an up-rating of the CB. Moreover, small modular-type CBs mounted in juxtaposition, are usually mounted in a small closed metal case. In this situation, mutual heating, when passing normal load currents, generally requires them to be derated by a factor of 0.8.

F ig. 4.62. Examples of tables for the determination of derating/uprating factors to apply to CBs with

uncompensated thermal tripping units, according to temperature

Example What rating ( I n) should be selected for a C60 N? protecting a circuit, the maximum load current of which is estimated to be 34 A installed side-by-side with other CBs in a closed distribution box in an ambient temperature of 50 0C A C60N circuit breaker rated at 40 A would be derated to 35.6 A in ambient air at 0 50 C (see F ig. 4.62.). To allow for mutual heating in the enclosed space, however, the 0.8 factor noted above must be employed, so that, 35.6·0.8 = 28.5 A, which is not su itable for the 34 A load. A 50 A circuit breaker would therefore be selected, giving a (derated) current rating of 44⋅0.8 = 35.2 A. § § §

Compensated thermal-magnetic tripping units These tripping units include a bi-metal compensating strip which allows the overload trip-current setting ( I r or I rth) to be adjusted, within a specified range, irrespective of the ambient temperature. For example: In certain countries, the TT system is standard on LV distribution systems, and domestic (and similar) installations are protected at the service position by a circuit breaker provided by the supply authority. This CB, besides affording §


97

Designing an electrical installation (Beginner Guide) protection against indirect-contact hazard, will trip on overload; in this case, if the consumer exceeds the current level stated in his supply contract with the power authority. The circuit breaker ( ≤ 60 A) is compensated for a temperature range of 0 0 - 5 C to + 40 C.

Fi g. 4.63. Derating of Masterpact NW20 circuit breaker, according to the te mperature §

LV circuit breakers at ratings ≤ 630 A are commonly equipped with compensated 0 0 tripping units for this range (- 5 C to + 40 C).

Electronic tripping units An important advantage with electronic tripping units is their stable performance in changing temperature conditions. However, the switchgear itself often imposes operational limits in elevated temperatures, so that manufacturers generally provide an operating chart relating the maximum values of permissible trip-current levels to the ambient temperature (see F ig. 4.63.). Selection of an instantaneous, or short-time-delay, tripping threshold F igur e 4.64. below summarizes the main c haracteristics of the instantaneous or shorttime delay trip units. Selection of a circuit breaker according to the short-circuit breaking capacity requirements The installation of a LV circuit breaker requires that its short-circuit breaking capacity (or that of the CB together with an associated device) be equal to or exceeds the calculated prospective short-circuit current at its point of installation.


98

Designing an electrical installation (Beginner Guide) The installation of a circuit breaker in a LV installation must fulfil one of the two following conditions: either have a rated short-circuit breaking capacity Icu (or Icn) which is equal to or exceeds the prospective short-circuit current calculated for its point of installation, or if this is not the case, be associated with another device which is located upstream, and which has the required short-circuit breaking capac ity. §

§

Fi g. 4.64. Different tripping units, instantaneous or short-time-delayed

In the second case, the characteristics of the two devices must be co-ordinated such that the energy permitted to pass through the upstream device must not exceed that which the downstream device and all associated cables, wires and other components can withstand, without being damaged in any way. This technique is profitably employed in: associations of fuses and circuit breakers or associations of current-limiting circuit breakers and standard circuit breakers. 4.6.4.2. The selection of main and principal circuit breakers A single transformer The circuit breaker at the output of the smallest transformer must have a short-circuit capacity adequate for a fault current which is higher than that through any of the other transformer LV circuit breakers. If the transformer is located in a consumer’s substation, certain national standards require a LV circuit breaker in which the open contacts are clearly visible.


99

Designing an electrical installation (Beginner Guide) Example (see F i g. 4.65.)

What type of circuit breaker is suitable for the main circuit breaker of an installation supplied through a 250 kVA HV/LV (400 V) 3-phase transformer in a consumer’s substation? I n transformer = 360 A I sc (3-phase) = 8.9 kA A Compact NS400N with an adjustable tripping-unit range of 160 A - 400 A and a short-circuit breaking capacity ( I cu) of 45 kA would be a suitable choice for this duty.

Fi g. 4.65. Example of a transformer in a

consumer’s substation

Several transformers in parallel (see F i g. 4.66.) The circuit breakers CBP outgoing from the LV distribution board must each be capable of breaking the total fault current from all transformers connected to the busbars: I sc1 + I sc2 + I sc3. The circuit breakers CBM, each controlling the output of a transformer, must be capable of dealing with a maximum short-circuit current of (for example) I sc2 + I sc3 only, for a short-circuit located on the upstream side of CBM1. From these considerations, it will be seen that the circuit breaker of the smallest transformer will be subjected to the highest level of fault current in these circumstances, while the circuit breaker of the largest transformer will pass the lowest level of shortcircuit current. The ratings of CBMs must be chosen according to the kVA ratings of the associated transformers. Note: The essential conditions for the successful operation of 3-phase transformers in parallel may be summarized as follows: the phase shift of the voltages, primary to secondary, must be the same in all units to be paralleled; the open-circuit voltage ratios, primary to secondary, must be the same in all units; the short-circuit impedance voltage ( Zsc%) must be the same for all units. For example, a 750 kVA transformer with a Zsc = 6% will share the load correctly with a 1,000 kVA transformer having a Zsc of 6%, i.e. the transformers will be loaded automatically in proportion to their kVA ratings. For transformers having a ratio of kVA ratings exceeding 2, parallel operation is not recommended. F igur e 4.67. indicates, for the most usual arrangement (2 or 3 transformers of equal kVA ratings) the maximum short-circuit currents to which main and principal CBs (CBM and CBP respectively, in fi gure 4.66.) are subjected. It is based on the following hypotheses: the short-circuit 3-phase power on the HV side of the transformer is 500 MVA, §

§

§

§

§

§

§


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Designing an electrical installation (Beginner Guide) the transformers are standard 20/0.4 kV distribution-type units rated as listed, the cables from each transformer to its LV circuit breaker comprise 5 metres of singlecore conductors, between each incoming-circuit CBM and each outgoing-circuit CBP there is 1 metre of busbar, the switchgear is installed in a floormounted enclosed switchboard, in an 0 ambientair temperature of 30 C. Moreover, this table shows selected circuit breakers of M-G manufacture recommended for main and principal circuit breakers in each case. § §

§

§

Fi g. 4.66. Transformers in parallel

Example (see F ig. 4.68.) Circuit breaker selection for CBM duty: .), I n for an 800 kVA transformer = 1.126 A I cu (minimum) = 38 kA (from fi gure 4.67 the CBM indicated in the table is a Compact NS1250N ( I cu = 50 kA) Circuit breaker selection for CBP duty: The short-circuit breaking capacity ( I cu) required for these circuit breakers is given in the fi gure 4.67. as 56 kA. §

§

Fi g. 4.67. Maximum values of short-circuit current to be interrupted by main and principal circuit breakers

(CBM and CBP respectively), for several transformers in parallel


101

Designing an electrical installation (Beginner Guide) A recommended choice for the three outgoing circuits 1, 2 and 3 would be current limiting circuit breakers types NS400 L, NS250 L and NS 100 L. The I cu rating in each case = 150 kA. These circuit breakers provide the advantages of: Absolute discrimination with the upstream (CBM) breakers, Exploitation of the “cascading” technique, with its attendant economy for all downstream components. § §

Fi g. 4.68. Transformers in parallel

4.6.5. Protection of circuits according GL The rating of switches must correspond at least to the current rating of the fuse protecting the circuit in question and they must have a making/breaking capacity in accordance with category AC-21 or DC-21. Where the sequence busbar-switch-fuse is chosen, the making/breaking capacity must conform to category AC-23 or DC-23. Each supply line run from the main switchboard must be provided with a circuit breaker with overcurrent and short-circuit protection or with a fuse for each non-earthed conductor and an all-pole switch, or with a co ntactor with control switch. Where fuses and switches are employed, the sequence busbar-fuse-switch is to be used. The specified sequence may be changed where motor switches of service category AC-23 are used as switches, provided that the switches are weldproof even in the event of a short circuit. Fuse links must have an enclosed fusion space. They must be made of ceramic or other material recognized by GL as equivalent. The fusible link must be embedded in heat-absorbent material. Fuses may be used for overload protection only up to a rating of 315A. Exception to this rule are subject to approval by GL.

4.6.6. Protection of circuits according Bureau Veritas 1. Each separate circuit shall be protected against short-circuit and against overload.


102

Designing an electrical installation (Beginner Guide) 2. Each circuit is to be protected by a multipole circuit-breaker or switch and fuses against overloads and short-circuits. 3. Circuits for lighting are to be disconnected on both non-earthed conductors, single pole disconnection of final sub-circuits with both poles insulated is permitted only in accommodation spaces. 4. The protective devices of the circuits supplying motors are to allow excess current to pass during transient starting of motors. 5. Final sub-circuits which supply one consumer with its own overload protection (for example motors), or consumers which cannot be overloaded (for example permanently wired circuits and lighting circuits), may be provided with short-circuit protection only. 6. Steering gear circuits are to be provided with short-circuit protection only.

4.7. Coordination between circuit breakers 4.7.1. Cascading Definition of the cascading technique By limiting the peak value of short-circuit current passing through it, a currentlimiting CB permits the use, in all circuits downstream of its location, of switchgear and circuit components having much lower short-circuit breaking capacities, and thermal and electromechanical withstand capabilities than would otherwise be the case. Reduced physical size and lower performance requirements lead to substantial economies and to the simplification of installation work. Advantages of cascading The limitation of current benefits all downstream circuits that are controlled by the current-limiting CB concerned. The principle is not restrictive, i.e. current-limiting CBs can be installed at any point in an installation where the downstream circuits would otherwise be inadequately rated. The result is: simplified short-circuit current calculations, simplification, i.e. a wider choice of downstream switchgear and appliances, the use of lighter-duty switchgear and appliances, w ith consequently lower cost economy of space requirements, since light-duty equipment is generally less voluminous. § § § §

4.7.2. Discriminative tripping (selectivity) Selectivity (discrimination) is the ability of an electrical system to interrupt a circuit suffering a short-circuit from the system to maintain safe operation of the remaining consumers.


103

Designing an electrical installation (Beginner Guide) Discrimination is achieved by automatic protective devices if a fault condition, occurring at any point in the installation, is cleared by the protective device located immediately upstream of the fault, while all other protective devices remain unaffected (see F ig. 4.69.).

Fi g. 4.69. Absolute and partial discrimination

Discrimination between circuit-breakers A and B is absolute if the maximum value of short-circuit current on circuit B does not exceed the short-circuit trip setting of circuit breaker A. For this condition, B only will trip (see F ig. 4.70.). Discrimination is partial if the maximum possible short-circuit current on circuit B exceeds the short-circuit trip-current setting of circuit breaker A. For this maximum condition, both A and B will trip (see F ig. 4.71.).

Fi g. 4.70. Absolute discrimination between CBs A and B


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Fi g. 4.71. Partial discrimination between CBs A and B

Discrimination based on current levels: Protection against overload (see F ig. 4.73.a ) This method is realized by setting successive relay tripping thresholds at stepped levels, from downstream relays (lower settings) towards the source (higher settings). Discrimination is absolute or partial, according to the particular conditions, as noted in the above examples. As a rule, discrimination is achieved when: I I rA < 2 ; rmA > 2 I r I rmB

The discrimination limit is I rmA. Discrimination based on stepped time delays: Protection against low level short-circuit currents (see F ig. 4.72.b ) This method is implemented by adjusting the time-delayed tripping units, such that downstream relays have the shortest operating times, with progressively longer delays towards the source. In the two-level arrangement shown, upstream circuit breaker A is delayed sufficiently to ensure absolute discrimination with B (for example: Masterpact electronic). Current-level discrimination Current-level discrimination is achieved with circuit-breakers, preferably limiters, and stepped current-level settings of the instantaneous magnetic-trip elements: The downstream circuit-breaker is not a current-limiter. The discrimination may be absolute or partial for a short-circuit fault downstream of B. Absolute discrimination in this situation is practically impossible because I scA ≈ I scB, so that both circuit breakers will generally trip in unison. In this case discrimination is partial, and limited to the I rm of the upstream circuit breaker. The downstream circuit breaker is a current limiter. Improvement in discriminative tripping can be obtained by using a current limiter in a downstream location, e.g. for circuit breaker B. § §

§


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Designing an electrical installation (Beginner Guide) For a short-circuit downstream of B, t he limited level of peak current I B would operate the (suitably adjusted) magnetic trip unit of B, but would be insufficient to cause circuit breaker A to trip.

F ig. 4.72. Discrimination (selectivity)

Fi g. 4.73. Downstream limiting circuit breaker B

Note: All LV breakers (considered here) have some inherent degree of current

limitation, even those that are not classified as current limiters. This accounts for the curved characteristic shown for the standard circuit breaker A in fi gure 4.73. Careful calculation and testing is necessary, however, to ensure satisfactory performance of this arrangement. The upstream circuit-breaker is high-speed with a short-delay (SD) feature. These circuit-breakers are fitted with trip units which include a non-adjustable mechanical short-time-delay feature. The delay is sufficient to ensure absolute discrimination with any downstream high-speed CB at any value of short-circuit current up to I rms (see F ig. 4.74.). §


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Fi g. 4.74. Use of a “selective” circuit breaker upstream

Example Circuit breaker A: Compact NS250 N fitted with a trip unit which includes a SD feature, I r = 250 A, magnetic trip set at 2,000 A. Circuit breaker B: Compact NS100N, I r = 100 A. The Merlin Gerin distribution catalogue indicates a discrimination limit of 3,000 A (an improvement over the limit of 2,500 A obtained when using a standard tripping unit). Time-based discrimination This technique requires: The introduction of “timers” into the tripping mechanisms of CBs. CBs with adequate thermal and mechanical withstand capabilities at the elevated current levels and time delays envisaged. Two circuit breakers A and B in series (i.e. passing the same current) are discriminative if the current-breaking period of downstream breaker B is less than the non-tripping time of circuit breaker A. § §

Fi g. 4.75. Discrimination by time delay

Discrimination at several levels An example of a practical scheme with circuit-breakers Masterpact (electronic protection devices). These CBs can be equipped with adjustable timers which allow 4 time-step selections, such as:


107


§

the delay corresponding to a given step is greater than the total current breaking time of the next lower step, The delay corresponding to the first step greater than the total current-breaking time a high-speed CB (type Compact for example) or o f fuses (see F ig. 4.75.). Main Bus C.B.

G

C.B.

G

F2

Bus I

Bus II C.B. F1

M Fi g. 4.76. Selectivity in ships systems

Main Bus

80 kA

Bus I 50 kA

Bus II

10 kA

Fi g. 4.77. Natural selectivity (current selectivity)


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CHAPTER

5 5. Electrical machines 5.1. Induction motors 5.1.1. The basic functions of the motor-starters A motor starter consists of five basic functions: the disconnection, the interruption, the protection against the short-circuits, the protection against the overcharges and the commutation. The disconnection It is necessary to isolate, overall or partial, the circuits of their source of energy supply (main power supply) in order to be able to intervene on facilities for guaranteeing the security of the intervening parties. The function said of "disconnection" is assured by disconnectors. It can be integrated in multifunction equipments having, by conception, the faculty to the disconnection, such the switches-disconnectors, the circuit-breakers. The interruption Whereas an installation is in service, it is sometimes necessary to interrupt its electric power supply in full charge, this capable to serve as emergency stop. The function so-called "interruption" is assured by switches. She is also integrated in multifunction equipments, like the switches-disconnectors, the circuit-breakers. The protection against the short-circuit Facilities and the motors can be the seat of electric incidents or mechanical resulting in a fast and important elevation of the absorbed current. Whereas au starting a standard motor absorbs 6 to 8 times its rated current, a superior current of 10 to 13 times the rated current is a fault current. It is assimilated to a short-circuit current. In order to avoid the deterioration of facilities and equipments, the perturbations on the network supply and the risks of human accidents, it is indispensable to detect these short-circuit and to interrupt the concerned circuit quickly. The protective function against the short-circuit is assured by fuses or circuit-breakers. Note: The motor starters are part of the terminal circuits of an electrical installation. A defect in a terminal circuit must not disturb the other circuits of the installation. It is necessary to protect the installation against the consequences of the short-circuit in the terminal circuits while assuring the selectivity and the filiation of the magnetic protections of the installation.


109

Designing an electrical installation (Beginner Guide) The protection against the overcharges The mechanical overcharges and the shortcomings of the networks supply are the most frequent reasons of the overcharge supported by the motors. They provoke an important increase of the current absorbed by the motor, who conducted to an excessive warming-up of the motor, strongly reducing its lifetime, and capable to go until its destruction. It is therefore necessary to detect the overcharge of the motor. The protective function against the overcharges is assured by thermal protective relay, multifunction electronic relays, or special magnetic-thermal circuit-breakers so-called: "motor circuit-breakers". She is integrated in the motor starters-retarders and the electronic speed variators. A complementary protection can be achieved by protective relays with probes or by multifunction electronic relays, associated to thermal protection via PTC thermal probe integrated in the coils of the motor. Note: the starting time of a motor is bound closely to the features of the driven machine: off-load starting, on-weak-load starting or strong inertia starting, etc. The motors can absorb an important starting current during one variable time of some seconds to several decade seconds. The thermal protection relays are distributed in classes of starting point (release classes) permitting the adjustment of the thermal protection to the needs of the motor (to the motor needs). The commutation She’s role is to establish and to cut the power supply of the motor. The function socalled "commutation" is assured by electromagnetic contactors. In case of manual control, she can be assured by motor circuit-breakers or by switches, imperatively associated to release devices, on lack of power supply, and under some conditions by motor-starters or by electronic speed variators. Note: According to the nature of the commuted loads (motors, resistances, transforming, etc.), the cadence and the fashion of use, the poles of the contactor, are variously solicited, to the establishment as to the cut of the circuits. The choice of the caliber of a contactor is adapted to its use, according to the categories of employment.

5.1.2. The motor start solutions 5.1.2.1. D.O.L. solutions "Three products" solution

She rests on the association of three d istinct equipments: a magnetic circuit-breaker or a switch-disconnector with fuses integrating the functions: disconnection, interruption and protection against the short-circuit; a thermal protective relay integrating the protection against the overcharges; a contactor integrating the commutation function. She permits the realization of motor-starters on the whole beach of low power tension, whatever is their complexity or their specificity. The separation of the functions is an answer to some constraints of implementation and of exploitation; she assures a particularly comfortable maintenance. §

§ §


110


Example 1

magnetic circuit-breaker GV2, NS

+ contactors LC1 K, D, F and CV

+

thermal relays LR2 K, D, F

or

multifunction relays LT6


111

Designing an electrical installation (Beginner Guide) Example 2

switch-disconnector GS1

+ contactors LC1 K, D, F, V and CV

+ thermal relays LR2 K, D, LR9D, F or multifunction relays LT6


112

Designing an electrical installation (Beginner Guide) "Two products” solution

She is about the association of two distinct equipments: a motor circuit-breaker integrating the functions disconnection, interruption, protection against the short-circuit and the overcharges; a contactor dedicated to the function commutation. She permits the realization of direct on line motor-starter on the whole beach of low power tension, in a reduce clutter, and allow a comfortable maintenance. §

§

Example 3

+

circuit-breakers GV2 ME, GV2 P, GV3, GV7, NS

contactors LC1 K, D, F and CV

"One product" solution

This solution is achieved: either by a combined type equipment associating in a same ensemble one circuit breaker and a contactor; She permits the realization of the direct on line starter of small power motors in one optimized clutter. either by a "complete" type equipment regrouping in only one ensemble the all basic functions of the starter. She permits the realization of the direct on line starter of small and middle power motors in an optimized clutter, and guarantees the continuity of service (total coordination). §

§

Hand starter

motor circuit-breakers GV2 ME, GV7


113

Designing an electrical installation (Beginner Guide) Automatic starters

combined automatic circuit-breakers GV2 ME or GV2 P

contactors-circuit-breakers Integral 18, 32, 63

5.1.2.2. Star-delta for motor control This method of starting is only applicable to 3-phases motors whose delta connection corresponds to the main voltage and on which all 6 stator terminals are accessible. Star-delta starting should be used for motors starting on no-load or having a low load torque and gradual build-up: - the starting torque in star connection is reduced to on third of the direct starting torque, i.e. about 50 % of the rated torque; - the starting current in star connection is about 1.8 to 2 times the rated current. The transition from star to delta connection must occur when the machine has run up to speed. A too rapid build up in load torque would cause the stabilized run-up speed to bee too low and would therefore eliminate any advantage in this method of starting: this is the case with certain machines whose load torque depends on its speed (characteristic of centrifugal machines, for example). Romanian Electro Trade, Engineering & Consulting

114

Designing an electrical installation (Beginner Guide) Switching from star to delta connection must be complete within a minimum time. A control relay and a time delay auxiliary contact block perform this function.

Power diagram

Control diagram

Note: in accordance with the norms of facilities in force, every departure must be

protected against the short-circuit by fuses or a circuit-breaker.


115


5.1.2.3. Star-double star solution (Dahlander connection)

5.1.3. Variable speed drives for asynchronous motors (Altivar 38) 5.1.3.1. Applications

The Altivar 38 is a frequency inverter for three-phase asynchronous motors powered by a three-phase supply 360 V to 460 V in the power range 0.75 kW to 315 kW. The Altivar 38 has been designed for state-of-the-art applications in heating, ventilation and air conditioning in industrial and commercial buildings: ventilation; air conditioning; pumping. The Altivar 38 can reduce operating costs in buildings by optimizing energy consumption whilst improving user comfort. § § §


116

Designing an electrical installation (Beginner Guide) Its numerous integrated options enable it to be adapted to and incorporated into electrical installations and sophisticated control systems. The need for electromagnetic compatibility was taken into account at the outset of designing the drive. Depending on the drive rating, filters and chokes are either builtin or available as optional accessories. 5.1.3.2. Functions

The Altivar 38 is supplied ready for use in pumping and ventilation applications. It comprises a terminal which can be used to modify programming, adjustment, control or monitoring functions in order to adapt and customize the application to meet individual customer requirements. Specific functions for pumping/ventilation: - energy saving; - automatic catching a spinning load with speed det ection (catch on the fly); - adaptation of current limiting according to speed; - faster/slower, preset speeds; - integrated PI control, with preset PI references; - electricity and service hours meter; - motor noise reduction. §

Protection functions: - motor and fan thermal protection via PTC thermal probe; - protection against overloads and overcurrents in co ntinuous operation; - machine mechanical protection via jump frequency function; - protection via multiple fault management and configurable alarms. §

Easy to integrate into control systems: - 4 logic inputs, 2 relay outputs, 2 analog inputs and 1 analog output; - plug-in I/O connectors; - display of electrical variables and operating indicators; - an RS 485 multidrop serial link with Modbus protocol as standard in the drive. This serial link can be used to connect PLCs, a PC, communication gateways or one of the available programming tools. §

5.1.3.3. Options §

PowerSuite advanced dialogue solutions: 3 solutions are available, with plain text display in 5 languages (English, French, German, Spanish, Italian) and configuration memory: - Power Suite Pocket PC; - Power Suite software workshop; - Magelis display unit.

Customizing the application: - I/O extension cards; - application cards: pump switching, multi-motor function, multiple parameter settings and cycles; §


117

Designing an electrical installation (Beginner Guide) - communication cards for bus or network: METASYS N2, Ethernet, Fipio, Uni-Telway/Modbus, Modbus Plus, AS-i, Profibus DP, Interbus-S, CANopen, DeviceNet. - communication module for LonWorks bus. 5.1.3.4. Characteristics Torque characteristics (typical curves) The curves below define the available continuous torque and transient overtorque for both force-cooled and self-cooled motors. The only difference is in the ability of the motor to provide a high continuous torque at less than half the nominal speed.

1 self-cooled motor: continuous useful torque 2 force-cooled motor: continuous useful torque 3 transient overtorque 4 torque in overspeed at constant power

Motor thermal protection The Altivar 38 drive features motor thermal protection designed specifically for self-cooled or forced-cooled variable speed motors. This motor thermal protection is designed for a maximum ambient temperature of 40°C around the motor. If the temperature around the motor exceeds 40°C, thermal protection should be provided directly by thermistor probes integrated into the motor using one of the available option cards. 5.1.3.5. Special uses Switching the motor at the drive output The drive can be switched when locked or unlocked. If the drive is switched on-thefly (drive locked), the motor is controlled and accelerates until it reaches the reference speed smoothly following the acceleration ramp. The "flying restart" must be configured for this type of use and the "loss of motor phase" protection function must be disabled. Example: breaking of downstream contactor. : breaking safety circuit at drive outputs, "bypass" function, Typical appli cations switching of motors connected in parallel.


118


Operation with intermittent cycle and high switching frequency If the operating conditions are intermittent and the maximum cumalative running time is 36 s per 60 s cycle (load factor 60%), it is possible to operate at a high switching frequency without derating the power. Connecting motors in parallel The nominal current of the drive must be greater than or equal to the sum of the currents of the motors to be controlled. In this case, provide external thermal protection for each motor using thermal probes or relays. If the number of motors connected in parallel is ≥ 3, it is advisable to install an output filter between the drive and the motors or to reduce the switching frequency. If several motors are used in parallel, there are 2 possible scenarios: - the motors have equal power ratings, in which case the torque characteristics will remain optimised after the drive has been configured, or - the motors have different power ratings, in which case the drive configuration will be incompatible for the motors with the lowest power ratings and the overtorque at low speed will be considerably reduced. Ensure that the cables are the correct length. As the leakage currents are proportional to the total length of the cable between the drive and the motors, ensure L ≤ 100 m by L = l 1 + l 2 + l x + l 4.


119



120

Designing an electrical installation (Beginner Guide) 5.1.3.6. Connection diagrams


121


(1) Line choke recommended (2) Fault relay contacts for remote signalling of drive status (3) Internal + 24 V. If an external + 24 V supply is used, connect the 0 V on the external supply to the COM terminal, do not use the + 24 terminal on the

drive, and connect the common of the LI inputs to the + 24 V of the external supply. (4) Relay R2 can be reassigned (5) X and Y can be configu red between 0 and 20 mA independently for AI2 and AO1. Note: 1 All terminals are located at the bottom of the drive. All 2 Fit interference suppressors to all specific circuits near the drive or connected on the same circuit, such as relays, contactors, solenoid valves,

fluorescent lighting, etc.


122


(1) Line choke recommended (2) Fault relay contacts for remote signalling of drive status (3) Internal + 24 V. If an external + 24 V supply is used, connect the 0 V on the external supply to the COM terminal, do not use the + 24 terminal on the

drive, and connect the common of the LI inputs to the + 24 V of the external supply. (4) Relay R2 can be reassigned (5) X and Y can be configu red between 0 and 20 mA independently for AI2 and AO1. Note: 1 All terminals are located at the bottom of the drive. All 2 Fit interference suppressors to all specific circuits near the drive or connected on the same circuit, such as relays, contactors, solenoid valves,

fluorescent lighting, etc.


122



123



123


(1) Line choke recommended (2) Fault relay contacts for remote signalling of drive status (3) Internal + 24 V. If an external +24 V supply is used, connect the 0 V on the external supply to the COM terminal, do not use the + 24 terminal on the

drive, and connect the common of the LI inputs to the + 24 V of the external supply. (4) Use the "downstream contactor control" function with relay R2 (or with the logic output LO of one of the "I/O extension" cards, when connecting). (5) X and Y can b e configured between 0 and 20 mA independently for AI2 and AO1. Note: 1 All terminals are located at the bottom of the drive. Fit interference suppressors to all specific circuits near the drive or connected on the same circuit, such as relays, contactors, solenoid 2

valves, fluorescent lighting, etc.


124


(1) Line choke recommended (2) Fault relay contacts for remote signalling of drive status (3) Internal + 24 V. If an external +24 V supply is used, connect the 0 V on the external supply to the COM terminal, do not use the + 24 terminal on the

drive, and connect the common of the LI inputs to the + 24 V of the external supply. (4) Use the "downstream contactor control" function with relay R2 (or with the logic output LO of one of the "I/O extension" cards, when connecting). (5) X and Y can b e configured between 0 and 20 mA independently for AI2 and AO1. Note: 1 All terminals are located at the bottom of the drive. Fit interference suppressors to all specific circuits near the drive or connected on the same circuit, such as relays, contactors, solenoid 2

valves, fluorescent lighting, etc.



2-wire control Used to control the direction of operation by means of a maintained contact. Enabled by means of 1 or 2 logic inputs (one or two directions). This function is suitable for all one or two direction applications. Three operating modes are po ssible: - detection of the state of the logic inputs; - detection of a change in state of the logic inputs; - detection of the state of the logic inputs with forward operation always having priority over reverse. §

3-wire control Used to control the operating and stopping direction by means of pulsed contacts. Enabled by means of 2 or 3 logic inputs (non-reversing or reversing). This function is suitable for all non-reversing and reversing applicat ions. §

124


2-wire control Used to control the direction of operation by means of a maintained contact. Enabled by means of 1 or 2 logic inputs (one or two directions). This function is suitable for all one or two direction applications. Three operating modes are po ssible: - detection of the state of the logic inputs; - detection of a change in state of the logic inputs; - detection of the state of the logic inputs with forward operation always having priority over reverse. §

3-wire control Used to control the operating and stopping direction by means of pulsed contacts. Enabled by means of 2 or 3 logic inputs (non-reversing or reversing). This function is suitable for all non-reversing and reversing applicat ions. §


125


5.2. Connection diagrams for synchronous generators with slip rings U2

W2

V2

sliprings regulator rotor

F1

F2

stator

U1

V1

W1

brushlees type U2

V2

W2 Rot. Rect. rotor

regulator

Exc. Arm.

stator

U1

Exc. field

V1

W1


126


5.3. Connection diagrams for DC- Motors Shunt-connection

Series-connection

L+ L-

L+ LL

L M R

R

IA

I A1 CLOCK ROTATION

I

IE

E2

I

B2 E1

A1

(A2)

B2

D1

D2

(A2) (B1)

(B1)

L+ L-

L+ LL

L M R

I

IA A1

E2

B2

R I

IE E1

D1 B2

A1

I D2

ANTI CLOCK ROTATION

(A2)

(A2) (B1)

(B1)


127


Separate excited

Compound- connection L+

L+ L-

L-

L

L 2L+

R

2LR

IA

IA A1 B2

IE

IE

M

IA IE

F2

I

F1

CLOCK ROTATION

A1

B2

D1

D2

(A2) (B1)

E2

E1

(A2) (B1)

L

L

L-

L-

L IA

ANTI CLOCK ROTATION

A

(B1)

R

B2

L 2L IA IE

R IA

IE

F2 F

IE

M

2L-

D

A

B

I D

E2

E1

(A2) (A2) (B1)

5.4. Protection of motors according Bureau Veritas 1. Motors of rating exceeding 1kW and all motors for essential services are to be protected individually against overload and short-circuit. The short-c ircuit protection may be provided by the same protective device for the motor and its supply cable. 2. For motors intended for essential services, the overload protection may be replaced by an overload alarm. 3. The protective devices are to be designed so as to allow excess current to pass during the normal accelerating period of motors according to the conditions corresponding to normal use.


128

Designing an electrical installation (Beginner Guide) If the current/time characteristic of the overload protection device does not correspond to the starting conditions of a motor (e.g. for motors with extra-long starting period), provision may be made to suppress operation of the device during the acceleration period on condition that the short-circuit protection remains operative and the suppression of overload protection is only temporary. 4. For continuous duty motors the protective gear is to have a time delay characteristic which ensures reliable thermal protection against overload. 5. The protective devices are to be adjusted so as to limit the maximum continuous to a value within the range 105%-120% of the motor’s rat ed full load current. 6. For intermittent duty motors the current setting and the delay (as a function of time) of the protective devices are to be chosen in relation to the actual service conditions of the motor. 7. Where fuses are used to protect polyphase motor circuits, means are to be provided to protect the motor against unacceptable overload in case of single phasing. 8. Motors rated above 1kW are to be provided with: undervoltage protection, operative of the reduction or failure of voltage, to cause and maintain the interruption of power in the circuit until the motor is deliberately restarted or undervoltage release, operative on the reduction or failure of voltage, so arranged that the motor restarts automatically when power is restored after a power failure.

§

§

9. The automatic restart of a motor is not to produce a starting current such as to cause excessive voltage drop. In the case of several motors required to restart automatically, the total starting current is not to cause an excessive voltage drop or sudden surge current; to this end, it may be necessary to achieve a sequence start. 10. The undervoltage protective devices are to allow the motor to be started when the voltage exceeds 85% of the rated voltage and are to intervene without fail when the voltage drops to less than approximately 20% of the rated voltage, at the rated frequency and a time delay as necessar y.

5.5. Protection of generators Protection of generators according Bureau Veritas 1. Generators are to be protected against short-circuits and overloads by multipole circuit-breakers. For generators not arranged to operate in parallel with a rated output equal to or less than 50 kVA, a multipole switch with a fuse in each insulated phase on the generator side may be accepted.


129

Designing an electrical installation (Beginner Guide) 2. When multipole switch and fuses are used, the fuse rating is to be maximum 110% of the generator rated current. 3. Where a circuit-breaker is used: a) the overload protection is to trip the generator circuit-breaker at an overload between 10% and 50%; for an overload of 50% of the rated current of the generator the time delay is not to exceed 2 minutes; however, the figure of 50% or the time delay of 2 minutes may be exceeded if the construct ion of the generator permits this.

b) the setting of the short-circuit protection is t o instantaneously trip the generator circuit breaker at an overcurrent less than the steady short-circuit current of the generator. Short time delays (e.g. from 0,5s to 1s) may be introduced for discrimination requirements in “instantaneous” tripping devices. 4. For emergency generators the overload protection may, instead of disconnecting the generator automatically, give a visual and audible alarm in a permanently attended space. 5. After disconnection of a generator due to a overload, the circuit-breaker is to be ready for immediate reclosure. 6. Generator circuit-breakers are to be provided with a reclosing inhibitor which prevents their automatic reclosure after tripping due to a short-circuit. 7. Generators having a capacity of 1500 kVA or above are to be equipped with a suitable protective device or system which, in the event of a short-circuit in the generator or in supply cable between the generator and its circuit-breaker, will de-excite the generator and open the circuit-breaker (e.g. by means of differential protection). 8. Where the main source of electrical power is necessary for the propulsion of the ship, load shedding or other equivalent arrangements are to be provided to protect the generators sustained overload. 9. Arrangements are to be made to disconnect or reduce automatically the excess load when the generators are overloaded in such a way as to prevent a sustained loss of speed and/or voltage. The operation of such device is to activate a visual and audible alarm. A time delay of 5-20s is considered acceptable. 10. When an overload is detected the load shedding system is to disconnect automatically, after an appropriate time delay, the circuits supplying the non-essential services and, if necessary, the secondary essential service in a second stage. 11. Alternating current generators arranged to operate in parallel are to be provided with the reverse-power protection. The protection is to be selected in accordance with the characteristics of the prime mover. The following values are recommended: 2-6% of the rated power for the t urbogenerators 8-15% of the rated power for diesel generators. § §


130

Designing an electrical installation (Beginner Guide) The reverse-power protection may be replaced by other devices ensuring adequate protection of the prime movers. 12. Generators are to be provided with an undervoltage protection which trips the breaker if the voltage falls to 70%-35% of the rated voltage. The undervoltage release also prevents the closing of the circuit-breaker if the generator voltage does not reach a minimum of 85% of the rated voltage. The operation of the undervoltage release is to be instantaneous preventing closure of the breaker, but it is to be delayed for selectivity purposes when tripping the breaker.

Generator protection according GL Every generator must have the following protective devices: overload 110-150 % rated current, time delay 0 – 2 min.; short-circuit protection more than 150% of the rated current, but less than the permanent short circuit current I KD: - time delay up to 200 ms (DC) - time delay up to 200-500 ms (AC)

§ §

§

§

recommended automatic disconnection direct or in steps of the non-essential consumers. Minimum delay 5 sec and alarm output. On passenger ships this function is obliged. parallel operation: automatic disconnecting of non-essential consumers if one generator trips during parallel operation to avoid the overload of the remaining sets.


131

Designing an electrical installation (Beginner Guide) parallel operation: reverse power protection 4-10 % of the rated output of the diesel generators, 1-3 % for turbo generators. Delay 2-5 sec. low voltage protection 70 % - 30 % U rated. Short t ime delay acc. 0-2min; low frequency protection 90 % rated frequency, 5-10 sec. delay. synchronizer (if auto: also manual) Stand-by system - generators 45 sec - consumers -10 % f rated , delay 5 -10 sec After tripping due to overcurrent, generator power circuit-breakers must at once be ready for reclosing. Thermal trips are not suitable. Generator circuit-breakers must be fitted with a reclosing inhibitor to prevent automatic reconnection after a short circuit trip. For alternators with output ratings of less than 50 kVA fuses and on load switches are also permitted. Any alternator contactors are to have a tripping time lag (up to about 500 ms) and are to be designed to carry at least twice the rated alternator current. §

§ § §

5.6. Transformers A transformer is an electrical device that transfers energy from one electrical circuit to another by magnetic coupling without moving parts. It is often used to convert between high and low voltages and accordingly between low and high currents.

5.6.1. Basic principles A simple single phase transformer consists of two electrical conductors called the primary coil and the secondary coil . The primary is fed with a varying (alternating or pulsed direct current) electric current which creates a varying magnetic field around the conductor. According to the principle of mutual inductance, the secondary, which is placed in this varying magnetic field, will develop a potential difference called an electromotive force or EMF. If the ends of the secondary are connected together to form an electrical circuit, this EMF will cause a current to flow in the secondary. Thus, some of the electrical power fed into t he primary is delivered to the secondary. secondary coil primary coil

magnetic circuit


132

Designing an electrical installation (Beginner Guide) In practical transformers, the primary and secondary conductors are coils of wire because a coil creates a denser magnetic field (higher magnetic flux) than a straight conductor. A transformer winding should never be energized from a constant DC voltage source, as this would cause a large direct current to flow. In such a situation, in an ideal transformer, the current would rise indefinitely as a linear function of time. In practice, the series resistance of the winding limits the amount of current that can flow, until the transformer either reaches thermal equilibrium or is destroyed.

5.6.2. Circuit symbols Standard symbols §

§

§

§

§

Transformer with two windings and iron core.

Transformer with three windings. The dots show the adjacent ends of the windings.

Step-down or step-up transformer. The symbol shows which winding has more turns, but does not usually show the exact ratio.

Transformer with electrostatic screen, which prevents electrostatic coupling between the windings.

Power three-phase transformer

5.6.3. Transformer types Autotransformers An autotransformer has only a single winding, which is tapped at some point along the winding. AC or pulsed DC power is applied across a portion of the winding, and a


133

Designing an electrical installation (Beginner Guide) higher (or lower) voltage is produced across another portion of the same winding. Autotransformers are used to compensate for voltage drop in a distribution system or for matching two transmission voltages, for example 115 V and 138 V. For voltage ratios, not exceeding about 3:1, an autotransformer is less costly, lighter, smaller and more efficient than a two-winding transformer of a similar rating. Current transformers

Current transformers

A current transformer is designed to provide a current in its secondary which is accurately proportional to the current flowing in its primary. Current transformers are commonly used in electricity meters to facilitate the measurement of large currents which would be difficult to measure more directly. Care must be taken that the secondary of a current transformer is not disconnected from its load while current is flowing in the primary as in this circumstance a very high voltage would be produced across the secondary. Current transformers are often constructed with a single primary turn either as an insulated cable passing through a toroidal core, or else as a bar to which circuit conductors are connected. Voltage transformers The voltage transformers are utilized for the extension of measuring instruments. These are usual step-down electric transformers and are utilized for connection of voltmeters and for connection of the parallel coils (windings) of wattmeters, electric meters, relays etc. Pulse transformer A pulse transformer is a transformer that is optimised for transmitting rectangular electrical pulses (that is, pulses with fast rise and fall times and a constant amplitude). Small versions called signal types are used in digital logic and telecommunications circuits, often for matching logic drivers to transmission lines. Medium-sized power versions are used in power-control circuits such as camera flash controllers. Larger power versions are used in the electrical power distribution industry to interface low-voltage control circuitry to the high-voltage gates of power semiconductors such as triacs, IGBTs, thyristors and MOSFETs. To minimise distortion of the pulse shape, a pulse transformer needs to have low values of leakage inductance and distributed capacitance, and a high open-circuit inductance. In power-type pulse transformers, a low coupling capacitance (between the primary and secondary) is important to protect the circuitry on the primary side from high-powered transients created by the load. For the same reason, high insulation resistance and high breakdown voltage are required. A good transient response is


134

Designing an electrical installation (Beginner Guide) necessary to maintain the rectangular pulse shape at the secondary, because a pulse with slow edges would create switching losses in the power semiconductors. The product of the peak pulse voltage and the duration of the pulse (or more accurately, the voltage-time integral) is often used to characterize pulse transformers. Generally speaking, the larger this product, the larger and more expensive the transformer.

Pulse transformer


135


5.6.4. Vector- groups of Transformers VECTOR GROUP

POINTER DIAGRAM HV LV

CONNCTION HV

2V

1V

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

2V

1W

2W

2U

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

Dd0 1U

1W

2U

1V

0

2W

2V

Y 0 1U

1W

2U

1V

2W

2V

Dz0 2U

1U

2U

D 5

2W 1U

1W

1V

5

2W

1W

1V

LV

2W

Yd5 1U

1W

2V

1V

2U 2W

Yz5 1U

1W

2V


136

Designing an electrical installation (Beginner Guide) CONNECTION

POINTER DIAGRAM HV LV

1V

2W

HV

2U

Dd6 1U

1W

2V

1V

6

2W

1W

1V

2V

2W

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

1U

2U

1V

2V

1W

2W

2U

Dz6 1U

1W

1V

2V

2V

Dy11 1W

1U

1V

11

1U

2U

Yy6 1U

LV

2U

2W

2V

Yd11 1U

1W

2W 2U

1V

2V

Yz11 1U

1W

2U

the first letter (majuscule) primary connection the second letter (minuscule) secondary connection the third letter clock hour figure of vector group

2W

Y D Z y, d or z

star connection delta connection zig-zag connection star, delta, zig-zag connection

N or n 0,1,2, …11

sccessible neutral 0 delay of the LV beside HV stated in multiple of 30

: the clock hour figure of a vector group 11 correspond Example

at the delay

11·30° = 330°. Several transformers in parallel The essential conditions for the successful operation of 3-phase transformers in parallel may be summarized as follows:


137


§

§

the phase shift of the voltages, primary to secondary, must be the same in all units to be paralleled; the open-circuit voltage ratios, primary to secondary, must be the same in all units; the short-circuit impedance voltage ( Zsc%) must be the same for all units.

Characteristics: Connection

Yy

Yd

Yz

Transformation ratio

n2

m= m= m=

n1 3 n2

3

⋅

n1

3 n2 2

⋅

Dy

m = 3⋅

Dd

m=

n1 n2 n1

n2 n1

5.6.5. Important equations The voltage in the secondary coil depends on the voltage in the primary coil and the ratio of the number of turns in the secondary and primary coils. If we call: the voltage in and number of turns in the primary V p and N p the voltage in and number of turns in the secondary V s and N s then § §

Vs/Vp= Ns/Np

or, rearranging: Vs= Vp · Ns/Np

Similarly, the current in the secondary coil depends on the current in the primary coil and the ratio of the number of turns in the secondary and primary coils. However, the current is inversely related to the number of turns. If we call: the current in and number of turns in the primary I p and N p the current in and number of turns in the secondary I s and N s then § §

Is/Ip= Np/Ns

or, rearranging: Is= Ip · Np/Ns


138

Designing an electrical installation (Beginner Guide) The power in the primary is equal to the power in the secondary (a ssuming a perfect transformer). Ip ·Vp = Is · Vs

5.6.6. Protection of transformers according Bureau Veritas 1. The primary winding side of the power transformers is to be protected against short-circuit and overload by means of multipole circuit-breakers o r switches and fuses. Overload protection on the primary side may be dispensed with where it is provided on the secondary side or were the total possible load cannot reach the rated power of the transformer. 2. The protection against short-circuit is to be such as to ensure the selectivity between the circuits supplied by the secondary side of the transformer and the feeder circuit of the transformer. 3. When transformers are arranged to operate in parallel, means are to be provided so as to trip the switch on the secondary winding side when the corresponding switch on the primary side is open.

short circuit protection

primary wdg.

overload protection

secondary wdg


139


CHAPTER

6 6. Technical information 6.1. Degrees of protection provided by enclosures Degrees of protection against the penetration of solid bodies, water and personnel access to live parts IP

• • • code

The IP code comprises 2 characteristic numerals (e.g. IP 55) and may include an additional letter when the actual protection of personnel against direct contact with live parts is better than that indicated by the first numeral (e.g. IP 20C). Any characteristic numeral which is unspecified is replaced by an X (e.g. IP XXB). Additional letter: corresponds to protection of personnel against direct contact with live parts: A - with the back of the hand. B - with the finger. C - with a Ø 2.5 mm tool. D - with a Ø 1 mm wire.


140


1st

characteristic

numeral:

corresponds to protection of the equipment against penetration of solid objects and protection of personnel against direct contact with live parts Protection of the equipment Protection of personnel Non-protected machine No special protection 0

2nd characteristic numeral: corresponds to protection of the equipment against penetration of water with harmful effects

0

Non-protected machine

No special protection Protected against vertical dripping water, (condensation).

Protected against the penetration of solid objects having a diameter greater than or equal to 50 mm. Protected against the penetration of solid objects having a diameter greater than or equal to 12.5 mm. Protected against the penetration of solid objects having a diameter greater than or equal to 2.5 mm. Protected against the penetration of solid objects having a diameter > 1 mm.

Protected against direct contact with the back of the hand (accidental contacts).

1

Protected against direct finger contact.

2

Protected against dripping water at an angle of up to 15°.

Protected against direct contact with a Ø 2.5mm tool.

3

Protected against rain at an angle of up to 60°.

Protected against direct contact with a Ø 1mm wire.

4

Protected against splashing water in all directions.

5

Dust protected (no harmful deposits).


5

6

Dust tight.


6

Protected against water jets in all directions. Protected against powerful jets of water and waves. Protected against the effects of temporary immersion. Protected against the effects of prolonged immersion under specified conditions.

1

2

3

4

Ø 50mm

Ø 12.4mm

Ø 2.5mm

Ø 1mm

7

8


141


6.2. Degrees of protection against mechanical impact IK • • code The IK code comprises 2 characteristic numerals (e.g. IK 05) corresponding to a value of impact energy. h (cm) 00

Energy (J)

Non-protected

01

7.5

0.15

02

10

0.2

03

17.5

0.35

04

25

0.5

05

35

0.7

06

20

1

07

40

2

08

30

5

09

20

10

10

40

20


142


6.3. Minimum required degrees of protection on ships (Bureau Veritas) Condition in location

Example of location

Danger of touching live parts only Danger of dripping liquid and/or moderate mechanical damage

Dry accommodation spaces, dry control rooms Control rooms, wheel-house, radio room Engine end boiler rooms above floor

Increased danger of liquid and/or mechanical damage

Steering gear rooms Emergency machinery rooms General storerooms Pantries Provision rooms Ventilation ducts Bathrooms and/or showers Engine and boiler rooms below floor Closed fuel oil separator rooms

Switchboard, control gear, motor starters IP20

Luminaires

Heating appliances

Cooking appliances

Socket outlets

IP20

IP20

IP20

IP20

IP20

Accessories (e.g. switches, connection boxes) IP20

Generators

Motors

Transformers

X(1)

IP20

IP22

X

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP22

IP44

IP44

IP22

IP22

IP22

IP22

IP22

IP22

X

IP44

IP44

IP22

IP22

IP22

IP22

IP22

IP22

X

IP44

IP44

IP22

X

IP22

IP22

IP22

IP22

X

IP22

IP44

IP22 IP22

X X

IP22 IP22

IP22 IP22

IP22 IP22

IP22 IP22

IP22 X

IP44 IP44

IP44 IP44

X

X

IP22

X

X

X

X

X

X

X

X

X

X

IP34

IP44

X

IP55

IP55

X

X

IP44

X

IP34

IP44

X

X

IP55

IP44

X

IP44

IP44

IP34

IP44

X

X

IP55


143


Increased danger of liquid and mechanical damage

Closed lubricating oil separator rooms Ballast pump rooms Refrigerated rooms Galleys and laundries Public bathrooms and shower Shaft or pipe tunnels in double bottom

IP44

X

IP44

IP44

IP34

IP44

X

X

IP55

IP44

X

IP44

X

IP55

IP55

X

IP44 (2) IP44

IP34

X

IP44 (2) IP44

IP34

IP44

X

IP55

IP55

IP44

X

IP44

IP44

IP34

IP44

IP44

IP44

IP44

X

X

IP44

IP44

IP34

IP44

X

IP44

IP44

Danger of IP55 X IP55 IP55 IP55 IP55 X IP56 IP56 liquid spraying, presence of cargo Holds for X X IP55 X IP55 IP55 X IP56 IP56 dust, general cargo serious mechanical damage, aggressive Ventilation X X IP55 X X X X X X fumes trunks Danger of Open decks IP56 X IP56 X IP55 IP56 X IP56 IP56 liquid in massive quantities (1) The symbol “X” denotes equipment which it is not advised to install. (2) Electric motors and starting transformers for lateral thrust propellers located in spaces similar to ballast pump rooms may have degree of pr otection IP22


144


Increased danger of liquid and mechanical damage

Closed lubricating oil separator rooms Ballast pump rooms Refrigerated rooms Galleys and laundries Public bathrooms and shower Shaft or pipe tunnels in double bottom

IP44

X

IP44

IP44

IP34

IP44

X

X

IP55

IP44

X

IP44

X

IP55

IP55

X

IP44 (2) IP44

IP34

X

IP44 (2) IP44

IP34

IP44

X

IP55

IP55

IP44

X

IP44

IP44

IP34

IP44

IP44

IP44

IP44

X

X

IP44

IP44

IP34

IP44

X

IP44

IP44

Danger of IP55 X IP55 IP55 IP55 IP55 X IP56 IP56 liquid spraying, presence of cargo Holds for X X IP55 X IP55 IP55 X IP56 IP56 dust, general cargo serious mechanical damage, aggressive Ventilation X X IP55 X X X X X X fumes trunks Danger of Open decks IP56 X IP56 X IP55 IP56 X IP56 IP56 liquid in massive quantities (1) The symbol “X” denotes equipment which it is not advised to install. (2) Electric motors and starting transformers for lateral thrust propellers located in spaces similar to ballast pump rooms may have degree of pr otection IP22


144


LEXICON §

§

§

§

§

§

Breaking Capacity: a presumed current value that a switching device must be capable of breaking under the recommended conditions of use and behavior. Farthing fault: fault due to the direct or indirect contact of a conductor with the earth or the reduction of its insulation resistance to earth below a specified value. Fault: accidental modification affecting normal operation. Ir: rated current corresponding to the RMS value of the current that the device must be capable of withstanding indefinitely under the recommended conditions of use and operation. Overvoltage: any voltage between a phase conductor and the earth or two neutral phase conductors where the peak value exceeds the highest voltage acceptable for the equipment. Overvoltage factor: ratio between the overvoltage’s peak value and the peak value of the maximum voltage acceptable by the device.


LEXICON §

§

§

§

§

§

§

§

§

§

§

§

§

Breaking Capacity: a presumed current value that a switching device must be capable of breaking under the recommended conditions of use and behavior. Farthing fault: fault due to the direct or indirect contact of a conductor with the earth or the reduction of its insulation resistance to earth below a specified value. Fault: accidental modification affecting normal operation. Ir: rated current corresponding to the RMS value of the current that the device must be capable of withstanding indefinitely under the recommended conditions of use and operation. Overvoltage: any voltage between a phase conductor and the earth or two neutral phase conductors where the peak value exceeds the highest voltage acceptable for the equipment. Overvoltage factor: ratio between the overvoltage’s peak value and the peak value of the maximum voltage acceptable by the device. Rated value: value generally set by the manufacturer for given operating conditions for a component, a mechanism or piece of equipment. Re-ignition: resumption of current between the contacts of a mechanical switching device during a breaking operation, within a quarter cycle after passing to 0 current. Re-striking: resumption of current between the contacts of a mechanical switching device during a breaking operation, after a quarter cycle after passing to 0 current. Short-circuit: an accidental or intentional connection through a resistance or relatively low impedance, of two or more points on a circuit normally existing at different voltages. Switching device: device intended to establish or interrupt current in an electrical circuit. Switchgear: general term applicable to switching devices and their use in combination with control, measurement, protection, and command devices with which they are associated. Ur: rated voltage corresponding to the RMS. value of the voltage that the device must be capable of withstanding indefinitely under the recommended conditions of use and operation.


145

Designing an electrical installation (Beginner Guide) Disconnector: - mechanical connection device which in an open position guarantees a satisfactory isolating distance under specific conditions; - intended to guarantee safe isolation of a circuit, it is often associated with an earthing switch; - used for opening : at no load; for closing : at no load and at short-circuit, depending on the case; for isolating . §

Earthing: - specially designed switch for switch connecting phase conductors to the earth; - intended for safety in case of work on the circuits, it relays the de-energized active conductors to the earth; - used for opening : at no load; for closing : at no load and at short-circuit, depending on the case. §

§

System Earthing Arrangements

Mains electricity systems are categorised in the many European countries (Finland, UK, etc.) according to how the earthing is implemented. The common ones are TN-S, TN-C-S and TT. Note that in these descriptions, “system” includes both the supply and the installation, and “live parts” includes the neutral conductor. Description of letter First letter: T The live parts in the system have one or more direct connections to earth. I The live parts in the system have no connection to earth, or are connected only through a high impedance. Second letter: T All exposed conductive parts are connected via your earth conductors to a local ground connection. N All exposed conductive parts are connected via your earth conductors to the earth provided by the supplier. Remaining letter(s): C Combined neutral and protective earth functions (same conductor). S Separate neutral and protective earth functions (separate conductors). Valid systems types in the 16

TN-C TN-S TN-C-S TT

th

Edition IEE regs:

No separate earth conductors anywhere – neutral used as earth throughout supply and installation (never seen this). Probably most common, with supplier providing a separate earth conductor back to the substation. [Protective Multiple Earthing] Supply combines neutral and earth, but they are separated out in the installation. No earth provided by supplier; installation requires own earth rod (common with overhead supply lines).


146

Designing an electrical installation (Beginner Guide) IT

Supply is e.g. portable generator with no earth connection, installation supplies own earth rod.

Switch: - mechanical connection device capable of establishing, sustaining and breaking currents under normal circuit conditions eventually including overload currents in service; - intended to control circuits (opening and closing), it is often intended to perform the insulating function. In public and private MV distribution networks it is frequently associated with fuses; - used for opening : at no load and under load; for closing : at no load, under load and at short-circuit; for isolating : depending on the case. §

Contactor: - mechanical connection device with a single rest position, controlled other than by hand, capable of establishing, sustaining and breaking currents under normal circuit conditions, including overvoltage conditions in service; - intended to function very frequently, it is mainly used for motor control; - used for opening : at no load and under load; for closing : at no load, under load and at short-circuit, §

Circuit-breaker: - mechanical connection device capable of establishing, sustaining and breaking currents under normal circuit conditions and under specific abnormal circuit conditions such as during a short-circuit; - general purpose connection device. Apart from controlling the circuits it guarantees their protection against electrical faults. It is replacing contactors in the control of large MV motors; - used for opening : at no load, under load and at short-circuit; for closing : at no load, under load and at short-circuit; for isolating : depending on the case. §

Maximum permissible current: I z This is the maximum value of current that the cabling for the circuit can carry indefinitely, without reducing its normal life expect ancy. The current depends, for a given cross sectional area of conductors, on several parameters: - constitution of the cable and cable-way (Cu or Al conductors; PVC or EPR etc. insulation; number of active conductors); - ambient temperature; - method of installation; - influence of neighbouring circuits. §

Overcurrents An overcurrent occurs each time the value of current exceeds the maximum load current IB for the load concerned. This current must be cut off with a rapidity that depends upon its magnitude, if permanent damage to the cabling (and appliance if the overcurrent is due to a defective load component) is to be avoided. Overcurrents of relatively short duration can however, occur in normal operation; two types of overcurrent are distinguished: §


147

Designing an electrical installation (Beginner Guide) Overloads These overcurrents can occur in healthy electric circuits, for example, due to a number of small short-duration loads which occasionally occur co-incidentally; motor starting loads, and so on. If either of these conditions persists however beyond a given period (depending on protective-relay settings or fuse ratings) the circuit will be automatically cut off. Short-circuit currents These currents result from the failure of insulation between live conductors or/and between live conductors and earth (on systems having low-impedance-earthed neutrals) in any combination: - 3 phases short-circuited (and to neutral and/or earth, or not); - 2 phases short-circuited (and to neutral and/or earth, or not); - 1 phase short-circuited to neutral (and/or to earth). §

§

Factor of maximum utilization (k ) u In normal operating conditions the power consumption of a load is sometimes less than that indicated as its nominal power rating, a fairly common occurrence that justifies the application of an utilization factor (k u) in the estimation of realistic values. This factor must be applied to each individual load, with particular attention to electric motors, which are very rarely operated at full load. In an industrial installation this factor may be estimated on an average at 0.75 for motors. For incandescent-lighting loads, the factor always equals 1. For socket-outlet circuits, the factors depend entirely on the type of appliances being supplied from the sockets concerned. §

Factor of simultaneity (k s) It is a matter of common experience that the simultaneous operation of all installed loads of a given installation never occurs in practice, i.e. there is always some degree of diversity and this fact is taken into account for estimating purposes by the use of a simultaneity factor (k s). The factor k s is applied to each group of loads (e.g. being supplied from a distribution or sub-distribution board). The determination of these factors is the responsibility of the designer, since it requires a detailed knowledge of the installation and the conditions in which the individual circuits are to be exploited. §

§

Modern circuit breakers are equipped with the following devices:

SSN - instantaneous magnetic safety trip (not adjustable); SN - instantaneous magnetic safety trip (adjustable); B - thermal overcurrent trip (adjustable); BN - thermal overcurrent trip (not adjustable); SK - thermal overcurrent trip (not adjustable, temperature compensated); A - shunt trip; U - undervoltage trip; M - multifunction (electronic device). §

RMS (Root Mean Square) - is the effective value of a varying voltage or current. It is the equivalent steady DC (constant) value which gives the same effect.


148

Designing an electrical installation (Beginner Guide) V peak =

2 ⋅ V RMS

V RMS

= 1.4 ⋅ V RMS

= 0.7 ⋅ V peak

RCD (Residual Current Devices) An RCD is a simple fitting designed to help prevent electric shock and other accidents due to faulty electrical appliances or wiring. An RCD can detect changes in the proper flow of electric current (when a flex or cable is cut, for instance, or an electrical tool malfunctions). Within milliseconds of this happening, the RCD automatically disconnects the power supply to the equipment before you can be electrocuted or further damage can be done. §

Coordination The coordination of protection devices involves combining, in a selective way, a short circuit protection device (fuses or magnetic circuit breakers) with a contactor and an overload protection device. Its objective is to break any abnormal current, in plenty of time, without any danger to personnel whilst providing adequate protection of the equipment against an overload or a short circuit current. Type 1 – IEC 947-4-1 – In a short circuit condition, the contactor or starter must not present any danger to personnel or installations and may not be able to resume operation without repair or the replacement of parts. Type 2 – IEC 947-4-1 – In a short circuit condition, the contactor or starter must not present any danger to personnel or installations and must be able to resume operation. The risk of contact welding is permissible if they can be easily separated. Total, ensuring reliability of operation – IEC 947-6-2 – In the event of a short circuit, no damage or risk of welding is permissible on the equipment constituting the motor starter. Operation can be resumed without any maintenance. §


149


BIBLIOGRAPHY [1] Schneider Electric, “Electrical Installation Guide” According to IEC international standards (Technical series), Edition 2005 http://www.electricalinstallation.merlingerin.com/guide/electrical_installation.htm

[2] Germanicher LLOYD, “Short circuit current calculation” [3] Bureau Veritas, “Short circuit current calculation”


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Designing an Electrical Installation Beginner Guide

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