Inspection and Testing

LEARNER WORK BOOK

Inspection And Testing Learner Work Book

Name: Group: Tutor:

Inspection and Testing REV4.1 1

LEARNER WORK BOOK

Inspection and Testing REV4.1

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Table of Contents Foreword ........................................................................................................4 Inspection and Testing Unit Overview .........................................................5 Practical skills you will need to demonstrate....................................................... 5 Knowledge Requirements .................................................................................. 5

Purpose of Inspection and Testing ..............................................................6 Test frequency ................................................................................................... 9

Electrical test instruments ..........................................................................11 Calibration and instrument accuracy ................................................................ 11 Instrument types............................................................................................... 12 Testing your meter ........................................................................................... 14

Initial Verification .........................................................................................16 The importance of paperwork........................................................................... 17 Information needed .......................................................................................... 19 Scope of the inspection .................................................................................... 21

Initial inspection checklist ..........................................................................21 Sequence of Tests .......................................................................................42 Recording circuit details ................................................................................... 42 Recording the test results................................................................................. 42 Test sequence.................................................................................................. 44 Test 1 - Continuity of protective conductors...................................................... 45 Test 2 - Continuity of ring final circuit conductors ............................................. 49 Test 3 – Insulation Resistance ......................................................................... 53 Test 4 - Protection by electrical separation....................................................... 56 Test 5 - Polarity ................................................................................................ 58 Test 6 – Earth electrode resistance .................................................................. 60 Test 7 – Earth loop impedance (Zs) ................................................................. 62 (Inc. prospective fault current – Ipf) .................................................................. 62 Test 8 – Operation of residual current devices ................................................. 65

Periodic Inspection and Testing.................................................................69 General Requirements ..................................................................................... 69 Routine checks................................................................................................. 69 Sequence of tests ............................................................................................ 71

Unsatisfactory Test Results........................................................................73


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Foreword In this unit you will learn about Inspection and testing. Inspection and testing is an immensely important subject to grasp and is relevant to every electrical installation. It is carried out during the erection of an installation and forms its completion. Inspection and testing is also carried out periodically to ensure a system is still in compliance with the latest edition of BS7671. The results of testing are documented as proof that the installation is safe to use. It is a legal requirement that the statutory document Electricity at Work Regulations 1989 is adhered to and that installations are safe to use and do not cause any danger. The non-statutory documents; BS7671, the Onsite Guide and Guidance Note 3 (Inspection and Testing) are not legal requirements but by following them we are deemed to be complying with the Electricity at Work Act. This unit examines the requirements for inspecting and testing of an installation when it is brand new, when additions or alterations have been made to it and when it has been in use for some time.

This workbook is to be accompanied by PowerPoint “Inspection and Testing”


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LEARNER WORK BOOK

Inspection and Testing Unit Overview Practical skills you will need to demonstrate To achieve the learning outcome the candidate must be able to: Carry out an initial inspection of an installation Select correct instruments to carry out tests Complete the correct sequence of tests Record the test results obtained Carry out functional testing of an installation Fill in recognised certificates of completion

Knowledge Requirements To achieve the learning outcome the candidate must know: How to carry out an initial inspection How to correct any deviations found during inspection How to use various test instruments The importance of the sequence of tests How to carry out functional testing How to document inspection and testing What to do if you discover unsatisfactory test results


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Purpose of Inspection and Testing Inspection and testing is not just carried out because it is someone’s job or that it is what the client wants. It is a legal requirement in the domestic sector. In the commercial and industrial sector it falls under the Electricity at Work Act and is harnessed by most companies who have the legal obligation to protect their premises and personnel. The purpose of inspection and testing is to provide, so far as is reasonably practicable, for: The safety of persons and livestock against the effects of electric shock and burns Protection against damage to property by fire and heat arising from an installation defect, and Confirmation that the installation is not damaged or deteriorated so far as to impair safety, and The identification of installation defects and non-compliance with the requirements of the Regulations which may give rise to danger. Inspection and testing is carried out: • • • •

During and or on completion of a new installation. When minor works such as additions or alterations are carried out To satisfy the periodic inspection of a companies’ premises. To satisfy the requirements of Part P of the building regulations.

Building Regulations Part P of the Building Regulations (England and Wales) was introduced by the Government on January 1st 2005. It is designed to reduce accidents caused by faulty electrical installations and to prevent incompetent installers from leaving electrical installations in an unsafe condition. Part P applies to the following situations: Dwelling houses and flats Dwellings and business premises that have a common supply eg shops that have a flat above Common access areas in blocks of flats such as corridors or staircases Shared amenities in blocks of flats such as laundries or gyms In or on land associated with dwellings – such as fixed lighting or pond pumps in gardens Outbuildings such as sheds, detached garages and greenhouses Approved Document P is called ‘Electrical Safety’ and will be complied with if the standard of electrical work meets the ‘Fundamental Requirements of Chapter 13 of BS7671:2008’.


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LEARNER WORK BOOK Section P1 of Part P states: ‘Reasonable provision shall be made in the design, installation, inspection and testing of electrical installations in order to protect persons from fire and injury’.

In your own words describe how people are protected from fire and injury whilst using an electrical installation.

Section P2 of Part P states: ‘Sufficient information shall be provided so that persons wishing to operate, maintain or alter an electrical installation can do so with reasonable safety.

In your own words describe what information relating to safety can be provided to persons wishing to use an electrical installation.


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Electrician and bathroom fitter prosecuted for breach of Part P of the building regulations An electrician in Newcastle and a bathroom fitter from bath and Somerset are to be the first to be successfully prosecuted for offences under Part P of the building regulations. Able Electrical based in Newcastle and the company’s director John Waugh, an electrician with 28 years experience, admitted 23 counts of breaching building regulations and was fined total of £16,000. Able Electrical carried out rewiring on a property that, according to Newcastle magistrates' court, could have resulted in death or serious injury. Waugh admitted to 23 offences including falsely claiming to be registered with the NICEIC, failing to notify work to Building Control, installing cables under the landing floor in a poor manner, using old wires which are no longer covered by current regulations and not using Residual Circuit Breakers for sockets. Newcastle Council Building Control brought charges against Able Electrical after the householder called in an NICEIC registered

electrician to inspect Able’s work and found that the property needed a complete rewire and tests could not be carried out for safety reasons. Jim Speirs, director general of the NICEIC said: “It is unacceptable for an electrician with this level of experience to have carried out work to such a poor standard that lives are put at risk. “A professional and competent electrician or installer would have no problem in becoming registered with a competent person scheme, and would therefore have no reason to falsify their status. The NICEIC takes misuse of its name and logo seriously and we will always prosecute any persons falsely claiming registration with our schemes.” In a second incident, Bath & North East Somerset Council Building Control brought charges against bathroom fitter Roger Martin Drinkwater for contravening Building Regulations with regard to the installation of an electric shower in a replacement bathroom at a private property.

The defendant pleaded guilty to charges that included using a method of wiring not in accordance with the British Standard, and failing to advise the complainant that the incomplete shower should not be used and that it was awaiting checking. He was fined £1,000 for the Part P offence and £250 each for the remaining offences of failing to give a Building Notice to Building Control prior to commencement of the work, and failing to give notice of commencement and completion of certain stages of the work. The court also ordered the defendant to pay £1,066 in costs. Jim Speirs continued: “It is vital that anyone carrying out electrical installations are qualified to do so, and have a practical understanding of current wiring and building regulations. These prosecutions under Part P are evidence that building control bodies and scheme operators are taking compliance with Part P seriously, and will not tolerate false claims of competent scheme registration and substandard, dangerous working practices.”

Class discussion. Firstly read and then discuss the above article and consider the people involved. Should these workmen be prosecuted? Why lie about being part of the NICEIC? Write down the key points below.


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Test frequency Initial inspection and testing is necessary on all newly completed installations. In addition, because all electrical installations deteriorate due to a number of factors such as damage, wear and tear, corrosion, excessive electrical loading, ageing and environmental influences, periodic inspection and testing must be carried out at regular intervals determined by the following: Legislation requires that all installations must be maintained in a safe condition and therefore must be periodically inspected and tested. Licensing authorities, public bodies, insurance companies and other authorities may require public inspection and testing of electrical installations. The installation must be checked to ensure that it complies with BS 7671. It is also recommended that inspection and testing of installations should occur when: There is a change of use of the premises Any alterations or additions to the original installation Any significant change in the electrical loading of the installation Where there is reason to believe that damage may have been caused to the installation. The table below details the maximum period between inspections of various types of installation.


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LEARNER WORK BOOK Now answer the questions below

1 In you own words state the four purposes of inspection and testing

2. When and why should inspection and testing be carried out?

3. State the main aim of Part P of the building Regulations

4. Explain why installations need to be periodically re-tested

5. When is it recommended that electrical testing of installations be carried out?

6. How often should a pub be inspected and tested?


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LEARNER WORK BOOK

Electrical test instruments BS EN 61010 covers basic safety requirements for electrical test instruments, and all instruments should be checked for conformance with this standard before use. Older instruments may have been manufactured in accordance with BS 5458 but, provided these are in good condition and have been recently calibrated, there is no reason why they cannot be used. Guidance note GS38 stipulates test leads, including probes and clips, must be in good order and have no cracked or broken insulation. Fused test leads are recommended to reduce the risk of arcing under fault conditions. Instruments may be analogue (i.e. fitted with a needle that gives a direct reading on a fixed scale) or digital, where the instrument provides a numeric digital visual display of the actual measurement being taken. Insulation and continuity testers can be obtained in either format whilst earth-fault loop impedance testers and RCD testers are digital only.

Calibration and instrument accuracy To ensure that the reading being taken is reasonably accurate, all instruments should have a basic measurement accuracy of at least 5 per cent. In the case of analogue instruments a basic accuracy of 2 per cent of full-scale deflection should ensure the required accuracy of measured values over most of the scale.

Calibration label

All electrical test instruments should be calibrated on a regular basis. The time between calibrations will depend on the amount of usage that the instrument receives, although this should not exceed 12 months in any circumstances. Instruments have to be calibrated in laboratory conditions against standards that can be traced back to national standards; therefore this usually means returning the instrument to a specialist test laboratory. On being calibrated the instrument will have a calibration label attached to it stating the date the calibration took place and the date the next calibration is due. It will also be issued with a calibration certificate detailing the tests that have been carried out and a reference to the equipment used.

The user of the instrument should always check to ensure that the instrument is within calibration before being put to use.


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LEARNER WORK BOOK A further adhesive label is often placed over the joint in the instrument casing stating that the calibration is void should the seal be broken. A broken seal will indicate whether anyone has deliberately opened the instrument and possibly tampered with the internal circuitry. Instruments that are subject to any electrical or mechanical misuse (e.g. if the instrument is subject to an electrical short circuit or is dropped) should be returned for re-calibration before being used again. Electrical test instruments are relatively delicate and expensive items of equipment and should be handled in a careful manner. When not in use they should be stored in clean, dry conditions at normal room temperature. Care should also be taken of instrument leads and probes to prevent damage to their insulation and to maintain them in a good, safe working condition.

When using an instrument out on site, the accuracy of the instrument will probably not be as good as the accuracy obtained under laboratory conditions. Operating accuracy is always worse than basic accuracy and can be affected by battery condition, generator cranking speed, ambient temperature, instrument alignment or loss of calibration

Instrument types Low resistance Ohmmeters Where low resistance measurements are required when testing earth continuity, ring circuit continuity and polarity, then a low reading ohmmeter is required. They are only used on isolated circuits.

0.50Ω

This may be a specialised low-reading ohmmeter or the continuity scale of a combined insulation and continuity tester. Whichever type is used it is recommended that the test current should be derived from a source of supply not less than 4 V and no greater than 24 V with a short circuit current not less than 200 mA and give a reading to two decimal places. Instruments manufactured to BS EN 61557 will meet the above requirements. Errors in the reading obtained can be introduced by contact resistance or by lead resistance. Although the effects of contact resistance cannot be eliminated entirely and may introduce errors of 0.01 ohm or greater, lead resistance can be eliminated either by clipping the leads together and zeroing the instrument before use, where this facility is provided, or alternatively measuring the resistance of the leads and subtracting this from the reading obtained.


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LEARNER WORK BOOK Insulation resistance Ohmmeters Insulation resistance should have a high value and therefore insulation resistance meters must have the ability to measure high resistance readings (typically 200MΩ). The test voltage required for measuring insulation resistance is given in BS 7671 Table 71A as shown below.

>200MΩ

Circuit Nominal Voltage to earth

Test Voltage dc v

SELV & PELV

250 V

Up to and including 500 v with the exception of the above supplies

500 V

Above 500 V

1000 V

Minimum Insulation Resistance (MΩ Ω) 0.25 1.0 1.0

SELV = Separated extra low voltage - Not exceeding 50V A.C. or 120V Ripple Free D.C. PELV = Protective extra-low voltage

The photograph above shows a typical modern insulation and continuity tester that will measure both low values of resistance for use when carrying out continuity and polarity tests and also high values of resistance when used for insulation resistance tests. This type of instrument and test is only ever carried out on an isolated circuit Instruments of this type are usually enclosed in a fully insulated case for safety reasons and have a range of switches to set the instrument correctly for the type of test being carried out i.e. continuity or insulation. The instrument also has a means of selecting the voltage range required e.g. 250 V, 500 V, 1000 V. Other features of this particular type of instrument are the ability to lock the instrument in the ‘on’ position for hands-free operation and an automatic nulling device for taking account of the resistance of the test leads. Earth-loop impedance testers Earth-loop impedance testers of the type shown in the photograph have the capability to measure both earth-loop impedance and also prospective short-circuit current, depending on which function is selected on the range selection switch.

0.50Ω

The instrument also has a series of LED warning lights to indicate whether the polarity of the circuit under test is correct or not. The instrument gives a direct digital read-out in Ohms of the value of the measurement being taken at an accuracy of plus or minus 2 per cent and to two decimal places.


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LEARNER WORK BOOK RCD testers Instruments for testing residual current devices, such as the one shown in the photograph have two selection switches. One switch that should be set to the rated tripping current of the RCD (e.g. 30 mA, 100 mA etc.) and the other set to the test current required i.e. 50 per cent or 100 per cent of the rated tripping current or 150 mA for testing 30 mA RCDs when being used for supplementary protection. Half cycle tests can be selected to ensure full protection.

40mS

All-in-one tester A modern innovation by manufacturers is the production of an ‘all in one’ instrument that has the ability to carry out the most common tests required by the Regulations. These are: Continuity tests (including polarity tests) Insulation resistance tests Earth-loop impedance tests RCD tests Measurement of prospective short circuit current. The photograph below shows an example of this type of instrument, which by manipulation of the function and range switches will perform, all of the above tests.

Testing your meter In order to carry out effective testing it is not just a case of unpacking your meters and carrying on with the tests. It is important that you regularly check your instruments to make sure they are in good and safe working order. Before using any of your instruments make sure that all test probes and conductors to be tested are scrupulously clean to avoid incorrect test results Check the leads for damage Check the battery levels by zeroing or nulling the lead resistance Ensure you get visual confirmation of the expected test values. (Open leads display a high resistance value. Closed leads display a low resistance)


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1 Describe the general aim of the standard BS EN 61010.

2. Describe what is meant by instrument calibration

3. What is the recommended calibration period and how can we check if an instrument is calibrated?

4. Name three tests we carry out with a low reading Ohmmeter and how accurate must the meter reading be?

5. What three voltage settings are available on an insulation resistance tester?

6. There are two selector switches on an RCD tester. What are they for?


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Initial Verification The following notes provide a detailed description of the procedures required to carry out an initial inspection of an electrical installation. Substantial reference has been made to the lEE Wiring Regulations (BS7671), the On Site Guide and lEE Guidance Note No.3 and it is recommended that wherever possible these documents are referred to should clarification be required. The most important considerations prior to carrying out any inspection and test procedure are that: All the required information about the installation is available The person carrying out the procedure is competent to do so That all safety requirements have been met Forward planning is also a major consideration and it is essential that suitable inspection checklists have been prepared and that appropriate certification is available for completion. It is also important to realize that a large proportion of any new installation will be hidden from view once the building fabric has been completed and therefore it is preferable to carry out a certain amount of visual inspection throughout the installation process: e.g. conduit, cable tray or trunking is often installed either above the ceiling or below the floor and once the ceiling or floor tiles have been fitted it is difficult and often expensive to gain access for inspection purposes. The same applies to testing and it may be advisable to carry out tests such as earth continuity during construction rather than after the building has been completed.

Incorrectly terminated SWA

It must be remembered however that when visual inspection and / or tests are carried out during the construction line, the results must be recorded on the appropriate checklists or test certificates.

It is also worth noting that although the major part of any inspection will be visual other human senses may be employed: e.g. a piece of equipment with moving parts may generate an usual noise if it is not working correctly or an electrical device which overheats will be hot to touch as well as giving off a distinctive smell. The senses of hearing touch and smell will assist in detecting these.


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The importance of paperwork When an installation is complete the persons responsible for the work must report to the owner that it is complete and ready for service. This is presented in the form of an electrical installation certificate that must be separately signed to verify the design, the construction and the inspection and test aspects to confirm that the installation complies with BS7671. The installer should also compile an operational manual for the installation, which will include all the relevant data, including: A full set of circuit and schematic drawings, All design calculations for cable sizes, cable volt drop, earth-loop impedance, etc. Leaflets or manufacturers' details for all the equipment installed, As fitted' drawings of the completed work where applicable, A full specification, Copies of the electrical installation certificate, together with any other commissioning records, A schedule of dates for periodic inspection and testing, The names, addresses and telephone numbers of the designer, the installer, and the inspector / tester. The certificate we will take a look at is the NICEIC’s (National Inspection Council for Electrical Installation Contracting) domestic installer form. This would be supplied to a client who had requested work to be done on domestic premises

The certificate could be used in a court of law to prove the competence of the electrical tester should anything happen at a later date. If we were to certify an electrical installation that would later result in damage or harm to persons or property we would require proof that we carried out a full inspection and test in accordance with BS7671 which would satisfy the Electricity at Work Act. The legalities of our responsibilities are that we are guilty until proven innocent. So having correct paper work and test records could save your neck!


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Information needed Before carrying out the initial inspection (and test) of an installation it is essential that the person carrying out the work be provided with the following information: 1. The maximum demand of the installation expressed in amperes per line together with details of the number and type of live conductors both for the source of energy and for each circuit to be used within the installation, (e.g. single-line two-wire a.c. or three line four-wire a.c. etc). 2. The general characteristics of the supply such as: The nominal voltage (Uo) The nature of the current ( I ) and its frequency (Hz) The prospective short circuit current at the origin of the installation (kA) The earth fault loop impedance (Ze) of that part of the system external to the installation. The type and rating of the over current device acting at the origin of the installation. If this information is not known it must be established either by calculation, measurement, inquiry or inspection. 3. The type of earthing arrangement used for the installation e.g. TN-S, TN-C-S, TT etc. 4. The type and composition of each circuit (i.e. details of each sub-circuit, what it is feeding, the number and size of conductors and the type of wiring used). 5. The location and description of all devices installed for the purposes of protection, isolation and switching (e.g. fuses/circuit breakers etc). 6. Details of the method selected to prevent danger from shock in the event of an earth fault (This will invariably be protection by earthed equipotential bonding and automatic disconnection of the supply). 7. The presence of any sensitive electronic devices which may be susceptible to damage by the application of 500 volts d.c when carrying out insulation resistance tests.

The above information may be gained from a variety of sources such as the project specification, contract drawings, as fitted drawings or distribution board schedules. If such documents are not available, then the person ordering the testing should be approached


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Sample taken from an NICEIC certificate The initial information will be recorded in the boxes below


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Scope of the inspection BS 7671 states that as far as reasonably practicable, an inspection shall be carried out to verify that: All equipment and materials used in the installation are of the correct type and comply with the appropriate British Standards or acceptable equivalent All parts of the installation have been correctly selected and installed No part of the installation is visibly damaged or otherwise defective The installation is suitable for the surrounding environmental conditions.

Initial inspection checklist The visual inspection shall include the checking of the following items where relevant to the installation and where necessary, during erection of the equipment. This means that some of the visual inspections can be carried out during erection of the equipment and therefore need not be re-inspected. Initial Inspection at a glance: 1. Connection of conductors 2. Identification of conductors 3. Routing of cables within mechanical protection 4. Selection of conductors for current carrying capacity and volt drop. 5. Connection of single – pole devices in the line conductor only 6. Correct connection of equipment 7. Presence of fire barriers and suitable seals 8. Methods of protection against electric shock (earthing) 9. Prevention of detrimental influences 10. Presence of appropriate devices for isolation and switching

11. Presence of under – voltage protective devices 12. Choice of setting of protective devices 13. Labeling of protective devices, switches and terminals 14. Selection of equipment appropriate to external influences 15. Adequacy of access to switchgear and equipment 16. Presence of warning signs and danger notices 17. Presence of diagrams, charts, instructions and similar information 18. Erection methods

Pick two different inspection checks from above and try to describe what is required .

Remember, if any of the initial verification checks require you to remove covers then you will need to carry out safe isolation, otherwise you will contravene the Electricity at Work Act 1989. The key point with all electrical work is that you maintain yours and everyone’s safety when carrying out such work. Inspection and Testing REV4.1 21

LEARNER WORK BOOK 1. Connection of conductors Questions to ask ourselves: • Are terminations electrically and mechanically sound? • Is insulation and sheathing removed only to a minimum to allow satisfactory termination?

Every connection between conductors or between conductors and equipment must be electrically continuous and mechanically sound. We must also make sure that all connections are adequately enclosed but accessible as required by the regulations. Loose connections can lead to many dangerous events from electric shock to fire. Note: Before attempting to re-secure any electrical accessory you must ensure that the supply has been isolated.

Dangers: Movement of the socket outlet may dislodge circuit connections and contact exposed conductors. Work to this standard generally means connections are also loose. Can lead to arcing; overheating; electric shock; fire. Remedy:

Dangers: Constant use of this main isolator with a loose supply connection can catch fire through arcing and overheating. Remedy:


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LEARNER WORK BOOK 2. Identification of conductors Questions to ask ourselves: • Are conductors correctly identified in accordance with BS7671? • Are switch wires identified as live at both terminations? A check should be made that each conductor is identified in accordance with the requirements of BS7671 Table 51A and Table 51B. Although numbered sleeves or discs may be used in special circumstances, the most common form of identification is by means of coloured insulation or sleeving. It should be noted in particular that only protective conductors should be identified by a combination of the colours green and yellow.

Harmonised colours of conductors to BS7671:2008

Dangers: Old switch wire colours not identified as live at two way switch so could present a danger when switch is replaced. Remedy:


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3. Routing of cables within mechanical protection Questions to ask ourselves: • Are cables installed so that external influences from mechanical damage, corrosion or heat etc have been considered? • Are covers and lids in place to prevent unauthorised access?

Cables should be routed out of harms way and protected against mechanical damage where necessary. Permitted cable routes are clearly defined in the 'on site guide' or alternatively cables should be installed in earthed metal conduit or trunking. Danger:

Remedy: Install cables away from terminations and ensure they are protected from mechanical damage

Dangers: Unprotected single insulated conductors may get snagged or damaged by persons or equipment. Remedy:

Single core insulated cables should only be installed where they are afforded mechanical protection. Name five types different types of installation where they are properly protected.


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LEARNER WORK BOOK 4. Selection of conductors for current carrying capacity and volt drop Questions to ask ourselves: • Are conductors selected for current carrying capacity and voltage drop in accordance with the design requirements? Where practicable the size of cable used at the consumer unit should be checked for current carrying capacity and voltage drop based upon information provided by the installation designer. Incorrect ratings can lead to equipment failure and overheating of conductors. How can we determine that the size of the conductor is correct for the intended use of the circuit?

The maximum permitted voltage drop allowable from the nominal voltage is 3% for lighting and 5% for power. This value is from the origin of the installation to the furthest point of utilisation. At 230V that is 6.9V for lighting and 11.5V for power. If we know the conductor size the procedure to measure voltage drop is simple. 1. For each circuit - when isolated – the L and N conductors are joined at the furthest point and the resistance of the loop measured at the distribution board. 2. We then calculate the approximate length of the circuit. Circuit length in metres = 29.4 x R x S Where R = loop resistance value and S = cable cross sectional area in mm² Example: the loop resistance of a lighting circuit, shorted out at the furthest point is found to be 0.7Ω. If the c.s.a of the cable is 1.0 mm², what is the circuit length? L = 29.4 x 0.7 x 1 = 20.6 metres. The voltage drop may then be determined by reference to appendix 4 of BS 7671. 1.0 mm² is listed as dropping 44mV/a/m Therefore if the above circuit is carrying a current when fully loaded of 5A, the voltage drop will be: Vd = Ib x L x mV/a/m = 1 000

5 x 20.6 x 44 = 4.53 Volts 1 000


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LEARNER WORK BOOK 5. Connection of single pole devices in the line conductor only Questions to ask ourselves: • Are single pole devices and switching devices connected in the live conductor only? • Are there only live conductors terminated into switches and circuit protection? This is verification of polarity. A check must be made that all single pole devices are connected in the line conductor only. Where neutrals are used to switch devices the equipment or circuit remains live when the circuit is seemingly isolated. Note: Before attempting to re-secure any electrical accessory you must ensure that the supply has been isolated.

L

Load

N

Dangers: A fault or an overload will cause the fuse to operate but the equipment will still remain live but not operational. Electric shock risk Remedy:

E

Danger: L

Load

N

E

Remedy: Disconnect the neutrals from the switch and connect the live conductors into the switch terminals


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LEARNER WORK BOOK 6. Correct connection of equipment Questions to ask ourselves: • Are all accessories and items of equipment correctly connected? • Do all terminals have the correct conductors connected into them? Accessories and equipment should be checked to ensure they have been connected correctly including correct polarity. Incorrect connection of equipment can lead to damage to the equipment or fire.

Danger:

Remedy: Isolate circuit and re-wire strappers with a three core and earth and re-connect the switch

Danger:

Remedy: Disconnect and re-connect socket conductors into the correct terminals


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LEARNER WORK BOOK 7. Presence of fire barriers and suitable seals Questions to ask ourselves: • Are fire barriers present where required and protection against thermal effects provided? • Where cables pass through walls and floors are the access holes sealed? • Are correct termination methods used for cable entries? • Where there is a danger of overheating conductors have they been protected by heat resistance sleeving or barriers? A check must be made (preferably during construction) that fire barriers, suitable seals and/or other means of protection against thermal effects have been provided as necessary to meet the requirements of the regulations. Suitable fire barriers need to be installed where cables pass through floors and walls. Due to there being an entry to pass the cable through it would provide a path for fire to travel through. Expanding foam or transient blocks are the main form of seal used. Where conduit, trunking or ducting does not exceed an internal csa of 710mm² it need not be sealed internally as it passes through walls and floors. Where this dimension is exceeded it needs to be sealed against the spread of fire. Dangers: An electrical fire within this trunking would escape through the open cable entries. Also carries an electric shock risk. Remedy:

Danger:

Remedy: Disconnect and circuit conductors. Remove conduit and re-terminate using a 25 to 20mm reducer. Reconnect the conductors.


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LEARNER WORK BOOK 8. Methods of protection against electric shock Questions to ask ourselves: • What methods have been used to provide basic and fault protection? • Are all live parts correctly protected from contact of persons or livestock? • Are all barriers in place so contact with live parts is not possible? • Are all points of earth termination on accessories and equipment connected to earth? • Have all exposed conductive parts been connected to earth? • Have all extraneous conductive parts been connected to earth?

A check must be made that the requirements of the regulations have been met for the method of protection used. Failure to comply with BS7671 could result in an electric shock. Basic Protection BS7671 defines it as: “Protection from electric shock under fault free conditions.” Basic protection is protecting from touching parts that are live under normal use. This generally corresponds to contact of persons or livestock with live parts. The unfortunate being receives maximum shock voltage. We are granted basic protection by: Insulation Although protection by insulation is the usual method of protection against direct contact other methods can be used. However, where insulation should be present it should be checked to ensure that no live conductors have been left exposed. Barriers / Enclosures Where live parts are protected by barriers or enclosures (e.g. bare bus-bars enclosed in a metal bus-bar chamber) they should be checked to ensure that all covers have been fitted and all fixing devices are secure. Obstacles Protection by obstacles provides protection only against unintentional contact with live conductors. If this method is used the area should be accessible only to skilled persons or persons under supervision. Out of reach Placing live parts out of reach can also provide protection against direct contact although increased distances may be necessary where long or bulky conducting objects are likely to be handled in the vicinity.


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Danger:

Remedy: Isolate CU. Remove supply busbar. Replace with correct model and ensure it is shrouded

Dangers: Access to live parts via poorly fitting terminal shroud. Electric shock Remedy:

Fault protection BS7671 defines it as: “Protection against electric shock under single fault conditions”

Methods of fault protection are given in BS7671 as: Automatic disconnection of supply. Use of class II equipment. Non-conducting location. Earth-free local equipotential bonding Electrical separation. Where persons or livestock come into contact with an exposed conductive part that has become live under fault conditions they should be protected by the part being earthed. Examples of exposed conductive parts include metal trunking, metal conduit or exposed metal parts of an appliance such as an electric kettle. Should the insulation of any of the live parts within the kettle become defective then the metal casing may become live and anyone touching the kettle would be at risk of receiving a dangerous electric shock.


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The most commonly used method of fault protection is automatic disconnection of the supply (ADS) and it these requirements that should be checked at the initial inspection stage. Earthing arrangements; earthing conductors; main protective bonding conductors; circuit protective conductors and supplementary bonding conductors should all be checked to ensure that they have been correctly installed and are of the correct size and are correctly labelled. Danger:

Remedy: Remove brass light switch and replace with a plastic one or use the earth terminal point on the switch cover

Protection against both basic and fault protection Separated extra low voltage (SELV) is the most common method of providing protection against both. Requirements for this type of system include: An isolated source of supply - e.g. a safety-isolating transformer to BS3535. Also numbered BS EN 60742. Electrical separation, which means no electrical connection between the SELV circuit and higher voltage systems. No connection with earth or the exposed conductive parts

There must be no connection to earth and precautions must he taken to ensure, as far as possible, that earth faults will not occur. Such precautions would include the use of flexible cords without metallic sheaths, using double insulation, making sure that flexible cords are visible throughout their length of run, and so on. Perhaps the most common example of a separated circuit is the bathroom transformer unit feeding an electric shaver. By breaking the link to the earthed supply system using the double wound transformer, there is no path to earth for shock current.


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LEARNER WORK BOOK 9. Prevention of detrimental influences Questions to ask ourselves: • Are wiring systems installed such that they can have no harmful effect on non-electrical systems? • Are systems of different voltages are segregated where necessary? Account must be taken of the proximity of other electrical services of a different voltage band and of non-electrical services and influences. E.g. fire alarm and emergency lighting circuits must be separated from other cables and from each other and category 1 and category 2 circuits must not be present in the same enclosure or wiring system unless they are either segregated or wired with cable insulation suitable for the highest voltage present. This is due to the magnetic influence low voltage circuits can have on extra low voltage circuits. This can appear as false signals or “noise” on telephone lines for example. Voltage Bands / Circuit Categories Category 1 Circuits operating at low voltages (50 to 600 volts AC) and supplied from the electrical mains. Category 2 Any data, telecommunication, intruder alarm systems and circuits operating at extra low voltage. (not exceeding 50 volts AC and 120 volts DC) Category 3 Any fire detection system, emergency lighting or alarm

Dangers: Circuit with different categories are in close proximity which can lead to interference or false signals Remedy:

Dangers: Circuits with different categories are in close proximity which can lead to interference or false signals Remedy:


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LEARNER WORK BOOK 10. Presence of appropriate devices for isolation and switching Questions to ask ourselves: • Are there appropriate devices for isolations and switching correctly located and installed? • Are there suitable means for isolating circuits and equipment?

BS7671 requires, that effective means suitably positioned and ready to operate shall be provided so that all voltage may be cut off from every installation, every circuit within the installation and from all equipment, as may be necessary to prevent or remove danger. This means that switches and/or isolating devices of the correct rating must be installed as appropriate to meet the above requirements. It may be advisable, where feasible, to carry out an isolation exercise to check that effective isolation can be achieved. This should include switching off, locking-off and testing to verify that the circuit is dead and no other source of supply is present.

11. Presence of under voltage protective devices Questions to ask ourselves: • Where under voltage may give rise for concern are there protective devices present? • Are there contactor control circuits with manual starting where required? Suitable precautions must be taken where a loss (no volt) or lowering of voltage and subsequent restoration of voltage could cause danger. The most common situation would be where a motor driven machine stops due to a loss of voltage and unexpectedly re-starts when the voltage is restored. Precautions such as the installation of a motor starter containing a contactor must be employed. To overcome the dangers a control circuit is employed and the use of a manual stop and start station that requires a manual input to reset the circuit once it has failed.

State one example of where automatic re-energisation may cause danger to persons or property and explain the possible consequences.


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LEARNER WORK BOOK 12. Choice of setting of protective devices Questions to ask ourselves: • Are protective and monitoring devices correctly chosen and set to ensure protection against overload and faults? Protective devices are employed in a circuit to detect over current and fault current. Their sole purpose is to disconnect should their rating be exceeded. If a device is installed that is of insufficient rating this may lead to conductors and / or equipment over heating and resulting in damage to the circuit.

Dangers: Radial circuits incorrectly added to 32A MCB. Overloaded conductors and subsequent damage to insulation and equipment. Electrical fire Remedy:

13. Labelling of protective devices, switches and terminals Questions to ask ourselves: • Are all protective devices, switches (where necessary) and terminals correctly labelled? • Do devices and accessories display their source or supply and duty so we know where and what we are isolating? A check should be carried out to ensure that labels and warning notices as required by B7671 have been fitted e.g. labelling of circuits, MCBs, RCDs fuses and isolating devices with their circuit designation. Periodic inspection notices advising of the recommended date of the next inspection and warning notices referring earthing and bonding connections. The connection of the bonding wires to the pipes has to be made with a proper clamp to BS 951 complete with the label “SAFETY ELECTRICAL CONNECTION - DO NOT REMOVE.”

Consider a domestic dwelling. What identification would you expect to see above the protective devices in the consumer unit? Give three different examples


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LEARNER WORK BOOK

What information can we place on isolators, sockets and light switches in commercial premises?

Keeping circuits in order What BS7671 says: Regulations 314-1 (i) and (ii) state that every installation shall be divided into circuits to avoid danger and minimise inconvenience and to facilitate safe operation, inspection, testing and maintenance. Regulation 514-1-2 “states as far as reasonably practicable, wiring shall be arranged or marked that it can be identified for inspection, testing, repair or alteration of the installation.”

One way this can be achieved is to correctly terminate the conductors in sequence so that mistakes cannot be made when an attempt to identify a circuits’ conductors. Each protective device in a consumer unit is classified as a “way”. For example, there might be 6 ways in one consumer unit and a number in a sequence (usually from the main isolator). The neutral and earth bars in this consumer unit will also be numbered in sequence. If a lighting circuit is supplied from way 3 the neutral and earth conductors should also be connected into way 3 on the neutral and earth bars. If the conductors are not connected in sequence this can cause confusion for the test engineer.

What dangers might be associated with disconnecting a circuit from a consumer unit?

We can usually identify a circuit by tracing the individual conductors from each terminal to the point in the cable where the sheath is stripped to. If single core cable is used they might be wired into a conduit. However, conduit is often used for more than one circuit making identification even more difficult. Ideally the installer would apply identification on each conductor therefore making the process of removing a circuit a lot easier.


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LEARNER WORK BOOK

Class activity Read the evidence thoroughly then work through the initial numbered inspection requirements. Then use the circuit chart below to complete the task. We are required to inspect and test a six-way MCB consumer unit. • Contained within it are four MCBs rated at 6A, 16A, 32A and a 45A (all type B) in sequence from left to right. • The main switch is on the right hand side of the consumer unit. • There is a circuit chart present that displays the information from left to right; Lighting; Immersion heater; Ring main; Shower. • Connected into the 45A MCB is one 10mm² flat twin and earth. • There is also a 25mm conduit carrying 9 x 2.5mm² stranded conductors, three browns, three blues and three green / yellows. Two of the browns connect into the 32A MCB. • A brown 1.5mm² solid conductor connects into the 6 amp MCB and we trace that to a flat pvc/pvc cable. • The neutrals and earths do not correspond with the way numbers allocated to the protective devices

Circuit Chart 2.1.3.L1 Way

Description

Rating

Type

Live mm²

CPC mm²

1. Record the way numbers of the circuits and record their description of load and rating i.e. Way 1 - Lift Motor- 20 A Type B 2. Record all of the live and circuit protective conductor sizes connected 3. How would you identify the neutrals and earths of each of the circuits and put them in the correct sequence?


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LEARNER WORK BOOK

14. Selection of equipment appropriate to external influences Questions to ask ourselves: • Have all items of equipment and protective measures been selected in accordance with the appropriate external influences? All equipment must be selected as suitable for the environment in which it is likely to operate. Items to be considered are taken from Chapter 52 of BS7671: Ambient temperature: A wiring system and its components shall be selected so as to be suitable for the highest and lowest ambient temperature. Presence of external heat sources: A wiring system shall be selected and erected so as to avoid harm from heat sources such as the sun or hot pipe work. This shall be achieved by shielding; selecting a system suitable for such conditions; placing sufficiently away from the heat source. Presence of water and subsequent corrosion: A wiring system shall be selected and erected so that no damage will be caused by condensation or moisture. This may be overcome by selecting accessories using the IP chart. Ingress of foreign bodies: A wiring system shall be selected and erected to minimize the ingress of dust and other matter. This may be overcome by selecting accessories using the IP chart shown in the Tables from BS7671 and the On Site Guide or on the next page. Impact: A wiring system shall be selected and erected so as to minimise mechanical damage from impact, abrasion, penetration, compression or tension. This shall be afforded by the mechanical characteristics of the wiring system or the use of extra mechanical protection. Vibration: A wiring system shall be selected and erected so as to be suitable to withstand the effects of vibration. This shall be afforded by using secure fixings suitable for the situation. Flora and Fauna: A wiring system shall be selected and erected so suitable to withstand the effects of flora (mould growth) and fauna (insects, birds and small animals) This shall be afforded by using shielding or equipment suitable for the environment. Radiation: A wiring system shall be selected and erected so suitable to withstand the effects of radiation from the sun and ultraviolet rays. Using shielding or equipment suitable for the environment shall afford this. Building use and structure: A wiring system shall be selected and erected so as to be suitable to withstand the effects of building stresses or movement. This shall be afforded by using secure fixings suitable for the situation so that no stress is put onto the wiring system.


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LEARNER WORK BOOK 15. Adequacy of access to switchgear and equipment Questions to ask ourselves: • Are all means of access to switchgear and equipment adequate? BS7671 requires that every piece of equipment that requires operation or attention must be installed so that adequate and safe means of access and working space are provided. Why is adequate access important with regard to switch gear and equipment?

Danger:

Remedy: Create a cut in the ceiling to allow removal of consumer unit cover or lower the entire trunking and consumer unit installation

16. Presence of warning signs and danger notices Questions to ask ourselves: • Are danger notices and warning signs present where required? A check should be carried out to ensure that warning notices as required by BS7671 have been fitted e.g. labelling of circuits, MCBs, RCDs fuses and isolating devices of the voltages present within enclosures. Notices displaying that only authorised personnel may enter switch rooms and open enclosures etc.

State three places where you think a warning sign might need to be placed


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LEARNER WORK BOOK 17. Presence of diagrams, charts, instructions and similar information Questions to ask ourselves: • Are diagrams, instructions and similar information relating to the installation available?

All distribution boards should be provided with a distribution board schedule that provides information regarding types of circuits, number and size of conductors, type of wiring etc. This should be attached within or adjacent to each distribution board. Dangers: No chart or identification is present. Isolating a specific circuit for testing and inspection or maintenance is not an easy task. Unintentional isolation of supplies. Remedy:

By displaying a circuit chart, usually mounted inside the door, it would display information about the circuit such as: • • • • •

The consumer unit designation Zs at the consumer unit Where this consumer unit is supplied from and the size of its protective device/s. Size of the supply cable Way reference for each final circuit

• • • • •

Circuit description Type of wiring Over current protection type and rating Circuit cable sizes Number of points served

Other information might relate to the operation of specific equipment. An operator in a factory may benefit from the information supplied to minimise danger, confusion and delay.


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LEARNER WORK BOOK 18. Erection methods Questions to ask ourselves: • Have all wiring systems, accessories and equipment been selected and installed in accordance with the requirements of BS7671, and are fixings for equipment adequate for the environment? Correct methods of installation should be checked, in particular fixings of switchgear, cables, conduit etc, which must be adequate and suitable for the environment. Danger:

Remedy: Assuming there is enough slack on the cable ensure the sheath enters the enclosure and secure it in place so it will not fall out.

Chapter 52 of BS7671 deals with the selection and erection of wiring systems. Below are the main points. • • • • • •

Non-sheathed cables to be enclosed in conduit, ducting or trunking. Prevention of damage by condensation or water ingress, and drainage points if necessary. Ingress of solid foreign bodies to be minimised. Wiring systems to be selected and erected to minimise mechanical damage. Wiring systems buried in floors to be sufficiently protected against damage. Cables under floors or above ceilings and cables concealed within walls or partitions should be no less than the 50mm minimum requirement from the surface.


40


1 List seven items of information needed prior to commencing an initial verification

2. Give a brief description of the scope of an initial verification

3. What are the main objectives when inspecting the connection of conductors?

4. Why is it important to ensure single pole devices are only connected in the live conductor?

5. Where do fire barriers need to be installed in an electrical installation?

6. How are we granted basic and fault protection from electric shock?


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LEARNER WORK BOOK

Sequence of Tests Testing can be hazardous, both to the tester and to others who are within the area of the installation during the test. The danger is compounded if tests are not carried out in the correct sequence. Some tests require the supply to be on. Some tests will prove the operation of the circuit. The person designated to do the testing carries a huge responsibility to verify that the circuits will not cause danger to property, persons or livestock therefore BS7671 states the order we should carry out the tests. For example, it is of great importance that the continuity of protective conductors is confirmed before the insulation resistance test is carried out. A continuity test confirms the circuit under test is correctly identified whilst the high voltage used for insulation testing could appear on a circuit still being installed and could result in an electrician receiving an electric shock whilst up a ladder. Again, an earth-fault loop impedance test cannot be conducted before an installation is connected to the supply, and the danger associated with such a test before verifying polarity or insulation resistance will be obvious. What BS 7671 says •

Regulation 711-01-01 states ‘Every installation shall, during erection and/or on completion before being put into service, be inspected and tested to verify, so far as is reasonably practicable, that the requirements of the Regulations have been met. Precautions shall be taken to avoid danger to persons, livestock, and to avoid damage to property and installed equipment during inspection and testing’

•

Regulation 713 lists the sequence in which tests should be carried out. If any test indicates a failure to comply, that test and any preceding test, the results of which may have been influenced by the fault indicated, must be repeated after the fault has being rectified

Recording circuit details To aid the testing process a record must be made first of the final circuits. See the left half of the certificate part on the next page for the information that is needed prior to commencing testing. Recording these details logs on a document the installation and can this can then be used as a reference of “as installed” circuits.

Recording the test results Not every reading we take needs to be recorded but there are specific ones that do. See the right half of the certificate part on the next page for the information that is needed to document the testing. Recording these results is written proof that the installation has been tested in accordance with BS7671.


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LEARNER WORK BOOK

Test sequence Having carried out the initial inspection, the following items where relevant, must be tested in the same sequence as stated in BS7671 and shown below. Some tests will be carried out before the supply is connected, whilst others cannot be performed until the installation is energised. The list below shows the correct sequence of testing to reduce the possibility of accidents to the minimum.

X

Test Sequence at a glance:

With the supply isolated: 1. Continuity of protective conductors, including main and supplementary equipotential bonding 2. Continuity of ring final circuit conductors 3. Insulation resistance 4. **Insulation of site built assemblies 5. Protection by electrical separation 6. **Protection by barriers or enclosures provided during erection 7. **Insulation of non-conducting floor and walls 8. Polarity

With the supply energized: 9. Earth fault loop impedance 10. Prospective fault current 11. *Earth electrode resistance 12. Operation of residual current devices

* Using an earth loop tester only ** Specialist testing and will not be discussed in this unit

Consider that the insulation resistance test failed on a ring main. Explain what would be the dangers of carrying on with the testing sequence.


LEARNER WORK BOOK

Test 1 - Continuity of protective conductors (Including main and supplementary protective bonding) Why do we do this test? All protective and bonding conductors must be tested to ensure that they are electrically safe and correctly connected. Regulations state that every protective conductor, including each bonding conductor, shall be tested to verify that it is electrically sound and correctly connected. The test method 1 described below checks the continuity of the protective conductor and will also measure R1 + R2 which, when corrected for temperature, will enable the designer to verify the calculated earth fault loop impedance Zs. Testing the operation of switching circuits during this test will also confirms polarity which is that live conductors are connected correctly. How do we do this test? There are two methods for completing this test (Method 1 and Method 2) but only one of them is necessary. For this test you need a low reading ohmmeter Method 1 Before carrying out this test the leads should be ‘nulled out’. If the test instrument does not have this facility, the resistance of the leads should be measured and deducted from the readings. The live conductor and the protective conductor are linked together at the consumer unit or distribution board. The Earth Continuity - Method 1 ohmmeter is used to test between the live and earth terminals at each outlet in the circuit. The measurement at the circuit’s extremity should be recorded and is the value of R1 + R2 for the circuit under test. On a lighting circuit the value of R1 should include the switch wire at the luminaires. This method should be carried out before any supplementary bonds are made. Operate switches to confirm polarity and see that they affect the reading. Method 2 One lead of the continuity tester is connected to the consumer’s main earth terminal. The other lead is connected to a “wandering” lead, which is used to make contact with protective conductors at light fittings, switches, spur outlets etc. The resistance of the test leads will be included in the result; therefore the resistance of the test leads must be measured Earth Continuity - Method 2 and subtracted from the reading obtained if the instrument does not have a nulling facility. In this method the protective conductor only is tested and this reading R2 is recorded on the installation schedule. This method is also used to test the main and supplementary protective bonding conductors. The ohmmeter leads are connected between the points being tested, between simultaneously accessible extraneous conductive parts i.e. pipe work, sinks etc. or between simultaneously accessible extraneous conductive parts and exposed conductive parts (metal parts of the installation). Inspection and Testing REV4.1

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LEARNER WORK BOOK

Picture the test Method 1 – Continuity of CPC, R1+R2 and Polarity

5. Place meter across live and earth at each accessory

7. Operate switch with meter connected to confirm polarity

3. Light switch on 2. Live (R1) and earth (R2) linked 6. Take readings and record the highest value

1. Isolate supply

4. Zero lead resistance and set to Ω Inspection and Testing REV4.1 46

LEARNER WORK BOOK

Picture the test Method 2 – Continuity of CPC (R2)

4. Take readings at each earth point and record the highest value

3. Zero lead resistance and set to Ω

1. Isolate main supply

2. Connect “wandering” lead to the earth bar and one meter lead Inspection and Testing REV4.1 47

LEARNER WORK BOOK

Picture the test 2. Disconnect bonding conductors from main earth terminal

Continuity of Main Protective Bonding Conductors


5. Take reading ensuring less than 0.05 Ω 4. Place one lead on conductor and other on the clamp connection (Use wandering lead if necessary)


Class Discussion: Mike is obtaining values which seem too high than the expected values. What could explain higher than expected readings whilst carrying out earth continuity tests? Record the key points below.


LEARNER WORK BOOK

Test 2 - Continuity of ring final circuit conductors Why do we do this test? A test is required to verify the continuity of each conductor including the circuit protective conductor (cpc) of every ring final circuit. The test results should establish that the ring is complete and has no interconnections. The test will also establish that the ring is not broken and that the polarity is correct. Incorrectly wired ring main

How do we do this test? There are three steps for completing this test (Step 1, Step 2 and Step 3). For this test you need a low reading ohmmeter. 1. Step 1 - End to End The live, neutral and protective conductors are identified and their resistances are measured separately. A finite reading confirms that there is no open circuit on the ring conductors under test. The resistance values obtained should be the same (within 0.05 ohms) if the conductors are the same size. If the protective conductor has a Step 1 – End to End reduced csa, the resistance of the protective loop will be proportionally higher (typically 1.67 x higher) than that of the live or neutral loop. If these relationships are not achieved then either the conductors are incorrectly identified or there is a loose connection at one of the accessories. 2. Step 2 – Live and neutral cross coupled The live and neutral conductors are then connected together so that the outgoing live conductor is connected to the returning neutral conductor and vice versa. The resistance between live and neutral conductors is then measured at the db and then at each socket outlet. The readings obtained from those sockets wired into the ring will be substantially Step 2 – Live and neutral cross coupled the same and the value will be approximately half the resistance of the live or the neutral loop resistance. Any sockets wired as spurs will have a proportionally higher resistance value corresponding to the length of the spur cable. 3. Step 3 – Live and earth cross coupled Step 2 is then repeated but with the live and cpc cross-connected. The resistance between live and earth is then measured at each socket. The highest value recorded represents the maximum R1 + R2 of the circuit and is recorded on the test schedule.

Step 3 – Live and earth cross coupled


LEARNER WORK BOOK

Picture the test Continuity of Ring Final Circuit Conductors Step 1 – End to End

2. Disconnect ring conductors to be tested

4. Place meter on one loop of the ring (L, N or E)


6. Repeat test for each loop

5. Take reading and record

1. Isolate supply Inspection and Testing REV4.1 50

LEARNER WORK BOOK

4. Place one meter lead on outgoing live leg and incoming neutral leg

Picture the test Continuity of Ring Final Circuit Conductors Step 2 – Live to Neutral Cross Coupled


5. Place other meter lead on incoming live leg and outgoing neutral leg

7. Test live and neutral at each socket. Each value should be the same

2. Ring conductors still disconnected from Step 1

6. Take reading

1. Isolate supply


LEARNER WORK BOOK

6. Take reading and record (R1+R2)

Picture the test Continuity of Ring Final Circuit Conductors Step 3 – Live to Earth Cross Coupled

4. Place one meter lead on outgoing live leg and incoming earth leg

5. Place other meter lead on incoming live leg and outgoing earth leg

7. Test live and earth at each socket. Each value should be the same

2. Ring conductors still disconnected from step 2


1. Isolate supply Class Discussion: John ponders his test results after carrying out the three ring continuity tests. How will the readings be affected when tests are taken at all sockets on a circuit that is wired with unintentional spurs or interconnections? How can the circuit be corrected?


LEARNER WORK BOOK

Test 3 – Insulation Resistance Why do we do this test? Insulation resistance tests are to verify, for compliance with BS 7671, that the insulation of conductors, electrical accessories and equipment is satisfactory and that electrical conductors and protective conductors are not short-circuited, or do not show a low insulation resistance (which would indicate defective insulation). In other words, we are testing to see whether the insulation of a conductor is so poor as to allow any conductor to ‘leak’ to earth or to another conductor. How do we do this test? There are two main methods for testing IR 1. Testing between individual circuits and their conductors 2. Testing between multiple circuits and their conductors We will be looking at testing individual circuits and their conductors but an appreciation of testing multiple circuits will be given in the power point. The test equipment to be used would be an insulation resistance tester meeting the criteria as laid down in BS 7671 using the appropriate d.c. test voltage as specified in Table 61A. Before testing ensure that: Pilot or indicator lamps and capacitors are disconnected from circuits to avoid an inaccurate test value being obtained. Voltage sensitive electronic Insulation resistance test on a lighting circuit equipment such as dimmer switches, electronic starters for fluorescent lamps, emergency lighting, RCDs etc. are disconnected so that they are not subjected to the test voltage. There is no electrical connection between any live and neutral conductor (e.g. lamps left in) To illustrate why we remove lamps, consider the diagram here. The coil that is the lamp filament is effectively creating a short circuit between the live and neutral conductors. Besides this the lamp is only designed to operate with 230 volts and would be damaged if subjected to 500 volts.

Test voltage applied to lamp filament

As mentioned above we can test multiple circuits at once but some electricians prefer, or find it quicker, to test between individual circuits and their conductors. On lighting circuits where two way / intermediate circuits are used tests have to be repeated with switches in the opposite position so that all strappers are tested. An insulation resistance value of not less than 1.0 megohms complies with BS7671. Where an insulation resistance value of less than 2 megohms is recorded, the possibility of a latent defect exists. If this is the case further investigation is required to uncover the source of the non-compliant reading.


LEARNER WORK BOOK

Picture the test Insulation Resistance between Individual Circuit Live Conductor and Neutral

5. Place one meter lead on neutral bar

3. Ensure all loads are removed and switches are on

8. Operate two way / intermediate switches and repeat test

6. Place other meter lead on live on the load side of the MCB

1. Isolate main supply 2. Isolate final circuit supply

4. Set to MΩ (500V)

7. Take reading and record Inspection and Testing REV4.1

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LEARNER WORK BOOK

Picture the test Insulation Resistance between Individual Circuit Live Conductor and Earth


5. Place one meter lead on the earth bar


6. Place other meter lead on live on the load side of the MCB

7. Take reading and record 9. Repeat test across neutral to earth and


2. Isolate final circuit supply


Class Discussion: Lance has been given the task of testing some existing circuits on a db. It is discovered that several readings are between 0 and 2 megohms. What can possibly cause these non-compliant readings? Lance is not fully confident with his testing knowledge. Discuss the entire scope of this event and list the key points below.


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Test 4 - Protection by electrical separation Why do we do this test? This test is an insulation test across separate circuits in the installation. A fault may be present whereby two lives from two different circuits could be damaged and be in contact with each other. Energising one circuit would energise both circuits. The test would confirm whether there is a low insulation resistance between two or more conductors from different circuits. How do we do this test? The test should be made between all conductors in the installation to ensure that at no point in that installation are there any problems that would cause danger. We therefore test between all lives, lives to neutral, lives to earth and neutrals to earth of each and every circuit. The entire test can be completed at the circuit’s supply.

1

2

3

4

Tests shown between four circuits’ lives and one earth

1

2

3

4

Tests shown between four circuits’ lives

As you can imagine this entails a lot of testing.

How might this test be completed in a quicker time? Are there any methods we can use to ensure all conductors are tested but in less time than testing them individually? Use the space below for your diagrams.


LEARNER WORK BOOK

Picture the test Protection by Electrical Separation (Insulation Resistance) between Individual Circuit Live Conductors

5. Place one meter lead on one circuit live



9. Repeat test on each and every other circuit and their conductors

3. Disconnect all final circuit conductors

6. Place other meters lead on another circuit’s live


7. Take reading



LEARNER WORK BOOK

Test 5 - Polarity Why do we do this test? The purpose of the test is to confirm that all protective devices (such as fuses and circuit breakers) and single-pole switches are connected to the line conductor and that the line terminal in socket outlets and the centre contact of screw-type lamp holders are also connected to the live conductor. (Also a check should be made to ensure that the polarity of the incoming supply is correct, otherwise the whole installation would have the wrong polarity). How do we do this test? In essence, having established the continuity of the cpc using method 1 in an earlier test we should have operated light switches and observed their affect on the continuity readings. This means we have already carried out this test. For ring circuits, if the tests required by Steps 2 and 3 (ring circuit continuity) have been carried out, the correct connections of live, neutral and cpc conductors will have already been verified and no further testing is required For radial circuits the R1 + R2 and R1 + Rn measurements should also be made at each point, using this method. To carry out this test (if we forgot to do it) temporarily link out the circuit live and earth conductor at the distribution board and then make our test across the live and earth terminals at each item in the circuit under test. Remember to operate lighting switches before during out the test. The diagram on the right shows the test for polarity of an Edison Screw lighting circuit.

Polarity test of Edison screw lamp holder


LEARNER WORK BOOK

Picture the test 5. Place meter across live and earth at each accessory

Polarity, Continuity of CPC and R1+R2

7. Operate switch with meter connected to confirm polarity

3. Light switch on

6. Take readings and confirm continuity

2. Live (R1) and earth (R2) linked

1. Isolate supply

4. Zero lead resistance and set to Ω Inspection and Testing REV4.1 59

LEARNER WORK BOOK

Test 6 – Earth electrode resistance Why do we do this test? Where the earthing system incorporates an earth electrode as part of the system (such as TT sysytems), the electrode resistance to earth needs to be measured. Some of the types of accepted earth electrode are: • Earth rods or pipes • Earth tapes or wires • Earth plates • Underground structural metalwork embedded in foundations • Lead sheaths or other metallic coverings of cables • Metal pipes The resistance to earth depends upon the size and type of electrode used and we want as good a connection to earth as possible. The connection to the electrode must be made above ground level via an inspection pit like the one showed above. We should carry out the test during dry periods as moisture in the ground will not provide the “worst case” test result. Typical values should be around 100-200 ohms How do we do this test? We measure earth electrode resistances to earth using an earth electrode resistance tester that consists of three test leads and two test spikes. Before this test is undertaken, the earthing conductor to the earth electrode must be disconnected either at the electrode or at the main earthing terminal. This will ensure that all the test current passes through the earth electrode alone and not via parallel paths. However, as this will leave the installation unprotected against earth faults, switch off the supply before disconnecting the earth. The test requires that we drive two temporary test spikes into the ground by the electrode. They are: A current spike at a distance away approximately 10x the length of the rod under test (20m for a 2m rod) A potential spike approximately midway between the current spike and the electrode We then take three readings. 1. Once the spikes are in place as above we take the first reading 2. We then move the potential spike 10% towards the electrode under test and take another reading 3. And a final reading 10% away from the initial middle spike location We then take an average of these three readings to obtain the electrode resistance. As long as the average reading does not deviate from any of the three readings by more than 5% this can be accepted as the electrode resistance value. If they do deviate by more than 5% then the current spike must be moved a greater distance and then re-tests must be carried out. Inspection and Testing REV4.1 60

LEARNER WORK BOOK


Picture the test Earth Electrode Resistance

8. Move potential spike 10% away from the electrode and take 3rd test

7. Move potential spike 10% towards the electrode and take 2nd test

9. Take an average of the three readings ensuring no deviation of more than 5% exists from any reading

6. Take the 1st reading 2. Disconnect the main earthing conductor from the electrode

3. Drive the current spike in a distance of 10x the electrode length away from the electrode

4. Drive the potential spike in half the distance between the electrode and current spike

5. Attach the electrode lead


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Test 7 – Earth loop impedance (Zs) (Inc. prospective fault current – Ipf) Why do we do this test? When designing an installation, it is the designer’s responsibility to ensure that, should a live-to-earth fault develop, the protection device will operate safely and within the time specified by BS 7671. Although the designer can calculate this in theory, it is not until the installation is complete and live that the calculations can be checked. It is necessary therefore to determine the earth-fault loop impedance (Zs) at the furthest point in each circuit and to compare the readings obtained with either the designer’s calculated values or the values tabulated in BS 7671 or the IEE On-Site Guide. How do we do this test? With an earth fault loop impedance tester the value of earth-fault loop impedance (Zs) may be determined by: 1. Direct measurement of Zs at the furthest point in the circuit 2. Direct measurement of Ze** at the origin of the Diagram of Zs = Ze + (R1 + R2) circuit and adding to this the value of R1 + R2 measured during continuity tests 3. Obtaining the value of Ze from the electricity supplier and adding to this the value of R1 + R2 as above. **Ze is the loop impedance value external to the source of the circuit’s supply. Direct measurement of Zs Direct measurement of earth-fault loop impedance is achieved by use of an earth fault loop impedance tester, which is an instrument designed specifically for this purpose. The instrument operates from the mains supply and therefore can only be used on a live installation so great care must be taken. Earth-fault loop impedance testers are connected directly to the furthest point of the circuit. It must be noted that parallel paths may be present which will affect the true circuit’s earth loop impedance reading. Measurement of Ze The value of Ze can be measured using an earth fault loop impedance tester at the origin of the installation. The instrument is connected using approved leads between the live terminal of the supply and the means of earthing with the main switch open or with all sub-circuits isolated. In order to remove the possibility of parallel paths, the means of earthing must be disconnected from the main protective bonding conductors for the duration of the test. Measurement of prospective fault current - Ipf (PFC and PSC) This test is made with an earth fault loop impedance tester set to amps. The prospective fault current, which is how much current will flow in the event of a fault, is divided into two parts: prospective earth fault current (live to earth fault); and prospective short circuit current (fault between live conductors). The PFC test is measured at the furthest point or can be calculated using the Zs value. The PSC test is made at the origin of the installation and its value, obtained in kA, is then compared to the fault current ratings of the protective devices. PFC and PSC tests are actually both carried out at the relevant point and the higher of the two values is recorded in the particulars of the origin of the installation. Inspection and Testing REV4.1 62

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Picture the test Earth Loop Impedance (Zs) on a Ring Final Circuit

6. Verify results by one of the approved methods

3. Place meter in furthest part of ring (mid-point)

5. Take readings and record

2. Circuit breaker on

1. Main isolator on

Live Circuit Test

4. Set to Ω and ensure display confirms correct polarity

The instrument is usually fitted with a standard 13 A plug for connecting to the installation directly through a normal socket outlet, although test leads and probes are also provided for taking measurements at other points on the installation Inspection and Testing REV4.1 63

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Verification of Earth loop impedance test results The values of earth loop impedance obtained (Zs) should be compared with one of the following: 1. BS7671 -Tables 41.2, 41.3 and 41.4 100% maximum measured values of earth loop impedance given in Tables 41.2, 41.3 and 41.4 of BS 7671 can be used which should not be exceeded when the conductors are at their normal operating temperature. These values are the “true” maximum design values for the circuit without parallel paths or variations in normal operating temperature of the circuit. If the conductors are at a different temperature when tested, the reading should be adjusted accordingly with either: i.

ii.

The rule of thumb As an approximation or ‘rule of thumb’ the measured value of earth-fault loop impedance (Zs) at the most remote outlet should not exceed 80% of the relevant value given in Tables 41.2, 41.3 and 41.4 of BS 7671. The verifier’s allowance factor The verifier (person responsible for the test results and certificates) may have a percentage factor which he applies to the measured values that will be specific the installation in which the circuit is installed.

2. Onsite Guide - Standard PVC circuits The measured loop impedances given in the IEE On-Site Guide have been determined to ensure compliance with the required disconnection times where conventional final circuits are installed (shown in Table 7.1 of the Onsite Guide). The values assume that the maximum operating temperature of the insulation is 70°C and the ambient temperature when carrying out the tests is at 10°C. At ambient temperatures other than this, correction factors shown below in table 2E should be applied. Ambient temperature ºC 0 5 10 20 25 30 Onsite Guide – Table 2E

Correction factors 0.96 0.98 1.00 1.04 1.06 1.08

The correction factor is given by: (1+0.004 (ambient temp – 10) Where 0.004 is the temperature coefficient per °C at 20°C for copper and aluminium E.g. If the ambient temperature is 25°C the measured impedance of a circuit protected by a 32A type B MCB should not exceed 1.16 x 1.06 = 1.23Ω

3. Design values Where the designer of the installation provides calculated values of earth-loop impedance, the measured values should be compared with these.

If the circuit being tested is protected by an RCD, the test procedure may cause the RCD to operate causing unwanted isolation of the circuit. Certain types of test instrument may be used that are specifically designed to overcome this problem; otherwise it will be necessary to measure the value of R1 and R2 with the circuit isolated and add this to the value of Ze measured at the incoming terminals


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Test 8 – Operation of residual current devices Why do we do this test? All RCDs are electromechanical devices that must be checked regularly to confirm that they are still in working order. This can be done at regular intervals by simply pressing the test button on the front of the device. Where an installation incorporates an RCD, Regulation 514-12-2 requires a notice displaying this information to be fixed in a prominent position at or near the origin of the installation. The integral test button incorporated in all RCDs only verifies the correct operation of the mechanical parts of the RCD and does not provide a means of checking the continuity of the earthing conductor, the earth electrode or the sensitivity of the device. This can only be done effectively by use of an RCD tester specifically designed for testing RCDs as described below. How do we do this test? The test must be made on the load side of the RCD between the live conductor of the protected circuit and the associated circuit protective conductor. The load being supplied by the RCD should be disconnected for the duration of the test. The test instrument is usually fitted with a standard 13 A plug top, and the easiest way of making these connections, wherever possible, is by plugging the instrument into a suitable socket outlet protected by the RCD under test. The correct test current is then selected and the test button is operated. We then repeat each test The test instrument operates by passing a simulated fault current of known value through the RCD and then measures the time taken for the device to trip Although different types of RCD have different requirements (time delays etc.), for general purpose RCDs the test criteria are as follows: 1. 50% of the rated trip current of RCD (½ x ∆) Should not trip in each half cycle 2. 100% of the rated trip current of RCD (1 x ∆) Should trip the RCD in each half cycle and within 200ms 3. 500% or the rated trip current of RCD (5 x ∆) Should trip the RCD in each half cycle and within 40ms 4. Functional test of actuator on and off 5. Operation of test switch Where ∆ is the device rating If the RCD is rated at 100mA or above only ½ times and 1x tests should be used


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Picture the test RCD on a Ring Final Circuit (1x∆ and 5x∆ only)

6. Reset RCD and repeat test with meter set to (5 x ∆)

3. Place meter in furthest part of ring (mid-point)

5. Take readings and record

2. Circuit breaker on

4. Set to rated trip current of RCD (1 x ∆)

1. Main isolator on

NOTE: Before we carry out the 1x∆ and 5x∆ tests and obtain the results we must first test at ½ x∆ to ensure the device does not operate. Following the 3 tests we test the actuator lever and push the test button. Inspection and Testing REV4.1 66

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Now answer the questions below

1. Why is it important to follow the correct sequence of tests?

2. State the four main tests that are carried out prior to the supply being energised.

3. Briefly, how do we carry out a method 1 earth continuity test on a 1 way lighting circuit?

4. Briefly describe the three step ring final circuit tests.

5. Why do we carry out insulation resistance tests?

6. What is the minimum value of insulation resistance for a 230V circuit?

7. What is the purpose of a polarity test?


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8. We can test polarity whilst carrying out other tests. What are they and how do we achieve it?

9. There are three ways of obtaining earth loop impedance (Zs) values. State how.

10. Explain the difference between i) Ze and Zs ii) PFC and PSC.

11. Once we have obtained values for Zs how can we check these values comply with BS7671?

12. What five tests do we carry out on a residual current device?


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Periodic Inspection and Testing General Requirements The regular inspection and testing of electrical installations is necessary because over a period of time the condition of all installations will deteriorate to some extent. This may be due to normal wear and tear, accidental damage, corrosion or other effects due to environmental influences, normal ageing or deterioration due to excessive electrical loading.

The Electricity at Work Regulations (1989) state that: ‘As may be necessary to prevent danger, all systems shall be maintained so as to prevent, as far as is reasonably practicable, such danger’ This means that all electrical installations must be maintained in a safe condition, and regular inspection and testing (periodic inspection) is an essential part of any such preventative maintenance program. In addition to statutory requirements other bodies such as licensing authorities, insurance companies, mortgage lenders etc. may also require periodic inspection and testing to be carried out on a regular basis. Other reasons for carrying out periodic inspection and testing are: To confirm compliance with the latest edition of BS7671 On a change of ownership of the premises On a change of use of the premises On a change of tenancy of the premises On completion of alterations or additions to the original installation Following any significant increase in the electrical loading of the installation Where there is reason to believe that damage may have been caused to the installation. In the case of an installation that is under constant supervision while in normal use, such as a factory or other industrial premises, periodic inspection and testing may be replaced by a system of continuous monitoring and maintenance of the installation provided that adequate records of such maintenance are kept.

Routine checks Electrical installations should still be routinely checked in the intervening time between periodic inspection and testing. In domestic premises it is likely that the occupier will soon notice any damage or breakages to electrical equipment and will take steps to have repairs carried out. In commercial or industrial installations a suitable reporting system should be available for users of the installation to report any potential danger from deteriorating or damaged equipment. In addition to this, a system of routine checks should be set up to take place between formal periodic inspections. The frequency of these checks will depend entirely on the nature of the premises and the usage of the installation. Routine checks are likely to include activities such as those listed on the next page.


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Routine check list •

Defect reports Check that all reported defects have been rectified and that the installation is safe

•

Inspection Look for: o Breakages o Wear or deterioration o Signs of overheating o Missing parts (covers/screws) o Switchgear still accessible o Enclosure doors secure o Labels still adequate (readable) o Loose fittings

•

Operation Check operation of: o Switchgear (where reasonable) o Equipment (switch off and on) o RCD (using test button)

All inspections should provide careful scrutiny of the installation without dismantling or with only partial dismantling where absolutely necessary. It is considered that the unnecessary dismantling of equipment or disconnection of cables could produce a risk of introducing faults that were not there in the first place. The frequency of periodic inspection and testing should aim to provide, as far as reasonably possible, the following: The safety of persons and livestock against the effects of electric shock or burns Protection against damage to property by fire or heat arising from an installation defect Confirmation that the installation has not been damaged and has not deteriorated to the extent that it may impair safety The identification of any defects in the installation or non-conformity with the current edition of the Regulations that may cause danger. In practical terms the inspector is carrying out a general inspection to ensure that the installation is safe. However, the inspector is required to record and make recommendations with respect to any items that no longer comply with the current edition of the Regulations. As with all inspections the person carrying out the work must be competent and have sufficient knowledge and experience of the type of installation to be inspected and tested. Enquiries should be made to the person responsible for the installation with regard to the provision of charts and/or diagrams to indicate the type of circuits, means of isolation and switching, and types and ratings of protective devices including a written record of all previous inspection and test results. The inspection is recorded on a periodic inspection report certificate and any limitations, which are parts of the installation that can not be checked, are added as a record incase of any future legal events Inspection and Testing REV4.1

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Sequence of tests Continuity Tests to be carried out between: • All main bonding connections • All supplementary bonding connections Note! When an electrical installation cannot be isolated, protective conductors including bonding conductors must not be disconnected Polarity Tests to be carried out: • Origin of installation • All socket outlets • 10% of control devices (including switches) • 10% of centre contact lamp holders Note! If incorrect polarity is found then a full test should be made on that part of the installation and testing on the remainder increased to 25 per cent. If further faults are found the complete installation must be tested Earth-fault loop impedance Tests to be carried out at: • Origin of installation • Each distribution board • Each socket outlet • Extremity of every radial circuit Insulation resistance If this test is to be carried out then test: • The whole installation with all protective devices in place and all switches closed • Where electronic devices are present, the test should be carried out between line and neutral conductors connected together and earth Functional Activities to be carried out: • All isolation and switching devices to be operated • All labels to be checked • All interlocking mechanisms to be verified • All RCDs to be checked both by test instrument and by test button • All manually operated circuit breakers to be operated to verify they open and close correctly Class discussion You are asked to carry out a periodic inspection at a small workshop. All goes well until you come to check the office that is attached to the factory, where you notice that about 20 staff are working on computers. No installation drawings exist for the office, as it turns out the office was built as an extension to the factory many years ago. 1. Can you identify the problems you will encounter in this job? 2. How would you deal with the situation? Inspection and Testing REV4.1

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1. Why is it necessary to carry out periodic inspections of installations?

2. State the four main reasons why periodic inspections are carried out.

3. When carrying out a periodic inspection what types of things do we look for?

4. Briefly describe how we complete a continuity test on a radial power circuit.

5. Briefly describe what we do to complete a polarity test.

6. Briefly describe what we do to complete earth fault loop impedance test.

7. Briefly describe what we do to complete insulation test.


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Unsatisfactory Test Results Continuity When testing the continuity of circuit protective conductors or bonding conductors we should always expect a very low reading, which is why we must always use a low-reading ohmmeter. Main and supplementary bonding conductors should have a reading of not more than 0.05 ohms whilst the maximum resistance of circuit protective conductors can be estimated from the value of (R1 + R2) given in Table 9A of the IEE On-Site Guide. These values will depend upon the cross-sectional area of the conductor, the conductor material and its length. A very high (end of scale) reading would indicate a break in the conductor itself or a disconnected termination that must be investigated. A mid-range reading may be caused by the poor termination of an earthing clamp to the service pipe e.g. a service pipe which is not cleaned correctly before fitting the clamp or corrosion of the metal service pipe due to its age and damp conditions. Polarity Correct polarity is achieved by the correct termination of conductors to the terminals of all equipment. This may be main intake equipment such as isolators, main switches and distribution boards or accessories such as socket outlets, switches or light fittings. Polarity is either correct or incorrect; there is nothing in between. Incorrect polarity is caused by the termination of live conductors to the wrong terminals and is corrected by reconnecting all conductors correctly. Single pole switches should only be connected in the live conductor and if the operation of switches during an earth continuity test does not affect the reading then this may imply the neutral has been connected in the switch and not the live Insulation resistance The value of insulation resistance of an installation will depend upon the size and complexity of the installation and the number of circuits connected to it. When testing a small domestic installation you may expect an insulation resistance reading in excess of 200 MΩ whilst a large industrial or commercial installation with many sub-circuits, each providing a parallel path, will give a much smaller reading if tested as a whole. It is recommended that, where the insulation resistance reading is less than 2 MΩ, individual distribution boards or even individual sub-circuits be tested separately in order to identify any possible cause of poor insulation values. An extremely low value of insulation resistance would indicate a possible short circuit between live conductors or a bare conductor in contact with earth at some point in the installation, either of which must be investigated. A reading below 1.0 MΩ would suggest a weakness in the insulation, possibly due to the ingress of dampness or dirt in such items as distribution boards, joint boxes or light fittings etc. Although PVC insulated cables are not generally subject to a deterioration of insulation resistance due to dampness (unless the insulation or sheath is damaged), mineral insulated cables can be affected if dampness has entered the end of a cable before the seal has been applied properly. Other causes of low insulation resistance can be the infestation of equipment by rats, mice or insects.


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Earth-fault loop impedance As explained previously the earth-fault loop path is made up of those parts of the supply system external to the premises being tested (Ze) and the live conductor and circuit protective conductor within the installation (R1 + R2), the total earth-fault loop impedance being Zs = Ze + (R1 + R2). Should the value of impedance measured be higher than that required by the design of the installation, then as we have no influence on the external value of impedance (Ze) we can only reduce the value of Zs by: Installing circuit protective conductors of a larger cross-sectional area or If aluminium conductors have been used, by changing these to copper. If the value were still too high to guarantee operation of the circuit protective device in the time required by BS 7671, then consideration would have to be given to changing the type of protective device (i.e. fuses to circuit breakers). Residual Current Devices (RCDs) Where a Residual Current Device (RCD) fails to trip out when pressing the integral test button this would indicate a fault within the device itself, which should therefore be replaced. Where a Residual Current Device fails to trip out when being tested by an RCD tester then it would suggest a break in the earth return path, which must be investigated. If the RCD does trip out but not within the time specified then a check should be made that the test instrument is set correctly for the nominal tripping current of the device under test.

1. What could a high reading during a continuity test indicate?

2. What value of insulation resistance is warrants further investigation?

3. What can cause values of insulation resistance lower than 1.0 MΩ?

4. How can we improve the value of earth loop impedance?


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