Electrical Installations - Guidelines

WFP Electrical Standards

Regulatory framework - Guidelines

2.1. TN-S system In a TN-S system, the exposed-conductive-parts are connected to a protective circuit named PE (separate conductor from Neutral). PE conductor is the preferential path for currents originated by a loss of insulation (ground fault currents).

2.2. TN-C system In a TN-C system, the neutral and the protective functions are combined in one conductor (named PEN). In this system, a fault current resulting from a l oss of insulation would flow through the same circuit of a short-circuit. This needs to be considered when designing fault protections (against indirect contact, see guidelines paragraph 6.1.2).

2.3. TN-C-S system In a TN-C-S system, TN-C and TN-S systems coexist. Therefore PE and N (neutral) conductors are partially combined. The same considerations as for TN-C systems can be applied. The use of this type of system in WFP electrical installations is not recommended.

2.4. TT system In TT systems the neutral conductor (N) of power supply neutral conductor is earthed, and the exposed-conductive-parts of the installation are connected to earth electrodes which are electrically independent of the earth electrode of the supply system. In case of insulation fault, the consequent fault current would circulate through the earth.

2.5. IT system In an IT system the power supply has all its live parts isolated from earth or one point connected to earth through a high impedance. The exposed-conductive-parts of the electrical installation are earthed independently or collectively. This type of system is generally used to ensure safe power supply even in case of a insulation fault (single), usually in medical facilities or other environments where the disruption of service due to a fault might have serious consequences. IEC recommends that, when this power system is used, the isolator or impedance device be constantly monitored.

2.6. Power systems around the world The classification of power systems, presented in previous paragraphs, is mainly focused on the preferential path to dissipate fault currents, and the main difference among them regards whether earth is considered a reliable path (TT and IT systems) or not (TN systems). As to the level of safety provided, they can be considered more or less equivalent (with the exception of TNC-S and IT systems which should not be used unless under specific requirements), even though each system has its own peculiarities that need to be taken into account when designing an electrical installation. The table below, indicative and not exhaustive, shows the variety of power systems used in some countries:

ITAROM0016DO0011F05

62



Country

National power system

LV user system

Belgium

TT

TT

Australia

TN-C

TN-C

China

TT

TT

France

TT

TT

Germany

TN-C

TN-C / TT

Ireland

TN

TT

Italy

TT

TT

Japan

TT

TT

Norway

IT

IT

Portugal

TT

TT

Spain

TT

TT

Switzerland

TN

TN-C / TN-S

United Kingdom

TN-C

TN-C / TN-S

United States

TN-C

TN-C / TN-S

The diversity of practices that can be observed in the table originates from the different evolution of electrical systems in different countries (and often i n different times), which was influenced by technical choices made when electricity started to be broadly distributed for commercial purposes. It must be also considered that an international trade of electricity developed only recently, bringing along the need for a stronger harmonization of systems. At the same time, the methodologies for the prevention of electrical accidents evolved in close relationship with the development of studies on electricity and effects of current on the human body. At the beginning of commercial distribution the main precaution was insulation toward earth; then, the practice of connecting all metallic (conductive) parts to earth was g radually introduced. The principle at the basis of this practice was and still is ensuring “equi -

potentiality” of conductive parts (according to IEV, “the state wh en conductive parts are at a substantially equal electric potential”), in order to avoid currents that may originate from a difference in potential (voltage) of two conductive parts. As a further development, the

“earthing” practice was integrated with active protection measures (i.e. the use of RCD breakers).

Until the 50’s, TT was the most common system, and connection to earth was either realized through specific devices or through water pipelines (this practice is now generally forbidden). Afterwards, technical authorities and electrical distributors of different nations decided to adopt different systems, in relation to the situation and development of existing infrastructures, existing technical context and el ectrical market conditions – for example, TT systems may provide better segregation (in terms of responsibility also) among sections of the systems managed and owned by different entities (di stributor and user).

ITAROM0016DO0011F05

63



3. Standards and codes Even though the technical principles underlying electrical design are invariable, each power system requires specific considerations for equipment and wiring both from a quantitative and qualitative standpoint. Moreover, the acceptable level of safety may be set by conventions and regulatory authorities, where existing, at different levels. In all this, while electrical infrastructures were evolving on separate, although sometimes parallel, tracks in several countries, a number of different national and international standards and codes have been developed in the last century, for manufacturers and installers to comply with. As mentioned in the introduction to the guidelines, the harmonization process is still developing for infrastructures, standards, and codes, and even though there is now general agreement on fundamental principles, there are a number of areas in which harmonization is still far to reach, and for which production of equipment has to be diversified on the basis of the target market. To give an example, equipment produced for the American market would not be compatible with the European market for three main aspects: 

Different plug type



Different Voltage



Different frequency

Moreover, production in these two regions complies with different technical standards, for which compatibility has to be specifically checked (e.g. the reference standard on enclosure protection from dust and water is different, and therefore a conversion table is needed). To start the study of standards and codes, it is necessary to distinguish the meaning, for electrical matters, of the two terms. A standard, according to the Merriam-Webster On-line Dictionary, is “something set up and established by authority as a rule for the measure of quantity, weight, extent, value, or

quality”. Substantially, standards set performance parameters and criteria for evaluation, as well as test methods and procedures for materials and entire systems, throughout the whole supply chain from manufacture of equipment to design, erection and operation & maintenance practices for installations.

On the other hand a code is defined, in the same dictionary as above, “a systematic statement of the body of law; especially: one given statutory force; a system of principles or rules.” This definition provides a measure of the similarities and differences between the concepts of standard and code. Practically, in technical and particularly in electrical matters, the term ‘standard’ has come to indicate those docu ments whose adoption and use remains fully

voluntary. On the contrary, the term ‘code’ indicates those documents whose adoption and use may be voluntary or mandated by law. Moreover, standards usually do not prescribe only one way to engineer a product or procedure, whereas that is not necessarily the case with codes. It is evident that the border between the two definitions is subtle; for example, the U.S. National Fire Protection Association (NFPA) document named NFPA-70 is known by most experts as the National Electric Code, but it is developed as a consensus standard until its use is mandated by individual legislative bodies. Similarly, the standards published by the IEC, are consensus-based standards. In particular, the Technical Standard series 60364 for Low Voltage electrical installations, whose main purpose is to form the basis for th e development of national codes, was indeed used as a basis for the preparation of most European national codes, but it can be adopted as a code ‘as is’ by legislative bodies, should need be. The study and publication of electrical standards and codes arose in the 1880s with the commercial introduction of electrical power. Many conflicting standards existed for the

ITAROM0016DO0011F05

64



selection of wire sizes and other design rules for electrical installations, so the first codes were an attempt to both establish some sort of consistency as to the level of safety provi ded, and facilitate trade. Electrical codes, often referred to as wiring regulations, are intended to protect people and property from electrical shock and fire hazards, setting best practice reference and ensuring reliability of systems. They are usually more prescriptive than standards, and their target audience is mainly composed by designer or installer technicians. Finally, electric codes usually refer to existing standards for materials and equi pment, so that their characteristics and performance can be practically identified and their selection and use in electrical installations is consistent with the code’s prescription s. As a consequence, in a specific context and/or geographical area there has to be consistency throughout electrical infrastructures, power supply characteristics and system, standards, codes, and legislation. This concept is well explained in the following quote from a study published by U.S. National Electrical Manufacturers Association (NEMA):

“To be effective, an electrical installation code must be suitable for the existing electrical infrastructure, be suitable for the electrical safety system employ ed in a country, and be capable of being uniformly interpreted, appli ed, and enforced. It must also have compatibility with standards applicable to products whose installation, use, and maintenance is intended to be governed by the code.” (N.E.M.A. - Underwriters laboratories inc., Electrical Installation Requirements: a global perspective, April 1999). The quoted document provides an interesting comparison of the two main families in which wiring codes and regulations may be classified: the codes which are derived from the IEC standards, and the North-American National Electric Code. This division is, obviously, coherent with the distribution of power supply characteristics as can be seen in the r elevant picture ahead. The difference between IEC derived codes and NEC, and coordinated standards, has indeed its roots in a difference between power supply characteristics (Voltage and Frequency), and is as well a consequence of the fact that the United States developed a capillary electrical infrastructure earlier than Europe did. For example, in the 60’s when Europe evolved from 110 to 240 V due to the increase in power demand by final users, the U.S. chose to not upgrade the existing infrastructures. A significant difference between the two standards can be found in their origin and purpose: the NEC is a code developed in the United States and intended to be a manual for designers and installers in that country, a manual that at a later stage was adopted by other countries, for various reasons varying from geographi cal contiguity to cultural influence. At the same time the IEC standards were, since the beginning, an attempt to coordinate national regulations and electrical markets under an international standard. This observation marks an important difference in the approach that the two documents (n.b. IEC is actually a set of several different documents) adopt towards some technical solutions. The IEC standards have all the characteristics of a classification and coordination document, which proposes basic principles and suggests a set of possible technical solutions. The NEC code is a practical guide, which was originally developed for a specific electrical infrastructure and market, with all their peculiarities. However, it must be noted that IEC standards and NEC share the same basic principles (as expressly stated in the preface of NEC 2011 edition), and therefore the same level of intrinsic safety and reliability, as far as they are applied consistently and in the appropriate context. The picture below shows the geographical distribution of IEC and NEC, and it is possible to observe how closely it relates to the distribution of power supply voltage and frequency, if compared to the relevant picture in paragraph 1).

ITAROM0016DO0011F05

65



Picture 2 (source Legrand)

The picture above is published on the internet document “International electrical standards and regulations”, which can be consulted on -line at the following link: www.legrand.com/files/fck/file/pdf/guide-international.pdf It provides useful guidance and information on the differences among national electrical codes around the world, especially for small installations.

3.1 International Electro-technical Commission (IEC) The International Electro-technical Commission is an international organization that prepares and publishes standards. It held its inaugural meeting on 26 June 1906, following discussions between the British Institution of El ectrical Engineers, the American Institute of Electrical Engineers and other National institutions, which had begun at the 1900 Paris International Electrical Congress.

Today IEC is one of the world’s leading organizations for the preparation and publicat ion of International Standards for all electrical, electronic and related technologies, counting 83 member countries and 83 countries participating to the affiliate country program, a form of participation which is designed to help industrializing countries get involved (figures in May 2015). It closely cooperates with the International Standard Organization (ISO) and the International Telecommunication Union (ITU) to ensure coordination and complementarity of respective international standards. The IEC is one of the bodies recognized by the World Trade Organization (WTO) and entrusted by it for monitoring the national and regional organizations agreeing to use the IEC's international standards as the basis for national or regional standards as part of the WTO's Technical Barriers to Trade Agreement. Joint committees have been established in order that the standards combine all relevant knowledge of expert working in related areas. In addition, it works with several other major standard development organizations, including the IEEE

ITAROM0016DO0011F05

66



(Institution of Electrical and Electronics Engineers) with which it signed a cooperation agreement in 2002, later amended in 2008 to include joint development work. As mentioned above IEC international standards are consensus-based, prepared and reviewed regularly by dedicated committees, in which member countries are represented. In Europe, the CENELEC (European Committee for Electro-technical Standardization) has encouraged the harmonization of national codes on the basis of IEC documents. For Low Voltage electrical installations the reference standard is the IEC 60364 series, and national codes have been structured with the same system of sections and chapters, for easy reference, even though they may contain additional provisions to cater for historic national practice and to simplify field use and determination of compliance by electrical tradesmen and inspectors. National codes and guides are aimed at complying with fundamental principles and requirements of the IEC 60364, and provide rules and guidance for technicians installing and inspecting electrical systems. A couple of examples of European national regulations: 



In the United Kingdom, wiring installations are regulated by the Institution of Engineering and Technology Requirements for Electrical Installations: IEE Wiring Regulations, BS 7671: 2008. The first edition was published in 1882. The 17th edition (issued in January 2008) is completely harmonized with IEC 60364 and includes new sections for micro-generation and solar photovoltaic systems. In Germany, DKE (the German Commission for Electrical, Electronic and Information Technologies of DIN and VDE) is the organization responsible for the promulgation of electrical standards and safety specifications. DIN VDE 0100 is the German wiring regulations document harmonized with IEC 60364.

A similar situation can be found in France, Italy, Spain, etc. and for this reason all European codes can be used to design, erect, operate and maintain electrical installations in full compliance with IEC standards. Beyond Europe, Australia and New Zealand have developed the standard AS/NZS 3000 for

electrical installations, published under the name of “Australian/New Zealand wiring rul es”, in which the preface states: “During preparation of this Standard, reference was made to IEC 60364, Electrical installations of buildings (all parts) and acknowledgment is made of the

assistance received from this source”. The picture below shows the worldwide presence of IEC in terms of full/associate membership and affiliate country programme.

ITAROM0016DO0011F05

67


full members


Picture 3 (source: Wikipedia)

associate members Affiliates

3.2 NFPA-70: USA and Canada The first electrical codes in the United States originated in New York in 1881 to regulate installations of electric lighting. Since 1897 the National Fire Protection Association, a private non-profit association formed by insurance companies, has published an electrical code under the name of NFPA-70, which is now commonly known as the National Electrical Code (NEC). States, counties or cities often include the NEC i n their local building codes by reference along with local amendments or additional provisions. The NEC is m odified every three years, and it is prepared as a consensus code considering suggestions from interested parties. The proposals are studied by committees of engineers, t radesmen, manufacturer representatives, fire fighters, and other invitees. Due to the fact that the construction industry in the U.S. commonly uses timber construction, the NEC framework places attention on the fire prevention aspects of electrical standards. NEC is used in various jurisdictions in the U.S. and in a number of other countries, and has been translated in Japanese, Korean and Spanish. The diffusion of NEC around the world can be seen in picture 2. It is worth noting that, as mentioned in previous paragraph the United States participated in the foundation of IEC and have full membership in the IEC. Therefore, the preface of NEC 2011 edition references the fundamental principles of IEC 60364 international standard:

“Art. 90.1 Purpose. Clause D: Relation to Other International Standards: The requirements in this code address the fundamental principles of protection for safety contained in section 131 of IEC Standard 60364-1, Electrical Installations of Buildings.” Since 1927, the Canadian Standards Association (CSA) produces the Canadian Electrical Code (CEC). It deals with broadly similar objectives as the US code, but they differ occasionally in technical detail. As part of the North American Free Trade Agreement (NAFTA) program, US and Canadian standards are progressively converging toward each other, i n a harmonization process.

ITAROM0016DO0011F05

68



In 2006 edition, CEC references IEC 60364 (Electrical Installations for Buildings) and, in analogy with NEC, states that the code addresses the fundamental principles of electrical protection in IEC Standard Section 131.

4. IEC 60364 and NFPA-70 4.1 Comparative analysis This section intends to provide general information about the similarities and differences between IEC 60364 and NFPA-70, and is based on the 1999 study from the U.S. National Electrical Manufacturers Association (NEMA), “Electrical Insta llations Requirements: A Global

Perspective”. Although the study is based on the editions of the standards which were available in 1999, the general aspects that are discussed below retain their validity for the latest editions. The study from NEMA remains the best reference for a detailed analysis and comparison between NFPA-70 and IEC 60364, but it is important to remind that some specific articles/clauses in both IEC and NEC might have b een updated in the meantime. The NEC is a stand-alone document, cohesive and comprehensive, with a prescriptive style and mandatory language. It is intended to be a code, i.e. a practical guide for designers, installers, and surveyors, providing technical solutions to ensure that performance objectives are met. Even though there are no normative (mandatory) references in it, there is a close relationship with and reliance on provisions in product standards. The code rules take into consideration known performance capabilities and required construction features of electrica l construction material and utilization equipment. On the other hand, electrical products must be evaluated and certified not only for risks to life and property, but also against conformity to the installation and use provisions of the NEC. It covers electrical installations from the Point Of Delivery (POD – utility supply) to terminal circuits and power outlets, including some requirements for utilization equipment. The code also covers installations for MV and in hazardous locations (explosive atmosphere). As opposed to the practical nature of NEC, the IEC institution aims to achieve i nternational consistency on standards and specifications. IEC publications consist of a series of different documents that evolve into International Standards (IS) only when approved by all members of the relevant technical committee. These international standards are intended to serve as the basis for the development of national requirements. The note to chapter 13, part I of IEC 60364 series, states:

“Where countries not yet having national regulations for electrical installations deem it necessary to establish legal requirements for this purpose, it is recommended that such requirements be limited to fundamental principles which are not subject to frequent modification on account of technical development. The contents of Chapter 13 may be used as

a basis for such legislation.” These fundamental principles cover the need for protection against various hazards that may occur due to the use of electricity. IEC IS 60364 part 1 is broadly performance-based and it is not intended to be used directly by designers, installers, or verification bodies, but rather for use as a guide for development of national wiring rules. In g eneral, all IEC standards tend to be more performance-based then prescriptive, in order to ensure flexibility and compatibility with the different contexts of member countries. The variety of documents produced by IEC goes into different levels of detail, setting principles, rules, specifications, and standards. Reference is made to other IEC or ISO standards for the performance and characteristics that materials and equipment need to ensure, in order that the installations as a whole can ensure IEC performance requirements and prescriptions.

ITAROM0016DO0011F05

69



While IEC IS 60364 covers low voltage electrical installations, MV installations and explosive atmospheres are discussed in other publications by IEC. When it comes to practical differences, NEC refers to American Wire Gauge (AWG) system to classify the size of wires, while IEC Standards u se cross-sectional area expressed in square millimetres. Enclosures performance under NEC is classified according to NEMA or UL standards (Underwriters Laboratories inc.) according to hazard classification for locations defined in the NEC, while IEC Standards reference to IP grade rating according to IEC 60529. Levels of performance might be similar, but to have a perfect match it is necessary to have consistency throughout the whole supply chain. The following text is part of the comparative study prepared by Underwriters Labs. Inc. and published by NEMA, and provide valuable additional information on the background and applicability of both standards, with regard to e xisting infrastructures:

“A significant difference in electrical system cha racteristics that has influenced electrical safety rules is the difference in voltage for the majority of utilization circuits. In North America and a number of other countries, typical household and other general purpose receptacle circuits operate at 120 V, ac. In European countries and some other parts of the world, 240 V, ac, (between conductors and to ground) is the norm. The higher voltage makes it easier to disconnect earth faults in TN systems without use of residual current devices (RCDs). However, the higher circuit voltage can create higher touch voltages. Together with the permitted variations in supply system grounding (earthing) rul es, a necessity is created to devote more attention to prevention of shock hazards due to indirect contact (with accessible parts that may become live due to a fault). One important consideration in development of new national electrical installation requirements, is the type of existing infrastructure and electrical supply systems. In areas where the general purpose utilization circuits operate at 120 V, ac, the NEC may be more appropriate. Even if these circuits operate at 240 V and the supply systems are of TNS or TNCS type, the NEC could be applied with modifications to some parts of the Code, mainly in Article 210 sections on branch circuit voltages. The Code also accommodates IT and TNC systems. In the event the existing branch circuit conductors have metric dimensions and the common conductor sizes and overcurrent device ratings of the IEC standards are employed, some adjustments in the NEC would be necessary, mostly for uni t conversions. However, from the standpoint of uniform application and enforcement, the NEC, with its comprehensive requirements, would be a more appropriate base document for development of national wiring rules. Countries wi th IT, TNC, TNS, and TNCS systems could adopt Chapter 13 on fundamental principles as the guiding principles and adopt the NEC as the national installation and wiring rules, or they could use IEC 60364 as a basis for development of their national rules. In areas of the world where TT premises wiring systems exist, the IEC 60364 documents may be more suitable for promulgating national wiring rules. The NEC specifically prohibits TT supply systems. The IEC 60364 documents contain the requirements for the additional safety

features, which are necessary for TT supply systems.” 4.2 Conclusions As mentioned in the foreword, this study is derived from the preliminary research that was done in the context of the WFP Electrical Standards Project, with the purpose of identifying those standards and regulations that could ensure the objectives of consistent performance for electrical installations in WFP premises, facilities and operations. The guidelines provide greater detail on the objectives, considerations and recommendations that have been developed in the project, however the reasons that drove the adoption of the fundamental principles and requirements of the IEC 603 64 series of Technical Standards, while

ITAROM0016DO0011F05

70



leaving flexibility for the choice of national/local electrical codes, are grounded in this report and should be clearer to the reader at this point. For readers who may be interested in more detailed analysis and information on the subject, a specific bibliography is included here below.

5. Bibliography “Electrical codes, standards, recommended practices and regulations”, by Robert J. Alonzo, ed. Elsevier, 2010;

“International electrical standards and regulations: an overview of electrical installations”, Legrand (downloadable on Legrand website);

“Electrical installation requirements: a global perspective - National Electrical Manufacturers Association (USA) By Underwriters Laboratories Inc. Principal Investigator Paul Duks - April 1999 (downloadable on NEMA website).

ITAROM0016DO0011F05

71



Annex 2: Fundamental Principles for Electrical Safety

1. Introduction The hazard presented by electricity for human beings and animals is mainly due to the current that can flow through their body. An electric current is generated by a difference in potential energy (voltage) between two parts, causing a flow of electrons between the two parts when these are somehow connected with a conductive element. When a human or animal body comes into contact with two parts that have a different potential, it becomes the conductor for the electric current that is generated. To better understand this concept, it is useful to observe birds that sit quietly on HV electric cables: although it may seem a dangerous location, nothing happens so long as they are in contact with only one cable. On the other hand, should any living body come into contact with two HV cables with different potentials, consequences would certainly be lethal.

It is therefore evident that electrical safety lies primarily in “equi -potentiality”. If all the conductive surfaces (or objects) that a person can reach at a given time are at the same voltage, no harm is possible. However, this does not mean that electric shock is only possible touching two different energized objects. In fact, in electrical installations, live parts are maintained at a certain voltage, or potential difference, with respect to earth voltage which is considered neutral. This means that if any conductive part (including a person or an animal), comes into contact with a live part and the earth – at the same time - a current will flow through the part (or the body). Considering this, one of the main methods for protection is insulation. For example, if a person is equipped with special boots that provide insulation from the ground, that person will be able to touch a live part without being exposed to a dangerous current. Of course, the effectiveness of insulation depends on the voltage, and therefore insulation layers and protective equipment need to be adequate to the voltage (both for thickness and inherent insulation capacity). Moreover, air itself can conduct electricity when the voltage between two parts is too high with respect to the distance between them; the sudden establishment of a current in free air, with light and heat production, is known as “arc flash” phenomenon. Air conductivity can be influenced by the humidity, as water is a good conductor. And water conductivity is the reason for which the contact with electricity, in presence of water, is even more dangerous. The amount of current that can be caused by a voltage depends on the resistance of the circuit. The resistance is the opposite of conductivity, as it measures the opposition that the current will face while flowing through a specific circuit. The energy that is l ost to overcome this opposition is dissipated into heat. When a person comes into contact with two live parts at a different voltage, or with a live part and the earth, the body becomes the circuit through which the current flows. The mechanisms by which the tissues are damaged will be analysed ahead, but it is important to understand that the resistance of the whole circuit, including the body, affects the amount of current to which the body is exposed. The energy of the current, that can be calculated multiplying the current intensity by the applied voltage, will be dissipated into the body due to its intrinsic resistance, mainly through heat.

ITAROM0016DO0011F05

72



2. Types of contact As mentioned in the guidelines, when a person comes into contact with electricity, this cont act is classifiable as indirect or direct. Indirect contact is the contact with a conductive part that is not normally live, but has become live due to a fault in insulation of live parts (e.g. the metallic enclosure of an electric device). In this case the voltage applied to the contacts depends on the entity of the fault. On the other hand, direct contact is the contact with a conductive part that is normally live, and therefore it happens at the full voltage of the system: in case of contact with a live part and the ground, the voltage would be the single phase voltage E (110 – 240 V). Instead, with two different live parts, the voltage is the threephase voltage V (240 – 400 V, roughly 1.7 times the voltage toward earth). Examples of types of contact are shown in the picture below:

In general, considering all other factors unchanged, the most dangerous contact condition is the one show in the picture on the right, as the only resistance offered to the current flow is the resistance of the body, and the body itself therefore absorbs all the energy. In the first two conditions, the intensity of the current, and consequently the amount of energy that the body would absorb, depends on the overall resistance of the circuit through the ground (earth resistance). It must be noted that, if there is an alternative circuit with a minor resistance, the electric current will be divided between the two circuits in reverse proportion to the resistance. This means that, if a metallic enclosure is connected to the ground through a low resistance conductor (ground or earth conductor), in case of a fault causing a voltage on the enclosure a greater amount of current would preferentially pass through the conductor rather than through a body. The intensity of the current that would flow through the body in case of indirect contact is therefore greatly reduced if the electrical systems and metallic enclosures are properly connected to earth (Earthing or Grounding).

3. Step potential The ground (earth) under our feet is usually considered to be at 0 V. As already mentioned, power lines, radio antennas and most electrical systems are usually earthed (grounded) by connecting the neutral point and the metallic parts to metal rods driven into the ground. This is necessary to ensure that the voltage of live parts (or potential difference with the ground) is limited to the design voltage, and at the same time to ensure that all the conductive parts, which are not live under normal conditions, are equipotential to the ground. In case of an insulation fault, the voltage is dissipated to earth through the connection.

ITAROM0016DO0011F05

73



If a person walks barefoot on the ground with his/her feet spread apart, there should be 0 V between the two feet. This normal state can be temporarily modified if a loss of insulation occurs in HV power lines or transforming stations, or if a HV conductor reaches the ground, or if lightning strikes. When an energized part contacts the ground directly or through another conductor, this condition is referred to as a ground fault. The voltage on the ground creates a radial potential field, with voltage decreasing with distance. The decrease rate is defined ground potential gradient. Step voltage is the difference between ground potentials at a step distance. When the step voltage is different from zero, it can cause electric shock, and therefore the ground surrounding HV power lines or equipment can become potentially dangerous when a fault occurs.

4. Effects of electric current An electric shock is defined as a sudden violent response to an electric current flowing through any part of a person's body. When flowing through the human body, electricity can cause a number of effects, both temporary and pe rmanent. An electric current may have consequences on blood, blood vessels, nerve cell s, tissues; consequences on cerebral activity and/or such systems as cardiovascular, central nervous system, auditory, visual and other ones, may be permanent. Internal injuries and haemorrhage might not be immediately apparent, and therefore any incident involving electricity requires medical care. The intensity of a current is related to the electrical charge flowing per second (Coulomb/second) and it is measured in Amperes, or milli -Amperes (1/1000 Ampere). Three primary factors affect the severity of the shock that a person receives when he/she becomes part of an electrical circuit: 

Amount of current flowing through the body (measured in milli-Amperes)



Path of the current through the body



Duration of exposure to the current

Other factors that may affect the severity of th e shock are: 

Voltage (the effect of voltage is also related to the current that it can generate)



The presence of moisture in the environment



The phase of the heart cycle when the shock occurs



The general health of the person prior to the shock

Effects can range from a barely perceptible tingle to muscular contractions, severe burns and immediate cardiac arrest. However, the more frequent and important effects that a current can cause are mainly four: Tetany Persistent muscular contraction, known as tetany, happens when the intensity and duration of the current is sufficient to override the voluntary impulses that control muscles. Usually this happens for pulsating currents greater than 10 mA for women and 15 mA for men. Under certain conditions the contraction lasts until the current stops, and this phenomenon is called tetany. As a consequence, the victim may not be able to leave the electrified part. Depending on the duration of the contact and on the path of current through the b ody, tetany can cause

ITAROM0016DO0011F05

74



difficulty in breathing, convulsions, respiratory and cardiac arrest and loss of consciousness. Tetany is involved in 10 % of deaths due to electrocution. Respiratory arrest Respiratory arrest can happen when tetany affects the muscles that take part in respiration, usually with currents above 20-30 mA, det ermining suffocation and loss of consciousness. Respiratory arrest contributes to approximately 6% of deaths due to electrocution. Ventricular fibrillation Electric nerve impulses, in normal condition, control the cardiac muscle. If an electrical current overrides these impulses, it can disrupt the normal heart rhythm, and cause ventricular fibrillation. This phenomenon is normally caused by currents greater than 70-100 mA, and is found in 90% of electrocution deaths, concurring wi th other effects. Burns As mentioned above an electric current, flowing through the human body, dissipates its energy in the form of heat (Joule effect). This effect is usually stronger on the skin where the surface contact resistance is higher, so much that it is sometimes possible to clearly identify the so

called “entry and exit” wounds (burns). Even though the terms “entry and exit” can be misleading if used for an electric current, which is usually bi-directional, by locating the wounds it is possible to determine the path of the current through the body in order to assess the possible damage to internal tissues. Burns can also be caused by arc flashes, and usually a flash burn where no current entered the body tends to be diffuse and relatively uniform. It is possible to represent the likelihood of the different effects of an electric current (AC between 15 and 100 Hz) flowing through the human body, on the basis of its intensity and duration of exposure (Left hand to feet current).

Picture 1

1. Perception level, light shock without consequences 2. No dangerous physiopathological effect 3. Tetany and burns, low likelihood of ventricular fibrillation 4. High likelihood of ventricular fibrillation

ITAROM0016DO0011F05

75



Of course, due to the multiplicity of factors involved, the injuries that may result from any given amperage can be predicted only on a statistical basis. The following table presents another general relationship for 50-60 Hz AC, hand-to-foot shock of one second's duration: Current level (Milliamperes)

Probable Effect on Human Body

1 mA

Perception level. Slight tingling sensation. Still dangerous under certain conditions.

5mA

Slight shock felt; not painful but disturbing. Average individual can let go. However, strong involuntary reactions to shocks in this range may lead to injuries.

6mA - 16mA

Painful shock, begin to lose muscular control. Commonly referred to as the freezing current or "let-go" range.

17mA - 99mA

Extreme pain, respiratory arrest, severe muscular contractions. Individual cannot let go. Death is possible.

100mA - 2000mA

Ventricular fibrillation (uneven, uncoordinated pumping of the heart.) Muscular contraction and nerve damage begins to occur. Death i s likely.

> 2,000mA

Cardiac arrest, internal organ damage, and severe burns. Death is probable.

To have an idea of the quantities involved, the reader can consider that a normal power outlet for office/residence is capable of supplying up to 16 Amp of current, which corresponds to 16,000 mA, therefore 8 times the level that can cause cardiac arrest according to the study above. That is why specific protection measures are necessary. Different effects with AC/DC electricity It must be noted that the consequences of DC on the human body are usually different from those due to AC. Due to an intrinsic capacitor effect of cells, dry human skin tends to oppose greater resistance to AC than to DC, and DC is usually less dangerous than AC of the same intensity. This is especially true for low frequencies in the interval from 15 to 100 Hz, as in this range the AC pulsation is most likely to cause tetany and its collateral effects like respiratory

problems, convulsions, suffocation. DC’s most common damage, instead, is caused by tissues’ overheating due to Joule effect. Another important factor is the path that the electric current follows through the body, and the organs or tissues that it finds on its way. For example, the most dangerous path, with regard to the likelihood of ventricular fibrillation, is left hand to right hand. When evaluating the resistance that the human body would oppose to an electric current, it is important to consider all the following factors: Voltage: there is experimental evidence that the skin’s resistance decreases when the applied voltage increases, up to becoming negligible over 100 V. Status of the skin: the presence of humidity (even sweat) or of abrasions/wounds in the contact area determines a reduction of resistance. On the contrary, wh ere the skin presents callouses, the resistance increases. Contact surface: increasing the surface that is interested by the contact, the resistance decreases.

ITAROM0016DO0011F05

76



Contact pressure: a higher contact pressure brings along a lower resistance (e.g.: when a fault occurs in a tool that has to be firmly hold, like a drill, the likelihood of tetany in the

hand’s muscles is higher, and it can be Contact duration: usually a prolonged contact reduces the resistance, unless other phenomena happen, like when the skin is charred by excessive heat causing, on the opposite, a significant increase in resistance. Path of the current: while the picture no. 1 refers to a left-hand-to-feet path, in order to compare a current of the same intensity wi th a different path, some coefficients (F) have been introduced. Therefore, if I is the current intensity, the value Ieq or equivalent intensity is defined as I / F, where F can be: 

Left-hand-to-right-hand: F = 0.4



Right-hand-to-feet: F = 0.8



Back-to-right-hand: F = 0.3



Back-to-left-hand: F = 0.7



Chest-to-right-hand: F = 1.3



Chest-to-left-hand: F = 1.5

Obviously, these factors have been calculated considering ventricular fibrillation as the most dangerous effect. It is evident therefore that the factor value is directly affected by th e likelihood that a certain path will cross the cardiac area.

5. Safety curve In the international context and for el ectrical design purpose, the threshold usually assumed as the safety curve, is shown in the picture below, representing the relationship between time and current intensity on a stat istical basis. The safety curve is intermediate between the “b”

curve defining the threshold for electric shock, and the “c1” curve defining the threshold for ventricular fibrillation): Picture 2

ITAROM0016DO0011F05

77



For practical reasons, and for design purpose, it is preferable to refer to the safety curve in terms of time-voltage curve. The relation between the two curves is given by the Ohm law, considering Rb as the resistance of the human body, and REB the resistance between the body and earth. It is prudentially assumed a path hands-to-feet, with feet laying on the ground. REB resistance is assumed equal to 1,000 Ohm in indoor environments and to 200 Ohm outdoor. Picture 3

Basically, the safety limit for AC voltage even in case of prolonged contact (5 seconds for study purposes) is set at 50 V indoor and 25 V outdoor. In DC, the safety limit is 120 V indoor and 60 V outdoor. These limits, which are considerably lower than usual voltage i n power supply throughout the world (usually 120 / 240 V) require additional measures, on top of those required for the systems to work, in order to ensure effective protection from indirect/direct contacts.

6. Conclusions As discussed in the relevant paragraphs, voltages and currents capable of causing severe burns and cardiac arrest, are normally p resent in all circuits of any electrical system. Standard power supply voltage is always above 100 V, and in order to allow proper operation of the electrical systems, terminal circuit breakers are usually set to 10 – 16 Amps (minimum), well beyond the threshold that cause damage to the human body. This happens because circuit breakers are intended to protect the electrical systems and, as a standard, cannot prevent electric shock or electrocution. For this reason, protection from el ectric shock needs to be

ITAROM0016DO0011F05

78



considered separately from other design issues, ensuring a proper combination of time and current thresholds for the intervention of breakers, in close coordination with the earth connection system, in order to guarantee safety. A special type of circuit breakers has b een designed to protect persons from electric shock. These are known under different names such as Residual Current Devices (RCD), or Ground Fault Breakers (GFB), or Electric Leakage Circuit Breakers (ELCB). These devices, notwithstanding the different names, are all based on the same principle; i.e. they are designed to stop any current that is not coming back into normal circuits (live phases or neutral conductor) on the assumption that, as a consequence of a fault, the missing current is flowing through the earth or through an accidental contact. The safety threshold for their intervention has been established at 30 milli -Amps, a current limit that is related to th e safety limit for voltage, in coordination with a time of intervention that is limited to few milliseconds, so that no permanent damage is possible, apart from a quick shock. The use of these breakers is a good additional protection measure on to p of insulation and earth connection, to prevent electrocution. IEC pre scribes mandatory use of these breakers under certain conditions (see guidelines, protection from electric shock). In some cases, but only in TN systems and on the basis of a proper design of the system, short circuit breakers can provide the same level of safety of RCDs. In conclusion, on the basis of what was discussed in this document, three levels can be defined for electrical safety: Level 1, consisting of the so-called “passive” protective measures, intended to ensure equi potentiality and/or prevent users from touching li ve parts (insulation, earth connection, segregation of electric equipment); Level 2, consisting of the so-called “active” protective measures, intended to intervene i n case of insulation faults or any other fault through disconnection of power supply (earth connection in coordination with breakers); Passive and Active protection measures need to be built-in into the electrical infrastructure, and therefore pertain to the Design and Erection phases. Level 3, not less important than the other two for electrical safety, is the establishment of correct operation and maintenance procedures, and the provision of complete and correct training to operators and information to users. Operators (internal staff or maintenance contractors) need to be properly trained to work in a safely manner and ensure safety for users through regular maintenance procedures. Users need to be aware of the potential electrical risk in the areas they can access; and capable of recognizing a potentially dangerous si tuation (e.g. an exposed conductor), to promptly inform maintenance staff and request intervention. Finally, they should be not allowed into technical rooms/areas, where access should be restricted to technicians (see also guidelines paragraph 11.4, basic safety precautions for users).

ITAROM0016DO0011F05

79



Annex 3: Priority of Loads and Power Supply Sources

1. Load priority As discussed in the guidelines, Chapter 7, requirements for power supply should be analysed prior to commencing design of an electrical installation. In particular, requirements for safety, security and business continuity should be considered carefully, providing for the most appropriate back-up systems. (See for example WFP IT requirements, at the end of this Annex). Power outages can happen as a conseque nce of a fault in the electrical installation or in the external power distribution network, while some maintenance actions may require planned service interruptions. Both types of event require specific provisions in terms of backup equipment, with automatic or manual change-over. It is suggested to classify the different loads (circuits and/or equi pment) in priority categories. The factors to be considered in order to assign priorities and decide on back-up requirements are: 1. Reliability of primary power supply (likelihood and expected duration of power outages, based on historical data) 2. Type of service provided by the equipment or system (safety, security, mis sion critical, or none of them) 3. Consequences and tolerable length of break in power supply (see guidelines paragraph 7.16 for the classification of automatic supplies according to change-over ti me) 4. Optimization of available resources Consequences of a power outage can be of different types: damage to equipment, loss of data, service disruption (office temporarily unable to deliver). Of course, pr ovisions to prevent/recover data loss should be made in coordination with the IT unit, and include the design and installation of IT equipment and systems that go beyond the scope of this document (e.g. data back-up, etc.). Four categories can be identified for priority of loads, each of them with recommended provisions for continuity; in a single installation, multiple priorities can co-exist, and therefore the structure of circuits should be designed in accordance with priority requirements: in general, loads in the same priority category should be grouped and served through dedicated circuits. The suggested four categories, from lower to higher priority, are:

Priority 4: Discretionary load (No back-up) This category includes systems and/or equipment that can tolerate planned and unplanned power outages. These loads can suffer interruptions in service (short to long break) due to maintenance or failures, with limited or no consequences. No back-up equipment is needed.

Priority 3: Essential load (Stand-by source) This category includes systems and/or equipment that can tolerate unplanned short-break power outages and planned long-break power outages. For unplanned power outages, the ITAROM0016DO0011F05

80



service disruption is limited to the duration of change-over time with the back-up source. Due to the nature of its function, the standby source should be readily available and therefore the use of diesel generators is preferable. Standby equipment should be properly maintained and regularly tested to ensure availability. Change-over should be automatic. Elevators, HVAC systems, ordinary lighting, security systems are typical loads requiring a stand -by power source in case of temporary unavailability of the primary source, depending on the availability of resources.

Priority 2: Critical load (Uninterruptible power supply) This category includes systems and/or equipment that can only tolerate planned power outages (long break), for maintenance operations. No unplanned break is possible, whatever the length. These loads require Uninterruptible Power Supply, provided either by local devices or through a centralized UPS system, because a break in power supply, although short, causes damage, increased danger, or loss of data. Typi cal examples are medical equipment, IT infrastructure equipment, computers, emergency lights, and fire equipment. Since UPS devices have a limited autonomy (depending on battery capacity), a standby alternative source is anyway necessary to supply the loads in case of prolonged power outages. UPS and standby source together form the back-up system.

Priority 1: Mission-critical load (Redundant uninterruptible power supply) This category includes systems and/or equipment that cannot tolerate break i n service, either short or long, planned or unplanned, without serious consequences. Power supply has to be designed to allow maintainability without power supply interruption. This is usually the case for large and/or important data centres and communication hubs. These loads need to be equipped with at least two redundant uninterruptible power supplies. This solution is expensive and requires careful design, to avoid bottlenecks that would affect the actual redundancy capability. It is important to note that, in locations where grid power supply is particularly unstable, or there are voltage fluctuations, the use of automatic change-over with diesel generators requires careful consideration, and the installation of automatic voltage stabilizers might be required.

2. Considerations on autonomy UPS systems can only provide a short-term autonomy, i n the range of minutes or hours (usually no more than one), in order to have a reasonable quantity of batteries and limit maintenance cost and space requirements. On the other hand, diesel generators can provide long-term autonomy, depending on fuel storage capacity or on regular fuel supplies. The capacity of tanks should be designed according to the required stand-alone autonomy, in agreement with the business owner, taking into consideration the following factors:   

Expectable duration of power outages on the primary power supply; Service requirements (office, 24/7 facility, data centre, etc.); Estimated time for procurement and delivery of additional fuel;

Looking at a worst case scenario, the overall autonomy of the back-up system (UPS device/s and generator/s) should be designed on the basis of specific load requirements. To give an example, for IT equipment the minimum autonomy requirement is the one that all ows to shut down equipment without damage or loss of data.

ITAROM0016DO0011F05

81



3. Conclusions The considerations discussed above pertain to the design and erection phases of an electrical installation. Of course, some adjustments to the priority categorization and to the arrangements for back-up could become necessary during operation and maintenance, and in this case the same considerations apply. Moreover, maintenance is essential to preserve the reliability of back-up systems. Standby systems, by nature, include ‘sleeping’ equipment and machines that are run just a few hours per year, but whose readiness is indispensable. As mentioned in the guidelines tests, drills and simulations should be performed regularly, to check all equipment (automatic change-over systems, automatic start of generators, UPS electronics, batteries, etc.). Maintenance personnel needs to be properly trained so to be able to take prompt and effective action in case of any emergency.

ITAROM0016DO0011F05

82


ITAROM0016DO0011F05


83


Annex 4: Diesel Generators


Power Rating

–

1. Definitions and ratings Power ratings of diesel generators help ensure that power needs are met and that generating equipment is protected from premature w ear. To choose the right rating, it is necessary to analyse the power supply requirements in terms of hours, peak load, and average load. The choice of the proper rating will ensure the optimum combination of installed cost and lifecycle cost of ownership. Standard ISO 8528-5:2013 defines terms and specifies design and performance criteria for A.C. diesel generators. It applies to generating sets for land and marine use, excluding generating sets used on aircraft or to propel land vehicles and locomotives. Under this standard, the following 4 ratings are defined: ESP - Emergency Standby Power The maximum power available during a variable electri cal power sequence, under the stated operating conditions, for which a generating set is capable of delivering in the event of a utility power outage or under test conditions for up to 200 hours or operation per year with maintenance intervals and procedures being carried out as prescribed by the manufacturer. The permissible average power output over 24 hours of operation should not exceed 70% of the ESP rating. LTP - Limited Time Running Power The maximum power available under the agreed operating conditions, for which the generating set is capable of delivering for up to 500 hours of operation per year with the maintenance intervals and procedures being carried out as prescribed by the manufacturer. PRP - Prime Running Power The maximum power which a g enerating set is capable of delivering continuously whilst supplying a variable electrical load when operated for an unlimited number of hours per year under the agreed operating conditions and with the maintenance intervals and procedures being carried out as prescribed by the manufacturer. The permissible average power output over 24 hours of operation should not exceed 70% of the PRP rating. COP

Continuous Operating Power

–

The maximum power which the generating set is capable of delivering continuously whilst supplying a constant electrical load when operated for an unlimited number of hours per year under the agreed operating conditions and with the maintenance intervals and procedures being carried out as prescribed by the manufacturer.

2. Environmental conditions: effect on power rating As discussed in the guidelines, all electrical appliances are designed for certain conditions under which they yield optimal performance. Any changes in these conditions can cause the appliances to run at a different efficiency. Generators are no exception to this: they are typically designed to run most efficiently at or near sea level under standard conditions of temperature and pressure (STP). Any fluctuation from STP conditions can i mpair generators

ITAROM0016DO0011F05

84



and cause decreased output. Under extreme circumstances, generators can cease to function. In general, the influence of environmental conditions b ecomes significant over 1500 m (5000 feet) above sea level, or with ambient temperatures over 38 °C (100 F) for a significant length of time. The effects of different parameters are discussed here below: Altitude: air pressure drops with altitude, reducing the air density. This can create problems with generator start up if not accounted for since air is crucial for ignition in any type of generator. Another effect is reduced heat di ssipation. In fact, the heat that is created during the combustion process needs to be dissipated into the environment to reduce engine temperature. At high altitudes, due to th e lower air density, heat dissipation occurs at a much slower rate than it would at sea levels, resulting in high engine temperatures for a sustained period of time. Overheating is a common problem in such cases. Temperature: High temperatures are also associated with lower air density and can cause similar ignition problems due to inadequate air supply. This can stress the engine, pushing to deliver the necessary power, but failing due to inadequate oxygen levels available for combustion. As a consequence, the engine gets ove rheated and suffers serious damage over time. Humidity: Humidity is the measure of water content in a given volume of air. In conditions of extreme humidity, water vapour in the air displaces oxygen. Low oxygen levels impair ignition, since oxygen is the element in air that is ignited in an engine for the burning of fuel.

3. Derating Generators As discussed above, fluctuations in environmental conditions reduce the capacity of the generator to perform at desired levels. In such cases, the design power rating of the machine should be reduced, to take into account the effect of environmental conditions. In simple words, to obtain the same power output, a bigger generator is needed. This technique, called 'Derating', is employed to determine the generator’s pe rformance under new ambient

conditions. Derating is defined by Wikipedia as “the technique employed in power electrical and electronic devices wherein the devices are operated at less than their rated maximum

power dissipation”. Since derating depends on technical parameters of the machine’s design and construction, different brands have different derating factors for estimating generator output under nonstandard ambient conditions. Therefore, most manufacturers advise to contact the dealer for information on the appropriate derating factors for a specific model. As a rule of thumb, a general formula can be used to estimate the output level of a generator in non-standard conditions. Averagely, the derating factor can be estimated between 2 and 4% for every 305 m (1000 ft) above sea-level, and 0.5% for every degree over 38 °C.

4. Load Considerations and conclusions As discussed above and in the guidelines, once the power demand of an installation is known the size of the generator must be chosen considering its main function (standby or prime source) and expected conditions and hours of operation. In general, it is typically advised that generators be run around 75% of their total capacity for maximum and continuous usage. In times of emergencies, however, they can be pushed to deliver up to 100% output, for short periods. From a maintenance point of view, this d oes not overload a generator and its life span is not negatively affected. In non-standard conditions, it is advised to derate the generator

ITAROM0016DO0011F05

85



(i.e. chose a bigger generator) as per the manufacturer’s derating formula and operate the unit accordingly so as to avoid overburdening the generator. In low temperatures, fuel can require winterization or use of additives. Regular maintenance and repairs must be taken care

of to enhance the life span of the generator, in accordance with manufacturer’s specifications. A trained technician or experienced electrical contractor should always be consulted prior to attempting any type of modications, in order t o ensure safety, reliability, and efficiency.

5. References 



IEEE Std 115, Guide for Test Procedures for Synchronous Machines ISO Std 8528-1 (2005), Reciprocating Internal Combustion Engine Drive Alternating Current, Generating Sets



NEMA Std MG1, Motors and Generators



UL 2200, Stationary Engine Generator Assemblies



IEC 60034, International Standard for Electrical Rotati ng Machines, 11th edition, 2004





Cat Application and Installation Guide for Electric Power Application s, Engine and Generator Sizing Publication LEXE0047, Understanding Generator Set Ratings

ITAROM0016DO0011F05

86



Annex 5: Most Common Protective Devices

1. Devices for protection from overcurrents Overcurrents due to overload or fault are usually limited through the use of automatic breakers or fuses. The difference between using a breaker and a fuse is that the breaker can be switched back on after the fault has been repaired, while the fuse has to be replaced. Thermal breakers are normally used to protect circuits and/or equipment from overload. The nominal current is the maximum current that i s allowed to flow through the circuit for an indefinite time. Currents with a higher intensity will cause the breaker to trip within a time limit that is indirectly proportional to the intensity of the current (the higher the value of current, the shorter the time). In some models, the threshold for intervention can be adjusted. Magnetic breakers are used to protect circuits and/or equipment from short circuit currents, which are usually higher (even tens of times) than overload currents. Due to the quantity of energy released in a short time, the intervention has to be practically instantaneous. The threshold for intervention is usually set at 8 – 10 times the nominal current, and in some models can be adjusted. Overload and short circuit protection can be combined in a single device known as thermalmagnetic breaker, or through the use of relays in large power distribution breakers, or through the use of fuses. The purpose of overcurrent protection is to limit the overcurrent to a safe intensity or duration (see relevant paragraph in the guidelines). These values are linked to the current-carrying capacity of the protected circuit, i.e. the energy that the conductor(s) would be able to absorb without damage. Therefore, the choice of overcurrent protection need to be coordinated with the cross-sectional area (or AWG size) of conductors, and with the method of installation (that can affect current-carrying capacity as well). Two additional important characteristic, for the selection of br eakers or fuses, are: 



The operating voltage range, i.e. the range of voltage values in which the device can be safely operated. Breakers and fuse should be selected on the basis of the operating voltage of the installation under normal service conditions; The breaking capacity, i.e. the value in kA of the maximum fault current that the breaker or fuse is able to interrupt. Breakers and fuses should always be selected in order to have a higher breaking capacity than the maximum fault current value expected on the protected circuit.

2. Devices for protection from earth leakage An earth leakage is usually caused by an insulation loss or another fault causing an exposedconductive-part to be unexpectedly energized and originating a current to earth. The current should be discharged through: 

Earth connection in TT systems



PE conductor in TN-S systems



PEN conductor in TN-C systems

ITAROM0016DO0011F05

87



Such a current can be detected and interrupted through the use of a specific type of breakers, known under the names of: 

Earth Leakage Circuit Breakers (ELCB)



Residual Current Devices (RCD)



Residual Current Circuit Breaker (RCCB)

These devices operate on the basis of the magnetic field originated by curr ents flowing through the conductors in the circuit, and can promptly identify and interrupt earth leakages. In single phase circuits, RCDs monitor live phase (L) and n eutral (N), or two or more live phases when the neutral is not present. In three-phase circuits, RCDs monitor the three live conductors (L1-3), and the neutral (N) when p resent. Since the circuits are closed, the sum of currents in all the live phases and the neutral should always be zero. RCDs compare the currents flowing through the conductors to detect a leakage, when the sum is different from zero. It is evident that if the protective and neutral functions are combined (like it happens in TN-C systems) RCDs would not be able to properly detect a leakage, because fault currents would flow through the neutral (PEN) conductor as well. For this reason, the use of TN-C systems is not recommended. Some models of RCD allow to adjust the value of leakage current or the delay before intervention, however common values for leakage current intensity are between 0.03 A and 0.5-1 A. It must be noted that 0.03 A (30 mA) RCDs, under certain conditions, ensure protection from the harmful effects of electric shock (see Annex 3 on electrical safety). RCD modules can be combined with magnetic and thermal module to form a single multifunctional breaker to provide multi-purpose protection.

3. Devices for protection from voltage faults Overvoltage may be caused by a fault in the power source, be it internal or external to the installation, or by atmospheric phenomena such as lightning. The effects on the installation can be mitigated through the use of voltage dischargers, which are dev ices capable of discharging to earth the excess of voltage over a determined threshold. Voltage fluctuations due to problems in the external distribution network can be controlled through specific electronic devices known as voltage stabilizers.

ITAROM0016DO0011F05

88



Annex 6: Selection of Designers and Installers

Foreword This document intends to present suggestions for the selection of electrical designers and installers (individuals and companies). Applicability and relevance of each of the qualification requirements should be carefully considered on a case-by-case basis, considering the context, purpose and terms of reference of the selection. The final list of requirements for each selection could include mandatory and desirable requirements. Once the requirements have been defined, qualification should be verified through document check as much as possible, and/or through interview. For the selection of companies, past employers can be interviewed to assess past pe rformance. Unfortunately, there are no international standards or qualification systems for electrical designers and installers. However, as mentioned in the guidelines, in countries where an official charter or qualification system exist for either designers and /or installers (individuals or companies), possession of the necessary registration/qualifications shall be a mandatory requirement. The requirements presented in this document are limited to technical capacity, and therefore do not cover all other mandatory and desirable qualifications that are necessary for hi ring staff and/or contracting companies. The reader should refer to HR and procurement processes and procedures for those aspects.

1. Qualification requirements for electrical designers For individuals 1. Education: Engineering degree or equi valent technical diploma (mandatory); 2. Professional license: Registration to a national charter or possession of a license as professional electrical designer (desirable, or mandatory if r equired by local legislation); 3. Work experience (mandatory): a. 5 years’ experience in electrical design (10 years if the qualification on point 1 is not at university level); b. Experience in the design of similar installations, to be assessed on the basis of: 

functions of the installation;



power demand;



number and type of power supply sources;



nominal voltage of supply;

4. Good knowledge of one or more of the electrical codes recommended by the guidelines (Paragraph 3.3), and/or the selected code for th e installation (mandatory);

ITAROM0016DO0011F05

89



For companies The company shall propose a team of designers for the project(s), and present i ndividual CVs of team members (Mandatory); The qualifications described above for a consultant shall be possessed by at least one of the team members, who shall take the rol e of Technical Director for design and will certify the conformity of the final design with the applicable code (Mandatory); Past experience of the company should include at least 3 similar projects, considering the factors listed above in point 1.3 (Mandatory). Where applicable, depending on the context, the company should be in possession of a quality certification (ISO 9001 or equivalent); Where applicable, possess of specific qualification, license, registration or recognition released by a national body, professional association or other institution/authority.

2. Qualification requirements for installers For individuals (electricians) 1. Education (mandatory): professional school for electricians or equivalent working knowledge according to point 2 below; 2. 5 years working experience in electrical installations (mandatory, 10 years if no school education present), such as: 

Low voltage electrical installations;



High voltage electrical installations;



Diesel generators;



Photovoltaic systems;



Wind turbines;

Note: the candidate should have experience in one or more of the categories above depending on the hiring manager’s requirements. 3. Working knowledge of one or more electrical codes (mandatory); 4. Ability to correctly size a circuit and identify the appropriate breaker for protection, given the load (mandatory); 5. Ability to assess power demand of electrical installations based on the nominal power of equipment (mandatory); 6. Ability to size a generator accordingly to load’s power demand and function, using the correct rating (mandatory). 7. Where applicable, possess of specific qualification, license, registration or recognition released by a national body, professional association or other institution/authority.

For companies 1. The company should be in possession of the Management System Certifications listed below (mandatory where applicable):

ITAROM0016DO0011F05

90



a. Quality, according to ISO 9001 or equivalent b. Environmental, according to ISO 14001 or equivalent c. Occupational Health and Safety, according to ISO 18001 or equivalent 2. The company shall propose at least one designer for the project(s), and present supporting documentation as to his/her qualification, which should fulfil the criteria listed for the selection of designers; this per son shall take the role of Technical Director for the electrical work and shall certify the correspondence of said work (installation) with the design and/or the applicable code (mandatory). 3. The company’s past experience should inclu de at least 5 similar projects 4. The company shall provide evidence of: a. compliance with local legislation on safety of workers where appli cable; or b. possession and application of safety procedures, use of personal protective equipment for workers, use of qualified personnel (see following point); 5. The company shall provide evidence of the qualifications of the personnel proposed for the works; such qualifications should be comparable with those listed at previous point 2 for individuals. 6. Where applicable, possess of specific qualification(s), license, registration or recognition released by a national body, professional association or other institution/authority.

ITAROM0016DO0011F05

91



Annex 7: IEC 60364 SERIES - Contents

60364-1

Part 1: Fundamental principles, assessment of general characteristics, definitions

60364-4-41

Part 4-41: Protection for safety – protection against electric shock

60364-4-42

Part 4-42: Protection for safety – protection against thermal effects

60364-4-43

Part 4-43: Protection for safety – protection against overcurrent

60364-4-44

Part 4-44: Protection for safety – protection against voltage disturbances and electromagnetic disturbances

60364-5-51

Part 5-51: Selection and erection of electrical equipment – common rules

60364-5-52

Part 5-52: Selection and erection of electrical equipment – wiring systems

60364-5-53

Part 5-53: Selection and erection of electrical equipment – insulation, switching and control

60364-5-54

Part 5-54: Selection and erection of electrical equipment – earthing arrangements and protective conductors

60364-5-55

Part 5-54: Selection and erection of electrical equipment – other equipment

60364-5-56

Part 5-56: Selection and erection of electrical equipment – safety services

60364-6

Part 6: Verification

60364-7-701

Part 7-701: Requirements for special installations or locations – locations containing a bath or shower

60364-7-702

Part 7-702: Requirements for special installations or locations – swimming pools and other basins

60364-7-703

Part 7-703: Requirements for special installations or locations – rooms and cabins containing sauna heaters

60364-7-704

Part 7-704: Requirements for special installations or locations – construction and demolition site installations

60364-7-705

Part 7-705: Requirements for special installations or locations – electrical installations of agricultural and horticultural premises

60364-7-706

Part 7-706: Requirements for special installations or locations – restrictive conducting locations

60364-7-707

Part 7-707: Requirements for special installations or locations – earthing requirements for the installation of data processing equipment

60364-7-708

Part 7-708: Requirements for special installations or locations – caravan parks, camping parks and similar locations

60364-7-709

Part 7-709: Requirements for special installations or locations – marinas

60364-7-710

Part 7-710: Requirements for special installations or locations – medical locations

60364-7-711

Part 7-711: Requirements for special installations or locations – exhibitions, shows and stands

ITAROM0016DO0011F05

92



60364-7-712

Part 7-712: Requirements for special installations or locations – solar photovoltaic power supply systems

60364-7-713

Part 7-713: Requirements for special installations or locations – furniture

60364-7-714

Part 7-714: Requirements for special installations or locations – external lighting installations

60364-7-715

Part 7-715: Requirements for special installations or locations – extra low voltage lighting installations

60364-7-717

Part 7-717: Requirements for special installations or locations – mobile or transportable units

60364-7-718

Part 7-718: Requirements for special installations or locations – communal facilities and workplaces

ITAROM0016DO0011F05

93



Annex 8: Colour Codes for Wiring The following table shows samples of colour codes that are currently adopted in countries around the world.

(Image source: Wikipedia)

ITAROM0016DO0011F05

94

Electrical Installations - Guidelines

Recommend Documents