The definition of an extraneous-conductive-part, as given in Part 2 of BS 7671 is:

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1 Contents 1. Introduction Summary 1. Introduction Extraneous-conductive-parts are referred to extensively in BS 7671, mostly in relation to equipotential bonding, but also with regard to electric shock protection measures such as SELV, placing out of reach and non-conducting location. The definition of an extraneous-conductive-part, as given in Part 2 of BS 7671 is: Extraneous-conductive-part. A conductive part liable to introduce a potential, generally earth potential, and not forming part of the electrical installation. The aim of this topic is to give a clearer understanding of the definition, and thereby to provide assistance where it is necessary to decide whether or not a specific item of metalwork or other conducting material is an extraneous-conductive-part. Before moving on, however, it is stressed that the term extraneous-conductive-part is a single, hyphenated noun, and it is quite wrong to omit the hyphens when writing the term. 2. For the purposes of this topic, the definition is considered in the following three portions: First portion: A conductive part Second portion: liable to introduce a potential, generally earth potential Third portion: and not forming part of the electrical installation First portion: A conductive part In most situations, only parts which are made of metal need to be considered as conductive parts. Metals have a conductivity which is very much higher than that of most other materials. (Conductivity is a measure of the conductance of a standard cube of a material, just as resistivity, the reciprocal of conductivity, is a measure of the resistance of a standard cube of a material.) Non-metallic materials, however, cannot be entirely ruled out. For example, whilst distilled water may be regarded as an insulator, water which is laden with salts or some other types of impurity is generally a conductor albeit a poor one compared with metals. NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 1 of 7

2 E169-9 E EXTRANEOUS-CONDUCTIVE-PARTS In some circumstances, water containing impurities may be considered to be an extraneous-conductive-part. This may be true, for example, of dirty water in a handbasin or bath, draining away through a non-metallic waste pipe, at the far end of which the column of water is in contact with Earth potential, perhaps due to a metallic underground pipe or tank. Similarly, a metallic fitting or section of metallic pipe connected to the non-metallic pipe in the above example, somewhere within the building, could be considered to be an extraneous-conductive-part due to contact with the Earthy water. The implications of this are dealt with in Topic B Second portion: liable to introduce a potential, generally earth potential In considering this portion of the definition, it is convenient to deal firstly with the source of a potential, and secondly with the question of whether the potential is liable to be introduced Source of potential As indicated by this portion of the definition, the potential which is generally of interest is earth potential, the electric potential of the conductive mass of Earth, which is conventionally taken as zero. This is normally the potential of an item such as a metal pipe which is buried in the ground, or a structural steel column which is fixed in a foundation in the ground. Other potentials, however, may also be of interest. For example, the metalwork of a non-electrical service or other facility entering the premises from another building, in which it may be bonded to the main earthing terminal, could, under earth fault conditions in the installation in that building, be at a potential related to the fault The question of whether the potential is liable to be introduced Electric shock to a person or livestock is caused by a potential difference (a voltage) applied between two or more parts of the body. Therefore, in order to be involved in causing electric shock, a conductive part must not only be at a potential, it must be liable to introduce that potential to a person (or livestock). That is to say, the conductive part must be liable to introduce its potential to a person (or livestock) who may simultaneously be in contact with another potential. 2 of 7 27 SEP 01 NICEIC TECHNICAL MANUAL NICEIC

3 Such other potentials may include the potential of: (i) An exposed-conductive-part (ii) an extraneous-conductive-part (where liable to be at a potential other than that of the conductive part under consideration); or (iii) a bare live part * Accessibility Generally, in order for a conductive part to be liable to introduce a potential to a person or livestock in contact with the potential of any of the parts listed in (i), (ii) and (iii) above, the conductive part must be accessible to be touched by such a person (or livestock). A conductive-part (exposed structural metalwork) accessible to a person who may be in contact with an exposed-conductive-part ON O Exposed-conductive-part (motor casing) Exposed structural metalwork The extraneous-conductive-part (structural metalwork) may introduce earth potential by being accessible to a person in contact with the exposed-conductive-parts of the pillar drill. Fig 1 * Applicable only where protection against direct contact by placing out of reach is used, where Regulation requires a distance of at least 2.5 m between a bare live part and an extraneous-conductivepart). NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 3 of 7

4 E169-9 E EXTRANEOUS-CONDUCTIVE-PARTS Exceptionally, an electrical installation designer may decide that in his or her opinion a particular conductive part is not accessible to be touched by anyone in contact with the potential of any of the parts listed in (i), (ii) and (iii) above, and that it is therefore not an extraneous-conductive-part. For example, the conductive part may be situated a long distance away from all such parts and/or may be of small dimensions, so that it cannot be gripped or contacted by a major surface of the body. However, in most types of building, conductive parts such as non-electrical services or exposed structural metalwork are generally accessible to be touched by persons who may be in contact with exposed-conductive-parts (including those of portable equipment) or extraneous-conductive-parts. Even where this is not initially the case, consideration should be given to the possibility that it may become true later during the life of the building, due to changes in the mechanical and/or electrical services installations or in the use of the premises. In addition, at a location such as a swimming pool location or agricultural premises, an item should not be ruled out as being an exposed-conductive-part simply because its distance from any exposed-conductive-part or extraneousconductive-part is too great to be spanned by the body of a single person or livestock. In such a location, a group of persons or livestock may form, whose bodies are in electrical contact with each other. Finally, the possibility should not be overlooked of a conductive part introducing a potential to a person through metalwork with which it is in electrical contact. This could occur by means of the metalwork being touched by a person who is also in contact with an exposed-conductive-part or an extraneous-conductive-part (or metalwork in contact with one of these). High resistance connection A conductive part may be precluded from falling within the definition of an extraneous-conductive-part because it is connected to the source of its potential through a high resistance. For example, a conductive part may be connected to Earth potential through the high resistance of a non-metallic part of a building structure. Where the resistance of the connection is sufficiently high, the conductive part is incapable of introducing its source potential (e.g. Earth potential) to a person or livestock who is simultaneously in contact with the conductive part and an exposed-conductive-part or extraneous-conductive-part. This is because the voltage drop across the high resistance connection, produced by the current flowing through it, is such that the touch voltage to which someone is exposed is within safe limits. 4 of 7 27 SEP 01 NICEIC TECHNICAL MANUAL NICEIC

5 Formula (1) may be used to assess whether Earth potential is liable to be introduced by a conductive part which is connected to the general mass of Earth through a resistance, as described above. The formula is intended to be used when a measurement has been made of the resistance (R CP ) between the conductive part concerned and the main earthing terminal of the installation. Allowance should be made, where appropriate, for the measurement not having been carried out under conditions of maximum moisture (ie worst case). If the resistance R CP (Ω) is such that: U 0 R CP > Z TL I B the conductive part can be considered not to be liable to introduce Earth potential. Where: (1) R CP U 0 I B Z TL is the resistance between the conductive part concerned and the main earthing terminal of the installation (Ω). is the nominal voltage to Earth of the installation (V). is the value of current through the human body (or livestock) which should not be exceeded (A). is the impedance of the human body (or livestock) (Ω). Values of Z TL and I B should be selected from PD 6519 depending on the conditions expected to arise in the installation concerned. For example, for a risk of electric shock to a person from hand-to-hand contact, where U 0 is 230 V (50 Hz), PD 6519 indicates that a suitable value to take for Z TL is 1,000 Ω in dry conditions. PD 6519 indicates that a suitable value to take for I B in an installation where the conditions of BS 7671 for automatic disconnection are fulfilled is between 0.5 ma (the threshold of perception) and 10 ma (the threshold of let-go) Third portion: and not forming part of the electrical installation This portion of the definition limits extraneous-conductive-parts to being items which do not form part of the electrical installation in question. Therefore, items which may be extraneous-conductive-parts include the following, which are referred to in Regulation of BS 7671 with regard to main equipotential bonding (provided such items are also covered by the other portions of the definition referred to in this topic): The content of PD 6519 is discussed in Topic E65-5. NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 5 of 7

6 E169-9 E EXTRANEOUS-CONDUCTIVE-PARTS water service pipes gas installation pipes other service pipes and ducting central heating and air conditioning systems exposed metallic structural parts of a building a lightning protection system In addition, any other non-electrical items which fall within the definition may be extraneous-conductive-parts. Although the definition precludes items forming part of the electrical installation from being extraneous-conductive-parts, items serving a purely non-electrical purpose within an installation should not be ruled out. Such items may include, for example, a metal support system or carrier for cables having non-metallic sheaths which is not required to be earthed. Moreover, items forming part of another electrical installation (that is, an installation supplied from a different origin or having a separate earthing arrangement) are not precluded from being extraneous-conductive-parts. Such items may include, for example, a metal conduit or the metal sheath or armouring of a cable entering the premises from another building. 3. Summary In order to decide whether or not a specific item is an extraneous-conductive-part, the following should be considered: Is the item conductive? In order to be conductive, an item must be formed of metal or some other conducting material (which may include a conducting liquid). Is the item liable to introduce a potential, generally earth potential? That is to say, is the item liable to introduce a potential to a person (or livestock) who may be in contact with another potential? To introduce a potential to such a person, the item must not only be at a potential, but must also be able to introduce the potential. Introduction of the potential requires two things. The first is that the item (or a conducting item in electrical contact with it) must be accessible to be touched by a part of such a person s body. The second is that any electrical resistance through which the item is connected to the source of its potential (such as the resistance connecting an item with Earth potential) must not be so great as to prevent the potential being introduced to a person (due to voltage drop in the resistance). 6 of 7 27 SEP 01 NICEIC TECHNICAL MANUAL NICEIC

7 Is the item part of the electrical installation? Only an item which does not form part of the electrical installation concerned may be an extraneous-conductive-part. However, items serving a purely non-electrical purpose within the installation should not be ruled out. Moreover, items forming part of another installation are not precluded. Topics referred to in this text: B37-45 BONDING: Metallic parts connected to non-metallic pipework E65-5 ELECTRIC SHOCK: The hazard, physiological effect of electric current through the body Topics not referred to in this text, which are related and may be of interest: D41-11 DIRECT CONTACT, PROTECTION AGAINST: Placing out of reach E161-9 EXTRA-LOW VOLTAGE (ELV): PELV (Protective extra-low voltage) E EXTRA-LOW VOLTAGE (ELV): SELV (Separated extra-low I17-9 INDIRECT CONTACT, PROTECTION AGAINST: Non-conducting location Requirements for electrical installations (BS 7671): Some of the most important requirements are found in: Definitions Part 2 NICEIC NICEIC TECHNICAL MANUAL 27 SEP 01 7 of 7