EXTRANEOUS-CONDUCTIVE-PARTS
Understanding the definition
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Contents
1.
Introduction
2.
Understanding the definition
3.
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.
Understanding the definition
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.’
2.1. 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.
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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 B37-45.
2.2. 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.
2.2.1. 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.
2.2.2. 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.
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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*
2.2.3. 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
412-05-02 requires a distance of at least 2.5 m between a bare live part and an extraneous-conductivepart).
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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.
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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 C P ) 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 C P (Ω) is such that:
U0
R CP > ------ – Z TL
IB
(1)
the conductive part can be considered not to be liable to introduce Earth potential.
Where:
RCP
is the resistance between the conductive part concerned and the
main earthing terminal of the installation (Ω).
U0
is the nominal voltage to Earth of the installation (V).
IB
is the value of current through the human body (or livestock) which
should not be exceeded (A).
ZTL
is the impedance of the human body (or livestock) (Ω).
Values of Z T L 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 T L 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).
2.3. 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 413-02-02 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):
†
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The content of PD 6519 is discussed in Topic E65-5.
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Understanding the definition
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).
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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
E65-5
BONDING: Metallic parts connected to non-metallic pipework
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
E161-9
E161-11
I17-9
DIRECT CONTACT, PROTECTION AGAINST: Placing out of reach
EXTRA-LOW VOLTAGE (ELV): PELV (Protective extra-low voltage)
EXTRA-LOW VOLTAGE (ELV): SELV (Separated extra-low
INDIRECT CONTACT, PROTECTION AGAINST: Non-conducting location
Requirements for electrical installations (BS 7671):
Some of the most important requirements are found in:
Definitions
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