FROM THE HELPLINE...
Questions commonly asked on the NICEIC Technical
Helpline.
Main equipotential bonding
Question: The main equipotential bonding conductors in a dwelling are 6 mm2. The
dwelling has a PME supply with a 100A cut-out fitted with a 60A fuse, with 25 mm2
copper tails and a 16 mm2 earthing conductor. The Ze is measured at 0.2 ½. The
main bonding is carried out using 6 mm2 insulated protective conductors.
Please advise whether the main bonding conductors need to be upgraded to 10 mm2
for the following alterations, additions or repairs:
•
Replacement of a damaged socket-outlet or switch
•
Addition of a spur to a ring final circuit
•
Addition of a new radial circuit
•
Replacement of a consumer unit
•
Repair work to the main bonding.
Answer: Replacement of a damaged socket-outlet or switch would normally come
under the umbrella of maintenance work and it would not be required by BS 7671 to
upgrade the main bonding conductors. For electrical work such as the addition of a
spur to a ring final circuit, addition of a new radial circuit or replacement of the
consumer unit, the electrical contractor should decide whether the 6 mm2 main
bonding conductors pose a risk. If the work being performed was, in fact, on the
main bonding, it would be appropriate to upgrade the main bonding conductors to 10
mm2.
Explanation: In the cited circumstances where PME conditions apply, BS 7671
requires main equipotential bonding conductors to have a cross-sectional area not
less than 10 mm2. The reason for this requirement is that currents, known as
circulating currents or diverted neutral currents, may exist in the conductors. Where
an installation uses a PME earthing facility as its means of earthing, the returning
current has two possible return paths: the supply PEN conductor and the general
mass of Earth.
Depending on the relative impedances of the two return paths, a certain amount of
neutral current may return through the general mass of Earth. (For further
information, see Topic E45-53, Earthing: Protective Multiple Earthing in the NICEIC
Technical Manual).
If the PME supply was provided by the electricity distributor after 1st October 1988,
the main bonding should be upgraded to
10 mm2. (Regulation 547-02-01 and Table 54H of BS 7671 refer.) Prior to that
date, the installation designer may decide that the deficiency does not pose a
significant risk to users, and carry out installation work. However, the deficiency,
and the reasons why the designer does not consider it to be a safety risk, should still
be recorded on the relevant certificate, such as the Minor Electrical Installation
Works Certificate if the alteration or addition does not include a new circuit or the
Electrical Installation Certificate if the work does include a new circuit
(Regulation 120-01-03 refers).
Socket-outlets behind domestic appliances
Question: Is it acceptable to supply an appliance such as a washing machine via a
low level socket-outlet in a kitchen, and is a separate local switch required above the
work top?
Answer: It is acceptable to supply an appliance via a low level socket-outlet in a
kitchen, and a separate local switch is not required above the work top.
Explanation: The socket-outlet is being used as a convenient connection device (ie
as a fused spur), provided the current consumption of the appliance does not exceed
13 A where a
BS 1363 plug and socket-outlet are being employed and the socket-outlet is not
considered to be providing the function of isolation.
Extract fan
Question: For an extract fan, where should the means of switching off for
mechanical maintenance be located?
Answer: Where a means of switching off for mechanical maintenance is required, a
device is often provided close to the fan to provide the functions of isolation and
switching off for mechanical maintenance. The device will be under the immediate
control of a person working on the fan (performing mechanical maintenance).
However, it is also acceptable to provide a means of isolation and switching for an
extract fan at the consumer unit. If the consumer unit is remote from the fan, as it is
likely to be, the means of isolation and switching - such as a circuit-breaker - must
be able to be locked in the off position or otherwise secured.
Explanation: In electrical installations, risk of injury may result from the mechanical
movement of electrically-actuated equipment such as an extract fan, and a means of
switching off for mechanical maintenance must be provided (Regulations
130-01-01 and 462-01-01 refer). The device must be suitably located and except
where the means of switching off is continuously under the control of any person
performing such maintenance, suitable provisions must be made so that precautions
can be taken to prevent any equipment becoming unintentionally or inadvertently
reactivated during mechanical maintenance (Regulations 462-01-02 and 462-01-03
refer).
Windowless bathroom or toilet
A particular difficulty can occur in such a room fitted with a timed-off extract fan
when it is required to perform mechanical maintenance, such as cleaning, on the fan.
The circuit arrangement will, in most cases, be such that the fan and the light are
supplied from the same circuit, and the fan comes on when the light is switched on
and overruns for a few minutes when the light is switched off. Switching off for
mechanical maintenance by switching off at the circuit-breaker in the consumer unit
will also cut off the supply to the light leaving the person about to clean the fan in
the dark. One solution is for the installation designer to provide a 3-pole device
configured to interrupt all 3 live conductors to the fan (phase, switched-phase and
neutral) leaving the light operational.
PME supplies in flats - distribution circuits
Question: A block of six flats is PME-supplied via a distributor’s board in a landlord’s
area. In the board, 6 x 63 A fuses provide the supply to individually metered flats.
Does the protective conductor for the distribution circuit have to have a minimum
cross-sectional area of 10 mm2 as it is also a bonding conductor? Or, does it have
to be sized in accordance with the incoming neutral to the distributor’s board which
could entail a larger size?
Answer: Requirements are placed by BS 7671 which give a minimum csa for the
protective conductor for the distribution circuit. However, the requirements placed by
BS 7671 may be modified by those of the electricity distributor.
Explanation: There are two very different electrical installations to be considered:
•
The installation in each of the six flats
•
The installation in the landlord’s area which includes the distributor’s board
and the six sub-main distribution cables
The installations in the six flats are subject to requirements which include those
placed by BS 7671. The installation in the landlord’s area is, additionally, subject to
the requirements, if any, placed by the electricity distributor. Electricity distributors
have different requirements in their guidelines for PME (Protective Multiple Earthing)
supplies. The recommended course of action is to check with the distributor, because
one distributor might accept the requirement placed by BS 7671 of, for example, 10
mm2, whereas another might require 50 mm2.
In each installation, main bonding conductors complying with Section 547 must
connect to the Main Earthing Terminal (MET) all the extraneous-conductive-parts
(Regulation 413-02-02 refers). In applying these requirements to a multi-occupancy
building, it is helpful to consider the following two definitions from Part 2 of BS 7671:
Electrical installation. (abbr: Installation). An assembly of associated electrical
equipment supplied from a common origin to fulfil a specific purpose and having
certain co-ordinated characteristics
Origin of an installation. The position at which electrical energy is delivered to an
electrical installation
The flats
With the aid of these definitions it can be seen that the requirements of Regulation
413-02-02 apply individually to each flat that has its own supply of electricity. This is
because the consumer’s electrical equipment of each such separately-supplied unit
(eg the consumer unit, wiring system and accessories) is an electrical installation as
defined in Part 2 of BS 7671.
The cross-sectional area (csa) required for the main bonding conductors of the
installation in each separate unit must be determined in accordance with Regulation
547-02-01. Where PME conditions apply, the regulation relates the main bonding
conductor csa to that of the neutral conductor of the supply to the particular
installation. This is the neutral conductor on the electricity distributor’s side of the
supply terminals, which does not form any part of the neutral conductor on the
consumer’s side of those terminals.
The above requirement of Regulation 547-02-01 also applies to the earthing
conductor of the installation. This is because the earthing conductor also performs
the function of a main equipotential bonding conductor. The cross-sectional area
requirements are given in Regulation 542-03-01 of BS 7671, which states that ‘Every
earthing conductor shall comply with Section 543 and, where PME conditions apply,
shall meet the requirements of Regulation 547-02-01 for the cross-sectional area of
a main equipotential bonding conductor...’ That is to say, the csa of the earthing
conductor must be not less than the minimum required for its function as an earthing
conductor or for its function as a main bonding conductor, whichever is the greater.
In our example, from Table 54H, if each distribution cable has phase and neutral
conductors of 35 mm2 or less, the protective conductor should have a minimum csa
of 10 mm2 (But refer to Regulation 542-03-01 if the earthing conductor is buried in
the ground).
The landlord’s installation
The same requirements as apply to the installation of each separately-supplied flat
also apply to the electrical installation in the common parts of the multi-occupancy
premises. The earthing conductor must be sized in accordance with Regulation 54702-01 for PME supplies, the distributor’s network conditions (see note at top of Table
54H,) and Regulation 542-03-01 for earthing conductors.
Further information is given in Technical Manual Topics B37-33 and B37-21.
Lightning Protection systems
Question: If a lightning system on a building is not bonded to the MET, is this a
Code 1 deviation on a Periodic Inspection Report (PIR) and can work be carried out
on an installation where the lightning protection system is not bonded?
Answer: A Code 2 recommendation is, normally, appropriate. Work can be carried
out on the electrical installation but the client should be advised to seek the advice of
a specialist lightning protection contractor and the departure should be noted on the
appropriate installation certificate.
Explanation: A Code 1 deviation means that urgent attention is required and the
safety of those using the installation may be at risk. A lightning protection system
that is not connected to the Main Earthing Terminal (MET) is extremely unlikely to
pose a risk of danger to those using the building. A Recommendation Code 2 would
be appropriate. Remedial action should be taken as soon as possible to improve the
safety of the installation to the level required by the national standard for the safety
of electrical installations.
Lightning Protection Systems.
The applicable Regulations (or parts of) from BS 7671 include:
Regulation No:
Relating to:
110-02-01
The Regulations do not apply to lightning
protection of buildings covered by BS
6651.
120-01-03
Any intended departure from the
Regulations requires special
consideration by the designer of the
installation and shall be noted on the
Electrical Installation Certificate specified
Part 7.
413-02-02
In each installation, main equipotential
bonding conductors complying with
Section 547 shall connect to the main
earthing terminal extraneous-conductiveparts of that installation including the
lightning protection system.
541-01-03
Where there is also a lightning protection
system, reference shall be made to BS
6651.