Handbook of Electrical Installation Practice - BeKnowledge

Selection of Wiring Systems 565
Statutory regulations
Certain statutory regulations, referred to in the IEE Wiring Regulations, limit the
range of cables from which the choice may be made, without removing the necessity
for taking into account other factors such as environmental conditions.
In the UK the Electricity at Work Regulations 1989, which apply to all places of
work, require that:
All conductors in a system which may give rise to danger shall either:
(1) Be suitably covered with insulating material and as necessary protected so as
to prevent, so far as is reasonably practicable, danger; or
(2) Have such precautions taken in respect of them (including, where appropriate,
their being suitably placed) as will prevent, so far as is reasonably practicable,
danger.
The Memorandum of Guidance produced by the Health and Safety Executive
refers the user to the IEE Wiring Regulations as giving guidance in installations up
to 1000V.
The guidance given in the Memorandum to the now repealed Factories Act (Electrical)
Special Regulations 1908–1944, that in most cases ‘protected’ meant surrounded
by earthed metal, has been replaced by the offering of the use of steel
trunking and conduit or the use of steel armoured cables as an example only. The
Electricity at Work Regulations allow lightly protected cables, such as PVC/PVC or
XLPE/PVC to be used in situations where the risk of damage is low, but the burden
of decision is on the specifier.
Coal mines have very special requirements and are not considered here. The Miscellaneous
Mines (Electricity) Regulations and the Quarry (Electricity) Regulations
have been replaced by the Electricity at Work Regulations and an approved Code
of Practice.This lays down that all cables operating at a voltage of 125V a.c. to earth
or between conductors should be entirely surrounded by earthed metal, which
should have a conductance to earth of not less than half the conductance of the
phase conductors. PVC/SWA/PVC cables are permitted to be used and are an excellent
choice in such situations, but in the smaller sizes the armour may not have the
necessary conductance. Cables with a copper strand in the armour may be specially
ordered, if the quantity is sufficient. Otherwise a cable with an extra core may be
used. In quarries, but not in mines, the lead sheath of a cable may be taken into
account.
Flexes should have a protective conductor with the same conductance as the
phase conductors and should be surrounded by earthed wire with a conductance of
half that of the phase conductor. Smaller protective conductors are permitted if a
suitable and reliable residual current device initiates disconnection.
The Cinematograph Regulations require cables for safety circuits, i.e. secondary
lighting, emergency lighting and alarms to be in a separate enclosure from ordinary
circuits and in particular from projection circuits. Cables must be enclosed in metal
or other rigid non-ignitable enclosures or have a rigid metal sheath. It is doubtful
whether PVC conduit complies with the former requirement; lead does not comply
with the latter although copper does.