Handbook of Electrical Installation Practice - BeKnowledge

106 Handbook of Electrical Installation Practice
of PVC. Non-armoured cables operating within this voltage range must be insulated
and sheathed with general purpose (GP) or heat-resisting elastomers such as vulcanised
rubber insulated GP cables or O&FR cables, both to BS 6007.
Flexible cords and cables, with a conductor cross-sectional area (csa) of not less
than 1.5mm 2 , may be the armoured type with oversheath of heavy-duty O&FR to
BS 6708. Such cables must be suitable for correct anchoring at accessories and
switchgear. Where operating on reduced low voltage, cables and cords must incorporate
a tough-rubber or PVC outer sheath that is water, oil, abrasion and impact
resistant and suitable for the ambient temperatures prevailing. Cables with PVC
insulation and sheathing to BS 6004 must not be used as flexible cables or cords. All
cables that are likely to be moved in normal use must be the flexible type.
PVC and XLPE steel-wire armoured cables are suitable for fixed wiring provided
they are properly supported throughout their length and terminated in appropriate
glands. Cable glands play another important role in providing a connection to earth
for the armouring and for this reason, if for no other, the gland must be sufficiently
tightened and kept free of undue strains.
The csa of the circuit protective conductor must be not less than that of the largest
associated phase conductor.
Where cables are laid underground either for site supplies or as part of the permanent
installation, their routes must be properly recorded on a map or site plan.
The metal armouring or sheath of cables must be adequately earthed irrespective
of whether a protective conductor is provided in the form of a separate core
within the cable. This will normally be achieved by the proper glanding of the cable
together with gland tag, nut, bolt and washers with a short length of suitably-sized
copper protective conductor to the earthing terminal within the switchgear. On no
account should an earthing and bonding clamp to BS 951, or other similar device,
be applied to the metal sheath or armouring for this or any other purpose. Such
misuse of the clamp is likely to damage the cable and would be considered unreliable
for earthing purposes.
Where site conditions are particularly arduous, the designer would wish to consider
the additional benefits of monitoring the protective conductor (see BS 4444:
Guide to electrical earth monitoring and protective conductor monitoring) of some
or all of the distribution and final circuits.
Transformer supply cabling
It is normal practice to use PVC insulated armoured cables to connect and MDA
to TAs: however, for flexibility some TAs may be fitted with an appliance inlet and
socket-outlet for connection and loop on of a flexible armoured supply cable. This
arrangement enables speedy re-siting of TAs with minimum inconvenience.
110V distribution equipment
A particularly important aspect of the sub-distribution within the concentrated
working areas of a building under construction is the availability of 110V socketoutlets.
This power is required for general lighting, safety lighting and small tool
operation. It is necessary to provide multiples of socket-outlets for this purpose, and
the following types of equipment are available.