Handbook of Electrical Installation Practice - BeKnowledge

536 Handbook of Electrical Installation Practice
who, if they consider the cable is suitable, will issue a certificate to the effect that
the cable, if used in the manner indicated in the certificate, will provide a degree of
safety not less than that required by BS 7671.
Such cables may be described as BASEC certified but do not totally comply with
a British Standard for cables and may not be described as complying with BS 7671.
Their use must be recorded, as a departure, in the Electrical Installation Certificate.
CONSTRUCTION OF WIRING CABLES AND FLEXIBLES
When selecting a wiring cable there are three components that must be considered:
conductor material, insulation and protective covering.
Conductors
Conductors of wiring cables are made of annealed copper, and for sizes of 16mm2 and above, also of aluminium. Apart from solid or single-wire conductors, which are
available up to 2.5mm2 in copper and for all sizes in aluminium, conductors are built
up from several wires twisted together to impart various degrees of flexibility to the
conductor. (Note: All sizes of conductor in mineral-insulated cables are of solid
annealed copper.) The wires used in ordinary cables are of such a diameter that the
cable is sufficiently pliable for convenient installation, but in no case should they be
subjected to continuous movement. Excessive vibration can cause breakage of the
wires, usually at or near terminations. Flexible conductors are built up of fine wires
and are available for applications where continuous movement or vibration occurs.
Where a cable route includes frequent bends or room for installation is limited, the
flexibility of stranded conductors is an advantage.
Insulation
Thermosetting compounds such as cross-linked polyethylene (XLPE) or ethlyene
propylene rubber (EPR) are now the most commonly used insulation materials for
normal uses. These materials have higher continuous operating temperatures than
PVC – 90°C rather than 70°C. PVC insulation is still used for some common types
of wiring cable and flexible cords. Mineral insulation and special types of rubber are
available where higher temperatures are expected.
Mechanical properties are usually the most important ones for insulation
of cables up to 1kV, and loss of strength and brittleness are the best signs to
look for when judging the state of a cable. This applies also to the insulation on
the tails of mineral-insulated cables, but not of course to the insulation of the cable
itself.
The higher temperatures permitted for thermosetting materials cannot always be
utilised if the choice of conductor size is dictated by considerations other than
current-carrying capacity, e.g. voltage drop. In such cases the higher temperatures
are of value where part of a cable route passes through a thermally adverse location
(groups, thermal insulation, high ambient temperature). In addition, these