4.3.1
- Cable types
When choosing a cable one of the most important
factors is the temperature attained by its insulation (see
{4.1.1}); if the temperature is allowed to exceed
the upper design value, premature failure is likely. In
addition, corrosion of the sheaths or enclosures may result.
For example, bare conductors such as busbars may be operated
at much higher temperatures than most insulated conductors.
However, when an insulated conductor is
connected to such a high temperature system, its own insulation
may be affected by heat transmitted from the busbar, usually
by conduction and by radiation. To ensure that the insulation
is not damaged:
either the operating temperature of the
busbar must not exceed the safe temperature for the insulation,
or the conductor insulation must be removed
for a suitable distance from the connection with the busbar
and replaced with beat resistant insulation (see {Fig 4.7}).
It is common sense that the cable chosen
should be suitable for its purpose and for the surroundings
in which it will operate. It should not be handled and installed
in unsuitable temperatures. P.V.C. becomes hard and brittle
at low temperatures, and if a cable insulated with it is
installed at temperatures below 5°C it may well become damaged.
[522] includes a series of Regulations
which are intended to ensure that suitable cables are chosen
to prevent damage from temperature levels, moisture, dust
and dirt, pollution, vibration, mechanical stress, plant
growths, animals, sunlight or the kind of building in which
they are installed. As already mentioned in {3.5.2},
cables must not produce, spread, or sustain fire.
[527-01] contains six regulations which
are intended to reduce the risk of the spread of fire and
are concerned with choosing cables with a low likelihood
of flame propagation (see BS 4066, BS 476, BS EN 50085 and
BS EN 50086). A run of bunched cables is a special fire
risk and cables in such a situation should comply with the
standards stated above.
Fig 4.7 Insulation of
a cable connected to hot busbar
BS 6387 covers cables which must be able
to continue to operate in a fire. These special cables are
intended to be used when it is required to maintain circuit
integrity for longer than is possible with normal cables.
Such cables are categorised with three letters. The first
indicates the resistance to fire alone (A,B,C and S) and
the second letter is a W and indicates that the cable will
survive for a time at 650°C when also subject to water (which
may be used to tackle the fire). The third letter (X, Y
or Z) indicates the resistance to fire with mechanical shock.
For full details of these special cables see the BS.
