Methods of cable installation
Methods of cable installation
We have seen that the
rating of a cable depends on its ability to lose the heat produced in it by the
current it carries and this depends to some extent on the way the cable is
installed. A cable clipped to a surface will more easily be able to dissipate
heat than a similar cable which is installed with others in a conduit,
lists twenty standard methods of installation, each of them taken into
account in the rating tables of the same Appendix. For example, two 2.5 mm²
single core p.v.c. insulated non-armoured cables drawn into a steel conduit
(installation method 3) have a current rating of 24 A A 2.5 mm² twin p.v.c. insulated and
sheathed cable, which contains exactly the same conductors, has a current
rating of 27 A when clipped directly to anon-metallic
surface. Cables sheathed in p.v.c. must not be subjected to direct sunlight,
because the ultra-violet component will leach out the plasticiser, causing the
sheath to harden and crack. Cables must not be run in the same enclosure (e.g.
trunking, pipe or ducting) as non-electrical services such as water, gas, air,
etc. unless it has been established that the electrical system can suffer no
harm as a result. If electrical and other services have metal sheaths and are
touching, they must be bonded. Cables must not be run in positions where they
may suffer or cause damage or interference with other systems. They should not,
for example, be run alongside hot pipes or share a space with a hearing
induction loop.
Special precautions may
need to be taken where cables or equipment are subject to ionising radiation.
Where a wiring system penetrates a load bearing part of a building construction
it must he ensured that the penetration will not adversely affect the integrity
of the construction.
The build-up of dust on
cables can act as thermal insulation. In some circumstances the dust may be
flammable or even explosive. Design cable runs to minimise dust accumulation:
run cables on vertically mounted cable ladders rather than horizontal cable
trays. When cables are run together, each sets up a magnetic field with a
strength depending on the current carried. This field surrounds other cables,
so that there is the situation of current-carrying conductors situated in a
magnetic field. This will result in a force on the conductor, which is usually
negligible under normal conditions but which can become very high indeed when
heavy currents flow under fault conditions. All cables and conductors must be
properly fixed or supported to prevent damage to them under these conditions.
