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.

HOW CHOOSING A CABLE ?????

Cable types

When choosing a cable one of the most important factors is the temperature attained by its insulation; 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 1.1}).

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 1.1 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.