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However, structural floors are nowadays designed to such close limits that the structural engineer may not permit the electrical engineer to have chases cut in the floor slabs. It is often necessary for conduits to cross each other in a floor and there are also other services, such as water and gas, which run in pipes laid in or over the floors. It is then almost inevitable that conduit has to cross one or other of these other services. It will be obvious that crossovers, whether of conduit and conduit, or of conduit and other services, are the places at which maximum depth is needed.

Its ignition temperature is sufficiently high for it not to be regarded as a flammable material, that is to say, a fault in the cable is unlikely to ignite the insulation. But if a fire from some other cause engulfs the cable, PVC will burn and give off dense smoke and acid fumes which can create a hazard greater than the original fire. To avoid misunderstanding here, it should be added that a cable fault, particularly if the insulation is damaged, can ignite adjacent material such as loose paper or timber, even if it does not ignite its own insulation.

Paper Paper-insulated cable was used for power distribution for nearly a century. It is too bulky to be used for the small cables of final circuits within buildings, or for most of the submains. The smallest practicable rating is 100A, and its chief use is for the Electricity Supply Company’s underground low-voltage and medium-voltage distribution. The conductor is either stranded copper or stranded aluminium, the latter becoming increasingly popular as its price advantage increases. Whichever is used, it is heavily stranded to give good flexibility, which is important in a cable of such comparatively large size.

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