Sheathing

We have described how a cable is made from conductor with insulation around it . Electrically , this is all that is needed to make a device to carry electricity from one place to another , but if the cable is to survive in use , it must also withstand mechanical damage , and the installation , which is enough to achieve electrical protection , is seldom strong enough to give adequte mechanical protection . Something further , therefore , has to be provided over the insulation , and it can either be made an integral part of the cable or provided by entirely separate means .
  For example , the steel armour of a thermoplastic PVC cable , which has been described above , gives mechanical protection and is part of the structure of the cable . Similarly ,the metal sheathing of MIMS cable gives the cable all the mechanical protection that is needed . Neither of these cables could be used without its built-in mechanical protection , and MIMS could certailny not even be made without it . In these cases , although one may try to dra a logical distinction between the function of insulation and that of sheathing , in practice the two must be done together . PVC insulated cable , on the other hand , is strong enough to be handled as it is during erection , but is too liable to mechanical damage to be left unprotected for long . THere are two practicable ways of giving it additional protection . One way is to install it in either doncuit or trunking and the other way is to put a sheath around the outside of the insulation . THe irst way gives protection by the use of particular method of wiring and the second way does it by makiing the cable asheathed cable , MEthod of wiring are discussed in the next chapter and the rest of this one is devoted to types of sheated cable .