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 particable rating is 100A , and its cheif use is for the Electricity Supply Company's underground low-voltage and medium -voltagedistribution .
  The conductor is either stranded copper or stranded aluminium , the latter becoming increasingly popular as its price adcantage increases . Whichever is used , it is heabily stranded to give good flexibility , which is important in a cable of such comparatively large size . Paper specially made for the purpose is used as an insulator . It is essential that it should have good mechanical properties to be suitable for this application . Paper itself is a hygroscopic , fibrous material , and has to be impregnated with an oily compound to make it fit for use in cables . The compound used is a heavy mineral oil mixed with resin . On its own impregnated paper , insulation would be too fragile to be used unprotected , and  lead sheath is therefore applied over the insulation . Further strengthening and protection can be applied according to the intended use of the cable and the physical wear to which it may be exposed . A very good strong protection is afforded by steel cable or tape .
  Figure 2.1 shows a single-core PVC - insulated steel wire armoured PVC-covered cable . This is conventionally referred to as a PVC /SWA?PVC cable . Figure 2.2 shows a three-core PVC-insulated steel wire armoured cable with a PVC covering . The abbreviationf for this is PVC/S/SWA/PVC cable . A considerable number of variations on this basic design is possible and , for any given application , a cable can olny be chosen with help of a cable manufacturer's catalogue .
  PVC is now used for the larger power and sub-main cables and has superseded paper insulated cables for these application . This construction of such cables is similar to that of paper insulated cables , and another example is shown in Figure 2.3 . THis particular cable would be describned as three-phase straight concentric  , which would be abbreviated system , where the armouring forms the CPC and the neutral conductor .