The type of a cable is basically decided based on the voltage level for which it is manufactured and the material used for the insulation such as paper, cotton, rubber etc. The classification of cables according to the voltage level is,
1- Low tension cables (L.T. cables) : These are used for the voltage levels upto 6.6 KV. The electrostatic stresses in L.T. cables are not severe hence no special construction is used for L.T. cables. The paper is used as an insulation in theses cables. Sometimes resin in also used which increases the viscosity and helps to prevent drainage.
 |
| Fig. 1 Single core L.T. cable |
The Fig.1 shows the cross-section of a single core L.T. cable. It consists of a circular core of stranded copper or aluminium. The conductor is insulated by impregnated paper. Over the paper insulation, the lead sheath is provided. Then a layer of compounded fibrous material is provided. Then armouring is provided and finally covered again with a layer of fibrous compounded material.
Many a times, L.T. cables are not provided with armouring, to avoid excessive sheath losses. The simple construction and the availability of more copper section are the advantages of L.T. single core cable.
2- Medium and high tension cables (H.T. cables) : The three phase medium and H.T. cables are three core cables. For voltages upto 66 KV, the three core cables i.e. multicore cables are used. These cables are classified as,
a. H.T. cables upto 11 KV level which are belted type.b. Super tension (S.T.) cables for 22 KV and 33 KV levels which are screened cables.
c. Extra high tension (E.H.T.) cables for voltage levels from 33 KV to 66 KV which are pressure cables.