Question

Determine the maximum working voltage of a single core lead sheath cable having a conductor diameter of 1 cm and and sheath inside diameter of 5 cm, if two insulating material with permittivity 2.5 and 4 and of maximum electric stress 50 KV/cm and 60 KV/cm respectively are used. state the arrangement of the insulating materials from conductor to the sheath and find the maximum electric stress for each material and draw the electric stress as function of radius.

Answer 1

It is a problem of grading of cables.

Here the material with highest product of dielectric strength and electric stress should be placed closest to core and then others should be placed in decreasing order.

So here material with dielectric strength 4 will be placed closest and therefore it is termed as material 1. The material with dielectric strength 2.5 will be placed in second and therefore it will be termed as material 2.

the further solution is given in following images.