38 



ELECTRICAL ENGINEERING 



Fig. 34 shows the same cable insulated with three layers of 

 material with dielectric constants Ki, K 2 and K z . 



The dielectric flux density at distance r cm. from the centre 

 of the core is 



and the stress is 



2 Q 



lines per square centimeter, 



-, dynes, 



where k has different values in the three dielectrics. 



In order to make the stresses in the various parts of the insula- 

 tion equal it would be necessary to place next to the conductor a 



FIG. 34. Graded insulation. 



material of high dielectric constant K\ and to gradually decrease 

 this constant in succeeding layers in inverse proportion to the 

 distance from the centre of the conductor. This is a very expen- 

 sive process and is not necessary since good results can be ob- 

 tained by using three or four layers of dielectric. 



If .R 3 and # 2 are the inside radii of the two outer layers, the 

 stress in the outer layer is 



_ 2<? 



; .... (64) 



and the drop of potential across it is 



C R i 20 



= I ^r=- WT = 



J R 3 K$T 



