Three insulating materials with same maximum working stress

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Three insulating materials with same maximum working stress and permittivities 2·5, 3·0, 4·0, are used in a single core cable, then location of the materials with respect to the core of the cable will be

1. 2·5, 3·0, 4·0
2. 3·0, 2·5, 4·0
3. 4·0, 3·0, 2·5
4. 4·0, 2·5, 3·0

Correct Option: C

When all the three materials are subjected to the same maximum stress,

g_max =	λ	=	λ	=	λ
	2πε₁r		2πε₂r₁		2πε₃r₂

or ε₁r = ε₂r₁ = ε₃r₂
Since r < r₁ < r₂,
Therefore ε₁ > ε₂ > ε₃
Thus, the dielectric material with highest permittivity should be placed near the conductor and other layer in the descending order.