If the insulation resistance of cable of length 10 km is 5 MΩ its insulation resistance for 70 km will be

If the insulation resistance of cable of length 10 km is 1 MΩ its insulation resistance for 70 km will be:

1. 0.255 MΩ
2. 0.556 MΩ
3. 0.714 MΩ
4. 0.955 MΩ

Correct answer: 3. 0.714 MΩ

For Case 1: R1 ∝ 1/L1 ; R2 ∝ 1/L2

R2/R1 = L1/L2

R2 = R1 * L1/L2 = 5 * 10 / 70 = 0.714 MΩ