Lets find out the answer with a question Below
Q. The insulation resistance of a cable of length 10 km is 1 ΜΩ. For a length of 100 km of the same cable, the insulation resistance will be
The insulation resistance is inversely proportional to the length of a cable.
Ri=p/2πL x In(ro/ri)
whereRi –> Insulation resistance
p -> Resistivity of the insulating material(ohm-meters)
L –> Length of a cable (meters)
ro –> Outer radius of a cable insulation (meters)
ri -> radius of conductor
In this example, R2/R1 = L1/L2R1 = 1 ΜΩ,
L1 = 10 km, L2 = 100 km
Thus, R21 x 10/100 = 0.1 ΜΩ
Hence insulation resistance decreases as length of a cable increases.