Insulation resistance is the ohmic resistance in an electric circuit offered by an insulating coating. cover, material or support to an impressed voltage, tending to produce a leakage or current through the same.

Consider a cable of inner and outer radii r₁ and r₂, length, l and of resistively of insulating material p.

Considering very thin layer of radial thickness dr at a radius r, the length through which the leaking current flow is dr and area of x-section provided to the flow of current is 2 πrl.

Hence insulation resistance of the layer under consideration

=  p dr
   2 πrl

Insulation resistance of the whole cable will be obtained by integrating the above expression between the limits r₁ and r₂

 Insulation resistance of the whole cable,



From the above expression it is obvious that the insulations resistance of the cable decreases with the increase in length.

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