Owing to the capacitance model existing between the insulator, conductor, and tower, the distribution of voltage along the insulator is not uniform, the insulator part nearer the conductor being the more highly stressed.

Equivalent Capacitance model:
Porcelain (String) Insulator = C eq + C 1eq + C 2eq
Composite Insulator = C + C 1 + C 2
Where
C = self-capacitance of disc or composite insulator
C 1 = capacitance insulator to earth
C 2 = capacitance insulator to line
R = leakage resistance
Methods of calculating this voltage distribution are available, but extra care may be given of the value owing to the shunting effect of the leakage resistance. Two models, one porcelain (string) insulator and second for composite insulator, illustrate the capacitance model distribution. This resistance depends on the presence of pollution of the insulator surfaces and is considerably modified by rain and fog.

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