The reference levels expressed in impulse crest voltage with a standard wave is not longer than a 1.2 X 50 µs wave. System insulation as calculated by suitable methods shall be equal or greater than the BIL.


The withstand voltage is the level that the insulator will withstand for a given length of time or number of applications without disruptive discharge occurring, i.e. a failure of insulation resulting in a collapse of voltage and passage of current (sometimes termed ‘sparkover’ or‘flashover’ when the discharge is on the external surface). The BIL is usually expressed as a per unit of the peak (crest) value of the normal operating voltage to earth.
Two models, one porcelain (string) insulator and second for composite insulator, illustrate the capacitance distribution. This resistance depends on the presence of pollution of the insulator surfaces and is considerably modified by rain and fog.
Owing to the capacitance 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. 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.

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