<p style="text-align:justify"><span style="font-size:14px">Whenever any sustained fault occurs in a transmission line, so often the disc of suspension insulator is required to be replaced or some repair is required to be carried out. Also, the scheduled maintenance requires the line to be made off. But we do not know the instant of the voltage waveshape at which the breaker was switched off. The transmission line will be charged by the voltage at the instant of switching off. This charge will, however, exponentially decay, but it takes long time as the d. c. resistance of line insulators is in terms of Megaohms.</span></p>

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<p style="text-align:justify"><span style="font-size:14px">Hence it is necessary to discharge the trapped charges in the capacitance of the transmission line. This is done by an earthing switch. After making a breaker off (or after the breaker opens due to signal received by a protective relay), an associated earthing switch is to be manually switched ON. Earthing switches on both sides of a transmission line are to be switched ON as a usual practice to discharge the trapped charges in the line.</span></p>