Transients or surges on the power system may originate from switching and from other causes but the most important surges are those caused by lightning. The lightning surges may cause serious damage to the expensive equipment in the power system (e.g. generators, transformers etc.) either by direct stroke on the equipment or by strokes on the transmission lines that reach the equipment as travelling waves. It is necessary to provide protection against both kinds of surges. The most commonly used devices for protection against lightning surges are:

(i) Earthing screen

(ii) Overhead ground wires

(iii) Lightning arresters or surge diverters

Earthing screen provides protection to power station and sub station against direct strokes whereas overhead ground wires protects the transmission lines against direct lightning strokes. Lightning arresters or surge diverters protect the station apparatus against both direct strokes and the strokes that come into the apparatus as travelling waves.

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