Lightning

Abstract

The first part of this paper deals with lightning as a physical phenomenon. The formation of thunderclouds is associated with convectional instability of the atmosphere, the separation of charge being effected by the resulting upward currents of air. Lightning strokes usually discharge negative electricity into the ground and consist of a preliminary “leader” stroke from cloud to ground followed by a powerful “return” stroke from ground to cloud. A complete flash usually consists of a sequence of such double strokes separated by time-intervals of the order of 10−2 sec. The distribution of flashes to earth under a storm centre is influenced by the distribution of space charge above the ground and by discontinuities of conductivity in the ground itself.The second part of the paper deals with the effects of lightning strokes to overhead transmission lines. It is concluded that direct strokes to line conductors are certain to cause flashover: interruption to supply may be minimized either by devices to clear such faults (Petersen coils, expulsion protective gaps), or by overhead earth wires, which will preventany strokes falling on the line conductors. Strokes to towers and earth wires are unlikely to cause flashover of a line insulated with 9 standard cap-and-pin units, if tower footing resistances are below 10 ohms; footing resistances low enough to prevent flashover with only 3 cap-and-pin units may be impossible to obtain.