OVERVIEW

Lightning. Lightning flashes are currents of electricity flowing from one cloud to another or between the cloud and earth. They always flow by the path of least resistance, just as water takes the path of least resistance when it flows down the hillside. Such paths down the hillside are not generally straight but, on the contrary, are very crooked. The same is true of the path taken by a bolt of lightning; this fact explains the zigzag path of the flash. Trees, high buildings, towers, transmission and distribution lines, and poles are paths of less resistance than air, and therefore lightning often strikes them and flows through them to ground, with damaging results (Fig. 16.1).

Figure 16.1: The damaging results to the underground electric system due to a nearby lightning strike on the overhead distribution system

Overhead lines have charges of electricity induced on them when charged clouds pass over them. Induced charges trapped on electric circuits can create abnormal voltages on the transmission and distribution lines when lightning strokes travel to ground, discharging the clouds. The abnormal voltages placed on the line will travel along the line until dissipated by surge arrestor operation, attenuation, and/or leakage or by failure of insulators, transformers, or other apparatus connected to the line.

Lightning-producing storms are very common. It is estimated that 44,000 thunderstorms rage daily over the earth, producing 100 flashes of lightning every second. Fig. 16.2 shows graphically the approximate number of lightning-producing storm days in various parts of...