Overvoltage Characteristics of Atmospheric Origin

•Over Voltage Characteristics of Atmospheric Origin may be explained with some figures of Lightning Strokes. The  Lightning flashes produce an extremely large quantity of pulsed electrical energy (See Fig. J-3)

–- of several thousand amperes (and several thousand volts),

–- of high frequency (approximately 1 megahertz),

–- of short duration (from a microsecond to a millisecond).

                                                   Fig. J-3: Example of lightning current

•Between 2000 and 5000 storms are constantly undergoing formation throughout the world. These storms are accompanied by lightning strokes which represent a serious hazard for persons and equipment. Lightning flashes hit the ground at an average of 30 to 100 strokes per second, i.e. 3 billion lightning strokes each year

•The table in Figure J-4 shows the characteristic lightning strike values. As can be seen, 50% of lightning strokes have a current exceeding 33 kA and 5% a current exceeding 65 kA. The energy conveyed by the lightning stroke is therefore very high.