Lightning has always been attracting mankind since the early times. Lightning is a natural electrical phenomenon consisting of a high current, short time discharge that neutralizes an accumulation of charge in the atmosphere. The discharge path can be between two different locations in a cloud, two clouds, a cloud and the earth (or any structure connected to the earth). The mechanisms by which such charge accumulations take place is not yet fully known but is related to the motions of large air masses that come across certain conditions of humidity, temperature, and pressure. When the electric held become excessive a breakdown or lightning flash takes place. As explained earlier, lightning strokes that terminate on or near to power lines create problems for power engineers. The real incentive to obtain additional knowledge about lightning came from the necessity of the electrical industry to protect against its effects. It has been proved that lightning is the greatest single cause of outages in the transmission sector. where the temperature is about -5°C and that the main positive charge center is located several kilometers higher up, where the temperature is usually below -20°C. Figure (4.2) shows such a cloud located above an overhead transmission line.
Fields of about 1000 V/m exist near the center of a single bipolar cloud in which charges of about 20 C are separated by a distance of about 3 km, and indicate the total potential difference between the main charge centers to be between 100 and 1000 MV. The energy dissipated in a lightning flash is therefore of the order of 1000 to 10,000 M]. Vertical separation of the positive and negative charge centers is about 2-5 km, and the charges involved are 10-3 0C. During an average lightning storm, a total of the order of kilo-coulombs of charge would be generated between the 0°C and the -40° C and the -40°C levels, in a volume of about 50km.