Electrical surges caused by thunderstorms can be fatal for electronic equipment like computers.
There are about 16 Million Thunderstorms per year worldwide, 1800 at any moment.
Storms typically lasts 30 minutes and billions of lightning bolts (flashes) occur every year.
Voltages associated with lightning flashes are in the range of 10 Million volts to 100 million Volts and currents of 10 kA to 100kA may occur.
The energy associated with a lightning bolt is typically 5 billion Joules.
In this section we shall consider surges on power lines due to lightning discharges.
It is commonly believed that surges on power lines are caused by direct lightning strikes. Surges are also caused on power lines whenever a strike takes place in their vicinity.
The situation in a thunderstorm before a lightning discharge takes place is depicted in Fig 1. The storm cloud becomes electrically polarised: inside the cloud winds and circulating air currents cause particles of rain and ice in the cloud to become charged. This is an example of tribo-charging, charging due to friction. Engineers are not that concerned about the mechanism that causes the cloud to become charged, or for that matter, whether the top of the cloud becomes positive with respect to its base, or negative. We have assumed the top of the cloud becomes positively charged in Fig 1.
Typical values of a test surge are-
Vp= 8kV, the peak value of the surge: 8000 volts
tr= 1us, the rise time of the surge: 1 millionth of a second
Vh= 4kV, the half value of the surge: 4000 volts
th = 50µs, the time for the surge to drop to half of its peak value: 50 millionths of a second