Metal oxide varistors as surge suppressors

High voltage transients and surges are produced due to lightning, inductive load switching and spikes or electrical noise on power lines. These voltage surges can be as high as 6 kV and the transient period can vary from a few microseconds to milliseconds. The frequencies of these surges vary randomly depending upon the source voltage. These surges, irrespective of their short duration, can cause severe damage to the electronic equipment which may adversely affect semiconductor devices, erode electrochemical contacts, break down insulators, and generate noise. To cope with the problems associated with transients, surge suppressor devices are commonly used. Among the surge suppressor devices the most reliable, economic and the latest are metal oxide varistors (MOVs). One may simply connect the surge suppressor between the two leads of the circuit to be protected. For a normal supply voltage, the surge suppressor will work almost like an open-circuit. When a high voltage appears, it will draw a large current and will not allow the voltage across itself to increase beyond a safe limit