Suntan some knowledge of Varistor
A varistor installed at an intermediate point of a power system, downstream from the service entrance, is exposed to a combination of surges: internal surges from load switching within the building, and residual surges from external lightning surges. The frequency of occurrence of these surges has been the subject of many sweys of which a recent publication is representative . In all these surveys, the steep rise in the number of occurrences of surges of lower amplitude is quite remarkable. In other words, a varistor attempting to clamp at lower surge levels will be required to divert a much greater number of surges than a varistor selected with only slightly higher voltage rating. The varistor with the lower clamping voltage will expend its puke rating faster than a varistor with higher clamping voltage.
