The internal core of surge protection device is a Zinc Oxide varistor. Zinc Oxide is composed of grains and the grains are separated by a grain boundary layer with a P-N junction semiconductor structure. These grain boundary layers determine the V-I characteristics of varistors. When the surge current of surge protection device exceeds its tolerance through the varistor, a part of the grain boundary layer will be damaged in the porcelain, which will lead to the decrease of the varistor voltage and the breakdown of the varistor in severe condition. With the increase of the number of discharge, the performance decreases gradually. When failure occurs, it is often short circuited. When a big surge burst through the surge protection device, the heat could transfer to the thermal fuse, Zinc Oxide pressure-sensitive devices have been damaged, if burst damage, natural discharge path is cut off, if the short circuit damage, because the heat did not reach the thermal fuse cut off the power supply, this time, we must rely on the fuse or breaker off power supply.