Time dependence of degradation in ZnO varistors

Starting from the results of a previous paper on the functional relationship of the degradation of ZnO-varistors and the influence factors (especially U, I,T) an equation system is derived to describe the time-dependence of the degradation. The solutions including the contribution of degradation-, recovery- and increasing effects of currents and temperature are given and discussed by special cases concerning the extent of the self-heating and the electric operation conditions in certain approximations. Their validity is confirmed by characteristic experiments on the varistor performance.