An assessment of ageing of oxide varistors exposed to pulse hazards using dielectric spectroscopy

The common use of zinc oxide varistors has brought about qualitative changes in the overvoltage protection of electronic and electric power equipment. This is due to the special properties of ZnO varistors, mainly the high current-voltage characteristic nonlinearity and the capacity to withstand strong current surges. In service conditions varistors are exposed to the sustained action of constant or variable voltage or momentary overvoltage pulses. As a result varistors may undergo degradation reflected in changes in their current-voltage characteristics. The changes are manifested in an increase in leakage current and are limited to a highly thermally activated low-current region where the performance of the varistor is determined by the parameters of the potential barriers. Thus one can conclude that the degradation is a result of processes leading to the movement of ions and the deformation of the potential barriers. Therefore, dielectric spectroscopy (testing of the dielectric response in a wide frequency band) could be useful for the investigation of degradation changes, and so this method is proposed in this paper.