Multi-factor ageing of metal oxide varistors

Localised overheating of zinc oxide varistor elements due to the transfer of external leakage currents arising from the incidence of partial electrical discharges (PD) onto their outer surface has been shown to cause premature failure by surface breakdown. The effects of PD cause modification to the intercrystalline barrier region resulting in an irreversible change in the V/I characteristics, an increase in the leakage current and localised heating. Thermal stressing also modifies the barrier height and causes an increase in leakage current. These combined effects accelerate the progression of ageing and reduces the efficiency of the varistor as a protective device. This paper describes multi-ageing experiments, discusses the results and their implications on varistor perfomance