Degradation phenomena due to humidity in low voltage ZnO varistors

Humidity-induced degradation in low voltage ZnO varistors was investigated. An experiment was conducted in which some varistors were put in humid condition with fixed relative humidity of 100% at 25 °C for a certain period of time. After the treatment, the samples showed serious degradation as their breakdown voltage decreased and their leakage current increased significantly. It was proposed that the diffusion and reaction of H+ generated by dissociation of water at the surface of ceramics induced the degradation due to humidity. When the degraded low voltage ZnO varistors were subject to 110% of maximum continuous operating voltage for 120 h, thermal runaway did not occur in service, and the voltage–current characteristics of these degraded samples could also be mostly restored by polishing process at the side surface of varistors. The experimental results indicated that the diffusion and reaction of H+ is confined to a surface layer of ceramic varistors.