Study of degradation by impulse having 4/10µs and 8/20µs waveform for MOVs (metal oxide varistors)

MOV technology is based on ceramics material science and its surge withstand capability is related to electrical or/and mechanical reasons. In addition, because DC, AC and various wave shapes based on the various standards are applied on MOVs, the degradation or failure analysis is very complicated. Therefore, the continuous study of mechanism of its degradation by surge is crucial. This study focused on the surge withstands capability of metal oxide various for 4/10μs, 8/20μs waveforms from varistor voltage change point of view. It was found that charge transfer value in an impulse per unit cross section area of MOVs is related to varistor voltage drop and its behaviour is different by waveforms. In order to find the reason, we estimated the impulse current flow distribution by identifying the parts having greater varistor voltage drop by spot electrode measurements and found that steep impulse is easy to flow in edge parts of a MOVs.