MOV Failure Modes and Microstructural Characteristics Under Operating Duty Tests With Multiwaveform Multipulse Currents

Low-voltage (LV) metal oxide varistor (MOV) is the main voltage limiting element employed in surge protective devices to protect sensitive equipment against lightning currents. MOV failures may threaten the security of electrical and electronic systems. In order to investigate the MOV failure modes under lightning strokes more realistically, operating duty tests with multiwaveform multipulse currents (indicated as “MODT”) are carried out on LV MOVs under simulated operating circumstances. To understand the mechanisms of MOV failure, microstructural examinations, including the elemental composition and phase constituent, are conducted on the tested samples with different failure modes. Experimental results show that the main failure modes for LV MOVs under MODT include coating crack, puncture, and cracking. The multiwaveform multipulse currents with larger amplitudes, shorter interpulse time, as well as when the subsequent steep currents occur before the intermediate stroke current, will result in more serious degradation on MOVs. The microstructural examinations on punctured MOVs show that the white intergranular layers become thicker, and the intergranular phase coats the zinc oxide (ZnO) grains. In addition, the decrease of ZnO grain size is observed both on the punctured and fractured MOV surfaces. Based on these microstructural characteristics, the mechanisms of different MOV failure modes are discussed.