Degradation Effects on Energy Absorption Capability and Time to Failure of Low Voltage Metal Oxide Varistors

Reliability of a surge protection device depends on the health of its protecting elements. Metal oxide Varistors (MOVs) are widely used in electric appliances and power distribution systems. They are known to degrade over time when they experience high-surges and long-duration transients. In this paper, more than 120 MOVs are subjected to nominal 8/20 μs unipolar and bipolar surges of 40 kA in different groups, up to different levels of degradations. Then, they are subjected to energy absorption capability (EAC) tests with ac currents, in a UL-certified lab. Their EAC, peak currents, and time to failures (TtFs) are measured and analyzed. Results show that although degradation due to surges might decrease EAC and TtF of the MOVs in a given current, the average EAC and TtF of degraded MOVs are increased for a certain applied over-voltage. This paper explains the reasons and proposes a model for a degraded MOV that shows a realistic behavior in a transient overvoltage (TOV) condition.