Electrical characteristics
 
Varistors remain non-conductive during normal operation. As a shunt mode device, varistors are often used to suppress line voltage surges when the voltage across them remains well below their "clamping voltage". Varistors can fail for two reasons.
Catastrophic failures have occurred due to unsuccessful attempts to limit very large surges caused by events such as lightning strikes, where the energy involved is many orders of magnitude higher than the varistor can handle. Subsequent current flow from the impact could melt, burn or even vaporize the varistor. This thermal runaway is due to the lack of uniformity at the junctions of individual grain boundaries, leading to the failure of the dominant current path under thermal stress when the energy in the transient pulse (usually measured in Joules) is too high (i.e. significantly exceeds the fabrication quotient of the "Absolute Maximum Ratings"). The potential for catastrophic failure can be reduced by overrating or using specially selected MOVs in parallel.

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