The Metal Oxide Varistor or MOV is a voltage dependent, nonlinear device that provides excellent transient voltage suppression. The Metal Oxide Varistor is designed to protect various types of electronic devices and semiconductor elements from switching and induced lightning surges.
When exposed to high transient voltage, the MOV clamps voltage to a safe level. A metal oxide varistor absorbs potentially destructive energy and dissipates it as heat, thus protecting vulnerable circuit components and preventing system damage. Varistors can absorb part of a surge.
A MOV contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes).
They can be connected in parallel for increased energy-handling capabilities.
MOVs can also be connected in series to provide higher voltage ratings or to provide voltage rating between the standard increments.
A Metal Oxide Varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its “clamping voltage”. If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed.
As Sivanand said on one article: “A Metal oxide varistor SHOULD be used to protect the circuit from HV spikes… It has become mandatory and can be seen in all power supplies that works off directly from mains.”