MOV: Metal Oxide Varistor (also known as a VDR - Voltage Dependent Resistor). Most surge suppressor "power strips" use one or more MOV as the active surge compression component.
MOVs may be connected between the mains Live and Neutral buses, and sometimes also between the Live and Ground buses, and the Neutral and Ground buses.
When the Voltage between any two buses (to which an MOV is connected) raises beyond a certain point (i.e. a Voltage "spike" or "surge"), the MOV's resistance drops substantially, shunting ("shorting" or "dumping") the excess voltage to ground. This prevents the "spike" from reaching, and possibly damaging, any connected electronic equipment.
MOV surge surpressors are often frowned upon for the following (and more) reasons:
MOVs have a finite life - they "wear out" (fail). When they fail, the equipment operator is left unaware that the connected equipment is no longer being protected by the suppressor.
MOVs shunt to ground. This raises the Voltage on the (safety) ground, which can damage interconnected equipment that is powered from another mains source.
MOVs can become damaged such that they "leak" current to the ground even when no surge is present. This can cause noise in electronic systems, and the tripping of GFI outlets/breakers.
MOVs can fail suddenly. When the failure is benign, a blown fuse/circuit breaker (and equipment shutdown) is the only immediate problem. On occasion, a failed MOV can catch fire, which can spread if the MOV is in a flammable housing (such as the typical plastic household surge suppressor power strip). For a minimum level of product safety on Transient Voltage Surge Suppressors (TVSS's) using MOV's, they should be listed by an NRTL (UL, ETL, or others) to the UL 1449 standard for surge suppression devices.
See also TVSS and Surge Protector.
For more detail, see Varistor - Wikipedia, the free encyclopedia
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