A metal oxide varistor (MOV) is a device used as a component in electrical equipment. The word varistor is short for variable resistor. A resistor is a circuit that does not conduct electricity well. This is useful for controlling how much electric current travels to certain parts of electronic devices. A metal oxide varistor will conduct low amounts of electric current very well, but will stop conducting when the current reaches high voltage levels.
This property of the metal oxide varistor makes it ideal for use in electrical surge protectors. At normal current levels, the MOV will simply pass along the electricity to the devices plugged into the surge protector. In the event of a power surge, however, the MOV will divert the current into itself, preventing it from reaching the attached devices. This will protect sensitive electronic components, such as computer chips, that can be damaged by excessively large voltages. In the event of a sudden power surge, the MOV can respond much more quickly than similar surge-protector components.
A metal oxide varistor is generally made of zinc oxide or a similar substance. These substances give the varistor its quality of passing along current at normal household levels. When electrical current rises to a certain point, called the breakover point or breakover voltage, the tiny zinc oxide particles begin conducting the current among themselves only. This prevents the high-voltage current from passing into the rest of the surge protector or the devices plugged into it.
The metal oxide varistor is not a perfect solution to power surges. They are cheap to manufacture, which means they are often used in inexpensive surge protectors. The downside is that if they stop a power surge, they can effectively burn out, but the user may have no idea this has happened. When the next power surge occurs, the attached equipment will be unprotected. Better, but more expensive, surge protectors will have an indicator light that displays the MOV’s status.
There is also a risk of fire when a metal oxide varistor absorbs high voltage and burns out. For this reason, the U.S. Consumer Product Safety Commission and similar agencies in other countries require safeguards in surge protectors. This is also why computer manufacturers do not simply place MOVs in their computers. The best solution is to have a surge protector containing multiple components and fuses in series with an MOV. If the varistor fails, the other components will still protect the attached devices from damage.