Potential Hazards of Metal Oxide Varistor (MOV)

Metal Oxide Varistor is often used as a surge and spike protector in electronic circuits. They function by shunting the extra current as their resistance drop markedly beyond the threshold voltage. As they are designed to conduct significant power for very short duration of the order of microseconds, they do not work for sustained over voltages. An example is a disconnection of the neutral conductor in a three phase system. For a home power supply of 230 V, loss of neutral connection can result in the voltage between two phase lines, that is 440 V reaching the appliances. When sustained high voltage is applied to an MOV, the device can catch fire due to the high dissipation of energy as heat. That is an example of catastrophic failure of MOV devices in surge suppressors. Thermal fuses connected in series with the MOV can be helpful in preventing catastrophic failure. Blown fuse disconnects flow to the MOV.

Another type of damage to the MOV is when the conducted current is not enough to cause catastrophic failure, but can degrade the device and reduce its life expectancy. If the excessive current conducted by an MOV leads to catastrophic failure in open circuit condition, the load will be connected without any surge protection. The device user can be unaware that the surge suppressor has failed! Though MOV provides surge protection, sustained and harmful over voltages below the device threshold may be ignored and cause damage to the circuit for which protection was aimed. MOV specifications include the voltage range that they can tolerate without damage. An ideal surge protector system should have built-in over voltage protection. This depends on a voltage-sensing circuit and relay for disconnecting the mains supply when the voltage reaches a danger threshold.