MOV = Metal Oxide Varistor or Voltage Dependant Resistor

A MOV device is constructed of a zinc oxide paste. It is cheap and simple to manufacture. It performs very well in absorbing high speed energy by quickly varying its resistance so as to draw down excess currents. The MOV should thus be used as per manufacturers instructions.

One key instruction is to NOT assemble devices in parallel as individual MOV's cannot be made to share current. It is best to purchase larger single block MOV devices to cope with the anticipated peak current versus the required life span.

Typical manufacturers instructions for suitable use include a MOV life v/s surge graph.

Click on image for larger version

Appropriate usage for MOV devices is to protect the insulation of motors, cabling and main switchboards from being damaged. Importantly the activation voltage should not be set too low where the MOV sustains high repetitions of current conduction. The reason is that 3 failure modes are possible under high energy currents passing through a MOV device. These failure modes are:

1. Hairline permanent short circuit melts develop in the MOV
2. Resistive tracks due to charring of the Zinc Oxide under extreme currents. This may result in the device catching fire.
3. Open Circuit tracks leading to a non performing condition in the MOV

The failure mode frequency is increased when too low an operating voltage level is chosen for the MOV.

Note: This is NOT a recommended device for protecting electronics.