Metal Oxide Varistor Failure Without Fuse

Hi,

I am using Metal Oxide varistor in parallel with application circuit, with out fuse. If the MOV fails due to aging what happens to my crcuit? Will it pass the current to the circuit or the circuit also gets damaged due to shorting?

It depends on how it fails, doesn't it? Shorted? Open? A lead falls off and shorts something on your board? Obviously if it fails open, it's just like it's not there. If it shorts, then the result depends on what's driving it.

Hi Barry,

Thanks for your reply.. Some times when the MOV connected parallel to AC line and neutral fails it does not allow any current to the circuit. But once it is replaced the circuit behaves normal.

Rgards,
Mokthar

Hi,

Usually a MOV is connected in parallel to your application.
On overvoltage it becomes conductive...if it fails it should become low impedance causing a short circuit.
If you then apply power to your circuit..overcurrent may occur.
To avoid damage (wires, connectors, PCB traces) you should use a fuse in series to the application.
The fuse trips and disconnects the application from power.
But if you omit the fuse and don't have any other current limiter installed then I expect this will cause damage to wires, connectors, PCB traces...

It depends on where the damage happens whether your application is protected or not.
It's a problem of correct design.

I assume the datashett of the MOV, or application notes from the MOV manufacturer will show you how to correctly design a protection circuit.

Klaus

Hi Klaus,

Thank u.