Testing a MOV (Metal Oxide Varistor) with a DMM—Is It Reliable for Fault Detection?

Had an issue with equipment that was blowing a fuse.  Began troubleshooting and discovered it was a MOV. MOV is not a output, it's an voltage suppressor.  How was that possible? How can you test a MOV? Can you test it with a DMM?

Hi Christopher-With a DMM yo u can only check to see if an MOV is shorted. To properly test it you'll need a current-limited variable voltage bench supply.  The supply will need to generate a high enough voltage to exceed the rated clamp voltage of the MOV.  Simply set the current limit down low- maybe 10 milliamps or so, and then begin raising the voltage.  You should see little, if any, current increase until you approach the clamp voltage. A couple things to keep on mind- First, MOV's aren't generally as precise as zener diodes, and their turn-on characteristic is more gradual then a zener would be .  .Also, after an MOV absorbs a bunch of surges it can lose the ability to handle a lot of current. It's often more trouble to test for this an its worth.  If you see an MOV that looks like it has been hot it's best to replace it.-Rick 

Hi Christopher.  It's a bit imprecise, but would probably do what you want: use a Megger (insulation tester).  I have one that has three ranges: 250 / 500 / 1000 volts.  I've found that an MOV will give deflection approximately inversely proportional to its voltage, so a 275V MOV gives no or hardly any deflection on the 250V range, but around half on the 500V range.  I've picked up one shorted MOV with this.  Basically a Megger is the current limited power supply that Rick is talking about, and probably the cheapest way to get one.

Don't even bother. If there is any chance that it has been stressed, just replace it. There are so many failure modes, and so many things that may shorten there life, if you want on going transient protection, don't risk it even if testing indicates it may be OK.See this article MOV degradation
Cheers,Richard