Electrical – Identify Varistor TVR 20471

You will need to know more information about your source of the "spike", normally your specification would tell you that you can sustain 1000V and the generator has the ability to deliver say 250A. So understanding that you would know that the impedance of the generator is 1kV/250A= 4 ohms.

With that information your information says that your normal voltage is 115Vac (rms). You need to know what tolerance you have on that voltage, lets say 20%. So your worst case voltage is going to be 115V*1.2= 138Vac (rms).

In order to give you some margin pick a MOV with a Max continuous voltage of 150Vac(rms). For example the V14H150P. At this point based on the datasheet the MOV will start to conduct between 216V-264Vac. I would assume worst case of 264V. Since you know your spike is 1000V and the protection will start at 264V and the spike generator impendace is of 4 ohms, then you can figure out the current to sink by the MOV

(1000V-264V)/4 ohms = 184A (note that even you generator is capable of 250A you will only pull 184A).The problem here is as current through the MOV increases, so does the voltage. So while this happens your higher voltage will pull less current until it reaches an equilibrium. So with 184A check the Fig of Peak current to Maximum peak voltage. At 184A voltage is 420V. But (1000-420V)/4 =145A. At 145A the clamping voltage is 400V. So in short your protective device will clamp down the 1000V down to 400V only, and you can't just lower the working voltage as you run the risk of having it conducting during normal operation.

Spec your transformer to support 400V spike and add TVS diodes on the secondary to further reduce the spike. Or add some common mode and differential inductance and add a second stage of protection with TVS diodes to further reduce the impact.

The MOV is probably toast. They fail shorted. You can measure it with an ohmmeter.

The rating (normal operation) is 250VAC (assumed to be RMS voltage of a sine wave), which is \$\sqrt{2}\cdot 250 \$, or about 354V peak.

The 390V refers to the (nominal) peak voltage when the MOV is just starting to turn on (1mA). It's not clamping much, but it could get hot if that condition persisted, especially with DC. It could be as much as 429V or as little as 351V.

When it is clamping a heavy transient (50A) the voltage can be as much as 650V.

So if you anticipate surges no greater than 50A you can use it to protect a circuit that is capable of withstanding 650V peak.

Perhaps worth noting that MOVs "wear out" and a series of heavy transients can lead them to failing shorted. Of course if the surge is large enough they can fail open because the lead wires have been blown off etc.

Silicon devices such as bipolar TVS devices can provide sharper clamping without the wear-out mechanism (they will still fail- often shorted- if overloaded, of course).