Varistor Buffer Room – How Much is Required?

One of the main differences between a TVS and a Varistor is that the voltage tolerance of a TVS is much tighter than a Varistor. They also don't appear to have the "wear" factor that Varistors have.

Varistors that are repeatedly subjected to transients appear to have their clamping voltage lowered as a result of the stress. Eventually, they become conductive at the operating voltage and then fail catastrophically. That is: they get very hot, then turn black, then eventually catch fire.

Many of the better AC power bars that have Varistor clamps within also have a thermal cutoff device mounted between two Varistors. The thermal cutoff device disables the power bar when a Varistor fails.

In terms of what happens to a transient suppression device when it is exposed to voltages in excess of their rated clamp voltage: they get hot. The will, in fact, get hot enough to fail.

When a Varistor fails because of over-temperature, it generally becomes restively-conductive. That is, it turns black, then catches fire. Your over-current device may or may not operate, depending on how low the resistance of the Varistor has dropped.

When a Tranzorb fails because of over-temperature, it generally becomes a short-circuit. This causes sufficient current to flow to cause your over-current device to open.

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.