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Varistors Introduction (Vishay Document Number: 29079):
BEHAVIOUR OF THE CIRCUIT WITH VARISTOR VDRS07H250 INSERTED

On opening switch S, the peak voltage developed across the varistor is: V = Cmax. * I^ß = 600 V.

The thyristors in the bridge can withstand this voltage without damage.

The total energy returned to the circuit is 200 mJ. Of this 200 mJ. 15.1 mJ is dissipated in the heater, and 184.3 mJ is dissipated in the varistor. The varistor can withstand more than 10⁵ transients containing this amount of energy.

https://www.vishay.com/docs/29079/varintro.pdf