thermistors could be used as means of protecting a transformer..using an NTC type thermistor in pararell to the input of the transformer could terminate overvoltages..basically, an NTC thermistor's resistance decreases as temperature rises..and at high surges (in kV) temperature tends to rise..now, as the temperature rises, the thermistor become less resistive (almost zero)..this of course provide a return path to ground for the burst..so, the transformer remains protected..