Thermally protected varistors

Thermally protected varistors are a combination of varistors (MOV)
and temperature protection components (thermal fuses). With aging and
Increasing exposure to overvoltages degrades varistors and
the leakage current increases. In addition, leakage currents mean losses and they
corrode grounding systems. If the varistor is in the range of the
Threshold voltage in operation it becomes very hot. This presents a fire hazard. For
maximum protection, a thermal fuse is inserted, which in the event of overtemperature
triggers. This separates the varistor from the rest of the circuit or even
the whole application disconnected from the power supply.