The EPCOS ETFV ThermoFuse varistor series combines a disk varistor with a ThermoFuse. The ThermoFuse is connected to a terminal lead of the disk varistor by a solder bridge. If a sustained supply-frequency overvoltage occurs, the varistor absorbs the overvoltage and heats up the in the process. If the melting point of the ThermoFuse solder is exceeded, it opens and the disk varistor is permanently disconnected from the power line. This disconnection assures that the varistor is not overloaded and short circuited, thereby preventing a catastrophic varistor failure.