EPCOS developed the ETFV series Varistors to satisfy the requirements of a TVSS module to UL 1449, Section 37 (abnormal overvoltage limited current requirement). The UL 1449 standard specifies that the ThermoFuse, in the thermally protected varistors, must be triggered by a current of 0.125A. During an extreme overvoltage, the solder melts and the ThermoFuse opens.