The EPCOS ETFV series has a third terminal lead that can be used for signal displays in addition to the two terminal leads for the disk varistors. The monitor lead my be combined with an LED display to indicate that the device is being protected. If an overvoltage occurs at a TVSS module and the LED display lights up, the user receives an optical message that the module no longer has effective overvoltage protection. The module must then be replaced.