Damage to equipment each year from overvoltages costs firms millions. A recent thermal fused varistor consisting of a disk varistor (voltage-dependent resistor) and a thermal fuse provides better overvoltage protection than standard fuses alone. Applications include power supplies, X-ray machines, CAT scanners, and AC-panel protection modules.

Thermal fused varistors satisfy the requirements of transient-voltage surge-suppressor (TVSS) modules to UL 1449 Section 37 (abnormal overvoltage limited current requirement). The standard specifies that the thermal fuse must be triggered by a 0.125-A current.

Voltage surges or overvoltages come from several sources. For example, direct lightning strikes and lightning electromagnetic pulses cause overvoltages. Switching operations from load rejections, resonances, and shorts-to-ground also generate voltage surges. In addition, surges can come from transients or switching electromagnetic pulses (SEMP). These travel along power and data lines and are caused by short circuits and switching inductive loads such as motors and solenoids.

Unlike other thermal-fused varistors, which have internal fuses, the EPCOS ThermoFuse Varistor (ETFV) sports an external one. Thus, switching is not impaired and there is no current flashover. One ETFV lead connects to a thermal fuse with a solder bridge. Typically, the varistor absorbs voltage surges within a certain range and thereby protects the equipment. However, during a sustained overvoltage, the varistor begins to heat up. The thermal fuse opens when the varistor temperature exceeds the solder's melting point. This action permanently disconnects the varistor, protecting it from blowing up from an exceptionally large surge. ETFV structural measures ensure the thermal fuses trigger under precisely specified conditions.

ETFVs use 14, 20, and 25-mm disk varistors that absorb up to 33% more surge current than standard disk varistors of the same diameter. Applied voltages range from 115 through 460 V for the 14 and 20 mm series.

The 25 series absorbs surges up to 20 kA 8/20 ms and can handle up to 750 V, which covers most supply voltages. It absorbs surges about 100% larger than those handled by the ETFV 20 mm.

Another ETFV lead can be connected to an LED for signaling. When the thermal fuse opens, the LED lights up, warning that the protective module needs replacing.