Metal Oxide Varistors (MOVs) are commonly used to suppress transients in many applications such as: Transient Voltage Surge Suppressors (TVSS), Uninterruptible Power Supplies (UPS), AC Power Taps, AC Power Meters or other products. Lightning, inductive load switching, or capacitor bank switching, are often the sources of these over-voltage transients. Under normal operating conditions, the AC line voltage applied to an MOV is not expected to exceed the MOV's Maximum ACRMS Voltage Rating or Maximum Continuous Operating Voltage (MCOV). Occasionally, over-voltage transients may occur that exceeds these limits. These transients are clamped to a suitable voltage level by the MOV provided the transient energy does not exceed the MOV's maximum rating. MOVs can also be subjected to continuous abnormal voltage conditions rather than short duration transients. If an MOV is subjected to a sustained abnormal over-voltage, limited current condition (as is required in UL1449), the MOV may go into thermal runaway resulting in overheating, smoke, and potentially fire. For end products to comply with UL1449, some level of protection must be afforded to the MOV to prevent this failure mode. That protection has traditionally been a thermal fuse or Thermal Cut-Off (TCO) device.