MOV has been a common solution for overvoltage circuit protection, it’s cost effective and ideal for transient surge protection. However, a prolonged or repetitive overvoltage event will generate an overcurrent which can lead to thermal runaway. The thermally protected alternative solutions on the market are either bulky or expensive.
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.
Events such as lightning surges or switching transients exceeding an MOV’s current ratings can begin to decrease its varistor voltage, resulting in increased standby/off-state leakage and watt loss heating. Additionally, increased system voltage swells (TOVs) applied for durations longer than microseconds can lead to physical and chemical changes within the multiple grain boundaries of the MOV. With decreased varistor voltage, increased leakage and compromised ability to dissipate heat caused by these events, the MOV is more likely to enter into a thermal runaway condition, thus ending the MOV’s operational life.
Fuzetec FCMOV is a combination of a PPTC resettable fuse and a MOV. Combining overcurrent and overvoltage circuit protections, this creates a device which will open if subjected to continuous over current, thus protecting the MOV and greatly reducing the risk of catastrophic failures. Also since the PPTC is a direct current sensing device, it provides a more accurate and reliable mechanism compare to other temperature sensing devices. It helps to extend the useful life of the MOV by preventing high current surges from damaging it. This should extend the life of the device. Because the PPTC is a resettable device, it will reset once the current fault is removed. In the case of temporary faults, this can greatly reduce the need for repairs.