EC635 - Littelfuse

EC635 Designing with thermally protected TMOV Varistors in TVSS Applications

NOTES Designing with thermally protected TMOV Varistors in TVSS Applications Introduction 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, o.

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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. overheat and potentially cause the TVSS device to overheat resulting in smoke, outgassing and eventually fire. For example, in a standard U.S. 120V AC Line application, two 120V AC power lines (180° out of phase) are commonly fed f.