The Models TC-105 and TC-100 Duct High Temperature Limits feature a helix bimetal element that senses rapid increases in duct temperature, such as those caused by fire. The high temperature limit then operates an SPST switch that opens with a rise in temperature. A manual reset button reactivates the temperature limit after the high-temperature condition has cleared. These duct high temperature limits are indispensable in specialized HVAC applications and Building Automation Systems, BAS.