Fixed temperature heat detectors Operation

This is the most common type of heat detector. Fixed temperature detectors operate when the heat sensitive eutectic alloy reaches the eutectic point changing state from a solid to a liquid.

Thermal lag delays the accumulation of heat at the sensitive element so that a fixed-temperature device will reach its operating temperature sometime after the surrounding air temperature exceeds that temperature.

The most common fixed temperature point for electrically connected heat detectors is 58°C (136.4°F). Technological developments have enabled the perfection of detectors that activate at a temperature of 47°C (117°F), increasing the available reaction time and margin of safety.