The Heat detector contains a pair of matched negative temperature coefficient thermostats mounted on a printed circuit board. One is exposed and in thermal contact to the surrounding air whilst the other is insulated.

The Rate of Rise Principle
If the air temperature increases rapidly, the change of temperature results in a change in resistance between the thermostats. When the ratio of resistance exceeds a factory preset level, an alarm is initiated.

Grade 1 and Grade 2
Detectors are classified according to the fixed upper temperature range and response times, with Grades 1 at 60°C and 2 at 65°C designed for most normal applications.