Which statement best describes fixed-temperature heat detectors' operating principle?

• A. They respond to smoke density
• B. They respond when ambient temperature reaches their set point
• C. They respond to gas concentration
• D. They respond to light changes

Answer: (B)

Fixed-temperature heat detectors activate when the surrounding temperature reaches a predetermined threshold. Inside the device is a temperature-sensitive element (such as a fusible link or a bimetallic switch) that stays stable until the ambient temperature hits the set point, at which point it triggers the alarm. This means they respond to an absolute temperature, not how quickly the temperature is rising. They don’t react to smoke density, gas concentration, or light changes, which is why they’re chosen for environments where those factors could cause false alarms. They’re reliable for steady high-heat conditions but may be slower to detect fires that start with a gradual temperature rise.