What does thermal lag refer to in fixed heat detectors?

• A. The time to reach the activation point after heating
• B. The time to reset after alarm
• C. The rate of heat transfer
• D. The ambient humidity effect

Answer: (A)

Thermal lag is the delay between when heat starts to affect the area and when the fixed heat detector actually triggers. These detectors have a set activation temperature, and they respond only once the ambient temperature near the sensor reaches that point. Because the detector and its surroundings have some thermal mass, heat must diffuse to the sensing element and raise it to the activation temperature, causing a brief pause before alarm. This is different from how quickly heat is transferred overall or from humidity effects, and it isn’t about resetting after an alarm. The key idea is the time it takes for the temperature at the sensor to reach the detector’s fixed activation point after heating begins.