Which detector is auto resetting?

• A. Bimetallic detector
• B. Heat detector
• C. Ionization detector
• D. Photoelectric detector

Answer: (A)

Auto resetting means the device returns to its normal state on its own after the triggering condition ends. A bimetallic detector uses a strip made of two metals with different expansion rates. When heat reaches the set point, the strip bends and opens the circuit, triggering the alarm. When the temperature drops, the strip straightens back to its original shape and the contacts close again, rearming the detector automatically. The other detectors shown are smoke detectors that respond to smoke via ionization or light, and they do not rely on a mechanical reset—their reset behavior depends on the system design or power cycling rather than a physical return to a ready state.