Which component forms the sensing element in a Kidde continuous-loop fire detection system?

• A. Fiber-optic loop with glass fiber.
• B. Copper thermistors in a tube.
• C. Pressure-based switch in a steel tube.
• D. Inconel loop whose resistance changes with temperature.

Answer: (D)

The sensing element is the loop itself: a length of Inconel formed into a continuous loop that changes resistance as its temperature rises. In Kidde systems, a small current is run through this loop and the electronics monitor resistance along the entire length. When a fire heats any part of the loop, its resistance increases, and that change is detected to trigger the alarm. This is different from other approaches—fiber-optic would rely on light transmission changes, copper thermistors in tubes are for thermistor-based point detectors, and a pressure-based switch responds to pressure rather than a resistive temperature sensor in a loop.