Which fire detection system uses a continuous loop whose electrical resistance changes with temperature along the loop?

• A. Fenwal double-loop system.
• B. Lindberg gas-tube system.
• C. Kidde continuous loop fire detection system.
• D. Photoelectric smoke detector.

Answer: (C)

In a continuous loop fire detection system, the sensing element is laid out along the area to be protected as a long loop. The loop’s electrical resistance changes with temperature along its length. The fire detection electronics continuously monitor that loop impedance; when heat from a fire raises the temperature enough, the resistance crosses a preset threshold and an alarm is triggered. This design allows the detector to sense heat anywhere along the loop’s path, providing broad coverage with a single continuous sensing element.

This matches a Kidde continuous loop fire detection system, which is built around a resistive, temperature-responsive loop. Other options don’t fit the description: a gas-tube system relies on a pressure-change mechanism in a gas-filled tube, not a resistance-changing loop; a photoelectric smoke detector uses optical sensing rather than a resistive loop; a Fenwal double-loop system uses two separate loops for redundancy rather than the single continuous loop whose resistance changes with temperature along its length.