Which type of fire detector circuit can continue to function with either one open or one short in the detector loops?

The correct choice is the four-wire thermal switch. This type of fire detector circuit is designed to operate with a degree of redundancy. Essentially, the four-wire configuration allows the system to maintain functionality even if one wire in the loop is compromised, either through an open circuit or a short circuit.

In contrast to two-wire systems, which rely on a single pair of wires for power and signaling, the four-wire system separates the power and signal wires. This separation means that if there is a failure in one part of the circuit, the remaining wires can still facilitate the necessary communication or power supply. This enhances the reliability of the fire detection system, ensuring it can still detect a fire hazard under certain fault conditions.

In other types of detectors, such as two-wire thermal switches, photoelectric detectors, or ionization detectors, the reliance on fewer conductors makes them more susceptible to the complete failure of the system if any single point is compromised. Thus, the four-wire thermal switch is the optimal configuration for maintaining operational integrity even when faced with certain issues in the wiring.