When comparing two-wire and four-wire smoke detectors, which statement is true?

When comparing two-wire and four-wire smoke detectors, which statement is true?

• A. They require a separate auxiliary power source.
• B. They receive power from the monitored circuit.
• C. They use three wires.
• D. They are battery-powered only.

Answer: (B)

The key idea is that these detectors are part of a supervised, monitored fire alarm circuit. In practice, the detector’s electrical power is provided through the system that the panel monitors. In two-wire detectors, power and signaling share the same two conductors, so the detector is powered by the monitored circuit. Four-wire detectors add separate conductors for auxiliary power, but the detector remains integrated with the monitored system for signaling and supervision. The other statements aren’t universally true: not all configurations require a separate auxiliary power source, three wires isn’t the standard for either type, and these devices aren’t battery-powered only. So, they receive power from the monitored circuit.

The key idea is that these detectors are part of a supervised, monitored fire alarm circuit. In practice, the detector’s electrical power is provided through the system that the panel monitors. In two-wire detectors, power and signaling share the same two conductors, so the detector is powered by the monitored circuit. Four-wire detectors add separate conductors for auxiliary power, but the detector remains integrated with the monitored system for signaling and supervision. The other statements aren’t universally true: not all configurations require a separate auxiliary power source, three wires isn’t the standard for either type, and these devices aren’t battery-powered only. So, they receive power from the monitored circuit.