Two-wire smoke detectors and four-wire detectors: which statement is true?

• A. Two-wire detectors require an auxiliary power source.
• B. Two-wire detectors do not require a separate or auxiliary power source.
• C. Four-wire detectors monitor only.
• D. Two-wire detectors cannot be used for emergency systems.

Answer: (B)

Two-wire detectors use the same two conductors for both power and signaling back to the control panel. The panel provides the low-voltage supply on that loop, and each detector taps into it to operate, so no separate or auxiliary power source is needed at the detector itself.

That’s why the statement is true: two-wire detectors do not require a separate power source. The other options don’t fit because a) they claim an extra power source is required, which isn’t the case; b) four-wire detectors involve a separate power and signaling path, so “monitor only” isn’t an accurate description; and c) two-wire detectors can and are used in emergency life-safety systems.