What type of signal must duct smoke detectors report as in monitored facilities?

Duct smoke detectors in monitored facilities are designed to enhance fire safety by detecting smoke within the ductwork of heating, ventilation, and air conditioning systems. These detectors must report a supervisory signal to the monitoring system. A supervisory signal is used in the context of a system's maintenance and operational status and indicates that the device is functioning properly and is actively monitoring for smoke.

In a monitored environment, the supervisory signal functions as a way of confirming that the detector is operational and can fulfill its role in detecting smoke. This ongoing communication is vital for ensuring the reliability and effectiveness of fire safety systems within a building, particularly in areas where smoke can travel through ducts and potentially spread fire.

When an alarm signal is generated due to detected smoke, it would indicate an immediate emergency response action, whereas the supervisory signal is more about ensuring the ongoing operational healthcare of the smoke detectors themselves. This distinction underscores why the supervisory signal is the correct answer; it reflects the necessity for continual monitoring and readiness, rather than a direct response to a hazard.