Final answer:
A supervisory signal to the Fire Alarm Control Panel (FACP) regarding gravity tanks is triggered by conditions such as low water level, changes in water temperature, or abnormal valve conditions, which indicate potential issues in the fire suppression system's readiness.
Step-by-step explanation:
In fire alarm systems, a supervisory signal to the Fire Alarm Control Panel (FACP) regarding gravity tanks usually occurs when a condition such as a low water level, a significant change in water temperature, or an abnormal condition of the tank's control valves is detected. Since gravity tanks are part of the water supply system for fire suppression, FACP monitors these parameters vigilantly.
When the water level drops below a preset threshold, the FACP is alerted so that maintenance or immediate action can be taken to ensure the system remains operational and effective in the event of a fire. Similarly, unexpected changes in valve positions can indicate tampering or failure, which would also trigger a supervisory signal.