What signals do water level supervisory signal initiating devices send?

Water level supervisory signal initiating devices are designed to monitor and report changes in the water level within a system, such as a fire protection water tank. When the water level reaches predetermined thresholds, these devices send signals to indicate the condition of the water level. Specifically, they will send one signal when the water level is high, indicating that the tank is full or close to capacity, and another signal when the water level is low, signaling that the tank is reaching a critical level.

This dual signal system is crucial for ensuring proper maintenance of the fire protection system, as it provides real-time information about water availability. By alerting operators or automated systems to both high and low water levels, the supervisory device helps prevent situations where the system may not have sufficient water to function effectively in the event of a fire, or where excess water could create other hazards.

The other options do not accurately reflect the operational purpose of these devices. For example, sending two signals when the level returns to normal lacks specificity in the monitoring process. Similarly, signaling only when the level changes does not account for the distinct criteria of high and low water levels, and maintenance notifications are not directly related to water level changes in the same manner.