What type of alarm is activated by a flow of water in an automatic sprinkler system?

The correct answer is the water motor alarm. This type of alarm is specifically designed to activate when there is a flow of water through the sprinkler system. When the water flow is detected, it drives a water motor that operates an alarm bell or horn, providing an audible alert to notify occupants or personnel of a potential fire situation.

In automatic sprinkler systems, the water motor alarm serves as a critical safety feature, ensuring that any discharge of water due to the sprinkler heads activating will trigger an immediate warning. This is essential for prompt response to fire conditions, facilitating evacuation and activation of firefighting protocols.

Other alarm types listed do have their respective functions but are oriented toward different operations. For example, pressure type alarms typically respond to changes in water pressure rather than flow, while vane type alarms detect flow in a more mechanical manner but are often used in contexts other than automatic sprinkler systems. The pressure switch alarm also monitors pressure variations but is not directly linked to the flow of water in the same way that a water motor alarm is. Thus, the water motor alarm stands out as the appropriate response to the described situation.