Water Flow Switch in Fire Alarm system

A water flow switch is the interface between the building's fire suppression system and the building's fire alarm system. It detects water flowing in the sprinkler system and causes the fire alarm system to sound the evacuation. 

When the building sprinkler system activates, the water flow switch has three main functions:
It alerts the fire department that help is needed
It notifies the occupants of the building to evacuate

It shows the firefighters on their arrival that water is being used, and, by sounding the outside horn, where to connect their hoses