The common type of fire fighting system used in most of the buildings is automatic wet pipe sprinkler systems. In case of fire, the alarm valves act as check valves and also give an alarm. The alarm is usually through the water-going audible alarm bell or through the signal from a pressure or flow switch.
Pre-action alarm systems have two signals in order to start water flow through a sprinkler system. It is especially of great use in any case of a false alarm or a mechanical failure. This is recommended in areas where sprinklers are necessary only in case of an actual fire, so there is no water damage to any expensive instruments in the building, due to inadvertent opening of sprinklers.