A pre-action sprinkler system consists of a sprinkler system and an electrical detection system with pressurised air in the sprinkler pipework and water held back by the pre-action control valve. In the event of the operation of the electrical detection system the pre-action valve operates and allows water into the sprinkler pipework.
Pre-action systems differ from the normal wet systems in that the operation is initiated by an automatic detection system linked to the sprinkler system, and not by the sprinkler itself. Discharge of water can only occur, when, in addition to the detection system a sprinkler head operates. The operation of the detection system allows water to flow from the control valve into the sprinkler distribution network. A typical pre-action system is shown in the demonstration. When the detection device senses a fire, it opens the main valve, allowing water to flow through the pipes before the sprinklers are set off. When the heat activates the sprinklers, water is discharged immediately, as in a wet-pipe system.
In some instances, the pre-action system may be set up with a double interlock whereby pressurised air or nitrogen is added to system piping. The purpose of this feature is two-fold: first to monitor piping for leaks and second to hold water from the system piping in the event of inadvertent detector operation. The most common application for this system type is in freezer warehouses.
Pre-action systems are usually employed in areas that are at risk for serious water damage due to damaged sprinklers and/or piping.
They operate faster than dry systems but tend to be more expensive.