Pre-action systems are specifically designed to provide controlled protection. These systems allow fast detection and extinguishing. This type of system is very often used for the protection of data centers. There are single interlock and double interlock pre-action systems.
There are three general types of fire sprinklers systems, wet-pipe, dry-pipe, and pre-action. A wet-pipe system contains water under pressure at all times and utilized closed sprinkler heads. A dry-pipe system is used in areas where temperatures drop to less than 40 degrees Fahrenheit as the water in that temperature will be frozen, or in areas that require maximum protection against inadvertent operation of the sprinkler system. Dry-pipe system contains no water in the above ground piping prior to system activation, and is charged with air under pressure. The pre-action system, similar to the dry-pipe, is charged with air under pressure.
In a pre-action system the water supply is held back by a pre-action valve. This valve is connected to a supplemental detection system. Water will not enter the pipe until the detection system is activated. Once activated, the valve is released and allows water into the sprinkler piping. Water will not come from the system, until sufficient heat causes the individual sprinkler head to activate (after the pre-action valve activates, the system functions the same as a wet-pipe system). This type of operation is known as a standard or single-interlock system.
The air pressure on the pre-action valve is constantly monitored. If the pressure changes (due to leak in pipe or other issue) an alarm will sound, however, the system will not activate under this condition. The valve will remain closed, preventing water running into the system until the detection system is activated.
Another type of pre-action system known as a double-interlock system will only operate when both of the supplemental detection system and a sprinkler head is activated.
These systems are commonly used in high value areas such as computer rooms, communications centers, and museums or flammable liquid handling, storage areas for valuable artifacts, aircraft hangars, high-hazard installations using water as extinguishing agent, data centers, libraries, archives and refrigerated spaces.
These systems are to be installed per the requirements of NFPA 13 and NFPA 72.