Pre-action fire sprinkler systems employ the basic concept of a dry pipe fire sprinkler system in that water is not normally contained within the pipes. But rather holding water from piping via pressurized air or nitrogen, pre-action systems restrain water with an electrically operated valve, known as a pre-action valve.
Pre-action sprinkler systems are specialized for use in locations where accidental activation is undesired, such as in museums with rare art works, manuscripts, or books; and Data Centers, for protection of computer equipment from accidental water discharge.
They are hybrids of wet, dry, and deluge systems, depending on the exact system goal. There are two main sub-types of pre-action systems: single interlock, and double interlock.
In some instances, the pre-action fire sprinkler system may be set up with a double interlock in which pressurized 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 system piping in the event of inadvertent detector operation. The most common application for this system type is in freezer warehouses.