The National Fire Protection Association (NFPA) defines a Preaction Sprinkler System as "a sprinkler system employing automatic sprinklers that are attached to a piping system that contains air that might or might not be under pressure, with a supplemental detection system installed in the same area as the sprinklers."

Preaction Systems are considered to be some of the most complicated types of sprinkler systems. Pre-Action Systems operate similarly to Dry Pipe Systems, but they have an electrically operated Preaction Valve that is controlled by a separate detection device such as a smoke alarm or heat detection device.

The detection system must be triggered first in order for water to be released into the system. When the sprinkler head operates, water then flows onto the fire.

In a Double Interlock System, pressurized air or nitrogen is added to the system, and both the detection system and sprinkler head must operate for water to be released into the system.

The National Fire Protection Association (NFPA) has the following to say about Deluge Sprinkler Systems:

• "Deluge systems are similar to preaction systems in that they use another type of detection for operation. However, the biggest difference is that deluge systems use open sprinklers or nozzles. Instead of getting water flow from individual heads that have operated, once water fills the system, water will flow from every sprinkler head. Much like a preaction system, a deluge valve will keep water from filling the system until the operation of another type of detection system, such as smoke detection. Once that detection system is activated, water not only fills the system but flows from the open sprinklers or nozzles."

Single Interlock Preaction Systems are most commonly used in areas where the risk of water damage needs to be heavily mitigated. Preaction Systems are commonly found in museums and areas that house computers and/or servers.

Double Interlock Systems have a somewhat limited application and are primarily used in areas subject to freezing. They have similar size limitations as Dry Pipe Systems.

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