A sprinkler system employing automatic sprinklers that are attached to a piping system that contains...

A preaction system is 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 areas as the sprinklers. The operation of the detection system opens a valve that permits water to flow into the piping system and to be discharged from any sprinklers that have operated. Preaction systems are used to protect areas where accidental activation is undesired, such as museums, libraries, or data centers.

References: NFPA 13, 2019 edition, section 3.3.22.1; NFPA 13 Handbook, 2019 edition, page 30; Types of Sprinkler Systems.