A pre-action system is a type of fire suppression system that minimizes damage to IT equipment stored in a data center when a false fire alarm event occurs. A pre-action system consists of sprinkler heads, pipes filled with pressurized air or nitrogen, and a separate water supply. The system is activated only when two conditions are met: a fire detection device (such as a smoke detector or a heat sensor) triggers an alarm, and the sprinkler heads detect a rise in temperature. When both conditions are met, the system releases the air or nitrogen from the pipes, and then fills them with water. The water is then discharged through the sprinkler heads that have been activated by the heat. A pre-action system prevents accidental water damage to IT equipment caused by false alarms, pipe leaks, or mechanical failures, as the pipes are normally dry and the water is released only when a fire is confirmed. A pre-action system also allows time for manual intervention or verification before the water is released, which can further reduce the risk of unnecessary water damage.
References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 9: Communication and Network Security, page 600. Official (ISC)² CISSP CBK Reference, Fifth Edition, Domain 4: Communication and Network Security, page 713.