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SYSTEM COMPONENTS

Pre-Action Sprinkler System
The Double Interlock

The sprinkler system designed for places where a false discharge is almost as dangerous as a fire — data centers, museums, archives, and cold storage.

By Samektra · April 2026 · 9 min read

What It Is

A pre-action sprinkler system is a dry-pipe arrangement where sprinklers are held closed (as in a dry system), the piping holds supervisory air, and an independent fire detection system must operate before water can enter the pipe. Only after both a detection-device alarm and a sprinkler fuse trigger will water discharge. This double-trigger arrangement makes accidental wetting almost impossible.

The system exists for one reason: the water is more dangerous than you'd think. A burst pipe or a damaged head in a server room, a museum archive, or a refrigerated storage area can ruin irreplaceable contents before anyone notices. Requiring a second, independent signal prevents that.

The Three Types

Non-interlock

Water admitted to the system on EITHER detection alarm OR sprinkler operation. Functionally identical to a dry system after detection trips. Used where detection is just an early warning.

Single-interlock

Water admitted on detection alarm ONLY. If a sprinkler fuses without a detector alarm, no water flows. If a detector alarms but no sprinkler fuses, the pipe fills and acts like a wet system. Most common type.

Double-interlock

Water admitted only when BOTH detection AND sprinkler operation have occurred. Requires an air-pressure-loss signal from the sprinkler side in addition to the detector alarm. Used in freezer rooms where an accidentally wet pipe would freeze.

Sequence of Operations

For a single-interlock preaction system in a data center:

  1. Smoke detector goes into alarm on the fire alarm panel.
  2. FACP signals the preaction valve solenoid to release.
  3. Preaction valve opens, water enters the previously dry sprinkler piping.
  4. Air pressure supervisory contacts drop (confirming water has reached the piping).
  5. The system is now a wet system. No discharge yet — sprinklers are still closed.
  6. If the fire continues and heat reaches a sprinkler fusible link, that single head fuses and discharges water at the fire only.

If the smoke detector alarm is a false trip, the system sits flooded until the maintenance team drains and resets it — but nothing has been damaged by water.

NFPA 25 Testing

QuarterlyAlarm test — trip the detection circuit and verify valve operation.§13.4.3
AnnualPartial flow trip test — similar to dry pipe valve testing.§13.4.3.2
3-YearFull flow trip test with control valve fully open.§13.4.3.2.2

▶ Watch on YouTube

See sprinkler system inspections and maintenance on What The Fire Code.

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References

1. NFPA 13 (2022), §8.3.5 — Preaction system types and requirements.

2. NFPA 72 (2022), §17.4 — Detection used for suppression actuation.

3. NFPA 25 (2023), §13.4.3 — Preaction and deluge valve ITM.

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Discussion (2)

You
MR
Mike R.Fire Inspector· 3 days ago

Great breakdown of the technical details. The NFPA 25 maintenance table is exactly what I needed for my ITM schedule.

▲ 8Reply
SL
Sarah L.Safety Officer· 1 week ago

Really clear explanation. Would love to see a companion video walkthrough of the inspection process.

▲ 5Reply