Single interlock or double interlock: which compliant preaction option should I choose?
Single interlock opens on detection. Double interlock requires both detection and sprinkler-side air pressure loss. Choose single interlock for faster response and double interlock for stronger protection against unwanted water entry.
- Single interlock: choose when you want faster system fill after detection.
- Double interlock: choose when you want the strongest protection against accidental water entering the sprinkler piping.
What single interlock means
A single interlock system uses detection to release the valve and admit water to the sprinkler piping before sprinkler operation.
Best fit for single interlock
- the hazard is water-sensitive,
- detection is reliable,
- faster preaction response is desired,
- the project does not require dual-event confirmation.
What double interlock means
A double interlock system requires both detection and a drop in supervisory air pressure before the valve opens.
Best fit for double interlock
- the protected space is highly water-sensitive,
- accidental damage from unwanted water entry must be minimized,
- the specification requires more conservative release logic.
Compliance point
Both options must be installed exactly as listed, approved, and designed. The chosen release logic should match the owner’s risk tolerance, the hazard, and the applicable code basis.
Frequently Asked Questions
Is double interlock always better?
Not always. It gives more protection against unwanted filling, but it is also more restrictive in release sequence.
Is single interlock faster?
Generally yes, because it fills on detection.
Which option is common for critical equipment?
Double interlock is commonly selected where accidental water entry is a major concern.
Technical references used
FireFlex DUAL single and double interlock datasheets; TOTALPAC N2 single and double interlock datasheets; VACTEC single and double interlock datasheets.