Pressure Switch
The Trigger
The small mechanical-electrical device that turns a pressure change into a start signal, an alarm, or a supervisory trouble message.
What It Does
A pressure switch contains a sealed diaphragm or Bourdon tube that flexes as pressure on one side changes. That mechanical motion throws a snap-action electrical contact. One pressure level maps to one switch state. Two pressure levels (set-point and reset differential) map to hysteresis — the switch closes at X, opens at Y, and won't chatter in between.
In a fire protection system, pressure switches do three very different jobs. Each job has a different set-point, a different code reference, and a different reason for existing.
The Three Jobs
Testing
Waterflow and supervisory pressure switches are tested quarterly — typically during the inspector's test or by operating the valve they monitor. Fire pump start-pressure switches are verified during the weekly (diesel) or monthly (electric) churn test: the controller records the pressure at start, which should match the set-point within a few psi.
▶ Watch on YouTube
See sprinkler system inspections and maintenance on What The Fire Code.
Watch on YouTube →References
1. NFPA 20 (2022), §10.5 — Pressure sensing lines and switches for fire pump start.
2. NFPA 72 (2022), §17.16 — Supervisory signal initiating devices.
3. NFPA 25 (2023), §13.3.3.5 — Testing of valve supervisory signal devices.
Was this article helpful?
Rate this article to help us improve
Discussion (2)
Great breakdown of the technical details. The NFPA 25 maintenance table is exactly what I needed for my ITM schedule.
Really clear explanation. Would love to see a companion video walkthrough of the inspection process.