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SYSTEM COMPONENTS

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.

By Stanislav Samek, Samektra · 6 min read · Last updated April 18, 2026
Waterflow Switch (left)

Has a blue vane paddle that inserts into the pipe through a saddle fitting. The vane deflects when water flows, pushing a rod into the switch body. Detects water movement — sends an ALARM signal.

Pressure Switch (right)

Has a threaded pipe connection (no paddle). Connects to a small-bore sensing line. A diaphragm inside flexes when pressure changes. Detects pressure level — sends ALARM, SUPERVISORY, or PUMP START depending on application.

How a Pressure Switch Works — The Mechanics

Inside every fire protection pressure switch is a diaphragm (thin flexible membrane) or a Bourdon tube (curved metal tube that straightens under pressure). System pressure acts on one side of the diaphragm through the threaded sensing port. On the other side, a calibrated spring pushes back. When pressure exceeds the spring force (setpoint), the diaphragm deflects far enough to trip a snap-action microswitch — changing the electrical contact from one state to the other.

1. Sensing

System pressure enters through the ½" or ¼" NPT threaded port via a small-bore copper or stainless sensing line tapped off the main piping. The line is typically ¼" to minimize trapped air.

2. Diaphragm Deflection

Pressure pushes against the diaphragm. The spring on the opposite side resists. When pressure reaches the setpoint, the diaphragm overcomes the spring and moves a push rod connected to the microswitch.

3. Contact Change

The SPDT microswitch snaps from normally-closed to normally-open (or vice versa). This electrical change is what the fire pump controller or FACP reads as "pressure low" or "pressure high." The snap-action prevents contact chatter.

⚠ Hysteresis (Deadband) Is Intentional

A pressure switch does NOT activate and deactivate at the same pressure. If the setpoint is 160 PSI, the switch might trip at 160 PSI but not reset until pressure climbs back to 170 PSI. This 10 PSI gap is called hysteresis or deadband — it prevents the switch from chattering on/off when pressure hovers near the setpoint. On jockey pump controllers, this deadband is the difference between the start and stop pressure. If the deadband is too narrow, the jockey pump short-cycles. Too wide, and pressure swings are excessive.

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

Fire pump start

NFPA 20 §10.5

Wired into the fire pump controller. When system pressure drops to the pump start pressure, the switch closes and signals the controller to run the motor. A second switch (pressure limit) provides backup supervision.

Waterflow alarm (alarm check valve)

NFPA 72 §17.13

On wet systems with an alarm check valve, a pressure switch monitors the alarm line between the clapper and the retard chamber. When the retard fills, the switch closes and sends a waterflow alarm to the FACP.

Low air / high air supervisory (dry and preaction)

NFPA 72 §17.16

Dual switches on the dry pipe valve trim monitor air pressure. If air drops below the normal band, a low-air supervisory signal is sent. If air rises above, a high-air signal.

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.

Pressure Switches Are NOT Just for Dry Systems

A common misconception is that pressure switches are only used in dry sprinkler systems. In reality, pressure switches appear in every type of fire protection system — but their role, signal type, and code reference change depending on the application:

Pressure Switch Applications by System Type
Wet Sprinkler SystemAlarmMounted on the alarm check valve alarm line. When a sprinkler head opens and water flows past the clapper, pressure builds in the alarm line, fills the retard chamber, and activates the pressure switch — sending a WATERFLOW ALARM to the FACP. This is how most wet systems detect sprinkler activation electronically.NFPA 72 §17.13
Dry Sprinkler SystemSupervisoryLow-air and high-air pressure switches monitor the air (or nitrogen) pressure inside the piping. If air pressure drops below the supervisory setpoint — indicating a leak, compressor failure, or cracked fitting — the switch sends a SUPERVISORY signal before the dry pipe valve trips accidentally. A separate alarm pressure switch detects when the valve actually trips and water enters the system.NFPA 72 §17.16
Pre-Action SystemSupervisory + AlarmSimilar to dry systems — air supervisory switches monitor piping pressure, while a separate alarm switch detects valve operation. In double-interlock pre-action, both the detection system AND the pressure drop must occur before water enters. The pressure switch confirms the mechanical side of the interlock.NFPA 72 §17.16
Deluge SystemAlarmPressure switches on deluge systems detect when the deluge valve opens and water pressure appears in the piping (which is normally empty and unpressurized). The switch confirms the valve has released.NFPA 72 §17.13
Fire Pump ControllerPump StartThe most critical pressure switch application. Mounted on the fire pump controller, it senses system pressure through a small-bore sensing line. When pressure drops below the pump start setpoint (typically 5-10 PSI below the jockey pump start), the switch signals the controller to start the fire pump. A second "pressure maintenance" switch may provide backup.NFPA 20 §10.5
Jockey Pump ControllerPump Start/StopTwo pressure switches (or one adjustable switch with deadband) control the jockey pump. The START switch activates when system pressure drops slightly from normal. The STOP switch deactivates when the jockey pump restores pressure. The gap between these two setpoints must be wide enough to prevent short-cycling but narrow enough that the fire pump doesn't start unnecessarily.NFPA 20 §10.7
Standpipe SystemAlarm / SupervisoryPressure switches on standpipe systems can monitor static pressure (supervisory) or detect pressure changes when a hose valve is opened (alarm). In dry standpipes, they function like dry sprinkler system supervisory switches.NFPA 14 / NFPA 72

The Key Distinction: Alarm vs Supervisory vs Pump Start

The same physical device (a diaphragm or Bourdon tube pressure switch) serves three completely different purposes depending on where it is installed and what it is monitoring. An alarm pressure switch on a wet system alarm line sends a fire alarm signal. A low-air supervisory switch on a dry system sends a supervisory signal. A pump start pressure switch sends a start command to the fire pump controller. Wiring them to the wrong zone type at the FACP is one of the most common installation errors — and one of the most dangerous.

Typical Pressure Switch Setpoints

Getting setpoints right is critical — too high and the switch triggers on normal pressure fluctuations (false alarms), too low and it misses real events. Here are typical field setpoints:

Fire Pump StartSystem pressure minus 10-15 PSIIf system is 175 PSI, pump starts at ~160 PSIMust start before pressure drops enough to impair sprinkler coverage
Jockey Pump Start5-10 PSI above fire pump startIf fire pump starts at 160, jockey starts at 170 PSIJockey handles small leaks; gap prevents fire pump from starting unnecessarily
Jockey Pump Stop5-10 PSI above jockey startIf jockey starts at 170, it stops at 180 PSIDeadband prevents short-cycling
Low Air Supervisory (Dry)10 PSI below normal air pressureIf normal air is 40 PSI, supervisory at 30 PSIWarns before pressure drops low enough to trip the dry pipe valve
Alarm Line (Wet)Per manufacturer specificationTypically 4-7 PSI above staticMust activate after retard chamber fills — confirms sustained flow, not a surge

Things You Might Not Know About Pressure Switches

The Same Switch, Three Different Jobs

A Potter PS-10 pressure switch is physically identical whether it is monitoring air pressure on a dry system, water pressure on an alarm line, or system pressure for a fire pump start. The difference is entirely in the setpoint calibration, the wiring destination (supervisory zone, alarm zone, or pump controller), and the code reference. One $75 switch — three completely different functions depending on where you install it.

Calibration Drift Is the Silent Killer

Pressure switch diaphragms fatigue over time, especially in hard-water areas where mineral deposits build up on the sensing port. A switch set for 160 PSI pump start that has drifted to 150 PSI means the pump starts 10 PSI late — which translates to lower pressure at the most remote sprinkler during the critical first minutes of a fire. NFPA 25 quarterly testing should catch drift, but only if the technician compares activation pressure to the documented setpoint.

The Retard Chamber Saves You From False Alarms

On wet systems, the alarm check valve pressure switch is NOT directly exposed to system pressure surges. The retard chamber sits between the clapper and the switch — it absorbs momentary pressure spikes (water hammer, jockey pump starts, thermal expansion) that would otherwise trigger false waterflow alarms. Only sustained water flow that fills the retard chamber reaches the switch. If the retard chamber drain plugs, surges reach the switch unchecked.

Nitrogen Systems Change the Game for Dry System Switches

Buildings converting from compressed air to nitrogen supervision on dry systems often need to recalibrate their low-air supervisory switches. Nitrogen behaves differently than air under temperature changes (less moisture, more stable pressure), so the normal operating pressure band shifts. A supervisory switch set for compressed air may false-alarm or under-report on nitrogen if not recalibrated.

Some Fire Pump Controllers Have Two Pressure Switches

NFPA 20 allows (and some AHJs require) a primary pressure switch plus a backup pressure switch on the fire pump controller. If the primary fails, the backup still starts the pump. The two switches are set at slightly different pressures — the primary at the normal start point and the backup 5 PSI lower. This redundancy is especially common in healthcare and high-rise installations.

A Stuck Pressure Switch Can Prevent the Pump From Starting

If a pressure switch contact welds shut (from arcing) or the diaphragm ruptures, the switch may permanently read either "pressure OK" or "low pressure." A switch stuck in "OK" will never signal the pump to start — even during a real fire with heads flowing. A switch stuck in "low" will try to start the pump continuously. Both are failure modes that only testing reveals.

Know Your Switches — Tamper vs Waterflow vs Pressure vs Transfer

These four switches are the most commonly confused components in fire protection. They look similar (red boxes on pipes), but each serves a completely different purpose. Getting them mixed up — especially during wiring — causes false alarms, missed signals, and compliance failures.

Side-by-Side Comparison
FeatureTamper SwitchWaterflow SwitchPressure SwitchTransfer Switch (ATS)
What it detectsSomeone closing a control valveA sprinkler head has opened — water is flowingPressure drop below setpoint — triggers fire pump startUtility power failure or voltage sag
Signal typeSUPERVISORYALARMSUPERVISORY (pump start) or ALARM (via waterflow)TROUBLE (power failure)
What happensAlert to building management — valve has been moved from normal open positionBuilding fire alarm + fire department dispatchAutomatic fire pump start when pressure drops below thresholdAutomatic transfer to generator/backup power so fire pump can still run
Installed whereOn every control valve (OS&Y, butterfly, PIV)On each sprinkler riser, downstream of the alarm check valveOn fire pump controller, connected to system pressure sensing lineBetween utility power and fire pump controller
Code referenceNFPA 72 §17.16 / NFPA 13 §8.16NFPA 72 §17.12 / NFPA 13 §8.16.1NFPA 20 §12.4 / NFPA 72 §17.16NFPA 20 §9.7 / NFPA 110
Test frequencySemi-annual — close valve 2 turns, verify signal at FACPQuarterly — open inspector's test, verify alarm within 90 secondsAnnual — verify start/stop pressure setpoints, calibrationAnnual — simulate power failure, verify transfer and retransfer

Quick Memory Aid

🔒
Tamper:"Is the valve open?" — monitors position, sends SUPERVISORY
💧
Waterflow:"Is water moving?" — detects flow, sends ALARM
📊
Pressure:"Is the pressure dropping?" — starts the fire pump
⚡
Transfer:"Did we lose power?" — switches to backup electricity

▶ Watch: How a Pressure Switch Controls a Jockey Pump

Source: Fire Protection · Open on YouTube ↗

Frequently Asked Questions

What does a pressure switch do in a fire protection system?
A pressure switch converts a pressure change into an electrical signal. Depending on where it is installed and how it is calibrated, the same physical device can serve as a fire pump start command, a waterflow alarm trigger, or a low-air supervisory signal on a dry system. The difference is entirely in the setpoint, the wiring destination (alarm zone, supervisory zone, or pump controller), and the code reference.
How is a pressure switch different from a waterflow switch?
A waterflow switch has a mechanical vane (paddle) that inserts directly into the pipe and deflects when water moves — it detects water movement. A pressure switch connects via a threaded sensing port with no paddle, and a diaphragm inside flexes when pressure changes — it detects a pressure level. Pressure switches are used where the piping is always full of water (alarm check valve alarm line, fire pump sensing line) or full of air (dry system supervisory).
What is hysteresis on a pressure switch?
Hysteresis (also called deadband) is the intentional pressure gap between where a switch trips and where it resets. If the trip point is 160 PSI, the switch might not reset until pressure climbs back to 170 PSI. This prevents the switch from chattering on/off when pressure hovers near the setpoint. On a jockey pump controller, the deadband is the difference between the start and stop pressures — too narrow causes short-cycling, too wide causes excessive pressure swings.
What are typical setpoints for fire pump and jockey pump pressure switches?
Fire pump start: system pressure minus 10-15 PSI (if system is 175 PSI, pump starts at ~160 PSI). Jockey pump start: 5-10 PSI above fire pump start (so jockey handles small leaks before the fire pump gets involved). Jockey pump stop: 5-10 PSI above jockey start. Low-air supervisory (dry system): 10 PSI below normal air pressure. Each gap exists to prevent unnecessary pump starts while still catching real pressure losses.
How do I know if a pressure switch has drifted out of calibration?
Diaphragms fatigue over time, especially in hard-water areas where mineral deposits foul the sensing port. NFPA 25 quarterly testing should catch drift — but only if the technician actually compares activation pressure to the documented setpoint instead of just confirming the switch "works." A fire pump start switch that has drifted 10 PSI low means the pump starts 10 PSI late, which translates to lower pressure at the most remote sprinkler during the critical first minutes of a fire.
Why is the same pressure switch model used in so many different applications?
A standard Potter PS-10 or similar fire-protection pressure switch is physically identical whether it is monitoring dry-system air pressure, wet-system alarm-line water pressure, or fire pump start pressure. The mechanical design is robust and application-neutral. What changes is the setpoint calibration and the wiring destination at the FACP — which is precisely why mis-wiring (alarm zone vs supervisory zone) is one of the most common installation errors.

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.

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