By Wu Peng, Senior Process Instrumentation Engineer · Last reviewed August 5, 2026
On a pressure switch, normally open and normally closed describe the contacts at zero pressure, exactly as the switch sits on the shelf. A normally open (NO) contact is open with no pressure applied and closes when pressure crosses the set point. A normally closed (NC) contact is closed with no pressure applied and opens at the set point.
The word normal trips people up because it does not mean normal plant operation. It means the de-energized, unpressurized reference state. Miss that distinction and you wire alarms that stay silent and pump controls that act in reverse. This guide defines the terms, maps the four possible contact behaviors, and gives selection rules for alarm and control circuits.
Contents
- What normal means
- The four behaviors
- Held open explained
- Choosing for alarms
- Choosing for control
- Changeover contacts
- Testing and wiring
- Switch lineup
- FAQ
What normal means
Normal is the state of the contacts when the switch is new in the box: no process pressure on the port, no power, nothing touching it. Electrical drawings follow the same convention. The contact symbol on a schematic shows the shelf state, not the state the contact happens to be in while the plant runs.
This matters because a pressure switch in service usually does not sit in its normal state. A switch monitoring a line that normally runs at 5 bar, with a set point of 3 bar, spends its whole working life actuated. Its NC contact is open all day. On the drawing it is still drawn closed, because the drawing shows zero pressure.
Two contact functions cover almost the whole market. Industry estimates put plain NO and NC at well over 95 percent of pressure switch applications, with changeover contacts serving the rest. The selection question is therefore narrow: which of two behaviors do you want when pressure crosses the set point, and which failure mode can your circuit tolerate.
The four behaviors
A pressure switch has two independent properties: the contact form (NO or NC) and the direction of actuation (the switch can act on rising or on falling pressure, depending on how it is set and plumbed). Cross them and you get four behaviors. Most confusion around these switches comes from discussing the contact form without stating the direction.
| Contact form | Acts on | What happens at set point | Typical use |
|---|---|---|---|
| NO | Rising pressure | Contact closes | High-pressure indication; energize an unloader or vent valve |
| NO | Falling pressure | Contact opens (returns to rest) | Low-pressure alarm; loss-of-suction trip. Held closed in healthy service |
| NC | Rising pressure | Contact opens | High-pressure cutout; breaks the compressor run circuit |
| NC | Falling pressure | Contact recloses | Pump or compressor cut-in as pressure falls back through the reset point |
The rising and falling strokes of one contact are separated by the deadband. A pump control uses the last two rows of the same NC contact: open at cut-out, reclose at cut-in.
Note what the table implies for ordering. A set point alone does not specify a switch. The purchase description has to state the contact form, the direction of action, and what the circuit should do when the contact changes state. One line covers it: opens on rising pressure at 4.5 bar, closes again at 3.0 bar.
Held open explained
Field jargon adds a wrinkle: technicians describe switches as held open or held closed. The phrase means the running process is holding the contact away from its shelf state. An NC contact on a line that runs above the set point is held open in normal operation. It is still a normally closed contact, because normal refers to the box, not the plant.
A furnace draft-proving switch is the classic confusion. It is a normally open switch. When the combustion blower starts, draft closes the contact and proves the fan is running, so during every heating cycle the switch sits closed.
Technicians who only ever see it closed start calling it normally closed, and the mislabel spreads. The control board itself relies on the true shelf state: it checks that the switch is open before the blower call, because a switch already closed at rest means a failed or jumpered contact.
Choosing for alarms
For alarm and trip circuits, choose the contact that is closed in the healthy condition and opens on the abnormal one. The reason is wire integrity. A closed healthy-state contact passes a continuous signal, so the circuit proves itself: if a wire breaks, corrodes off a terminal, or a plug backs out, the signal disappears and the system responds exactly as it would to the real abnormal condition. The failure is announced.
Now wire the same alarm through a contact that closes on the abnormal condition. The circuit sits dead all year. A broken wire looks identical to a healthy plant, and you discover the difference on the day the alarm was supposed to sound.
Printed guides still show sirens wired this way; the circuit works in the demo and fails silently in the field. This is why emergency stop buttons are normally closed, and why safety inputs on modern controllers expect a monitored, closed-when-healthy loop.
On a pressure switch, which physical contact gives you that loop depends on which side the failure sits. A high-pressure trip uses the NC contact: closed at healthy low pressure, opening on the rise. A low-pressure alarm uses the NO contact, held closed by normal line pressure and opening as pressure falls away. Same rule, opposite contacts.
Application example
Power plant, low-flow alarm points. A power station asked for a target-type flow switch on a DN50 PN16 line with two low-flow alarm points, at 81 L/min and 76 L/min, on a range of 1,200 to 10,000 L/h. It is a flow switch rather than a pressure switch, but the ordering question is the same one this guide covers: the specification is not complete until it states what each contact does as the process variable falls through the alarm point. We proposed a target flow switch covering both alarm settings.
Choosing for control
Control service reverses the logic. A pressure switch cycling a pump or compressor is not watching for a fault; it is the controller. Here the choice follows the load: pick the contact that energizes the motor circuit when pressure says run. A pump maintaining a tank uses a contact that closes as pressure falls to cut-in and opens as pressure rises to cut-out.
The gap between those two points is the differential, or deadband. Set cut-in at 3.0 bar and cut-out at 4.5 bar and the differential is 1.5 bar. Too narrow a band and the switch chatters, cycling the motor every few seconds around one pressure.
A wider differential means fewer switching cycles, which is also longer contact life. On our PC-100 miniature switch the deadband runs ±3 to 30 psi depending on range; refrigeration high-pressure cutouts are specified the same way, with figures like open at 600 psi, reclose at 300 psi printed on the nameplate.
Changeover contacts
Many switches do not force the choice at all. A changeover (SPDT) contact set brings out three terminals: common, NO and NC. One circuit closes while the other opens, and you pick the polarity at the terminal strip during wiring, not at the time of ordering. Our Y-500 diaphragm switch and Y-505 hydraulic switch each carry one normally open and one normally closed contact, rated 220 V AC at 6 A, so the same unit serves an alarm circuit one year and a control circuit the next.
Electronic switches take the last step. On an electronic pressure switch the outputs are solid-state and the NO or NC logic of each set point is a menu setting, along with the set point and deadband themselves. Nothing is decided by the mechanics; everything is configuration.
Testing and wiring
Identifying an unmarked switch takes a multimeter and two minutes. With the switch unpressurized, set the meter to continuity and probe the contact pair. A tone means you are on an NC pair; silence means NO. Then apply pressure past the set point, or actuate the mechanism, and confirm the reading flips.
If the meter shows continuity in both states, the contacts are welded or the switch has failed; if it shows continuity in neither, check that you are not probing across the common of an SPDT set.
On the controller side, remember that the input logic inverts once for an NC device. A healthy NC circuit holds the PLC input high, so the program examines the input for the closed state and trips when the signal disappears. Programmers new to safety circuits sometimes match the instruction to the device label instead of the wire state, and the interlock tests backwards on commissioning day. Draw the circuit in its shelf state, walk it to the running state, then write the logic.
Switch lineup
Every mechanical switch in our pressure switch lineup states its contact form and rating on the datasheet: the Y-500 for low ranges, the Y-505 for hydraulic pressures, the YL-802 digital vacuum switch with settable relay output, the PC series miniature switches, and the high-temperature switch with up to four set points. If the application needs a specific action on rising or falling pressure, say so in the inquiry and the contact arrangement comes configured, not improvised at the panel.
FAQ
Should a pressure switch be normally open or closed?
Choose by failure mode. Alarm and trip circuits should use a contact that is closed in the healthy state and opens on the abnormal condition, so a broken wire reads as an alarm. Control circuits pick the contact that energizes the load when the process calls for it, usually closing on falling pressure for pump cut-in.
How to tell if a switch is normally open or normally closed?
Test it at zero pressure with a multimeter on continuity. A tone across the pair means normally closed; no tone means normally open. Apply pressure past the set point and the reading should flip. Continuity in both states means welded contacts; in neither, you are probably across the wrong terminals of a changeover set.
What happens if you use a normally closed contact instead of normally open?
The circuit logic inverts: the load that should energize at the set point is energized everywhere except past the set point. A pump cycles in reverse, or an alarm sounds continuously and clears at the fault. On a changeover switch the fix is moving one wire from the NO terminal to the NC terminal; on a fixed-form switch it means the wrong part was ordered.
Is a pressure switch normally open or closed?
Neither by default; both forms are standard catalog items, and changeover types carry both at once. Process and general-purpose switches are most commonly supplied normally open, while safety cutouts are normally closed so the run circuit breaks on a fault. Automotive and HVAC trades follow their own conventions per device, so check the datasheet, not the habit.
Request a quote
Tell us the application: set point and range, rising or falling action, alarm or control circuit, and the load the contacts must switch. We will configure the switch to match, not hand you a shelf part. Reach our application engineers or use the form below.
Written and technically reviewed by Wu Peng and the Instranova engineering team.