The standard electrical symbol for a pressure switch consists of a basic switch contact (Normally Open or Normally Closed) actuated by a semicircle with an intersecting line or arrow, representing the physical diaphragm or piston responding to fluid pressure. However, the exact graphical representation shifts significantly depending on whether you are reading a North American NEMA schematic, a European IEC 60617 diagram, or an older JIS blueprint. Misinterpreting these regional variations or the 'normal' state of the contacts is a primary cause of miswired well pumps, air compressors, and HVAC safety cutouts.

The Complete Pressure Switch Symbol Reference Table

Use this reference table to identify the specific pressure switch symbol on your schematic. The 'Normal' state always refers to the switch's electrical state at zero applied pressure (atmospheric baseline), not the normal running state of your machinery.

Switch Type NEMA (US/Canada) Symbol IEC 60617 (Global) Symbol Practical Application
Normally Open (NO) NO contact with semicircle & horizontal line NO contact with semicircle & inward arrow High-pressure alarms; triggers a circuit only when pressure exceeds the setpoint.
Normally Closed (NC) NC contact with semicircle & horizontal line NC contact with semicircle & inward arrow Well pump controllers (e.g., Square D Pumptrol); closes circuit to run pump, opens to stop at max pressure.
Differential Pressure Single toggle with two opposing semicircle actuators Single toggle with two opposing inward arrows HVAC filter monitoring; compares pressure before and after a filter to detect clogs.
High-Pressure Cutout NC contact with semicircle, line, and 'H' or 'Max' label NC contact with inward arrow and 'P>' label Refrigeration compressor safety; breaks the control circuit if head pressure becomes dangerous.
Low-Pressure Cutout NO contact with semicircle, line, and 'L' or 'Min' label NO contact with inward arrow and 'P<' label Loss-of-charge protection; prevents compressors from running if refrigerant leaks out.

Regional Standard Variants: NEMA vs. IEC vs. JIS

Knowing which standard applies to your region and equipment origin prevents catastrophic wiring errors. A symbol that means 'pressure actuated' in one standard might look like a manual pushbutton in another.

  • NEMA (National Electrical Manufacturers Association): Dominant in the US and Canada. NEMA ICS 18 and ICS 2 standards typically depict the pressure actuator as a simple semicircle resting on the contact line, sometimes enclosed in a circle with the letter 'P'. The focus is on functional readability for electricians rather than strict geometric abstraction.
  • IEC 60617 (International Electrotechnical Commission): The global standard used across Europe, Asia, and modern international projects. IEC strictly separates the actuator symbol from the contact block. The pressure actuator is drawn as a semicircle with a distinct, inward-pointing arrow indicating the direction of force applied by the fluid. According to All About Circuits' switch symbol guides, this separation allows the same actuator symbol to be mechanically linked to multiple contact blocks (e.g., a single pressure switch triggering both a motor starter and a PLC input).
  • JIS (Japanese Industrial Standards) & Legacy UK: Older Japanese and British schematics often represent pressure switches using a rectangle containing the letter 'P', or a stylized bellows symbol (a zig-zag line). If you are servicing legacy imported CNC machines or older marine equipment, expect these non-standard geometric shapes.

Rows People Get Wrong: Decoding the 'Normal' State

The most frequent point of failure when reading pressure switch schematics is misunderstanding the word 'Normal'. In electrical theory, 'Normal' refers to the unactuated, shelf-state of the component. For a pressure switch, 'Normal' means 0 PSI (atmospheric pressure).

Common Wiring Mistake: Assuming a well pump pressure switch is 'Normally Open' because the pump is 'normally' running. A standard Square D FSG2 Pumptrol is actually a Normally Closed (NC) switch. At 0 PSI (a dry well tank), the internal spring forces the electrical contacts closed, sending 240V to the pump. As water pressure rises to the cut-out setting (e.g., 40/60 PSI), the diaphragm pushes against the spring, physically snapping the contacts open to stop the pump. If you wire it expecting an NO logic, your pump will run until it dead-heads and burns out the motor.

Another heavily misinterpreted row is the Differential Pressure Switch. Technicians often wire the two sensing ports backward. The 'High' port must connect to the upstream (dirty) side of a filter, and the 'Low' port to the downstream (clean) side. If reversed, the switch will trigger an alarm when the filter is brand new, and fail to trigger when it is completely clogged.

Safe Interpretation When Markings Are Faded or Missing

In industrial environments, UV exposure, oil mist, and heat often destroy the sticker diagrams on the inside of a pressure switch cover. If you are facing an unmarked Danfoss or Furnas switch, do not guess the NO/NC state based on the application. Use this field-verification protocol:

  1. De-energize and Lockout: Shut off the breaker and verify 0V across the line and load terminals using a CAT III rated multimeter.
  2. Bleed the System: Open a drain valve to drop the system pressure to absolute 0 PSI. This establishes the 'Normal' baseline.
  3. Continuity Test: Set your multimeter to the continuity (diode/beep) setting. Place probes across the two main power terminals.
    • If the meter beeps (reads < 1 ohm), it is a Normally Closed (NC) switch.
    • If the meter does not beep (reads OL), it is a Normally Open (NO) switch.
  4. Verify Actuation: If the system has a Schrader valve or test port, apply regulated shop air to the switch port while monitoring the multimeter. The continuity state should flip precisely at the pressure stamped on the physical adjustment spring (e.g., 45 PSI).

For further standardization details on motor control circuits and safety cutouts, refer to the NEMA Electrical Equipment Standards documentation.

Frequently Asked Questions

What does the circle with a 'P' mean on a pressure switch schematic?

A circle with a capital 'P' inside it, usually attached to the actuator line of a switch symbol, is the legacy NEMA and JIS shorthand for 'Pressure Actuated'. It differentiates the switch from temperature-actuated (circle with a 'T' or thermometer icon) or flow-actuated switches. In modern IEC 60617 schematics, this circle is largely obsolete, replaced by the strict semicircle-and-arrow geometry, but you will still see it on older US-manufactured HVAC and pump control panels.

How do I wire a 3-wire pressure switch if the symbol only shows 2 terminals?

Standard mechanical pressure switches (like a standard 40/60 PSI well switch) are 2-pole devices: they have Line 1, Line 2, Load 1, and Load 2. If you are looking at a physical device with only 3 wires (e.g., Black, White, Bare), it is not a 3-pole switch; it is a single-pole switch where the white wire is the neutral return for a 120V control circuit, or the third wire is simply the equipment grounding conductor. However, if you are dealing with an electronic pressure sensor (not a mechanical switch) with 3 wires, it is likely a 4-20mA loop device (Power+, Signal Out, and Common-) and requires a PLC analog input card, not a standard motor contactor.

Is a pressure switch symbol the same as a pressure transducer symbol?

No, and confusing them will destroy your control board. A pressure switch symbol shows discrete mechanical contacts (NO/NC) that snap open or closed. A pressure transducer or transmitter symbol features a circle with a 'P' and an analog wave or diagonal arrow pointing outward, indicating it outputs a variable voltage (0-5V) or current (4-20mA) proportional to the pressure. Wiring a 24V transducer output into a 240V motor starter coil circuit based on a misread symbol will instantly vaporize the transducer's internal PCB.