Pressure Switch Electrical Symbol Guide: NEMA & IEC

The standard US (NEMA/JIC) pressure switch electrical symbol is a set of parallel contact lines intersected by a diagonal pilot line, while the IEC standard uses a standardized rectangular contact block with a mechanical actuator line. If you are reading a schematic in North America, default to NFPA 79/NEMA conventions; in Europe and most of the globe, default to IEC 60617. Below is the definitive reference for identifying, interpreting, and replacing these components.

The Master Pressure Switch Symbol Reference Table

Use this table to map the schematic symbol on your print to the physical switch in your panel. 'Normally' refers to the state of the contacts at 0 PSI (unpressurized), not the normal running state of the machine.

Regional Variants: NEMA vs. IEC vs. Legacy UK

Electrical schematics are not universal. Misinterpreting a regional variant can lead to wiring a safety cutout backward, resulting in a catastrophic over-pressurization event.

North America: NEMA and NFPA 79

In the US and Canada, industrial schematics follow NFPA 79 and NEMA standards. The symbols are highly pictorial. A pressure switch looks like a physical switch being pushed by a line representing the pressure pipe. NEMA symbols are generally larger on the page and rely on the diagonal 'slash' through the contacts to indicate the actuating force.

Europe and Global: IEC 60617

The IEC 60617 standard strips away the pictorial elements. Contacts are strictly drawn as horizontal lines (gaps for NO, touching for NC), and the actuator is a strict vertical line with specific geometric add-ons (like a semicircle for pressure). IEC schematics are highly modular; the pressure actuator symbol might be drawn on page 1, while the actual contact it operates is drawn on page 4, linked only by a cross-reference tag (e.g., '-B12').

Legacy UK: BS 3939

Older British facilities may still use BS 3939. This standard is a hybrid, often using IEC-style contact lines but NEMA-style mechanical linkages. If you encounter a schematic drawn before 1990 in the UK, verify the legend block on the title page before assuming the contact states. BS 3939 is officially withdrawn, superseded by BS EN 60617 (the UK adoption of IEC), but legacy prints remain in active plant binders.

Rows People Get Wrong (And How to Fix Them)

• The 'Normal' Trap: 'Normal' in electrical schematics always means the unactuated, shelf-state of the device. For a pressure switch, 'Normal' means 0 PSI (atmospheric pressure). A well pump switch is 'Normally Open' because at 0 PSI, the contacts are open. When the tank drops to 20 PSI, the switch is still 'open' relative to its 40 PSI setpoint, but it is no longer in its 'normal' 0 PSI state. Do not confuse 'normal state' with 'normal running state'.
• High-Pressure Cutout vs. Low-Pressure Start: A low-pressure start switch (NO) closes to start the motor. A high-pressure safety cutout (NC) opens to stop the motor. If you replace an NC safety switch with an NO switch, the motor will run until the relief valve blows or the pipe ruptures. Always verify the actuator arrow direction on the schematic: an arrow pointing away from the contact line usually indicates pressure opens it (NC).
• Integrated Unloader Valves: Many 3-phase compressor switches (like the Condor MDR-3) feature a secondary symbol on the schematic for an 'unloader' contact. This is a tiny pilot valve that bleeds head pressure when the switch trips to open. If your schematic shows a dashed mechanical linkage to a secondary exhaust valve, you must buy a switch with an integrated unloader, or the motor will stall and trip the breaker on the next start cycle.

Safe Interpretation When Markings Are Faded or Missing

Physical pressure switches live in harsh environments. The wiring diagram sticker on the cover of a Square D 9013 will often flake off after a decade in a damp pump house. When you cannot read the symbol or the terminal markings, you must empirically determine the contact state.

1. De-energize and Lockout: Turn off the breaker feeding the switch. Verify 0V across the line and load terminals with a multimeter.
2. Bleed the System Pressure: Open a downstream valve or drain to drop the system pressure to absolute 0 PSI. Do not skip this. Testing continuity on a pressurized system will give you the 'actuated' state, not the 'normal' state, leading to a backwards diagnosis.
3. Set Multimeter to Continuity: Place probes across the two main power terminals (usually labeled 1 and 2, or Line and Load).
4. Interpret the Beep:
   • If the meter beeps (reads < 1 ohm) at 0 PSI, the switch is Normally Closed (NC).
   • If the meter is silent (reads OL or infinite) at 0 PSI, the switch is Normally Open (NO).
5. Apply Pressure (Optional Verification): If the system has a manual fill valve, slowly raise the pressure while watching the meter. The exact PSI where the state flips is your setpoint.

Decision Path: Selecting the Right Switch and Symbol

Stop guessing which replacement part to order. Use this decision matrix to match your schematic symbol and application to a concrete, proven part number.

Always cross-reference the physical part's data sheet with the schematic's ampacity requirements. A symbol tells you how the switch behaves; the data sheet tells you if it will survive the inrush current of your specific motor.