Pressure switch symbols on electrical schematics represent the physical state of the switch contacts (normally open or normally closed) and the actuation method (fluid or pneumatic pressure). The standard NEMA symbol uses a circle with a diagonal line and specific contact configurations, while the IEC standard uses a standardized graphical block with letter designations. Below is the complete reference to decode these symbols on your next pump, compressor, or hydraulic wiring diagram.
The Master Reference Table for Pressure Switch Symbols
The following table maps the most common pressure switch configurations to their NEMA (US/Canada) and IEC 60617 (Global/Europe) schematic representations. Use this as your bench reference when tracing control circuits.
| Switch Function | NEMA Actuator Symbol | IEC 60617 Actuator Symbol | Contact State (Shelf/0 PSI) | Typical Application & Model Example |
|---|---|---|---|---|
| Pressure Switch, NO (Closes on Rise) | Circle with bottom-left to top-right diagonal line | Square block with 'P' inside, arrow pointing up | Open | Air compressor cut-in (e.g., Square D 9013FSG) |
| Pressure Switch, NC (Opens on Rise) | Circle with diagonal line, contacts overlapping | Square block with 'P', arrow pointing down | Closed | High-pressure safety cutoff (e.g., Condor MDR3) |
| Pressure Switch, NO (Opens on Rise / Reverse) | Circle with diagonal line, NO contacts, reverse arrow | Square block with 'P', dual opposing arrows | Open | Low-pressure alarm triggering on pressure loss |
| Pressure Switch, NC (Closes on Rise / Reverse) | Circle with diagonal line, NC contacts, reverse arrow | Square block with 'P', specific reverse logic block | Closed | Hydraulic unloader valve activation |
| Differential Pressure Switch | Two circles with diagonal lines, linked mechanically | Two square blocks, 'ΔP' designation | Depends on setup | HVAC filter monitoring (e.g., Dwyer 1910) |
| Vacuum Switch (Closes on Vacuum Drop) | Circle with diagonal line, 'V' or vacuum bell symbol | Square block with 'P' and negative sign (-P) | Open | Pump suction loss protection |
Rows People Get Wrong (and How to Fix Them)
Even experienced electricians and controls technicians misinterpret specific rows in the table above when working under time pressure. Here are the most common schematic misreads and how to avoid them.
Mistake 1: Confusing "Normal" with "Running"
The most frequent error is assuming "Normally Open" (NO) means the switch is open while the compressor or pump is running. In schematic logic, "Normal" always refers to the de-energized, unpressurized shelf state (0 PSI). A standard well pump pressure switch (like the ubiquitous Square D 9013) is Normally Open. When the tank drops to 30 PSI, the switch remains open. It only closes when pressure drops below the cut-in threshold to start the motor. Always trace the symbol based on 0 PSI, not the operating state.
Mistake 2: Misidentifying the "Bow Tie" Actuator
On older NEMA ICS 1 schematics, you may see a symbol that looks like a bow tie or butterfly attached to the contact lines. Many assume this is a standard pressure switch. In reality, this is often a flow switch or a specific differential pressure switch depending on the drafter's convention. If you see the bow tie, check the title block or the device schedule on the drawing to confirm if it's measuring flow rate (GPM) or pressure differential (ΔP).
Mistake 3: Ignoring the Manual Reset Indicator
A standard diagonal line through the NEMA circle indicates automatic reset (the switch resets when pressure drops). However, if you see a small semi-circle or a triangle attached to the end of that diagonal line, it indicates a manual reset requirement. This is critical for safety circuits; if you are troubleshooting a lockout and the switch is manual-reset, simply cycling the power or dropping the pressure will not restore the circuit. You must physically press the reset button on the switch housing.
Regional Standards: NEMA vs. IEC vs. Legacy UK
Electrical schematics are not universally standardized. The symbol set you encounter depends heavily on the region where the machinery was manufactured or where the control panel was designed.
- NEMA (North America): Governed by NEMA ICS 1 standards. The defining feature is the circle with a diagonal line (often called a pennant or flag) representing the fluid/pneumatic actuator. Contacts are drawn as standard knife-blade symbols (parallel lines for NO, overlapping for NC). This is what you will see on 95% of US-manufactured air compressors and well pumps.
- IEC (Europe and Global): Governed by IEC 60617 and IEC 61082. IEC abandons the circle for a square or rectangular block. The actuator is identified by a letter code inside or adjacent to the block (e.g., 'P' for pressure, 'F' for flow). Contacts use the IEC standard horizontal line with a diagonal slash for NO, or a solid bridge for NC. You will see this on imported CNC machines and European packaging equipment.
- Legacy UK (BS 3939): Mostly superseded by IEC, but still present in older British plants and marine applications. It used a semi-circle for the actuator. If you are retrofitting a control panel built before 1990 in the UK, expect to see these legacy symbols mixed with early IEC drafts.
Always check the title block in the bottom right corner of the schematic to identify the drafting standard (e.g., "Symbols per IEC 60617"). Assuming a NEMA contact logic on an IEC drawing can lead you to wire a safety cutoff backward, creating a severe hazard. For comprehensive symbol libraries, refer to the All About Circuits standard symbol reference or the NEMA electrical control devices documentation.
Troubleshooting Faded or Missing Switch Markings
On jobsites, physical switch labels (like the data plate on a Furnas or Square D switch) often fade, peel off, or become covered in grease. If the schematic is also missing, you must determine if the switch is NO or NC safely. Never guess based on the application alone.
- De-energize and Lock Out: Turn off the breaker feeding the control circuit. Verify dead with a non-contact voltage tester and a multimeter.
- Bleed the System: Open the drain valve or bleed port to drop the system pressure to absolute 0 PSI. The switch must be in its "normal" unactuated state.
- Set Multimeter to Continuity: Use the continuity setting (the diode/sound wave icon) or the lowest Ohms range.
- Measure Across Line and Load: Place your probes on the main power terminals (usually marked L1/L2 or Line/Load).
- If the meter beeps or reads < 1.0 Ω, the switch is Normally Closed (NC).
- If the meter reads OL (Open Loop) or infinite resistance, the switch is Normally Open (NO).
- Verify Actuation (Optional but Recommended): If the system allows, manually apply pressure (using a hand pump or temporarily powering the compressor while monitoring) and watch the multimeter. The state should flip. If it doesn't, the internal diaphragm is likely ruptured or the contacts are welded.
Frequently Asked Questions
What does the circle with a diagonal line mean on a pressure switch symbol?
In NEMA schematic standards, the circle represents the physical housing or actuator mechanism of the switch, while the diagonal line indicates that the actuating force is fluid or pneumatic pressure (as opposed to a thermal element or mechanical lever). The line always points toward the contacts it controls. If the line touches the moving contact blade, it indicates the pressure directly actuates that specific contact.
How do I read a differential pressure switch symbol on a schematic?
A differential pressure switch measures the difference between two pressure points (e.g., across an HVAC filter or a pump strainer). On a NEMA diagram, this is drawn as two circles with diagonal lines, mechanically linked by a dashed line, acting on a single set of contacts. On an IEC diagram, look for a single square block with the designation "ΔP" (Delta P) and two input arrows. The switch will only change state when the pressure drop between the two ports exceeds the setpoint, regardless of the absolute line pressure.
Is a pressure switch symbol the same as a pressure transmitter symbol?
No. A pressure switch provides discrete on/off contacts (digital logic) and is drawn with standard NO/NC contact blades. A pressure transmitter (or transducer) outputs an analog signal (like 4-20mA or 0-10V) and is drawn differently. In NEMA, a transmitter is often a circle with the letters "PT" (Pressure Transmitter) inside. In IEC, it is represented by a circle (the "bubble") with "PT" and an analog output line, not discrete contact blades. Confusing the two will lead you to wire a 120V AC control circuit into a 24V DC PLC analog input card, instantly destroying the input channel.
What does a small triangle on the pressure switch actuator line indicate?
A small triangle or semi-circle attached to the actuator line (the diagonal line in NEMA, or the arrow in IEC) indicates that the switch requires a manual reset. This is a critical safety feature used in high-pressure cutoffs or boiler low-water cutoffs. When the fault condition clears (pressure drops back to normal), the switch will not automatically re-close the circuit; an operator must physically press a reset button on the device to restore power. For more on reading complex control diagrams, the Electrical Engineering Portal's guide to diagram symbols provides excellent visual breakdowns of these reset indicators.
