To identify the NEMA and IEC pressure switch symbols in a schematic, look at the actuator line intersecting the contact. NEMA uses a straight line with a perpendicular crossbar or zigzag, while IEC (per IEC 60617) uses a straight line with a specific semi-circular bow or flat bar. Both standards depict the switch in its unactuated, de-energized "shelf" state, not its running state. Getting this right prevents catastrophic miswiring in motor control and PLC logic circuits.
The Master Reference: NEMA vs. IEC Pressure Switch Symbols
The table below maps the exact graphical representations you will encounter on the bench or in the field. Use this as your primary lookup when tracing ladder logic or P&IDs.
| Switch Type | NEMA Symbol Description | IEC 60617 Symbol Description | Typical Application |
|---|---|---|---|
| Normally Open (NO) | Two parallel lines (contacts) separated by a gap. A straight horizontal actuator line crosses the gap, ending in a perpendicular crossbar (T-shape) pointing down. | Two parallel lines. A straight horizontal actuator line crosses the gap, ending in a flat horizontal bar or a small semi-circular bow pointing down. | Low-pressure cut-off for water pumps; starts motor when pressure drops. |
| Normally Closed (NC) | Two parallel lines with a diagonal line bridging them (closed contact). The straight horizontal actuator line ends in a perpendicular crossbar pointing up, away from the contact. | Two parallel lines bridged by a diagonal line. The straight horizontal actuator line ends in a flat bar or bow pointing up, away from the contact. | High-pressure safety cut-out; breaks circuit to stop motor when pressure exceeds limit. |
| Adjustable / Differential | Standard NO or NC base, but the actuator line features a diagonal slash or a small zigzag spring symbol near the crossbar to indicate adjustable setpoints. | Standard NO or NC base, with a small diagonal arrow crossing the actuator line, indicating an adjustable threshold. | Air compressors requiring a specific cut-in/cut-out band (e.g., 90 PSI on / 120 PSI off). |
| Vacuum Switch | Identical to pressure switch, but the actuator line ends in a crossbar with a small "V" or the word "VAC" adjacent to it. | Identical to pressure switch, but the actuator line features a downward-pointing arrowhead or a specific vacuum bow notation. | Pneumatic conveying systems; vacuum pump protection. |
Regional Standards: Which Symbol Set Applies to You?
Assuming a single standard applies globally is a fast track to misinterpreting a legacy machine import. Here is how the standards break down by region and era.
- NEMA / JIC (North America): Governed by the National Electrical Manufacturers Association and JIC (Joint Industrial Council) standards. If you are wiring a panel in the US or Canada, or working on machinery built by North American OEMs, you will see NEMA symbols. NEMA ladder logic reads top-to-bottom, left-to-right, and relies heavily on the physical layout of the schematic.
- IEC 60617 (Europe, Asia, International): The global standard. IEC symbols are more abstract and modular. If you are working on European imports (e.g., Siemens, ABB, Festo) or modern globalized PLC architectures, IEC is the default. IEC schematics often use a grid-coordinate system (e.g., page 4, row B) rather than physical layout mapping.
- Old UK (BS 3939): Largely superseded by IEC 60617 in the late 1990s, but you will still encounter BS 3939 symbols in legacy British manufacturing plants and older marine vessels. BS 3939 pressure switches often used a simple circle with a "P" inside rather than the modern actuator line. If you see a circle with a letter inside, suspect a pre-2000 UK diagram.
For a deeper dive into standard compliance, refer to the NEMA electrical control panel standards and comprehensive symbol libraries like All About Circuits wiring symbols.
The "Rows People Get Wrong" Field Notes
Confusion 1: Pressure vs. Temperature Actuators
Look closely at the actuator line. A pressure switch has a straight line ending in a flat bar or crossbar. A temperature switch (thermostat) has a wavy line, a zigzag, or ends in a small bulb/circle. If you wire a temperature switch logic into a pressure cutoff circuit because you misread the wavy line as a zigzag spring, your safety cutoff will never trigger.
Confusion 2: The Definition of "Normal"
In electrical schematics (both NEMA and IEC), "Normally Open" (NO) and "Normally Closed" (NC) refer to the unactuated, shelf state of the switch—meaning zero pressure applied. In process P&IDs, "normal" sometimes refers to the normal operating condition of the plant. Always interpret electrical ladder logic based on the de-energized, zero-pressure shelf state.
Confusion 3: Differential Gap Notation
A standard NO symbol just means "closes at setpoint." If the schematic includes a diagonal slash or arrow across the actuator line, it denotes an adjustable differential (deadband). If you replace an adjustable differential switch with a fixed-setpoint switch, your motor will short-cycle and burn out the contactor.
Decision Path: Specifying the Right Switch for Your Circuit
Use this decision matrix to terminate your design phase with a concrete part selection. Do not leave the specification open-ended.
| Application Scenario | Required Logic & Standard | Concrete Part Pick |
|---|---|---|
| 120V/240V AC Residential Well Pump Simple cut-in/cut-out control, direct line switching. |
NEMA, NO with adjustable differential, high current rating (20A+). | Square D 9013FSG2 (Standard NEMA well pump switch, 20-40 PSI adjustable). |
| 24VDC PLC Input for Hydraulic Press High-pressure safety cutoff, low voltage logic signaling. |
IEC, NC (for fail-safe E-stop logic), solid-state or low-current micro-switch. | IFM PN7094 (IEC-standard electronic pressure switch, 0-250 bar, PNP output). |
| 480V 3-Phase Industrial Air Compressor Heavy inductive load, requires contactor isolation. |
NEMA, NO pilot switch to trigger a motor contactor, heavy-duty diaphragm. | Furnas / Siemens Class 69 (NEMA 1 enclosure, pilot duty rated, use with a 30A contactor). |
Safe Interpretation When Markings Are Faded or Missing
In the field, you will frequently encounter physical pressure switches where the label is painted over, sun-baked, or missing entirely, and the original schematic is nowhere to be found. Here is how to safely identify the switch state and wiring without relying on the symbol.
- Isolate and Depressurize: Lock out / tag out the electrical supply. Critical: Bleed the pneumatic or hydraulic pressure from the line using a manual drain valve. A trapped 100 PSI air pocket can violently actuate the switch while your fingers are on the terminals.
- Set the Multimeter: Switch your DMM to the continuity (beep) or low-ohms (Ω) setting.
- Test the Shelf State: Place probes across the two main load terminals.
- If you read < 1 ohm (continuity) with zero pressure applied, it is a Normally Closed (NC) switch.
- If you read OL (open loop) with zero pressure applied, it is a Normally Open (NO) switch.
- Verify Actuation: If safe to do so, temporarily apply shop air via a regulated test hose while monitoring the meter. The state should flip. If it flips at 30 PSI and flips back at 35 PSI, you have confirmed an adjustable differential NO switch.
- Label Immediately: Once verified, use a Brady or Brother industrial label maker to print the verified state (e.g., "PS-101 / NO / 40-60 PSI") and affix it to the conduit body or switch housing.
For further reading on interpreting complex industrial diagrams and standardizing your facility's symbol library, consult the Electrical Engineering Portal's guide to electrical symbols. Standardizing your internal documentation to match either strict NEMA or strict IEC prevents the exact cross-wiring errors that lead to downtime.
