The standard NEMA/JIC symbol for a pressure switch is a circle intersected by a diagonal line with a perpendicular tick mark (resembling a flag), while the IEC 60617 standard uses a semicircle with a straight actuating line and an arrow. Knowing exactly which symbol applies to your schematic prevents miswiring critical pump, compressor, and boiler controls.
The Master Reference Table: Pressure Switch Symbols by Standard
Before tracing wires, identify the drafting standard used on your schematic. The table below maps the exact graphical representations to their electrical states across the three major standards you will encounter in the field.
| Standard | Symbol Description | Normal State | Typical Application |
|---|---|---|---|
| NEMA / JIC (NFPA 79) | Circle with diagonal line; perpendicular tick pointing UP/OUT | Normally Open (NO) | US/Canada industrial panels, well pumps, air compressors |
| NEMA / JIC (NFPA 79) | Circle with diagonal line; perpendicular tick pointing DOWN/IN | Normally Closed (NC) | Low-pressure cutoff safety interlocks, hydraulic limit stops |
| IEC 60617 | Semicircle with straight horizontal line; arrow pointing IN toward the line | Normally Open (NO) | European/Global PLC I/O diagrams, modern VFD integration |
| IEC 60617 | Semicircle with straight horizontal line; arrow pointing OUT away from the line | Normally Closed (NC) | Global boiler safety circuits, pneumatic exhaust valves |
| Legacy UK (BS 3939) | Square box with a diagonal line and a small 'P' inside or adjacent | Varies (check NO/NC tick) | Pre-2000s UK industrial plants, legacy marine switchboards |
Regional Standards: Which Symbol Applies to Your Region?
The symbol you should draw or expect to see depends entirely on the origin of the equipment and the governing electrical code of the installation site.
- North America (US & Canada): Equipment built for North America follows NFPA 79 (Electrical Standard for Industrial Machinery) and NEMA standards. Expect the circle-and-flag symbols. If you are wiring a Square D or Furnas panel, you are in NEMA territory.
- Europe, Asia, and Global IEC Markets: Machinery imported from or designed in regions following the International Electrotechnical Commission uses IEC 60617 graphical symbols. Expect the semicircle-and-arrow symbols. Siemens and ABB schematics default to this format.
- United Kingdom (Legacy): While the UK has largely adopted IEC standards, older facilities still running BS 3939 schematics will show the boxed 'P' variant. If you are retrofitting a plant built before 1995, keep a BS 3939 cheat sheet on hand.
Rows People Get Wrong: Common Misinterpretations
Misreading a pressure switch symbol rarely happens because the drafter made a mistake; it happens because the reader misinterprets the actuator modifiers or the definition of "normal."
1. Confusing "Normal" with "Running"
In schematic logic, "Normally Open" (NO) or "Normally Closed" (NC) refers to the switch's electrical state when the system is de-energized and at minimum (zero or atmospheric) pressure. It does not mean the state during normal machine operation. A well pump pressure switch set to 30/50 PSI is electrically "Normally Open" when the tank is at 0 PSI, but it spends most of its operational life closed while the tank sits at 50 PSI.
2. Missing the Differential Modifier
Many pressure switches have a differential gap (the difference between cut-in and cut-out pressure). In NEMA schematics, this is often indicated by two small parallel lines crossing the diagonal actuator line, or a small delta ($\Delta$) symbol. If you ignore this modifier, you might assume the switch is a simple single-point snap-action device and fail to program your PLC's deadband correctly, resulting in rapid short-cycling of your motor contactor.
3. Mixing Up Pressure and Temperature Switches
The circle-and-line symbol is the base for many process switches. The differentiator is the actuator line. A pressure switch uses a straight diagonal line. A temperature switch uses a wavy line or a small bulb symbol at the end of the line. A flow switch uses a paddle symbol. Always trace the line back to the process indicator.
Decision Tree: Selecting the Right Pressure Switch for Your Circuit
Do not guess which switch to buy based on the schematic symbol alone. Use this decision path to terminate your selection in a concrete, proven part number that matches the electrical and mechanical requirements of the symbol you are wiring.
| If Your Application Is... | And The Schematic Requires... | Then Specify This Exact Part |
|---|---|---|
| Residential / Light Commercial Well Pump (115V/230V) | NEMA NO, 30/50 PSI cut-in/cut-out, DPST | Square D 9013FSG2 (Standard 1/2" NPT, NEMA 1) |
| Shop Air Compressor (Single Phase, up to 175 PSI) | NEMA NO with unloader valve port, NC auxiliary for motor contactor | Condor MDR3 (11-16 bar range, 3-port flange, integrated unloader) |
| Industrial Hydraulic Power Unit (3-Phase, High Pressure) | IEC NO/NC convertible, IP65 rated, 5000 PSI max | Eaton/Vickers ST series (e.g., ST300A-020, adjustable 500-3000 PSI) |
| Steam Boiler Low-Water Cutoff / Pressure Limit | NEMA NC (failsafe open on low pressure), manual reset required | McDonnell & Miller 150 (Manual reset, ASME CSD-1 compliant) |
Safe Interpretation When Markings Are Faded or Missing
On jobsites, you will frequently encounter legacy pressure switches where the stamped schematic diagram on the cover is faded, painted over, or missing entirely. Do not assume the terminal markings (e.g., L1, L2, M) match the NO/NC state of the internal microswitch.
Follow this exact verification sequence to determine the switch state when markings are compromised:
- Isolate and Bleed: Turn off the breaker and verify dead with a CAT III multimeter. Bleed the process line to 0 PSI.
- Continuity Test (State 1): Set your multimeter to the Ohms/Continuity setting. Place probes across the suspected line and load terminals. If it reads < 1 $\Omega$, the switch is Normally Closed (NC) at zero pressure. If it reads OL (Open Loop), it is Normally Open (NO).
- Actuate Manually (State 2): Remove the cover. Locate the internal microswitch or contact block. Use a small flathead screwdriver to gently press the actuator pin or trip the lever. The multimeter should toggle from OL to < 1 $\Omega$ (or vice versa).
- Map and Label: Once verified, use a Brother P-Touch or Brady label maker to print a new terminal map and affix it inside the switch cover. Write the verified NO/NC state and the mechanical cut-in/cut-out setpoints directly on the label.
By relying on physical multimeter verification rather than faded silk-screened diagrams, you eliminate the risk of wiring a failsafe NC interlock as a NO run-command, which is a primary cause of uncontrolled compressor and pump runaways.
