The standard pressure switch symbol in US (NEMA/JIC) schematics features a circle intersected by a diagonal line with a pilot line, whereas the international (IEC) standard uses a square or semi-circle with a dashed pilot line. Understanding these symbols is critical for wiring air compressors, well pumps, and HVAC safety cutouts without causing dead shorts or motor burnout.
The Complete Pressure Switch Symbol Reference Table
Before tracing any wires, identify which standard your schematic follows. The table below maps the most common pressure switch configurations to their graphical representations and real-world applications. This data aligns with NEMA ICS 19 and IEC 60617 standards.
| Switch Type & Logic | NEMA / JIC Graphic (US) | IEC 60617 Graphic (EU/Global) | Shelf State (0 PSI) | Typical Application & Example Part |
|---|---|---|---|---|
| Normally Closed, Opens on Rise (NC-OOR) | Circle with diagonal cross-line; pilot line with arrow pointing toward the circle. | Square actuator with dashed mechanical linkage; crossbar on the contact. | Closed (Continuity) | Well pump cut-out (e.g., Square D Pumptrol 9013FHG12). Starts pump when pressure drops, stops when target is reached. |
| Normally Open, Closes on Rise (NO-COR) | Circle with diagonal cross-line; pilot line with arrow pointing away from the circle. | Square actuator with dashed linkage; NO contact symbol (gap). | Open (No Continuity) | High-pressure alarm or secondary compressor staging (e.g., Condor MDR 3 auxiliary contacts). |
| Normally Closed, Opens on Fall (NC-OO F) | Circle, diagonal line; pilot line marked with a 'Z' (zigzag) for pneumatic sensing. | Semi-circle actuator; dashed line with downward arrow notation. | Closed (Continuity) | HVAC low-pressure lockout. Shuts off compressor if refrigerant charge is lost. |
| Differential Pressure Switch | Two distinct pilot lines entering a single circular switch node. | Two dashed lines entering a single square actuator block. | Depends on delta-P | Filter monitoring or pump differential sensing (e.g., Dwyer 1900 Series). |
NEMA vs. IEC: Regional Standards and Variants
A frequent point of failure on the bench is applying IEC logic to a NEMA drawing, or vice versa. The two philosophies approach schematic drafting differently.
| Criteria | NEMA / JIC (North America) | IEC 60617 (Europe / International) | Old UK (BS 3939 - Superseded) |
|---|---|---|---|
| Actuator Shape | Circle | Square or Semi-circle | Circle with internal cross-hatching |
| Pilot / Process Line | Solid line, sometimes with a zigzag or arrow | Dashed line (indicating mechanical/pneumatic link) | Solid line with a distinct 'P' label |
| Contact Representation | Drawn adjacent to or inside the circle | Drawn separately, linked by a dashed mechanical line or reference tag (e.g., -B12) | Drawn adjacent, similar to NEMA |
| State Designation | Often relies on text (COR / OOR) or arrow direction | Relies on standard NO/NC contact gaps and crossbars | Relied heavily on text annotations |
If you are working on equipment imported from Europe (like many modern CNC machines or industrial air compressors), expect IEC formatting. The pressure sensor will be labeled as a square block (often tagged -BP for Block Pressure), and its corresponding electrical contacts will be drawn pages away in the control circuit, linked only by the tag name. For a deeper dive into cross-referencing these tags, consult the Electrical Engineering Portal's symbol database.
Rows People Get Wrong (And How to Avoid Miswiring)
Misinterpreting a pressure switch symbol usually results in a motor that runs backward, a compressor that blows its relief valve, or a pump that burns out from dry running. Here are the three most common schematic misreads.
1. The "Normally" Confusion (Shelf State vs. Running State)
The word "Normally" in electrical schematics refers to the unpressurized, shelf state of the device (0 PSI), not the state it spends most of its time in while the machine is running.
A standard well pump pressure switch is wired in series with the 240V motor. When the tank is completely empty (0 PSI), the switch must be closed so the pump turns on the moment power is applied. Therefore, it is a Normally Closed (NC) switch. As pressure rises to the cut-out setting (e.g., 50 PSI), the switch opens (OOR). Many DIYers assume a well pump switch is "Normally Open" because the circuit is open while the tank is full. Wiring a replacement switch based on this false assumption will result in the pump running continuously until the pressure relief valve pops.
2. Misreading the Pilot Line Arrow
In NEMA diagrams, the arrow on the pilot line indicates the direction of pressure change required to actuate the switch, not the direction of fluid flow.
- Arrow pointing toward the contact: Actuates on pressure increase (Rise).
- Arrow pointing away from the contact: Actuates on pressure decrease (Fall).
If you are wiring a low-pressure cutout for an HVAC system, you need a switch that opens when pressure falls. Ensure the schematic arrow points away from the contact node.
3. Ignoring the Differential Gap (Deadband) Notation
Schematics for industrial compressors often include a small text box next to the symbol reading ΔP = 2 bar or Diff = 20 PSI. This is the deadband. If your replacement switch (like a basic Square D Pumptrol) has a fixed differential, but the schematic calls for an adjustable differential (like a Furnas 69 series), the motor will short-cycle or fail to reach target pressure. Always match the deadband specification, not just the NO/NC logic.
Safe Interpretation When Markings Are Faded or Missing
On older equipment, sun-faded blueprints or missing schematic stickers inside the control panel are common. Never guess the switch logic based on the wire colors connected to it; 120V control circuits and 240V line-voltage circuits often use identical black/red wire colors in older panels.
The Bench-Test Mapping Procedure
When the schematic is illegible, use a digital multimeter (DMM) like a Fluke 87V and a shop air compressor to map the switch physically.
- De-energize and Isolate: Turn off the breaker, verify dead with a non-contact voltage tester, and disconnect the wires from the switch terminals. Bleed the system pressure to 0 PSI via a manual drain valve.
- Identify the Terminals: Most switches have Line (L1/L2) and Load (T1/T2) terminals, plus a possible auxiliary contact block. Set your DMM to Continuity (the diode/beep setting).
- Test the Shelf State: Place probes across L1 and T1. If it beeps, the switch is Normally Closed (NC). If it reads OL (Open Line), it is Normally Open (NO).
- Apply Test Pressure: Connect a regulated shop air hose to the switch's process port (use a brass fitting to avoid stripping the plastic port on cheap switches). Slowly increase pressure while watching the DMM.
- Map the Actuation: Note the exact PSI where the DMM transitions from beep to OL (or vice versa). This is your cut-in or cut-out setpoint. Release the air and note the PSI where it transitions back. The difference is your deadband.
By physically mapping the switch, you bypass the need for a faded schematic entirely. Document your findings with a label maker and stick it inside the panel door for the next technician. Always verify that the switch's voltage and current ratings (e.g., 30A at 240V AC for a 3HP pump) meet or exceed the motor's Full Load Amps (FLA) before re-energizing.
