The universal high pressure switch symbol depends entirely on the drafting standard governing your schematic. In ISA 5.1 process control diagrams, it is a circle tagged PSH (Pressure Switch High). In IEC 60617 and NEMA electrical ladder diagrams, it is depicted as a pressure-actuated contact (a circle with a 'P' and a switch mechanism) annotated with an H or a specific setpoint (e.g., 'Opens at 400 psi'). In HVAC refrigeration schematics, it is frequently abbreviated as HPS inside a square.

Below is the definitive reference for identifying, interpreting, and replacing these switches across different engineering disciplines.

The Core Symbol Reference Table

Use this table to decode the symbol on your P&ID (Piping and Instrumentation Diagram), electrical ladder logic, or fluid power schematic.

Standard Symbol Graphic Description Standard Tag / Label Typical Application
ISA 5.1 (Process) Circle with a horizontal line, connected to a process line. No internal switch graphic. PSH (Pressure Switch High) P&IDs, chemical plants, oil & gas process control.
IEC 60617 (Electrical) Circle with a diagonal line and a 'P', actuating a standard NO/NC contact line. -B or -P with 'H' annotation European/International electrical control panels, PLC I/O drawings.
NEMA ICS 19 (Electrical) Circle containing a 'P', with a mechanical linkage line to a switch contact. PS (often with setpoint text) North American industrial motor controls, HVAC ladder diagrams.
ISO 1219 (Fluid Power) Square with a diagonal arrow, actuated by a pilot line (not an electrical contact). Pressure Sequence Valve Hydraulic/pneumatic circuit diagrams (not electrical wiring).
HVAC/R Specific Square or circle, sometimes with a bellows graphic inside. HPS or HP Commercial refrigeration, chiller, and residential AC wiring diagrams.

Regional and Standard Variants Explained

Misinterpreting a symbol because you are applying the wrong regional standard is a primary cause of wiring errors and safety trips. Here is how the standards diverge in practice.

Pro-Tip for Panel Builders: If you are wiring a control panel in North America, default to NEMA ICS 19 symbols for your ladder logic. If the panel is destined for a CE-marked European facility, the OEM will mandate IEC 60617 symbols. Mixing NEMA and IEC contact symbols in the same schematic is a fast track to failing a UL or CE panel inspection.

ISA 5.1 vs. Electrical Schematics

The ISA 5.1 PSH bubble does not tell you if the electrical contact is Normally Open (NO) or Normally Closed (NC). It only tells the process engineer that 'when pressure gets too high, this instrument triggers.' To find out if it opens or closes the circuit, you must look at the corresponding electrical ladder diagram (using NEMA/IEC symbols) or check the instrument's specific datasheet.

The HVAC/R 'HPS' Shortcut

In residential and light commercial HVAC, draftsmen often bypass formal NEMA/IEC rules and simply draw a box labeled HPS. By industry convention in HVAC, an HPS is almost always a Normally Closed (NC) switch that opens on pressure rise. It is wired in series with the compressor contactor coil. When refrigerant head pressure exceeds the safe limit (e.g., 610 psi for R-410A), the switch opens, dropping the coil voltage and killing the compressor to prevent a catastrophic rupture.

Rows People Get Wrong (Common Confusions)

When reading or drafting these symbols, technicians consistently misinterpret three specific scenarios.

  1. Confusing PSH with PSHH: In ISA 5.1, PSH (Pressure Switch High) is typically a control-level alarm or a standard cutout. PSHH (Pressure Switch High-High) is an emergency safety trip, often wired to an independent Safety Instrumented System (SIS) or a hardwired emergency stop relay. Never wire a PSHH trip into a standard PLC logic loop; it must be hardwired for failsafe independence.
  2. Assuming NO vs. NC Defaults: A common mistake is assuming a high-pressure switch is Normally Open and closes on pressure rise. In 95% of safety applications, high-pressure switches are Normally Closed (NC) and open on rise. This ensures that if the wire breaks or the switch fails, the circuit opens and the equipment shuts down (failsafe design). If your schematic shows a NO contact for a high-pressure safety cutout, question the design.
  3. Misreading ISO 1219 Fluid Power Symbols: Maintenance techs sometimes see the ISO 1219 hydraulic pressure switch symbol (a square with pilot lines) on a combined mechanical/electrical print and try to wire it as an electrical contact. That symbol represents a mechanical hydraulic valve actuator, not an electrical microswitch. Look for the IEC circle-with-a-'P' for actual electrical wiring.

Decision Path: Identifying and Replacing an Unknown Switch

Use this decision tree to terminate your troubleshooting and select the exact replacement part or drafting symbol.

Condition / Scenario Action / Decision Concrete Pick / Output
Drafting a P&ID for a chemical process Use the ISA 5.1 bubble format. Tag as PSH-101 (or next available loop number).
Drafting an electrical ladder diagram (US) Use NEMA contact symbol, specify NC. Draw NEMA 'P' circle, label PS1 (NC, Opens at 400 psi).
Replacing a faded HVAC/R high-pressure cutout Verify NC operation, match flare/fitting size. Buy Johnson Controls P70A-167C (Standard 1/4" SAE flare, NC, R-410A compatible).
Replacing an industrial air compressor cutout Verify NO/NC, check voltage rating of contacts. Buy Furnas / Siemens 69BQ series pressure switch (rated for 120/240V AC motor loads).

Safe Interpretation When Markings Are Faded or Missing

In older mechanical rooms, the physical tag on a high-pressure switch is often corroded, painted over, or missing entirely. Never guess the setpoint or the contact state based on the physical size of the switch.

Safety Warning: High-pressure switches on commercial chillers or ammonia (R-717) systems can hold lethal pressures even when the compressor is off. Never attempt to remove a pressure switch from a system without first recovering the refrigerant, isolating the valve, and verifying the gauge reads 0 psi. Defeating or jumpering a high-pressure switch to 'get the system running' violates NEC Article 440 and can result in a catastrophic vessel explosion.

The Bench-Test Verification Protocol

When the symbol on the schematic doesn't match the physical switch, or the switch is unmarked, follow this exact procedure to map its behavior:

  1. Isolate and Disconnect: Lock out the compressor contactor. Disconnect the two wires from the switch terminals.
  2. Continuity Test (Static): Set your multimeter to continuity/ohms. Place probes on the switch terminals. If it reads < 1 ohm, it is currently closed. If it reads 'OL' (Open Loop), it is currently open. Note the current system pressure on the manifold gauge.
  3. Dynamic Trip Test: If the system is running and you are safely monitoring the cutout, watch the multimeter. When the pressure hits the trip point, does the meter snap to 'OL' (Opens on rise = NC) or snap to 0 ohms (Closes on rise = NO)?
  4. Map to Schematic: If the switch Opens on Rise (NC), it belongs in series with the contactor coil. If it Closes on Rise (NO), it is likely feeding a PLC input or an alarm light, not directly breaking the motor circuit.

By strictly adhering to the standard-specific symbols and verifying failsafe logic on the bench, you eliminate the ambiguity that leads to nuisance trips or, worse, failed safety cutouts during a high-head-pressure event.