Electrical pressure switch symbols translate physical fluid-pressure thresholds into logical electrical states on a schematic. Whether you are reading a NEMA ladder diagram for a residential well pump or an IEC fluid-power print for an industrial injection molding machine, the symbol dictates if the contacts are Normally Open (NO) or Normally Closed (NC) at zero pressure, and whether the setpoint is field-adjustable. Below is the definitive reference for interpreting these symbols and selecting the correct physical component.
Complete Electrical Pressure Switch Symbols Reference Table
The table below maps the most common pressure switch configurations across the three dominant drafting standards. Read the NEMA column for US/Canadian ladder logic, the IEC column for international machine schematics, and the ISO column for pure fluid-power (hydraulic/pneumatic) diagrams.
| Switch Configuration | NEMA Symbol (US/Canada) | IEC 60617 Symbol (Global) | ISO 1219 Actuator (Fluid Power) | Practical Meaning in Circuit |
|---|---|---|---|---|
| NO, Adjustable | NO contact with a diagonal arrow crossing the switch arm | NO contact box with a diagonal arrow on the actuator line | Spring-return pilot line with an adjustable diagonal arrow | Contacts close when pressure rises above the setpoint. Setpoint can be tuned via a screw. |
| NC, Adjustable | NC contact with a diagonal arrow crossing the switch arm | NC contact box with a diagonal arrow on the actuator line | Spring-return pilot line with an adjustable diagonal arrow, NC electrical output | Contacts open when pressure rises above the setpoint. Used for high-pressure safety cut-offs. |
| NO, Fixed | NO contact with a straight vertical pilot line (no arrow) | NO contact box with a straight actuator line | Fixed pilot line actuator, NO output | Triggers at a factory-set pressure. Cannot be adjusted in the field. |
| Differential (Deadband) | Two opposing arrows on the actuator spring symbol | Actuator line with a hysteresis loop symbol or dual arrows | Dual-spring pilot actuator symbol | Switch has distinct cut-in and cut-out pressures (e.g., 40 PSI close, 60 PSI open). |
Regional Standards: Which Symbol Set Applies to Your Panel?
Misidentifying the drafting standard is the fastest way to miswire a control panel. The physical switch might be identical, but the way it is drawn on the schematic changes based on the region and the industry.
- NEMA (National Electrical Manufacturers Association): Dominant in North America for general electrical controls. NEMA symbols focus on the electrical contact state. The pressure actuator is usually drawn as a simple line or spring intersecting the contact arm. Refer to NEMA ICS 2 standards for formal definitions.
- IEC 60617 (International Electrotechnical Commission): The global standard for machine tools and automated equipment. IEC symbols separate the electrical function (the contact box) from the actuating method (the pilot line). The diagonal arrow for 'adjustable' is strictly standardized in IEC literature.
- ISO 1219-1: Specifically for fluid power (hydraulics and pneumatics). ISO symbols focus heavily on the mechanical actuation method (e.g., a pilot pressure line with a spring return) rather than just the electrical output.
- Legacy UK (BS 3939): Obsolete since the 1990s but still found in older British plants. These symbols often used a semi-circle for the switch contact rather than the modern straight line. If you see semi-circles, assume the drawing predates 1995 and verify the physical switch state with a meter.
The 'Rows People Get Wrong' Notes
Even experienced technicians misread specific variations of pressure switch symbols. Watch out for these three common traps:
1. Resting State vs. Actuated State
The most critical rule in schematic reading: symbols always show the device in its unpressurized, resting state. A Normally Open (NO) pressure switch symbol means the contacts are open when the system is at 0 PSI. When the system pressurizes and hits the setpoint, the switch actuates and the contacts close. Never assume the symbol shows the 'running' state of the machine.
2. The Diagonal Arrow (Adjustable) vs. Straight Line (Fixed)
Technicians often ignore the small diagonal arrow crossing the actuator line. If the arrow is present, the switch has an adjustable setpoint screw. If the line is straight, it is a fixed, factory-sealed switch. Ordering an adjustable switch for a fixed-symbol application adds unnecessary cost and introduces the risk of field tampering.
3. Pilot Line vs. Electrical Line Confusion
In ISO 1219 fluid-power diagrams, a dashed line indicates a pneumatic/hydraulic pilot signal, while a solid line indicates an electrical conductor. A common mistake is treating a dashed pilot line feeding a pressure switch actuator as an electrical wire, leading to catastrophic wiring errors.
Safe Interpretation When Markings Are Faded or Missing
On aging equipment, the physical label on the pressure switch may be faded, painted over, or missing entirely. If the schematic is also unavailable, you must determine if the switch is NO or NC using empirical testing.
Testing Procedure:
- Disconnect the wiring from the switch terminals to isolate it from the circuit.
- Set your digital multimeter to Continuity or Resistance (Ω).
- Place probes across the two primary terminals.
- With the system fully depressurized (0 PSI), read the meter.
- If the meter reads < 1 ohm (or beeps), it is a Normally Closed (NC) switch.
- If the meter reads OL (Open Loop / Infinite), it is a Normally Open (NO) switch.
- Apply pressure using a hand pump or system compressor while monitoring the meter. The state should flip exactly at the cut-in threshold.
Decision Path: Selecting and Specifying the Right Switch
Use this decision matrix to translate your schematic requirements into a concrete physical part number. Do not guess; match the application environment to the switch architecture.
| If Your Application Is... | And The Schematic Requires... | Then Specify This Exact Part / Series |
|---|---|---|
| Residential well pump or light commercial water booster (up to 100 PSI) | NO, Adjustable, Differential (40/60 PSI deadband) | Square D 9013FSG2 (The industry standard, NEMA 1 enclosure, 1/4' NPT) |
| Industrial hydraulic power unit (up to 5,000 PSI) | NO or NC, Adjustable, Fixed deadband, high shock tolerance | Gems Sensors 3100 Series (Solid-state or snap-action, IP67, 1/4' NPT) |
| Shop air compressor (up to 175 PSI, motor cutoff) | NC, Adjustable, Differential, with unloader valve port | Condor MDR 3 (Includes integrated mechanical unloader for motor start protection) |
| Low-pressure pneumatic logic or vacuum conveying (0-30 PSI) | NO, Fixed, High sensitivity | SMC IS1000 Series (Precision pneumatic switch, metric fittings) |
Default Recommendation: If you are replacing an unlabeled switch on a standard 120V/240V residential or light-commercial water system and the schematic simply shows a generic adjustable NO pressure switch, buy the Square D 9013FSG2. It is the most widely available, universally compatible diaphragm-actuated switch in North America, typically priced between $25 and $40, and its internal 40/60 PSI differential spring is pre-set for standard bladder tank operations.
