Pressure Switch Symbol
Definition: The Pressure Switch symbol represents an electromechanical or electronic device that opens or closes an electrical circuit when the sensed fluid pressure reaches a preset threshold, depicted in schematic diagrams per IEC 60617 and ANSI Y32.2/IEEE 315 as a switch symbol combined with a pressure-sensing actuator indication.
Also known as: PSW, pressure-actuated switch, pressure-operated switch, pressure transducer switch, pneumatic pressure switch.
What the Pressure Switch symbol means
The Pressure Switch symbol in a circuit diagram indicates a switch whose contacts are actuated by fluid pressure (pneumatic or hydraulic) reaching a defined set point. Unlike a manually operated switch, the pressure switch opens or closes automatically in response to system pressure, providing protection, sequencing, or feedback control without operator intervention.
Pressure switches appear in wiring diagrams alongside their pneumatic or hydraulic counterparts: the electrical symbol shows the switch contacts and activation mechanism, while the fluid-power diagram shows the pressure-sensing connection. The symbol communicates that the downstream electrical circuit is enabled or disabled based on a pressure condition in the connected fluid system.
How to identify the Pressure Switch symbol
The pressure switch symbol is drawn as a diagonal line (the movable contact blade) connected between two fixed contact points (the terminal lines), with a small upward-pointing arrow or 'P' annotation to indicate pressure actuation. The diagonal blade is angled from lower-left to upper-right in the normally-open (NO) configuration, or shown bridging two contacts in the normally-closed (NC) configuration. In IEC-style diagrams the pressure actuator is shown as a short arrow attached to the contact blade; in ANSI-style diagrams the same arrow or 'P' label indicates pneumatic or hydraulic actuation.
Function in a circuit
A pressure switch monitors fluid pressure through an internal diaphragm, piston, or bourdon tube connected to the sensing port. When pressure rises to the switch's set point (cut-in or cut-out pressure), the sensing element deflects, mechanically actuating the electrical contacts to open or close the circuit. This action can start or stop a motor, trigger an alarm, activate a solenoid valve, or signal a PLC input, making the pressure switch an essential device for automatic pressure-based control and protection in pneumatic, hydraulic, and process systems.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-07 covers switching device symbols. A pressure switch is represented as a switch contact (IEC 60617-07) with a mechanical actuator symbol indicating pressure operation — a short arrow or 'P' annotation per IEC convention. The sensing port is shown as a line leading to the fluid system. |
|---|---|
| ANSI/IEEE 315 | IEEE 315-1975 (ANSI Y32.2) defines pressure-operated switches using the standard switch symbol with an attached actuator descriptor ('pressure' or 'P') on the operating mechanism. The ANSI symbol for the switch contact is a diagonal line between two terminals. |
| Key difference | IEC 60617 and ANSI Y32.2/IEEE 315 symbols for the pressure switch are functionally identical; the main visual difference is that IEC diagrams may show the actuator mechanism more explicitly with a labelled arrow, while ANSI diagrams use a 'P' or 'PRESS' annotation near the contact. |
Terminals / pins
| Pin | Name |
|---|---|
| in | In |
| out | Out |
Typical values
Set-point pressure range varies widely: HVAC low-pressure switches 0.05–0.5 MPa (0.5–5 bar); industrial hydraulic switches 0.5–35 MPa (5–350 bar); pneumatic control switches 0.05–2 MPa (0.5–20 bar). Contact rating typically 1–15 A at 120/240 V AC or 24–250 V DC.
Where the Pressure Switch symbol is used
- Air compressor automatic start/stop control, switching the motor off when tank pressure reaches cut-out and on at cut-in set point
- Hydraulic press circuits, confirming clamping pressure before initiating the work cycle via PLC input
- HVAC refrigeration systems, detecting low or high refrigerant pressure to protect the compressor
- Water pump systems, starting the pump when supply pressure drops below set point (pressure-tank systems)
- Pneumatic conveying systems, detecting blockages or low supply pressure and triggering alarms
- Fire suppression systems, sensing sprinkler line pressure drop to trigger the fire alarm panel
Example
In an air-compressor control circuit, the pressure switch symbol connects between the L1 supply line and the motor contactor coil. When tank pressure drops below 6 bar (cut-in), the NO contacts of the pressure switch close, energising the contactor to start the compressor motor. When pressure reaches 8 bar (cut-out), the contacts open, stopping the motor.
Key facts
- The Pressure Switch symbol is a switch contact symbol (diagonal line between two terminals) with a pressure-actuator annotation (arrow or 'P' label), per IEC 60617-07 and ANSI Y32.2/IEEE 315.
- Pressure switches are available in normally open (NO) and normally closed (NC) configurations; NO contacts close when pressure reaches the set point, NC contacts open.
- The standard designator for a pressure switch in electrical schematics is PSW or PS followed by a reference number (e.g. PSW1).
- The schematic symbol has two electrical terminals: In and Out (the switch contacts); a third sensing port connects to the fluid system (pneumatic or hydraulic).
- Pressure set points are measured in pascals (Pa), bar, or psi. Industrial pressure switches for pneumatic systems typically operate in the range 0.1–2 MPa (1–20 bar).
- Differential pressure switches monitor the difference between two pressures (e.g. across a filter) and actuate when the differential exceeds a set limit, indicating filter blockage.
- Pressure switches differ from pressure transducers: a pressure switch provides a binary on/off output, while a transducer provides an analogue output (0–10 V or 4–20 mA) proportional to pressure.
Diagrams that use this symbol
- compressor wiring diagram
- refrigerator compressor wiring diagram
- oil pressure sensor wiring diagram
- pressure sensor diagram
- oil pressure switch diagram
- air compressor wiring diagram
- oil pressure switch wiring diagram
- 220 volt pressure switch wiring diagram
Frequently asked questions
What does the pressure switch symbol look like?
The pressure switch symbol shows a diagonal contact blade between two terminal lines (the standard switch contact symbol) with a short upward arrow or 'P' label indicating pressure actuation. Normally-open (NO) versions show the blade not bridging the contacts; normally-closed (NC) versions show the blade touching both contacts.
What does the pressure switch symbol mean in a wiring diagram?
The pressure switch symbol means the circuit at that point is switched on or off by fluid pressure. When the connected pneumatic or hydraulic system pressure reaches the switch's set point, the contacts change state, starting, stopping, or signalling the connected electrical circuit.
What is the difference between a normally open and normally closed pressure switch?
A normally open (NO) pressure switch has open contacts at rest (below set-point pressure) and closes when pressure reaches the set point, completing the circuit. A normally closed (NC) pressure switch has closed contacts at rest and opens when pressure reaches the set point, breaking the circuit.
What standard defines the pressure switch symbol?
IEC 60617-07 (Switching Devices) covers the switch contact symbol used for pressure switches, with a pressure-actuator annotation. ANSI Y32.2 / IEEE 315-1975 provides the equivalent North American symbol. Both standards produce nearly identical representations.
What is the designator letter for a pressure switch?
The standard designator for a pressure switch is PSW or PS, followed by a sequential number such as PS1 or PSW2. In process and instrumentation diagrams (P&IDs), the ISA 5.1 standard uses the instrument tag format PS (Pressure Switch) with a loop number.
What are the pins on a pressure switch schematic symbol?
The electrical symbol shows two contact terminals: In and Out, which are the switch contacts that open or close based on pressure. The physical device also has a pressure-sensing port (connected to the fluid system) but this is shown in the fluid-power diagram rather than the electrical symbol.
How does a pressure switch differ from a pressure sensor or transducer?
A pressure switch provides a binary (on/off) electrical output when pressure crosses a set threshold, making it suitable for simple control and protection tasks. A pressure transducer or sensor provides a continuous analogue signal (e.g. 4–20 mA or 0–10 V) proportional to pressure, enabling measurement and closed-loop control.
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