The low pressure switch symbol you see on a schematic depends entirely on the drafting standard your facility uses. In North American electrical ladder diagrams (NEMA/IEEE 315), it is drawn as a standard switch contact with a diagonal-line bellows actuator, often tagged with 'P<' or 'LOW'. In European IEC 60617 schematics, the actuator is a square with a diagonal arrow. On process piping diagrams (P&ID), it is simply a circle tagged 'PSL'.
Master Reference Table: Low Pressure Switch Symbols by Standard
Before wiring a replacement cutout or tracing a fault on a compressor control board, identify which standard your schematic follows. Here is the complete breakdown of how low pressure actuation is represented across the three dominant engineering languages.
| Standard | Symbol Name / Tag | Visual Description | Typical Application |
|---|---|---|---|
| NEMA / IEEE 315 (US) | Pressure Switch (Low) | Standard NO/NC contact with a rectangular bellows actuator; a diagonal line crosses the rectangle, often with an arrow pointing away from the contact or labeled 'P<'. | HVAC control boards, well pump panels, industrial motor starters. |
| IEC 60617 (Global/EU) | Pressure Pilot (Low) | Switch contact with a square actuator containing a diagonal line and an arrow indicating the direction of actuation (usually pointing inward for pressure rise, outward for drop). | European-manufactured air compressors (e.g., Condor MDR series), PLC I/O diagrams. |
| ISA 5.1 (P&ID) | PSL (Pressure Switch Low) | A simple circle containing the letters 'PSL'. No electrical contact details are shown; it only represents the physical instrument in the piping system. | Process flow diagrams, oil/gas piping, chemical plant instrumentation. |
| JIC (Legacy US) | Pressure Switch | Similar to NEMA but often uses a simple semicircle or 'U' shape for the bellows actuator without the diagonal cross-line. | Older US machine tools and legacy hydraulic presses. |
Regional Variants and the 'Rows People Get Wrong' Trap
Knowing the symbol is only half the battle. The most dangerous mistakes happen when technicians misinterpret the state of the switch based on the symbol. Here are the specific rows and concepts people consistently get wrong when reading these diagrams.
The 'Normal' State Paradox
In electrical schematics, 'Normal' (as in Normally Open [NO] or Normally Closed [NC]) refers to the shelf state of the switch at 0 PSI, not the 'normal running state' of your machine.
- The Mistake: A technician sees a low-pressure cutout for an HVAC compressor (like a Johnson Controls P280) drawn as a Normally Closed (NC) contact. They assume 'NC' means the switch is closed when the system is running normally.
- The Reality: The P280 is electrically NC at 0 PSI (shelf state). When you pump the system up to 60 PSI (process normal), the bellows expands and the switch opens. If pressure drops to 20 PSI (fault state), it snaps back to its shelf state (closed) to trigger an alarm or lockout circuit. Always read the symbol as 'what happens when the pipe is empty and unpressurized'.
IEC Arrow Direction Confusion
On IEC 60617 diagrams, the diagonal arrow inside the square actuator indicates the direction of pressure change required to toggle the switch. An arrow pointing away from the switch contact means the switch actuates when pressure decreases (low pressure). An arrow pointing toward the contact means it actuates on pressure increase. Technicians frequently wire high-pressure cutouts into low-pressure circuits by ignoring this tiny arrow.
Safe Interpretation When Markings Are Faded or Missing
On legacy equipment—especially outdoor well pumps using Square D 9013-FSG2 switches or ancient industrial hydraulic presses—the paper schematics are often lost, and the physical switch cover markings are faded by UV exposure or oil mist. When you cannot rely on the symbol or the label, you must bench-test the physical component to determine its logic.
The Bench-Test Procedure for Unmarked Switches:
- De-energize and Isolate: Lock out the main breaker. Disconnect the line and load wires from the switch terminals. Verify dead with a CAT III multimeter.
- Set Up Continuity: Set your multimeter to the continuity/beep setting. Place probes across the two main power terminals (usually marked L1/T1 and L2/T2 on single-phase switches).
- Establish Baseline (0 PSI): With the system fully bled down to 0 PSI, note the meter reading. If it beeps, the switch is Normally Closed (NC). If it reads 'OL' (Open Loop), it is Normally Open (NO).
- Apply Pressure: Use a shop compressor and a regulated hose to slowly apply air pressure to the switch's port. Watch the multimeter and the pressure gauge simultaneously.
- Record the Actuation Point: Note the exact PSI where the meter changes state (beep to OL, or OL to beep). This is your cut-in or cut-out pressure. If it changes state at a low PSI (e.g., 15 PSI) and reverts at 30 PSI, you are holding a low-pressure actuation switch.
For authoritative guidance on standardizing these symbols in your own facility's documentation, refer to the NEMA Standards Portal for electrical ladder logic, or the ISA 5.1 Instrumentation Standards for process piping diagrams.
Low Pressure Switch Symbol FAQ
What is the difference between a generic pressure switch symbol and a low pressure switch symbol?
A generic pressure switch symbol (often just labeled 'PS' or 'P') only tells you that a switch is actuated by fluid pressure. It does not specify the direction of actuation. A specific low pressure switch symbol will include a modifier: in NEMA/IEEE, it is often an arrow pointing away from the contact or a 'P<' tag; in IEC, the internal arrow points outward; in P&ID, the tag explicitly reads 'PSL' (Pressure Switch Low) rather than just 'PS' or 'PSH' (High).
Does a low pressure switch symbol dictate Normally Open (NO) or Normally Closed (NC) wiring?
No. The symbol tells you the trigger condition (pressure dropping below a threshold), but the NO/NC designation is a separate part of the symbol. You will see low pressure switches drawn as both NO and NC depending on the safety logic. For example, a low-pressure alarm might use a NO contact (closes to sound the alarm when pressure drops), while a low-pressure motor lockout might use an NC contact (opens to break the contactor coil circuit when pressure drops, acting as a fail-safe).
How do I read a low pressure switch symbol on a P&ID vs an electrical ladder diagram?
On a P&ID (Piping and Instrumentation Diagram), the 'PSL' circle represents the physical instrument bolted to the pipe. It tells the pipefitter where to install it. It does not show you how to wire it. To wire it, you must cross-reference the P&ID tag (e.g., PSL-101) with the electrical ladder diagram, where you will find the actual NEMA or IEC switch contact symbol labeled 'PSL-101', which will show you whether the contact is NO or NC and which PLC input or relay coil it connects to.
What standard applies to my region for control panel wiring?
If you are in the United States or Canada, control panels are almost universally drafted using NEMA and IEEE 315 standards, and inspected to NFPA 79 or UL 508A. If you are in Europe, the UK, or working with imported machinery (like German-manufactured CNC routers), the schematics will use IEC 60617 symbols. Always check the title block in the bottom right corner of the schematic's first page; it will explicitly state the drafting standard (e.g., 'Drawn per IEC 60617' or 'JIC/NEMA Standard').
