The standard schematic symbol for a pressure switch consists of a basic switch contact (Normally Open or Normally Closed) actuated by a semicircle with a perpendicular line pointing toward the contact, representing the physical pressure diaphragm or bellows. In North America, this follows IEEE 315 and NEMA ICS 19 standards, while international diagrams rely on IEC 60617 conventions. Understanding these variations is critical before wiring well pumps, HVAC compressors, or industrial air systems.
Complete Reference Table: Pressure Switch Symbols by Standard
Before tracing wires on a control board or drafting a PLC ladder logic diagram, you must identify which drafting standard the original engineer used. The table below maps the most common pressure switch configurations to their specific NEMA/IEEE and IEC graphical representations, along with real-world applications.
| Switch Type & Actuation | NEMA / IEEE 315 Symbol Description | IEC 60617 Symbol Description | Typical Real-World Application |
|---|---|---|---|
| Normally Open (NO) Closes on pressure rise |
Standard NO contact (gap with no crossbar) actuated by a semicircle and line pointing at the movable contact. | NO contact (line gap) with a semicircle actuator attached via a dashed mechanical linkage line. | Well pump control (e.g., Square D Pumptrol 9013FHG). Closes to start the pump when tank pressure drops below the cut-in setpoint. |
| Normally Closed (NC) Opens on pressure rise |
Standard NC contact (gap with a perpendicular crossbar) actuated by a semicircle and line. | NC contact (overlapping angled lines) with a semicircle actuator and dashed mechanical linkage. | High-limit safety cutouts on air compressors or boiler steam limits. Opens the circuit to kill power if pressure exceeds safe thresholds. |
| Differential Pressure Actuates based on delta between two ports |
Two opposing semicircles (or a delta triangle) pointing at a central transfer contact (COM, NO, NC). | Two semicircles connected to a central pivot point, acting on a multi-pole changeover contact. | HVAC chiller oil pressure safety (e.g., Johnson Controls P70). Monitors the difference between oil pump discharge and crankcase pressure. |
| Manual Reset (High Limit) Requires physical button press to re-close |
NC contact with a semicircle actuator, plus a small manual reset lever symbol (often a small circle or 'R' flag) on the linkage. | NC contact with a semicircle and a distinct manual reset mechanical latch symbol (square block on the linkage). | Commercial refrigeration high-head pressure safety. Prevents automatic restarting after a dangerous pressure spike until a technician verifies the system. |
| Electronic Transducer 4-20mA or 0-5V output (Not a discrete switch) |
Often drawn as a circle with 'PT' (Pressure Transmitter) inside, outputting a dashed line to a PLC analog input module. | Circle with 'P' and an asterisk or analog wave symbol, connected to an analog input card. | VFD-driven booster pumps or precise pneumatic control where continuous pressure feedback is required rather than simple on/off hysteresis. |
Rows and Symbols People Get Wrong
When reading schematics, even experienced technicians misinterpret specific pressure switch symbols, leading to incorrect wiring or faulty ladder logic. Here are the most common points of failure.
The "Normal" State Paradox
The most frequent error involves misunderstanding the word "Normal" in Normally Open (NO) and Normally Closed (NC). In electrical schematics, "Normal" refers to the unactuated, shelf-resting state of the switch at zero pressure. It does not mean the normal running state of the machine.
For example, look at the NO row in the table above. A well pump pressure switch (like the ubiquitous Square D 9013FHG) is wired as a Normally Open switch. When the water tank is full and the system is sitting at 50 PSI (its "normal" resting state for the house), the switch is actually held open by the pressure. It only closes when pressure drops to 30 PSI to call for water. If you assume "Normally Open" means "Open during normal machine operation," you will fundamentally misunderstand the control logic.
Confusing Differential with Dual-Stage Switches
A differential pressure switch (Row 3) is often confused with a dual-stage or two-pressure switch. A differential switch has one electrical contact but two physical pressure sensing ports (usually labeled High and Low, or P1 and P2). The symbol shows two opposing semicircles because the diaphragm is being pushed from both sides. The switch actuates only when the difference between P1 and P2 exceeds the setpoint. If you wire P1 and P2 to the same manifold, the switch will never actuate, regardless of how high the system pressure climbs.
Missing the Manual Reset Latch
Safety cutouts (Row 4) feature a manual reset mechanism. On a schematic, this is indicated by a small latch symbol on the mechanical linkage line. If you replace a standard automatic-reset NC high-limit switch with a manual-reset variant (or vice versa) without checking the schematic symbol, you will either create a nuisance-tripping nightmare for the operator or bypass a critical safety lockout required by code.
Safe Interpretation When Markings Are Faded or Missing
On legacy equipment—like a 30-year-old Furnas air compressor or a sun-baked agricultural irrigation panel—the physical pressure switch cover may be missing, the schematic inside the lid is faded to white, and the switch itself has no visible NO/NC stamping. You must determine the switch state safely to replace it or troubleshoot the contactor coil circuit.
Pressure switches often control inductive loads (motors, compressors) directly or via contactor coils. Before opening any enclosure, de-energize the circuit at the main disconnect, apply Lockout/Tagout (LOTO), and verify zero voltage with a CAT III or CAT IV rated multimeter. Furthermore, mechanical pressure does not dissipate when you turn off the power. A compressed air tank or pressurized hydraulic accumulator holds lethal stored energy. Always safely bleed system pressure to zero before unthreading a pressure switch from its manifold.
Step-by-Step Continuity Verification
Once the system is electrically dead and mechanically depressurized (verified via a calibrated manifold gauge reading 0 PSI), use your multimeter to identify the switch type:
- Set your meter to Continuity or Ohms (Ω). A Fluke 117 or similar meter with an audible continuity beep is ideal for tight panel spaces.
- Isolate the switch. Pull the spade connectors off the switch terminals. Do not test through the circuit; parallel paths through a contactor coil will give you false low-resistance readings (often 15-50 ohms), masking the true state of the switch.
- Test at Zero Pressure. Place your probes across the two load-bearing terminals.
- If the meter reads OL (Open Loop) or infinite resistance, the switch is Normally Open (NO).
- If the meter reads < 1 Ohm and beeps, the switch is Normally Closed (NC).
- Apply Manual Actuation (if possible). If the system cannot be safely pressurized to test the actuated state, remove the switch cover. Use a non-conductive fiberglass alignment tool or a plastic zip-tie to gently press the diaphragm plate or toggle the mechanical linkage. The continuity state should invert. If it does not snap cleanly, the internal microswitch contacts are likely pitted or carbon-tracked and the unit must be replaced.
Identifying Electronic Transducers vs. Mechanical Switches
If you open a panel and find a cylindrical metal device with three wires instead of a plastic box with two heavy-gauge wires, you are not looking at a mechanical snap-action switch. You are looking at an electronic pressure transducer (Row 5 in the table). These require a 24V DC excitation voltage and output a 4-20mA or 0-5V analog signal to a PLC or VFD. Applying 120V AC line voltage to the signal wires of a Danfoss MBS 3000 transducer will instantly destroy the internal ASIC board. Always trace the three wires: one will be DC+ (often red or brown), one will be DC- / Common (blue or black), and the third will be the analog signal output (white or green). Consult the specific manufacturer datasheet before applying power.
