The Complete SPST Switch Symbol & Schematic Reference
The table below maps the exact visual characteristics of the SPST switch symbol across the most common drafting standards. Use this as your bench-side cheat sheet when deciphering wiring diagrams.
| Standard / Context | Visual Symbol Description | Text / ASCII Approximation | Typical Application |
|---|---|---|---|
| NEMA SPST-NO (Normally Open) | Straight horizontal line with a hinged lever (angled up) not touching the right terminal. | ---/ o | US residential lighting, basic industrial motor start circuits. |
| NEMA SPST-NC (Normally Closed) | Straight horizontal line with a hinged lever resting down on the right terminal. | ---\_o | Emergency stop circuits, interlocks, alarm triggers. |
| IEC 60617 SPST-NO | Gap in a horizontal line, with a bridging diagonal line angled up from the left pivot. | -- / -- | European/Global PLC schematics, modern automotive wiring. |
| IEC 60617 SPST-NC | Gap in a horizontal line, bridging diagonal line resting flat against the right contact. | -- \-- | Global safety relays, HVAC control boards. |
| Momentary SPST-NO | Standard NO symbol with a small arrow pointing toward the gap, indicating spring-return. | --/-> o | Pushbuttons, doorbells, reset switches, keyboard matrices. |
| Mercury / Tilt SPST | Standard SPST symbol enclosed in a small diamond or circle, sometimes with a tilt arrow. | --( / )-- | Legacy thermostats, tilt-sensors (largely replaced by solid-state MEMS). |
Regional Standards: NEMA, IEC, and Legacy UK Symbols
As of 2026, the global electrical industry largely operates on two dominant schematic languages. Knowing which standard applies to your region—and your specific equipment's origin—is critical for accurate interpretation.
NEMA (North America)
The National Electrical Manufacturers Association (NEMA) symbols dominate the United States, Canada, and Mexico. NEMA symbols are highly pictorial; the SPST switch symbol literally looks like a physical knife switch with a visible hinge dot and a lever. You will find these on almost all US residential blueprints, older industrial motor control centers (MCCs), and NEMA-standard equipment manuals. If you are working on a US-based 120V/240V branch circuit, expect NEMA.
IEC 60617 (Europe, Asia, and Global)
The International Electrotechnical Commission (IEC) standard 60617 uses a more abstract, minimalist approach. The hinge dot is often omitted, and the switch is represented simply by a line breaking a gap. IEC symbols are standard across the EU, UK, Australia, and most modern programmable logic controller (PLC) schematics globally. For a deep dive into component drafting, All About Circuits provides an excellent breakdown of how IEC symbols map to physical components.
Legacy UK (BS 3939)
If you are troubleshooting a panel in an older British facility (pre-1990s), you may encounter BS 3939 symbols. While largely superseded by IEC 60617 (which the UK adopted as BS EN 60617), legacy BS 3939 SPST symbols often feature a distinct 'loop' at the actuator end. Treat these with caution, as the surrounding wiring may also rely on deprecated UK color codes (e.g., red/yellow/blue for phases instead of modern brown/black/grey).
Common Misread Symbols and Faded Marking Safety
Even experienced electricians and makers misread switch symbols when schematics are poorly printed or when physical switch housings degrade. Here are the specific rows and scenarios people get wrong, and how to safely resolve them.
Rows People Get Wrong
- Confusing NO and NC States: The most common error is assuming 'Normally Open' means the switch is 'On'. In electrical theory, 'Open' means the circuit is broken (air gap). An SPST-NO switch is off at rest and requires physical force to turn on.
- SPST vs. SPDT (Single-Pole Double-Throw): An SPDT symbol looks like an SPST, but the lever can swing to two different target terminals (forming a Y-shape or a T-shape). If your symbol shows a common terminal branching to two distinct loads, it is SPDT, not SPST. Wiring an SPDT switch as an SPST by ignoring the third terminal can cause floating voltages or unintended backfeeding.
- Momentary vs. Maintained: Failing to notice the small return-arrow on a momentary SPST symbol. If you wire a maintained (latching) toggle switch into a circuit designed for a momentary pushbutton (like a PLC jog function), the machine will run continuously instead of stopping when the operator releases the button.
Safe Interpretation When Markings Are Faded or Missing
Physical toggle switches and pushbuttons often have their terminal diagrams stamped into the metal or molded into the plastic. Over years of use, heat, and UV exposure, these markings fade entirely. Never guess terminal assignments based on physical position alone.
If the switch controls a circuit over 50V AC or 120V DC, you must de-energize the circuit at the breaker panel before testing. Use a non-contact voltage tester and a multimeter to verify the circuit is dead. Lockout/Tagout (LOTO) procedures are mandatory in commercial environments. Local electrical codes (like the NEC) may require a licensed electrician for mains-level panel work.
The Multimeter Mapping Procedure:
- Isolate: Remove the switch from the circuit or ensure all wires are disconnected and the power is verified dead.
- Set Meter: Turn your multimeter to Continuity mode (the diode/soundwave icon).
- Probe and Toggle: Place probes on the suspected two terminals. Toggle the switch. If the meter beeps in one state and reads 'OL' (Open Loop) in the other, you have confirmed an SPST switch.
- Verify NO vs NC: If the meter beeps when the switch is in its natural, unpressed, or 'down' resting state, it is Normally Closed (NC). If it only beeps when you actively hold the toggle 'up' or press the button, it is Normally Open (NO).
SPST Switch Symbol FAQ
What does the symbol for SPST switch look like on a multimeter diagram?
On the internal schematic diagrams printed inside high-end multimeter manuals (like Fluke or Keysight), the SPST switch symbol is usually represented using the IEC 60617 standard. You will see it depicting the internal rotary selector switch ranges. It will appear as a simple line bridging a gap, often grouped in gangs (multiple SPST switches actuated by one rotary shaft) to show how turning the dial connects the shunt resistors and fuses to the probe inputs.
How is the SPST switch symbol different from a DPST switch symbol?
A DPST (Double-Pole Single-Throw) symbol is essentially two SPST switches drawn side-by-side, linked by a dashed mechanical line. This dashed line indicates that a single physical actuator (like a 240V water heater disconnect) mechanically throws both poles simultaneously. While an SPST breaks one hot wire (120V), a DPST breaks two hot wires (240V) at the exact same time, which is a critical safety requirement for split-phase and three-phase equipment.
Does the SPST switch symbol indicate which terminal is line and which is load?
No. The schematic symbol for an SPST switch is electrically symmetrical; it does not inherently dictate line (source) versus load (destination) orientation. However, in physical AC wiring practice (NEC guidelines), the 'line' hot wire should always connect to the bottom terminal of a standard single-pole toggle switch, and the 'load' wire to the top brass terminal. This ensures that when the switch is toggled 'off' (down), the internal mechanism drops away from the live source, reducing the risk of internal arcing across the gap.
Why does my European schematic show a different SPST symbol than my US textbook?
This is the difference between IEC 60617 (European/Global) and NEMA (US) drafting standards. US textbooks teach the NEMA pictorial style (a lever with a distinct hinge dot), while European schematics use the IEC abstract style (a simple diagonal line bridging a gap). Both represent the exact same physical electrical function. When designing PCBs or control panels for international export, most modern CAD software (like AutoCAD Electrical or EPLAN) allows you to toggle the symbol library between NEMA and IEC with a single click to match the target market's expectations.
