Single Pole Single Throw Switch Symbol: IEC, NEMA & Wiring Guide

The single pole single throw (SPST) switch symbol represents the most basic electrical control: one circuit path, two states (open or closed). In North American (NEMA/IEEE 315) schematics, it appears as a hinged lever breaking a horizontal line. In international (IEC 60617) diagrams, it is drawn as a diagonal line with a pivot dot. Below is the complete reference for identifying, interpreting, and sourcing SPST switches across global standards.

The Complete SPST Switch Symbol Reference Table

Use this table to decode the specific SPST variant you are looking at on a schematic. The symbols below assume the switch is in its unactuated (normal) state.

Regional Standards: NEMA vs. IEC vs. Old UK

When reading a schematic, your first task is identifying the drafting standard. Mixing these up can lead to misinterpreting whether a switch is normally open (NO) or normally closed (NC).

NEMA and IEEE 315 (North America)

The NEMA standard abstracts the physical 'knife switch' into its symbol. The lever represents the physical blade. A line breaking the horizontal bus indicates an open circuit (NO). If the lever is drawn resting on the contact point, it is NC. This standard dominates US residential wiring diagrams and older industrial schematics.

IEC 60617 (International / Europe)

The IEC standard abandons the physical knife-switch analogy for pure geometric abstraction. The pivot point is a solid dot, and the moving contact is a straight diagonal line. A small semicircle at the pivot denotes manual actuation. If the diagonal line points away from the stationary contact (a small dot or bar at the end of the line), it is NO.

Old UK (BS 3939)

Before harmonizing with Europe via BS EN 60617, the UK used BS 3939. You will only encounter this on legacy equipment (e.g., 1980s British manufacturing machinery). It looks similar to IEC but often used a small 'X' or cross-hatching at the pivot instead of a solid dot. Treat these as IEC equivalents, but verify with a multimeter before applying power, as drafting inconsistencies were common in that era.

Rows People Get Wrong (And How to Fix Them)

Even experienced technicians misread specific nuances of the SPST symbol. Here are the most common schematic traps and how to avoid them.

1. Confusing SPST with SPDT

The Mistake: Assuming a switch with a lever and two contact dots is an SPST with an extra terminal.
The Reality: If the lever can pivot to touch either of two separate contact dots, it is a Single Pole Double Throw (SPDT). An SPST symbol will only ever have one stationary contact point on the side opposite the pivot.

2. Ignoring the Actuator Line Style

The Mistake: Treating a pushbutton SPST and a toggle SPST as identical in practice.
The Reality: The actuator line tells you how the switch behaves when you let go. A dashed line connecting the button to the lever indicates a momentary switch (returns to normal state when released). A solid line indicates a maintained switch (stays in the toggled position). Wiring a momentary pushbutton into a circuit expecting a maintained toggle will cause your load to drop out the second you release the button.

3. Overlooking Inductive Load Derating

The Mistake: Sizing a physical switch based purely on the schematic's amperage note without checking the load type.
The Reality: A schematic might call for a '15A SPST Switch'. If that switch controls a resistive heater, a standard 15A toggle is fine. If it controls an inductive motor, the inrush current and arcing upon opening will destroy a standard 15A switch in weeks. Always check the physical switch's inductive rating (often 1/2 HP or 5A) versus its resistive rating.

Safe Interpretation When Markings Are Faded or Missing

On the jobsite or bench, you will frequently encounter physical SPST switches where the schematic is missing, the silkscreen is worn off, or the terminal stamps (like 'COM', 'L1', or 'NO') are illegible. Never guess the wiring based on physical pin placement alone.

The Multimeter Verification Procedure

1. Isolate: Disconnect the wires from the switch terminals to prevent reading parallel circuit paths.
2. Set Meter: Turn your multimeter to Continuity mode (the diode symbol with sound waves) or the lowest Ohms range.
3. Probe: Place one probe on each of the two suspected terminals.
4. Toggle and Read:
   • State A: Meter reads 'OL' (Open Line) or infinite resistance.
   • State B: Flip the actuator. Meter reads < 1.0 Ω and beeps.
5. Conclusion: If it transitions cleanly between OL and < 1 Ω across exactly two terminals, it is an SPST. If you find a third terminal that shows continuity in one state but not the other, you are holding an SPDT.

Decision Path: Picking the Right SPST Switch for Your Project

Schematics tell you the logic, but you still have to buy the physical part. Use this decision tree to select the exact SPST switch for your build, terminating in specific, proven part numbers.

By matching the schematic symbol standard (NEMA vs IEC) to the physical environment and load type, you eliminate the most common points of failure in basic control circuits. Always verify faded physical switches with a continuity test, and never exceed the inductive ampacity rating of your chosen component.