An electric switch symbol represents a mechanical or solid-state device that opens or closes a circuit, but the exact drawing depends heavily on whether you are looking at an IEC 60617 schematic, an ANSI/IEEE logic diagram, or a residential NEC-style architectural plan. Misreading these symbols is a primary cause of miswired control circuits and blown fuses on the bench. Below is the direct reference data you need to identify, interpret, and safely verify switch symbols across global standards.
Complete Electric Switch Symbol Reference Table
The following table maps the most common switch types to their schematic representations. Use this as your primary lookup when tracing a control board or drafting a wiring diagram.
| Switch Type | IEC 60617 Symbol Description | ANSI/IEEE 315 Equivalent | Common Practical Use Case |
|---|---|---|---|
| SPST (Single Pole Single Throw) | A single line with a hinged gap (knife switch style) breaking one conductor. | Identical hinged gap, sometimes drawn with a manual actuator line. | Basic power disconnects, simple LED toggles, emergency stop circuits (when wired NO). |
| SPDT (Single Pole Double Throw) | A hinged line originating from a common node, pointing toward two separate target nodes. | Similar, but the common terminal is often explicitly labeled 'C' or 'COM'. | Motor reversing circuits, selecting between two power sources, residential 3-way lighting. |
| DPST (Double Pole Single Throw) | Two parallel SPST symbols linked by a dashed horizontal line. | Two parallel hinged gaps with a mechanical linkage dash. | 240V appliance disconnects, isolating both Line and Neutral in control panels. |
| DPDT (Double Pole Double Throw) | Two parallel SPDT symbols linked by a dashed horizontal line. | Two parallel SPDTs with a mechanical linkage dash. | Polarity reversal for DC motors, switching between two separate dual-wire circuits. |
| Pushbutton NO (Normally Open) | A horizontal line with a push-actuator above it; the contact sits *under* the line. | Often drawn as a simple gap with a push-arrow; 'NO' explicitly labeled. | Start buttons, doorbells, momentary trigger inputs for PLCs and microcontrollers. |
| Pushbutton NC (Normally Closed) | A horizontal line with a push-actuator above it; the contact sits *over* the line. | Gap with a push-arrow and a diagonal cross-line indicating closed state. | Stop buttons, safety interlocks, limit switches on machine guards. |
| 3-Way (Residential Architectural) | Not a standard schematic symbol; drawn as an SPDT in logic diagrams. | Drawn as an SPDT. In architectural plans, represented by an 'S' with a '3' or traveler lines. | Stairway lighting, hallway circuits controlled from two physical locations. |
Regional Variants and the "Rows People Get Wrong"
Switch symbols are not universally standardized across all drawing types. The IEEE 315 standard governs most North American electronic schematics, while IEC 60617 dominates European and international industrial controls. Meanwhile, residential wiring in the US follows NFPA 70 (NEC) conventions, which prioritize physical layout over logical schematic accuracy.
The Rows People Get Wrong
Even experienced makers and junior electricians frequently misinterpret these three specific symbol variations:
- 3-Way vs. SPDT: In residential wiring, a "3-way" switch is electrically an SPDT. Do not look for a unique "3-way" schematic symbol on a control diagram; look for the SPDT symbol. Conversely, on an architectural blueprint, an SPDT symbol might just mean a standard single-pole switch if the drafter was lazy. Always check the legend.
- NO vs. NC Pushbuttons: The IEC symbol for Normally Open (NO) has the contact under the horizontal line (pushing the actuator down closes the gap). Normally Closed (NC) has the contact over the line (pushing down breaks the connection). People wire these backward constantly, resulting in machines that start when you press "Stop".
- Momentary vs. Maintained: A switch symbol featuring a small hook or latch mechanism at the end of the actuator line indicates a maintained switch (it stays in position when released). If there is no hook, it is momentary (spring-return). Ignoring this detail will cause you to buy the wrong replacement part for a control panel.
If you inherit a machine or panel with faded terminal stamps (common on older Allen-Bradley or Square D industrial controls), never guess the switch state based on physical button color or worn symbols. Green does not universally mean NO in retrofitted systems. De-energize the circuit, lock out the breaker, and use a multimeter (like a Fluke 117) in continuity mode. Probe the Common (C) and the suspected NO/NC terminals. Actuate the switch manually. If it beeps before pressing, it is NC. If it beeps only while pressed, it is NO. Never trust a faded stamp on a safety interlock.
Frequently Asked Questions
What is the electric switch symbol for a 3-way switch on a wiring diagram?
On a logical schematic or wiring diagram, a 3-way switch is drawn using the standard SPDT (Single Pole Double Throw) symbol: a single input line (the common terminal) branching into two output lines (the travelers). On an architectural floor plan, however, it is typically denoted by the letter "S" with a subscript "3" (S₃) or by drawing two switch symbols connected by a dashed line representing the traveler wires.
How do I read an electric switch symbol when the physical switch markings are faded?
When physical markings are illegible, the schematic symbol is your only reliable guide, provided the diagram matches the as-built reality. Locate the switch on the diagram and identify the Common (C), Normally Open (NO), and Normally Closed (NC) nodes. If the diagram is also missing or untrustworthy, you must perform a dead-circuit continuity test with a digital multimeter. Map the terminals by testing for continuity in both the actuated and unactuated states, then label the physical switch with fresh tape or an engraver before re-energizing.
Is the IEC electric switch symbol different from the ANSI/NEMA standard?
Yes, though they are conceptually similar. The IEC 60617 standard (used globally and in Europe) relies heavily on geometric shapes and strict line placements (like the horizontal line with contacts above or below for pushbuttons). The ANSI/IEEE 315 and NEMA standards (used primarily in North America) often use more literal representations, such as drawing a physical push-button head or explicitly writing "NO" and "NC" next to the contacts rather than relying solely on geometric positioning. Always check the title block of the drawing to see which standard the engineer used.
What does the dashed line connecting two switch symbols mean?
A dashed horizontal or vertical line connecting two or more switch symbols indicates a mechanical linkage or a "ganged" switch. This means that a single physical actuator (like one toggle lever or one relay coil) operates multiple separate electrical poles simultaneously. For example, a DPDT (Double Pole Double Throw) relay will show two separate SPDT switch symbols connected by a dashed line, meaning when the coil energizes, both isolated circuits change state at the exact same time.
