The Master Switch Symbol Reference Table
This table maps the most common switch types to their geometric representations in CAD software (like AutoCAD Electrical or EPLAN) and physical schematics. Note that while the visual geometry differs slightly between standards, the logical function remains identical.
| Switch Type | IEEE 315 / NEMA (US) Symbol | IEC 60617 (International) Symbol | Common Application |
|---|---|---|---|
| SPST (Single Pole, Single Throw) | Two dots connected by a hinged diagonal line. | A straight line broken by a diagonal slash with a hinge point. | Basic on/off lighting, simple disconnects. |
| SPDT (Single Pole, Double Throw) | One input dot, two output dots; hinged line connects input to one output. | One input line splitting into two angled paths with a moving contact. | US residential 3-way switches, motor reversing. |
| DPST (Double Pole, Single Throw) | Two parallel SPST symbols linked by a dashed mechanical line. | Two parallel broken lines with linked diagonal slashes. | 240V appliance disconnects, dual-pole safety interlocks. |
| DPDT (Double Pole, Double Throw) | Two parallel SPDT symbols linked by a dashed mechanical line. | Two parallel SPDT symbols with a mechanical linkage indicator. | US residential 4-way switches, DC motor polarity reversal. |
| Momentary NO (Normally Open) | SPST symbol with a spring-return arrow and 'NO' label. | Broken line with a spring symbol and 'NO' or '13/14' terminal marks. | Start buttons, doorbell switches, jog controls. |
| Momentary NC (Normally Closed) | SPST symbol with contact resting on the dot, spring-return arrow. | Continuous line broken by a wedge, spring symbol, 'NC' or '11/12' marks. | Stop buttons, emergency stops, safety limit switches. |
| Limit Switch | SPST/SPDT symbol with a roller or lever actuator graphic attached. | Standard switch symbol with a specific mechanical actuator line (roller/rod). | CNC machine home positions, conveyor belt end-stops. |
Regional & Standard Variants (IEC vs IEEE vs Old UK)
Assuming a single global standard is a fast track to miswiring a panel. The geometry of electrical switches symbols changes depending on the governing body and the era of the drawing.
IEEE 315 / ANSI Y32.2 (North America)
Dominant in the US and Canada. These symbols often rely on 'dots' to represent electrical junctions and physical contact points. NEMA standards heavily influence industrial control schematics here, favoring explicit mechanical linkage lines (dashed lines connecting multiple poles) to show that a single physical actuator moves multiple contacts simultaneously.
IEC 60617 (International / EU / Modern Global)
The IEC 60617 standard drops the 'dot' convention for switch contacts, relying instead on continuous lines and specific terminal numbering (e.g., 13/14 for NO, 11/12 for NC). IEC symbols are highly modular; a single physical contact block (like a Siemens 3SU1) is drawn exactly as it is wired, making troubleshooting significantly easier for technicians reading the schematic at the bench.
BS 3939 (Legacy UK)
Withdrawn in the 1980s in favor of IEC equivalents, but you will still encounter BS 3939 symbols in older UK industrial plants and maritime vessels. The primary difference is the use of distinct, often heavier, graphic strokes for isolators and a different convention for drawing thermal overload relays inline with switch contacts. If you are retrofitting a pre-1980 UK control panel, expect to translate these legacy symbols to modern IEC equivalents before ordering replacement contact blocks.
The 'Rows People Get Wrong' Field Notes
Schematics are only as good as the technician's ability to map them to physical hardware. Here are the most common misinterpretations I see on the jobsite.
Mistake 1: Confusing Momentary vs. Maintained Contacts
A standard SPST symbol without a spring-return graphic indicates a maintained (latching) switch. If you wire a maintained toggle switch into a circuit designed for a momentary pushbutton (like a PLC reset input), the machine will fault or latch into an unintended state. Always look for the small zig-zag spring line or the explicit 'momentary' text flag in the bill of materials.
Mistake 2: The Residential 3-Way / 4-Way Translation Error
DIYers frequently buy hardware based on schematic names rather than retail names. In US residential wiring, a '3-way switch' is electrically an SPDT switch. A '4-way switch' is electrically a DPDT switch wired internally as a crossover. If your schematic calls for an SPDT to control a light from two locations, you buy a 3-way switch at the hardware store, not a 3-pole industrial disconnect.
Mistake 3: Interpreting Faded Physical Limit Switch Markings
On industrial machinery, physical limit switches (like Allen-Bradley 802B or Schmersal units) get coated in oil and dust, obscuring the NO/NC terminal markings. Never guess based on the schematic alone; field wiring often deviates from the original prints.
Before probing any switch terminals, de-energize the circuit, apply Lockout/Tagout (LOTO), and verify zero energy with a Category III or IV rated multimeter. Once verified dead, set your meter to continuity (the diode/beep setting). Map the Common (COM) terminal to the NO and NC terminals by manually actuating the switch plunger or roller with a screwdriver. The terminal that beeps when actuated is NO; the one that beeps at rest is NC. Mark them with a silver paint pen immediately.
Frequently Asked Questions
What do the dots on electrical switches symbols mean?
In IEEE/NEMA schematics, a solid dot where two lines intersect indicates a physical, electrical junction (a splice or connection). If two lines cross without a dot, or if one line 'hops' over the other with a small semi-circle, they are electrically isolated and simply cross paths on the drawing. Note: IEC 60617 generally avoids intersection dots entirely, preferring to draw crossing lines at angles or with explicit terminal nodes to prevent confusion from faded print dots.
How do you read electrical switches symbols on a residential multi-way diagram?
Look for the traveler lines. In a multi-way lighting circuit, the power source enters the first SPDT (3-way) switch at the 'Common' terminal. The two remaining terminals (travelers) run to the next switch. If a DPDT (4-way) switch is in the middle of the run, the schematic will show the travelers crossing over internally when the switch is toggled, reversing the polarity of the traveler pair before they reach the final SPDT switch and the light fixture.
What is the symbol for a safety interlock or disconnect switch?
A safety disconnect or isolator is typically represented by an SPST or DPST symbol with an 'X' overlaid on the moving contact arm, or a manual handle graphic attached to the actuator line. In IEC standards, you will often see a specific 'Q' designator (e.g., Q1, Q2) next to the symbol, denoting it as a power circuit isolator rather than a control circuit switch (which uses 'S'). For more on standard reference designators, consult the Engineering ToolBox electrical reference charts.
Why does my schematic show a switch with a delayed-action symbol?
If you see a standard switch symbol with a small 'U' shaped delay graphic or a specific timer coil attached to the actuator, it represents a time-delay switch. This could be a thermal delay (like a time-delay relay on a motor starter) or a pneumatic dashpot. The symbol will specify whether the delay occurs on 'make' (closing the circuit) or 'break' (opening the circuit). Always verify the exact timing curve in the component datasheet before substituting a replacement part.
