The symbol for a selector switch on an electrical schematic is not universal. It changes entirely depending on whether the panel was drafted under IEC 60617 (International/European), IEEE 315 / NEMA (North American), or legacy British standards. At its core, a selector switch is a rotary or push-pull control device used to select between different circuit paths—most commonly seen in Hand/Off/Auto (HOA) pump controls or Forward/Reverse motor starters.
If you are looking at a North American NEMA ladder logic diagram, you will see a specific contact arc graphic with a rotary operator symbol. If you are looking at a modern IEC schematic, you will see a circle with a diagonal intersecting line and specific terminal numbering (like 13-14 for normally open). Below is the direct translation matrix to help you identify exactly what you are looking at.
The Master Selector Switch Symbol Reference Table
This table maps the physical switch type to its graphical representation in the two dominant global standards. Use this to decode the schematic taped inside your control panel door.
| Switch Type | IEC 60617 Symbol Geometry | IEEE 315 / NEMA Geometry | Contact Logic & Terminals | Typical Application |
|---|---|---|---|---|
| 2-Position Maintained (On/Off or Man/Auto) | Circle with a single diagonal line extending top-right. Solid dot on the right indicates maintained state. | Standard NO/NC contact symbol with a rotary 'pie-slice' operator graphic attached to the moving arm. | Maintained. IEC: 13-14 (NO), 21-22 (NC). Stays in position when released. | Enabling a control circuit, selecting local vs. remote speed control. |
| 3-Position Maintained (HOA: Hand/Off/Auto) | Circle with two diagonal lines (left and right). Dots on both ends indicate maintained in both extreme positions. | Two separate contact symbols stacked vertically, ganged to a single 3-position rotary operator graphic. | Maintained. Left pos closes Circuit A; Right pos closes Circuit B; Center opens both. | HVAC fans, water pumps, conveyor motors (Hand = manual run, Auto = PLC/thermostat control). |
| 2-Position Spring Return (Momentary Start/Stop) | Circle with one diagonal line. A small curved arrow points back to the center, indicating spring return to Off. | Contact symbol with a rotary operator graphic and a small curved return arrow drawn next to the arm. | Momentary. Returns to default (usually Off/NC) when the operator releases the knob. | Resetting faulted drives, momentary jog functions, acknowledge alarm buttons. |
| Key-Operated Selector (Maintained or Spring Return) | Standard selector symbol, but the diagonal line originates from a small 'keyhole' graphic (circle with a slotted rectangle) instead of a standard knob. | Standard contact with a rotary operator, but the operator circle contains a keyway notch graphic. | Same as above, but physically restricted. Often used for E-Stop resets or high-security mode selection. | Access control for high-voltage bypass, crane operation, fire pump controllers. |
Rows People Get Wrong (And Faded Schematic Survival)
Even experienced technicians misread selector switch symbols when the nuances of contact sequencing and mechanical return are misunderstood. Here are the most common traps and how to avoid them.
The 3-Position 'Center Off' Assumption
The most dangerous misinterpretation occurs with 3-position maintained switches (Row 2). Technicians frequently assume the middle position is always a safe 'Off' state that drops all power to the load. In 90% of HOA (Hand-Off-Auto) circuits, this is true: the center position opens both the manual contactor coil and the automatic relay coil. However, in specific pneumatic sequencing or reversing motor starters (Forward/Off/Reverse), a mechanically failed switch or a miswired replacement (like swapping an Eaton E50 with a different cam profile) can result in the center position inadvertently bridging a control circuit. Never assume the center position is electrically safe without verifying the specific cam truth table for that exact switch model.
Confusing Maintained vs. Spring Return
Look closely at the curved return arrow in the IEC and NEMA symbols. If the arrow is missing, the switch is maintained—it stays where you leave it. If the arrow is present, it is spring-return (momentary). Installing a maintained switch in a circuit designed for a spring-return 'Start' button will result in the motor running uncontrollably the moment the operator turns the knob, because the switch will not snap back to break the starting circuit.
Safe Interpretation When Markings Are Faded
In older panels, the ink on the schematic fades, and the crucial 'X' or solid dot indicating a closed contact becomes invisible. When you cannot read the symbol's contact state:
- De-energize and Verify: Turn off the main control breaker and verify dead with a tested CAT III multimeter.
- Identify the Commons: Look at the physical switch block behind the panel face. IEC switches use standard numbering: 13/14 are Normally Open (NO), 21/22 are Normally Closed (NC), and the 11/12 or 9/10 terminals are the Common (C) inputs.
- Map Continuity: Set your DMM to continuity mode. Place one probe on the Common terminal and the other on the NO terminal. Rotate the switch through all positions. The meter should only beep in the specific position indicated by the physical cam.
- Check for Welded Contacts: If the meter shows continuity on a contact that should be open in the 'Off' position, the internal contacts are likely welded shut from inductive arcing. Replace the contact block immediately (e.g., Siemens 3SU1 contact modules).
Regional Standards: IEC, NEMA, and Legacy UK Variants
To correctly interpret the symbol for a selector switch, you must first identify which drafting standard the engineer used. Mixing up IEC and NEMA symbols can lead to catastrophic wiring errors, particularly regarding coil representations and terminal numbering.
IEC 60617 (International, Europe, Modern UK, Australia)
The IEC 60617 standard relies on a strict alphanumeric terminal numbering system rather than purely graphical line art. The switch symbol itself is minimalist (circles and lines), but the context is provided by the terminal numbers. For example, if you see a selector switch symbol routing to terminals '13' and '14', you instantly know it is a Normally Open contact block, regardless of how the graphic is drawn. Furthermore, IEC schematics draw relay and contactor coils as rectangles.
IEEE 315 / NEMA (North America)
In the US and Canada, schematics follow IEEE 315 and NEMA conventions. The graphics are more pictorial. A selector switch is drawn with a distinct 'pie-slice' or rotary arc to visually differentiate it from a pushbutton (which uses a straight line with a perpendicular strike plate). NEMA schematics do not rely on terminal numbers like 13-14; instead, they use wire reference numbers (e.g., Wire 104 to Wire 105) and ladder logic rung numbers. NEMA draws relay coils as circles. If you see circular coils on your drawing, you are looking at a NEMA-style diagram, and you must interpret the selector switch symbols using the pictorial NEMA column in the table above.
Legacy BS 3939 (Old UK / Pre-1995 Plants)
If you are troubleshooting an older facility in the UK, you may encounter BS 3939 symbols. While largely superseded by IEC 60617, BS 3939 used distinct graphical variations, such as drawing the rotary operator as a solid black semi-circle rather than an outlined arc. If the schematic looks like a hybrid of pictorial graphics and early alphanumeric tags, treat it as legacy BS. For deeper study on cross-referencing these legacy and modern symbols, All About Circuits provides an excellent breakdown of historical wiring symbol evolution.
When retrofitting or expanding an existing panel, always draft your new additions using the exact same standard as the original schematic. Introducing IEC terminal-numbered switch symbols into an existing NEMA ladder logic diagram will confuse the next technician who has to troubleshoot the panel at 2:00 AM.
