A 3 position selector switch symbol represents a multi-circuit rotary control device used to toggle between discrete operational states, most commonly depicted in IEC schematics as a switch contact matrix showing positions 1-0-2 (or I-O-II). Whether you are reading a motor control ladder diagram or an HVAC sequence of operations, correctly interpreting these symbols prevents miswiring and potential equipment damage.
The Complete 3 Position Selector Switch Symbol Reference
The table below maps the most common 3-position switch functions to their schematic representations. Read the Contact State column as Left-Center-Right (L-C-R) to understand which internal circuits close in each physical position.
| Switch Function | IEC 60617 Symbol Logic | NEMA / NFPA 79 Designation | Typical Bezel Marking | Contact State (L-C-R) |
|---|---|---|---|---|
| Standard 3-Position Maintained | Cross with 3 intersecting nodes (1-0-2) | Circle with 3 intersecting lines / Ladder logic contacts | 1 - 0 - 2 | L: Pins 1-2 close C: All open R: Pins 3-4 close |
| Hand-Off-Auto (H-O-A) | Matrix showing manual bypass vs. auto logic | H-O-A schematic with interlocking contacts | HAND - OFF - AUTO | L: Manual circuit C: All open R: Auto/PLC circuit |
| Spring Return from Right | Arrow pointing left from the right position | Spring symbol on the right node (momentary) | I - O - <II | L: Pins 1-2 close C: All open R: Momentary close |
| Forward-Off-Reverse | Reversing contactor matrix with electrical interlock | F-O-R schematic with mechanical interlock notation | FWD - OFF - REV | L: Forward contactors C: All open R: Reverse contactors |
Rows People Get Wrong (and How to Read Them)
Even experienced panel builders misinterpret specific nuances of the 3 position selector switch symbol. Here are the most common pitfalls:
Mistake 1: Assuming "0" (Off) Means All Contacts Are Open
In a standard 1-0-2 switch, the center "0" position typically opens the main power or control circuits. However, many industrial selector switches (like the Schneider Electric Harmony XB5 series) feature an early-make or late-break auxiliary contact. This auxiliary contact might remain closed in the "0" position to signal a PLC that the switch is actively in the Off state. Always check the manufacturer's cam diagram, not just the simplified schematic symbol.
Mistake 2: Confusing IEC Terminal Numbering with NEMA Pin Numbering
IEC 60617 uses functional logic numbering. A normally open (NO) contact is always designated with a base number ending in 3-4 (e.g., 13-14), and normally closed (NC) ends in 1-2 (e.g., 21-22). NEMA and older US ladder diagrams often just use sequential physical pin numbers (1, 2, 3, 4) based on the specific switch block's layout. If you wire an IEC-designed panel using NEMA sequential pin logic, you will cross your control circuits.
Mistake 3: Missing the Spring-Return Arrow
The spring-return symbol (a small curved arrow pointing away from one of the outer positions) is frequently overlooked. If you treat a spring-return switch as a maintained switch in your code or relay logic, you will experience unexpected state drops when the operator releases the knob.
Regional Standards: IEC 60617 vs. NEMA vs. Legacy UK
Which standard applies to you depends on your region and the origin of the machinery you are servicing.
- IEC 60617 (Global/EU/Modern International): The modern baseline. It uses functional matrices and standardized terminal numbering (13-14 for NO). If you are building or exporting panels in 2026, this is the standard you must follow.
- NEMA / NFPA 79 (United States): NFPA 79 governs industrial machinery in the US. NEMA symbols tend to be more pictorial, representing the physical device rather than just the electrical logic. You will see circles with intersecting lines and explicit mechanical interlock dashed lines.
- Legacy UK (BS 3939): Deprecated and replaced by IEC equivalents, but still found in pre-2000 British control panels. BS 3939 used crossed circles to represent rotary switches. If you are retrofitting an old UK facility, expect to see these legacy symbols and plan for a full schematic update.
Safe Interpretation When Markings Are Faded or Missing
On jobsites, you will frequently encounter older panels where the plastic bezel is cracked, the engraving is worn off, or the schematic inside the door is missing. Do not guess the switch function based on the knob shape. Use a multimeter to map the internal cams safely.
- De-energize and Verify: Lock out and tag out (LOTO) the main breaker. Verify the circuit is dead using a tested CAT III or CAT IV multimeter.
- Set to Continuity: Switch your multimeter to continuity/diode mode (the setting that beeps when resistance is typically under 30 ohms).
- Identify the Common (C) Pin: Place one probe on a suspected common terminal. Rotate the switch to the Left position.
- Map the Throws: Touch the other probe to adjacent pins. When you hear a beep, note the pin number. Move the switch to Center, then Right, repeating the process.
- Check for Spring Return: Turn the knob to the far Right position, then release it. If it snaps back to Center, you have a spring-return switch. Map the momentary continuity accordingly.
By mapping the physical continuity, you bypass the need for faded symbols and create an as-built truth table for your troubleshooting.
Frequently Asked Questions
What does the 1-0-2 symbol mean on a 3 position selector switch?
The 1-0-2 symbol indicates a standard 3-position maintained rotary switch. "1" represents the left position (typically closing circuit A), "0" represents the center Off position (opening all main circuits), and "2" represents the right position (closing circuit B). It is the IEC standard designation for basic dual-circuit selection.
How do I wire a 3 position selector switch for Auto-Off-Man motor control?
In a Hand-Off-Auto (or Man-Off-Auto) setup, the "Off" position opens the control circuit to the motor contactor. The "Hand/Man" position routes 120V/24V control power directly to the motor contactor coil, bypassing automation. The "Auto" position routes the same control power through a PLC relay or sensor dry-contact before reaching the coil. Always use a switch with an electrical interlock or auxiliary contacts to ensure the PLC cannot backfeed the manual circuit.
What is the difference between a maintained and spring-return 3 position switch symbol?
A maintained switch symbol (like 1-0-2) shows three stable positions; the knob stays wherever the operator leaves it. A spring-return symbol includes a small curved arrow pointing from one of the outer positions back to the center or opposite side. This indicates a momentary contact that automatically snaps back when released, commonly used for "Jog" or "Reset" functions in motor starters.
