The 2 position selector switch symbol represents a rotary actuator that toggles between two discrete electrical states, such as ON/OFF, MAN/AUTO, or RUN/STOP. In standard schematics, it is drawn as a standard switch contact with a superimposed rotary knob indicator or a circle with a diagonal wiper line. Below is the definitive reference for identifying, wiring, and troubleshooting these components across global standards.
The Complete 2 Position Selector Switch Symbol Reference Table
This table maps the schematic symbols to their physical contact block configurations and real-world applications. Use this to decode prints from Rockwell Automation 800T Technical Data sheets or IEC-compliant panel drawings.
| Switch Type | IEC 60617 Symbol Description | NEMA / IEEE 315 Description | Standard Contact Block Config | Common Application |
|---|---|---|---|---|
| 2-Pos Maintained (ON-OFF) | Circle with diagonal line, no arrows | SPST/SPDT switch with rotary handle graphic | 1NO-1NC or 2NO | Main control power isolation, pump enable |
| 2-Pos Maintained (MAN-AUTO) | Circle with diagonal line, positions labeled 1-2 | Two-position switch with dotted mechanical link | 2NO (isolated circuits for PLC vs Hardwired) | HVAC fan control, VFD bypass systems |
| 2-Pos Spring-Return (Momentary) | Circle with diagonal line, arrow pointing to default | Switch with rotary handle and spring-return arrow | 1NO-1NC (returns to default state) | Motor jog, alarm reset, valve inching |
| 2-Pos Key-Release (Keyed) | Circle with diagonal line, key symbol overlay | Switch with keyhole graphic and rotary handle | 1NO-1NC or 2NO (key removable in one pos) | Security interlocks, crane master control |
Regional Standards: IEC vs. NEMA vs. Legacy UK
Interpreting the 2 position selector switch symbol correctly requires knowing which drafting standard the panel builder used. Misreading a NEMA print as an IEC print can lead to wiring a momentary circuit as a maintained one, causing runaway machinery.
- IEC 60617 (Global/Europe/Modern US): Uses a minimalist approach. The actuator is a circle with a diagonal line. The crucial differentiator is the presence of an arrow indicating a spring-return (momentary) action. If there is no arrow, it is strictly maintained.
- NEMA / IEEE 315 (North America): Tends to be more pictorial. The symbol often includes a literal drawing of a rotary handle or a dotted line connecting multiple switch poles to indicate they are operated by the same shaft. NEMA Electrical Graphical Symbols standards emphasize showing the mechanical interlock explicitly.
- Legacy UK (BS 3939): Largely superseded by IEC, but still found in older British industrial plants. It used a distinct cross-hatched rotary indicator and often relied on text labels (e.g., 'I' and 'II') rather than graphical arrows to denote switch states.
Rows People Get Wrong (and Faded Label Troubleshooting)
When working at the bench or in the field, the schematic is only half the battle. Here are the most common pitfalls when interpreting and wiring 2-position selector switches.
Mistake 1: Confusing MAN-AUTO with HAND-OFF-AUTO (HOA)
A classic error is assuming a 2-position MAN-AUTO switch has an 'OFF' state. It does not. In a MAN-AUTO setup, Position 1 (MAN) typically energizes a hardwired start/stop circuit, while Position 2 (AUTO) feeds a 24VDC signal to a PLC or VFD. If you need a true 'OFF' state to break the control circuit entirely, you must specify a 3-position HOA switch. Wiring a 2-position switch to act as an isolation device will leave the control circuit live in both positions.
Mistake 2: Ignoring the Physical Contact Block Stack
The schematic might show two independent NO (Normally Open) contacts closing in Position 2. However, physical contact blocks are modular. If you are using an Allen-Bradley 800T or IDEC TWTD series, you must physically stack a 2NO contact block on the rear of the operator. If you mistakenly install a 1NO-1NC block and wire it according to a 2NO schematic, your 'AUTO' position will prematurely open a critical holding circuit.
Troubleshooting Faded or Missing Markings
Industrial environments destroy faceplates. If your 2-position switch markings are faded and you cannot determine if it is maintained or spring-return, do not guess. Use a Fluke 87V or 117 multimeter in continuity mode. Turn the switch to Position 1 and measure across the suspected NO contacts. You should read < 1.0 Ω. Release the knob. If the resistance immediately jumps to OL (Over Limit / > 1 M-ohm), it is a spring-return (momentary) switch. If it stays at < 1.0 Ω until you manually twist it back, it is maintained.
2 Position Selector Switch FAQ
How do I read a 2 position selector switch truth table?
A truth table for a 2-position switch maps the physical knob position to the electrical state of the rear contact blocks. For a standard 1NO-1NC maintained switch: In Position 1 (Left), the NC contact is closed (< 1 Ω) and the NO contact is open (OL). In Position 2 (Right), the states flip. Always verify the truth table against the physical switch with a multimeter before wiring, as some manufacturers use 'clockwise' as Position 1, while others use 'counter-clockwise'.
What is the difference between maintained and spring-return 2 position symbols?
The difference lies in the actuator graphic. A maintained symbol (like a standard ON/OFF switch) shows a simple diagonal line or rotary handle. Once you turn it, it stays there. A spring-return (momentary) symbol includes a small arrow pointing toward the default resting position. This indicates an internal spring mechanism will force the switch back to its default state the moment the operator lets go of the knob, commonly used for 'Jog' or 'Reset' functions.
How do I safely test a 2-position switch with faded or missing markings?
First, execute Lockout/Tagout (LOTO) and de-energize the panel. Verify dead with a tested meter. Once safe, disconnect the load wires from the switch terminals to prevent back-feeding through parallel circuits. Set your multimeter to continuity or resistance. Probe the common (C) and normally open (NO) terminals. Rotate the switch. Map the exact pinout on a piece of masking tape and stick it to the panel door. If the switch feels 'mushy' or shows high resistance (> 5 Ω) when closed, the internal contacts suffer from carbon tracking and the entire operator/block assembly must be replaced.
