The two position selector switch symbol represents a manually operated, maintained-contact control device used to toggle a circuit between two distinct states—typically ON/OFF, AUTO/MAN, or FWD/REV. Unlike momentary pushbuttons, the contacts in a selector switch remain in their last position until the operator physically turns the knob or key. In North America, panel schematics predominantly use NEMA-style symbols, while IEC 60617 modular symbols dominate global exports and modern PLC-driven control panels.

The Master Reference Table: IEC vs NEMA Symbols & Pinouts

The table below maps the most common two-position switch functions to their respective schematic symbols and physical terminal pinouts. Use this as your primary translation layer when reading cross-regional electrical prints.

Function IEC 60617 Symbol Description NEMA Symbol Description Standard IEC Pinout (NO/NC) Common Use Case
Maintained ON-OFF Single-pole switch with manual actuator (cross/lever) and a mechanical detent (latch) marker. Manual switch symbol: a diagonal line intersecting a perpendicular bar, with a solid manual actuator arrow and no return spring. 13-14 (NO), 21-22 (NC) Main control power enable, pump start/stop.
Maintained AUTO-MAN Identical actuator to ON-OFF, but often drawn with two parallel contact blocks operated by a single cam. Multi-pole manual switch with ganged contacts indicated by a dashed mechanical interlock line. 13-14 (NO), 21-22 (NC) + 33-34 (NO) Bypassing a VFD/soft-starter, PLC vs. hardwired control.
Illuminated ON-OFF Standard switch symbol plus a small circle or light ray lines adjacent to the actuator block. Standard manual switch symbol with an integrated pilot light circle attached to the actuator. X1-X2 (Lamp), 13-14 (NO) Visual feedback for motor run status or heater enable.
Key-Release (2-pos) Switch symbol with a key actuator (drawn as a key profile) and a detent marker in both positions. Key-operated switch symbol: standard contact block with a key profile actuator and no spring return. 13-14 (NO), 21-22 (NC) Access control, safety bypass authorization, main disconnect.

Regional Standards: Which Symbol Applies to Your Panel?

Applying the wrong standard's logic to a schematic can lead to miswired control circuits, especially when dealing with auxiliary contacts. Here is how to identify which standard governs your print.

Pro-Tip for Panel Builders: Never assume a schematic is purely one standard. It is incredibly common to see IEC 60617 component symbols used on a drawing that follows NEC/NFPA 79 wire color codes (e.g., black for AC control, blue for DC control). Always check the title block for the governing standard.

NEMA (North America)

NEMA symbols are function-oriented. A two-position selector switch is drawn as a single, unified graphic representing the entire physical device. If the switch has multiple contact blocks (e.g., 2NO + 2NC), NEMA schematics will draw all contacts grouped together, connected by a dashed line to indicate they are mechanically ganged to the same knob. You will see this almost exclusively in US and Canadian NEMA ICS 19 compliant panels.

IEC 60617 (Global / EU / Modern PLC Panels)

IEC symbols are modular. The symbol is broken down into the actuator (the knob/key) and the contact blocks. A two-position maintained switch will show the manual actuator symbol (IEC 60617-02-03) combined with a mechanical latch symbol (IEC 60617-02-12). The contacts are drawn wherever they logically belong in the circuit (e.g., the NO contact in the motor starter coil rung, and the NC contact in the PLC input rung), linked only by the component tag (e.g., -S1).

Legacy UK (BS 3939)

Superseded by IEC 60617 in the 1980s, BS 3939 symbols still appear in older British plants. They look similar to early NEMA symbols but use distinct diagonal hashing to indicate maintained positions. If you are retrofitting a pre-1990 UK facility, expect to see these on original vellum prints.

Rows People Get Wrong: Faded Markings & Contact Logic

When troubleshooting an existing panel, the physical switch markings and the schematic rarely match perfectly due to field modifications, wear, or incorrect replacement parts. Here are the most common traps.

Mistake 1: Trusting the "I" and "O" Engravings for NO/NC Logic

The "I" (ON) and "O" (OFF) printed on the switch bezel indicate the actuator position, not the contact state. A maintenance technician might have replaced a failed 1NO/1NC contact block with a 2NO block and wired it backward. Rule: The physical knob position does not guarantee the underlying pinout. Always verify with a meter.

Mistake 2: Confusing Maintained (Selector) with Momentary (Pushbutton)

On a quick glance at an IEC print, a momentary pushbutton and a maintained selector look nearly identical. The difference is the detent marker. A maintained selector symbol features a small mechanical latch symbol (a small hook or solid block) next to the actuator stem. A momentary pushbutton features a dashed line indicating a spring return. Wiring a momentary switch where a maintained selector is specified will cause the circuit to drop out as soon as the operator releases the knob.

Mistake 3: Ignoring the Cam Profile (Break-Before-Make)

Standard two-position selectors use a "break-before-make" cam profile. When turning from Position 1 to Position 2, the Position 1 contacts open before the Position 2 contacts close. If your application requires overlapping signals (make-before-break) to prevent a PLC from seeing a momentary loss of input, a standard selector switch symbol will not suffice; you must specify a specialized overlapping cam block.

Safety Warning: Faded or Missing Markings
If the switch bezel is faded, painted over, or missing, never guess the position based on left/right or up/down orientation. Different manufacturers use different cam orientations (some are 90-degree throws, some are 45-degree offset). De-energize the panel, lock out the main breaker, and use a multimeter in continuity mode to map the pins before applying control power.

Decision Tree: Picking the Right Switch & Symbol for Your Build

Use this decision path to select the correct physical hardware and schematic symbol for your specific application. Follow the logic down to your concrete part selection.

Condition / Scenario Required Standard Recommended Product Line
Building a UL508A control panel for a US domestic client; panel uses 30mm knockouts. NEMA / ANSI Allen-Bradley 800T or Eaton XTCE
Building an export machine for the EU/Asia; requires CE marking and 22.5mm knockouts. IEC 60617 / EN 60947 Schneider Harmony XB4/XB5 or Siemens 3SU1
High-vibration environment (mining, heavy stamping presses); plastic bezels crack. IEC (Metal Bezel) Schneider Harmony XB4 (Die-cast metal)
Washdown environment (Food & Bev, CIP systems); requires IP69K rating. IEC / NEMA 4X Eaton RMQ Titan or ABB Pilot Devices

The Default Concrete Pick

If you are designing a standard industrial control panel, need a reliable 2-position maintained selector (ON/OFF or AUTO/MAN), and want a part that is globally available, reasonably priced, and fits standard 22.5mm knockouts, default to the Schneider Electric XB4BD25.

  • Part Number: XB4BD25 (IEC standard, 2-position maintained, black knob, 1NO+1NC contact block included).
  • Cost: Typically $18 to $24 USD from authorized distributors.
  • Why it wins: The XB4 line uses a metal bezel for durability, and its IEC modularity means if you need to change from 1NO/1NC to 2NO in the field, you simply unscrew the rear contact block and snap a new one on without replacing the front actuator. Its IEC pinout (13/14 for NO, 21/22 for NC) is the global benchmark.

Safe Interpretation & Verification Protocol

When you are handed a schematic with a two position selector switch symbol, but the physical panel has unmarked, faded, or non-standard switches, follow this verification protocol before energizing.

  1. De-energize and Lock Out: Shut off the control circuit breaker (typically 24VDC or 120VAC). Verify dead with a known-working CAT III multimeter.
  2. Locate the IEC/NEMA Tag: Find the component tag on the schematic (e.g., SS1 or -S1) and trace it to the physical switch on the panel door.
  3. Map the Pins (Continuity Test): Set your multimeter to continuity (the diode/beep setting). Place probes on the suspected NO terminals (IEC 13 and 14). Turn the knob to the "ON" or "I" position. The meter should beep. Turn it to "OFF" or "O"; the beep should stop.
  4. Verify the NC Block: Move probes to terminals 21 and 22. The meter should beep in the "OFF" position and go silent in the "ON" position.
  5. Check Illumination (If Applicable): If the symbol indicates an illuminated switch, locate the lamp terminals (usually X1 and X2 on IEC blocks). Verify the lamp voltage rating printed on the side of the block (e.g., 24V AC/DC or 120V AC) matches the control voltage feeding it. Applying 120V to a 24V LED block will instantly destroy the diode.

By strictly adhering to the pinout mappings and verifying physical continuity rather than trusting faded bezel engravings, you ensure the control logic matches the schematic intent, preventing dangerous unintended machine starts or PLC fault states.