The IEC 60617 selector switch symbol is built from a standard contact base (Normally Open or Normally Closed) actuated by a rotary mechanism. On electrical schematics, this rotary actuator is depicted as a solid diagonal line ending in a small solid circle or a cross, representing the physical knob. When a single selector switch controls multiple contact blocks simultaneously, a dashed mechanical linkage line connects the individual contact symbols to show they are ganged together on the same shaft.
Unlike simple pushbuttons, selector switches rely on a cam chart (or contact sequence table) to define exactly which terminals close in which rotational positions. Below is the complete reference for reading these symbols and mapping them to physical terminals like the Schneider Electric Harmony XB5 or ABB Tomset lines.
IEC 60617 Selector Switch Symbol & Contact Chart Reference
Before wiring a panel, you must cross-reference the schematic symbol with the manufacturer's cam chart. The IEC standard uses specific terminal numbering (based on IEC 60445) to identify contact functions. The tens digit indicates the contact sequence number, while the units digit indicates the contact type.
| Terminal / Symbol Element | IEC Designation | Function in Practice | Common Part Example |
|---|---|---|---|
| 13 – 14 | NO (Normally Open) | Open at rest/center; closes when cam lobe pushes the plunger. | Schneider ZB5 XB5 contact block |
| 21 – 22 | NC (Normally Closed) | Closed at rest/center; opens when cam lobe pushes the plunger. | ABB OC22G contact block |
| 5 – 6 / 7 – 8 | Changeover (CO) | Break-before-make or make-before-break transition contacts. | Eaton FAZ-R selector blocks |
| Rotary Actuator Line | Diagonal line + circle | Indicates manual rotary operation (selector knob). | N/A (Schematic only) |
| Dashed Linkage Line | Horizontal/Vertical dash | Shows multiple contacts are mechanically tied to one shaft. | N/A (Schematic only) |
Reading the Cam Chart (Truth Table)
A standard 3-position Hand-Off-Auto (H-O-A) selector switch cam chart uses an "X" or a solid dot to indicate a closed circuit. Blank spaces mean the circuit is open.
| Terminals | Contact Type | Pos 1 (Hand) | Pos 2 (Off) | Pos 3 (Auto) |
|---|---|---|---|---|
| 13 – 14 | NO | X (Closed) | Open | Open |
| 21 – 22 | NC | Open | X (Closed) | Open |
| 33 – 34 | NO | Open | Open | X (Closed) |
Regional Variants: IEC vs. NEMA vs. Old UK BS
While the IEC 60617 standard dominates global manufacturing, North American facilities and older British plants use different conventions. Misinterpreting these can lead to incorrect physical panel cutouts or schematic reading errors. For a deeper look at global symbol differences, the All About Circuits reference library provides excellent cross-standard comparisons.
| Feature | IEC (Global / EU / Modern UK) | NEMA (North America) | Old BS 3939 (Legacy UK) |
|---|---|---|---|
| Governing Standard | IEC 60617 / IEC 80617 | NEMA ICS 19 | BS 3939 (Superseded by IEC) |
| Schematic Symbol | Diagonal line with solid circle/knob | Often a circle with a cross or "SEL" text | Similar to IEC but with older hatching |
| Physical Mounting Size | 22mm or 16mm diameter cutout | 30mm diameter cutout (e.g., Eaton 10250T) | Varies, often imperial fractions |
| Terminal Numbering | Strict IEC 60445 (13-14, 21-22) | Often simple sequential (1-2, 3-4, A-B) | Alphabetical or sequential legacy |
| Cam Chart Notation | "X" or solid dot for closed | Lines drawn between terminals per position | Graphical cam lobe drawings |
Rows People Get Wrong & Faded Marking Troubleshooting
When reading manufacturer datasheets or tracing legacy panels, specific errors consistently cause wiring faults.
The "Rows People Get Wrong" Notes
- Maintained vs. Spring-Return (Momentary): A standard selector switch is maintained (it stays in the position you turn it to). However, many cam charts include a spring-return variant (often denoted by an arrow pointing back to the center position, like
Pos 1 → Pos 2 ← Pos 3). If you wire a spring-return switch assuming it is maintained, your "Auto" circuit will drop out the moment the operator lets go of the knob. - The "X" vs. The "Dot": In IEC cam charts, an "X" means the contact is closed in that exact position. Some older charts use a solid dot. If a row has an "X" in Position 1 and Position 2, it is a continuous close across both positions. Do not confuse this with a make-before-break transition.
- Ignoring the Mechanical Linkage Dashes: If a schematic shows three separate contacts but lacks the dashed line connecting their actuators, they are not a single selector switch. They are three separate devices. The dashed line is mandatory in IEC 60617 to indicate a single multi-deck selector switch.
Safe Interpretation When Markings are Faded or Missing
In industrial environments, UV exposure and chemical washdowns destroy the printed cam charts on the side of switch bodies. Never guess the pinout based on the knob's faceplate text (e.g., "Fwd/Rev"). The internal cam lobes dictate the truth table, not the plastic knob.
- De-energize and Verify: Shut off the control circuit breaker. Use a non-contact voltage tester and a multimeter to verify the circuit is dead (LOTO procedures apply).
- Isolate the Terminals: Disconnect the wires from the switch terminals to avoid reading parallel paths through contactor coils or PLC inputs.
- Map with Continuity: Set your multimeter to continuity mode (the diode/sound setting). Place probes on terminals 13 and 14. Rotate the switch through all positions. Note every position where the meter beeps (reads < 1 ohm).
- Draft a New Cam Chart: Repeat for all terminal pairs (21-22, 33-34, etc.). Draw your own truth table on masking tape and affix it inside the panel door for the next technician.
Frequently Asked Questions
What is the IEC symbol for a 3-position selector switch?
The IEC 60617 symbol for a 3-position selector switch consists of the individual contact symbols (NO/NC) stacked vertically or horizontally, linked by a dashed mechanical line. The actuator is a diagonal line ending in a solid circle, with three distinct position indicators (usually labeled I, II, III or 1, 2, 3) showing the angles at which the switch can be set. The specific contacts that close in each position are defined by the accompanying cam chart, not the schematic symbol itself.
How do I read a selector switch cam chart for IEC terminal numbering?
Look at the terminal numbers printed on the contact blocks. Under IEC 60445, the first digit is the sequence number, and the second digit is the function. Terminals ending in 3-4 (like 13-14, 23-24) are Normally Open. Terminals ending in 1-2 (like 21-22, 31-32) are Normally Closed. On the cam chart grid, find your terminal pair on the left axis, then look across the columns for Positions 1, 2, and 3. An "X" or dot in a column means that specific pair of terminals will have continuity (be closed) when the knob is in that position.
What is the difference between IEC and NEMA selector switch symbols?
The primary difference lies in the actuator depiction and terminal labeling. IEC 60617 uses a diagonal line with a solid circle to represent the rotary knob, and strictly enforces two-digit terminal numbering (13-14 for NO). NEMA ICS 19 often represents selector switches with a circle containing a cross or a simple "SEL" label, and frequently uses sequential single-digit numbering (1-2, 3-4) or letter designations (A-B, C-D) depending on the manufacturer (like Eaton or Allen-Bradley). Physically, IEC switches use 22mm/16mm cutouts, while NEMA switches typically require 30mm cutouts.
Can I replace a NEMA selector switch with an IEC model?
Electrically, yes, provided the voltage and current ratings (e.g., 10A at 600VAC) meet the NFPA 70 (NEC) requirements for the circuit. Mechanically, no. A standard 22mm IEC switch (like a Schneider XB5) will not fit the 30mm hole punched for a NEMA switch (like an Eaton 10250T) without an adapter plate. Furthermore, you must re-map the wiring to match the IEC 13-14 / 21-22 terminal scheme, as the physical pin locations and numbering will differ from the original NEMA block.
