Off On Switch Symbol Guide: IEC 60417 Meanings, Wiring & Selection

The universal off on switch symbol consists of a vertical line (“I”) for ON and a circle (“O”) for OFF. These derive from the binary digits 1 (closed circuit/power) and 0 (open circuit/no power), standardized globally under IEC 60417. If you are looking at a standard toggle or rocker switch, pressing the “I” side completes the circuit, while the “O” side breaks it.

The Complete Off On Switch Symbol Reference Table

Before wiring any panel or device, confirm the exact actuator markings. The following table covers the primary IEC 60417 designations you will encounter on modern electrical components.

Regional Wiring Standards: Translating Symbols to Wire Colors

Knowing that “I” means ON is only half the battle; you must know which wires to land on the switch terminals to make that symbol function safely. The physical switch doesn't care about wire colors, but your local electrical code does. Here is how the switch terminals map to regional wiring standards.

North America (NEC / UL)

• Line (Source): Black (120V) or Red/Black (240V). Connects to the brass or darker screw terminal.
• Load (To Device): Black or Red. Connects to the lighter brass screw.
• Neutral: White. Never switched on a standard single-pole; passes through to the load.
• Ground: Bare copper or Green. Connects to the green grounding screw on the switch yoke.
• Orientation Rule: Per NFPA 70 (NEC) 240.81, standard vertical toggle switches must be installed so that the UP position is ON. You will rarely see I/O symbols on standard US wall toggles for this reason.

Europe & Global (IEC 60446 / Harmonized)

• Line (Source): Brown. Connects to the terminal marked 'L' or 'In'.
• Load (To Device): Black or Grey. Connects to the terminal marked with an arrow or 'Out'.
• Neutral: Blue. Passes through the backbox via a WAGO or terminal block.
• Ground: Green/Yellow stripe. Bonds to the backbox and switch earth terminal (if Class I metal faceplate).

Old UK (Pre-2004 Installations)

Rows People Get Wrong: Common Symbol & Wiring Mistakes

Even experienced makers and apprentices misinterpret specific switch variations. Here are the most common errors when reading and wiring off on switch symbols.

1. Mistaking “O” for Ground or Zero-Volts: The circle does not mean “connect to ground” or “connect to the 0V DC rail.” It strictly denotes the physical actuator position that opens the circuit. Connecting a live wire to a terminal labeled “O” (if such a non-standard marking exists on a cheap import) will result in a dead short or reversed logic.
2. Illuminated Switches Without a Neutral: If you buy a rocker switch with an integrated neon or LED indicator that lights up when in the “I” position, check the pin count. A 2-pin illuminated switch uses the load itself as the return path for the indicator light. If the load is high-impedance (like an LED driver or a small fan), the indicator will glow dimly even when the switch is in the “O” (OFF) position. For a clean OFF state, use a 3-pin illuminated switch and wire the third pin to a dedicated neutral or 0V ground.
3. Center-Off Confusion (I - O - II): On a DPDT (Double Pole, Double Throw) switch, the center position is OFF. However, the “I” and “II” (or “O” and “I” on some motor controllers) do not mean ON and OFF. They mean Forward and Reverse, or Speed 1 and Speed 2. Always verify the internal pinout with a multimeter before wiring a 3-position switch.

Faded or Missing Markings: The Multimeter Verification Path

On older industrial equipment, marine panels, or salvaged components, the painted I/O symbols are often worn away. Never guess the orientation based on the actuator's physical tilt. Use this exact procedure to verify the true ON state.

1. De-energize: Ensure the circuit is completely dead. Verify 0V with a non-contact voltage tester and the DMM on AC/DC voltage mode.
2. Isolate: Disconnect at least one wire from the switch terminals to prevent reading continuity through the connected load (which will give false readings).
3. Probe: Place one DMM probe on the common (input) terminal and the other on the load (output) terminal.
4. Actuate: Flip the switch to Position A.
   • If the DMM reads < 1.0 Ω and beeps, Position A is ON (“I”).
   • If the DMM reads OL (Over Limit) or infinite resistance, Position A is OFF (“O”).
5. Mark: Once verified, use a silver or white industrial paint pen (like a Sharpie PEINT) to permanently redraw the I and O symbols on the actuator and panel face.

Switch Selection Decision Tree

Stop guessing which switch to buy for your project. Use this decision matrix to select the exact component based on your voltage, environment, and code requirements.

Final Recommendation: For general DC panel builds (solar, camper, marine), default to the Carling V-Series. The I/O symbols are physically molded into the actuator rather than painted, meaning they will never fade, and the internal silver-cadmium contacts handle the high inrush currents of DC motors and inverters without welding shut.