The I O on off symbol found on rocker switches and power buttons is a universal binary code: "I" means ON (closed circuit, current flows) and "O" means OFF (open circuit, current stops). These symbols derive from the binary digits 1 and 0 and are globally standardized under IEC 60417. When you press the "I" side of a switch, the internal contacts bridge the line and load terminals, completing the circuit.
The IEC 60417 Power Symbol Reference
Before wiring any switch, you must understand exactly what the actuator markings dictate. The International Electrotechnical Commission (IEC) maintains the definitive registry for these graphical symbols. Below is the complete reference table for power state indicators.
| Symbol | IEC Reference | Meaning | Circuit State | Common Application |
|---|---|---|---|---|
| | (Line) | 5007 | ON | Closed (Continuity) | Mains rocker switches, toggles, disconnects |
| O (Circle) | 5008 | OFF | Open (No Continuity) | Mains rocker switches, toggles, disconnects |
| ⏻ (Line in Circle) | 5009 | Standby / Power | Partial (Logic powered) | PC power buttons, appliance soft-switches |
| ⏼ (Line breaking Circle) | 5010 | Hard Power / Isolate | Open (Mains isolated) | Medical equipment, industrial control panels |
Rows People Get Wrong
- Confusing Standby (5009) with Hard OFF (5008): The "circle with a line" symbol on a PC or TV does not physically disconnect the mains voltage. It leaves internal logic boards energized to listen for a wake signal. If you are working inside the chassis, you must unplug the device; the standby button is not a safe isolation point.
- Assuming "I" means Input and "O" means Output: In networking or data routing, I/O means Input/Output. On a physical power switch, it strictly means 1 (ON) and 0 (OFF). Do not wire line voltage to the "I" terminal expecting it to be a dedicated "input" pin; standard SPST rocker switches are bidirectional.
Regional Wiring Variants for I/O Switches
While the I O on off symbol is universal, the wire colors you connect to the switch terminals vary drastically by region. When wiring a standard 120V/230V AC single-pole rocker switch, you must follow your local jurisdiction's color code. The table below assumes copper conductors in a standard branch circuit.
| Function | US / Canada (NEC) | EU / UK (IEC 60446) | Old UK (Pre-2004) |
|---|---|---|---|
| Line (Source) | Black | Brown | Red |
| Load (Switched) | Black or Red (with tape) | Brown or Black (with sleeve) | Red or Blue (with sleeve) |
| Ground / Earth | Bare or Green | Green/Yellow Stripe | Green/Yellow Stripe |
Troubleshooting Faded or Missing Markings
On workshop equipment and older DIY builds, the paint on rocker switches frequently wears off, leaving you with a blank actuator. Never guess the switch state based on the physical tilt of the rocker, as internal mechanisms can fail or be installed upside down.
The Multimeter Continuity Test:
- De-energize: Turn off the main breaker and verify zero voltage across the switch terminals.
- Set Meter: Switch your multimeter to Continuity mode (the diode/soundwave icon).
- Probe: Place one probe on the Line terminal and the other on the Load terminal.
- Toggle and Read: Flip the switch.
- If the meter beeps and reads < 1.0 Ω, the switch is in the "I" (ON) position.
- If the meter reads "OL" (Over Limit) or infinite resistance, the switch is in the "O" (OFF) position.
- Mark: Use a silver paint pen or engraving tool to permanently remark the actuator based on your verified test.
Frequently Asked Questions
Why is "I" on and "O" off on power switches?
The symbols originate from the binary number system used in early computing and digital logic, where "1" represents a closed, active circuit (ON) and "0" represents an open, inactive circuit (OFF). During World War II, engineers needed a language-neutral way to label equipment for international allied forces. The binary 1/0 concept was adopted and later formalized by the IEC in 1973 as a universal standard, replacing ambiguous localized words like "ON/OFF", "EIN/AUS", or "MARCHE/ARRÊT".
What does the circle with a line through it mean on a power button?
This is the IEC 5009 "Standby" or "Soft Power" symbol. Unlike a physical rocker switch with an "O" that physically breaks the electrical connection, a button with the ⏻ symbol usually triggers a solid-state relay or a logic signal. It tells the device's internal microcontroller to enter a low-power sleep state. The mains voltage is still present inside the device's power supply. Always unplug the device before performing internal repairs, even if the ⏻ button is pressed.
How do I wire a standard I O rocker switch to 120V mains?
For a standard 15-Amp, 120V AC branch circuit in the US using 14 AWG NM-B cable:
- Strip 3/4 inch of insulation from the black (Line) and white (Neutral) wires.
- Connect the incoming black Line wire to one of the switch's spade terminals (usually marked "L" or "1").
- Connect a black jumper wire (or a second black wire acting as the Load) to the other switch terminal (marked "Load" or "2").
- Wire-nut the incoming white Neutral wire directly to the load's white Neutral wire. Never switch the neutral wire; always switch the hot/line wire.
- Connect the bare copper ground wires together and pigtail them to the metal switch housing or panel ground lug.
- Torque all terminal screws to the manufacturer's specification (typically 12-14 in-lbs for standard 15A switches) to prevent arcing.
