The universal symbol for 'on' is a vertical line (I), and the symbol for 'off' is a circle (O). These markings derive directly from binary logic (1 and 0). When combined into a circle broken by a vertical line (⏻), the symbol indicates a soft power or standby state rather than full galvanic isolation. Below is the definitive reference for interpreting these markings across consumer electronics, industrial control panels, and schematic diagrams.
The Complete Reference Table for On/Off and Power Symbols
The following table maps the physical markings you will find on rocker switches (like the Carling V-Series), industrial disconnects, and consumer electronics to their governing international standards. Read the 'Graphic' column as the physical molding or silkscreen on the device.
| Graphic | Unicode | Standard Name | Practical Meaning | Governing Standard |
|---|---|---|---|---|
| I | N/A | On | Circuit closed; power applied to load. | IEC 60417-5007 |
| O | N/A | Off | Circuit open; power removed from load. | IEC 60417-5008 |
| ⏻ | U+23FB | Power / Standby | Soft power; device is in low-power mode but mains are still present internally. | IEC 60417-5009 |
| ⏼ | U+23FC | Sleep | Deep sleep state; wakeable via external interrupt or button press. | IEC 60417-5010 |
| ⏽ | U+23FD | Hard On/Off | Full galvanic isolation; physically breaks the line conductor. | IEC 60417-5011 |
Regional Standards and Variants (IEC vs. IEEE vs. NEMA)
While the I/O binary markings are globally recognized on physical hardware, the way these switches are represented in schematics and regulated by safety bodies varies heavily by region.
International and European (IEC 60417)
The International Electrotechnical Commission (IEC) standard 60417 is the undisputed global authority for physical switch markings. If you are designing a consumer product for the EU, UK, or global markets, your physical enclosures must use the exact proportions defined in IEC 60417-5007 through 5011. In IEC schematic diagrams, a switch is drawn as a simple line with a hinged contact, without the I/O text.
North America (IEEE 315 and NEMA)
In the United States and Canada, physical industrial switches (like Eaton NEMA 4X heavy-duty safety disconnects) still use the I/O moldings because they are universally understood by operators. However, electrical schematics follow IEEE 315 (Graphic Symbols for Electrical and Electronics Diagrams). IEEE 315 does not use I/O text in schematics; instead, it uses specific contact symbols (e.g., a normally open contact with a manual actuator line). Furthermore, NEMA standards dictate the physical enclosure ratings and the required physical force to actuate the disconnect, but defer to IEC for the actual symbol geometry on the handle.
Legacy UK (BS 3939)
Before harmonizing with IEC standards, the UK used BS 3939 for electrical symbols. You may still encounter legacy industrial panels in older British facilities where the 'Off' state was indicated by a horizontal line and 'On' by a vertical line, or where isolators were marked with a solid red circle for off and a green circle for on. If you are retrofitting a pre-1990s UK panel, do not assume modern IEC color codes apply; verify with a meter.
The 'Rows People Get Wrong' and Faded Marking Protocols
Even experienced technicians misinterpret specific switch symbols, leading to dangerous assumptions about circuit isolation.
The Rows People Get Wrong
- Confusing Standby (⏻) with Hard Off (⏽): This is the most dangerous mistake in electronics repair. The ⏻ symbol (circle with the line fully inside, not breaking the perimeter) means the device is in standby. The primary side of the power supply is still energized with lethal mains voltage. Only the ⏽ symbol (line breaking the top of the circle) guarantees the switch physically disconnects the line conductor.
- Reading 'O' as Zero: The 'O' is not the number zero; it is the letter 'O' for 'Off'. Conversely, 'I' is not the number one; it is the Roman numeral or letter 'I' representing a closed circuit (though the binary 1/0 mnemonic is the accepted origin). Never look for a '1' and '0' on industrial hardware; look for 'I' and 'O'.
- Toggle Switch Ambiguity: On vertical toggle switches, 'I' is almost always UP and 'O' is DOWN. However, on horizontal rocker switches, the orientation depends on the mounting. Always read the symbol, never assume 'up is on' if the switch is mounted sideways.
Safe Interpretation When Markings are Faded or Missing
Industrial environments, UV exposure, and chemical cleaners frequently obliterate silkscreened I/O markings on control panels. Never guess the state of an unmarked switch based on its physical position.
If a switch controlling a circuit >50V AC or >120V DC has missing markings, you must treat the load as energized. De-energize the upstream breaker, apply lockout/tagout (LOTO), and verify the absence of voltage using a Category III or IV rated multimeter (e.g., Fluke 117 or 87V) before touching the switch terminals.
The Continuity Test Method:
To map an unmarked switch safely, set your multimeter to continuity or resistance (Ω). Probe the common (C) and normally open (NO) terminals. Actuate the switch. The position that yields a reading of < 1.0 Ω is your 'On' (I) state. The position that reads 'OL' (Open Loop) is your 'Off' (O) state. Mark the panel immediately with an industrial engraving pen or rated adhesive label.
Frequently Asked Questions About On/Off Symbols
What does the circle with a line symbol for on off mean on a PC power supply?
On a PC ATX power supply, the circle with a line breaking the top (⏽, IEC 60417-5011) is a hard mechanical disconnect. Flipping this to 'O' physically breaks the AC hot line, removing all mains voltage from the primary side of the power supply. This is different from the front panel PC power button, which uses the standby symbol (⏻) and only sends a 5V logic signal to the motherboard to request power.
Why is the symbol for on off switch 1 and 0 instead of text?
The symbols are not actually the numbers 1 and 0; they are the vertical line (I) and circle (O). However, they were adopted during the early days of computing and digital logic because they perfectly map to binary states: 1 (high/closed/on) and 0 (low/open/off). Using language-agnostic geometric shapes ensures that a switch manufactured in Germany can be safely operated by a technician in Japan without requiring localized text translation on the physical hardware.
How do I read the symbol for on off on a 3-phase motor disconnect?
On heavy-duty 3-phase rotary disconnects (like those made by Allen-Bradley or Schneider Electric), the handle will feature a large 'O' and 'I', often accompanied by 'OFF' and 'ON' text for redundancy. The 'O' position aligns the internal contacts to the open state, isolating all three phases (L1, L2, L3) simultaneously. Some rotary switches also include a padlock hasp in the 'O' position to facilitate LOTO procedures, physically preventing the switch from being rotated to the 'I' position while maintenance is being performed.
