The universal on and off switch symbol uses a vertical line (“I”) for ON and a circle (“O”) for OFF. Established under the IEC 60417 standard, these binary-derived markings indicate a closed (conducting) circuit and an open (interrupted) circuit, respectively. While the physical markings on your wall switch or power supply are globally standardized, the schematic symbols used to draw those switches in wiring diagrams vary significantly between North American (ANSI/IEEE) and international (IEC) standards.
The Complete On and Off Switch Symbol Reference Table
Before pulling wire or designing a control panel, you need to know exactly what the physical markings and their schematic equivalents mean. The table below maps the physical switch symbols you see on hardware to their circuit states and standard schematic representations.
| Physical Symbol | Meaning & Circuit State | Standard / Origin | Common Application | Schematic Equivalent (IEC 60617) |
|---|---|---|---|---|
| I (Vertical Line) | ON / Closed Circuit (Current flows) | IEC 60417-5007 | Power supplies, industrial rockers, PC cases | Single-pole, single-throw (SPST) closed contact |
| O (Circle) | OFF / Open Circuit (Current interrupted) | IEC 60417-5008 | Power supplies, industrial rockers, PC cases | Single-pole, single-throw (SPST) open contact |
| I / O (Overlaid) | Power Toggle (Alternates between ON/OFF) | IEC 60417-5010 | Momentary pushbuttons, standby switches | Pushbutton with alternating contact (make/break) |
| UP / DOWN (No text) | ON (Up) / OFF (Down) - Physical orientation | NEC 240.81 / UL 60950 | Standard US/CA residential wall toggles (e.g., Leviton 15A) | Standard SPST switch symbol |
| I .. O (Side-by-side) | Rocker orientation guide | IEC / UL 62368-1 | Marine panels, automotive dashboards, power strips | N/A (Physical marking only) |
Regional and Standard Variants: IEC vs. NEC vs. Legacy Schematics
When reading a wiring diagram or designing a control board, it is critical to distinguish between the physical markings on the switch hardware and the schematic symbols used on paper. While the physical “I” and “O” are globally recognized, the schematic drawing standards are split by region.
Physical Hardware Markings (Global)
For physical hardware, the IEC 60417 standard is effectively universal. Whether you are buying a rocker switch from a US supplier like Carling Technologies or a European brand like Schurter, the “I” and “O” markings are mandated by international safety standards (and adopted by UL in North America) to prevent language-barrier accidents in industrial environments.
Schematic Symbols (North America vs. International)
The divergence happens on the blueprint. In the US and Canada, schematics typically follow ANSI/IEEE 315 or NEMA standards.
- ANSI/IEEE (North America): A switch is drawn as a gap in a line with a hinged lever (resembling a knife switch). A normally open (NO) switch shows the lever pointing away from the contact point.
- IEC 60617 (International/Europe): A switch is drawn as a simple gap with a diagonal or horizontal line bridging it, often using a small solid dot to indicate a fixed contact point.
- Legacy UK (BS 3939): Older British schematics used distinct graphical representations that predated IEC harmonization. If you are troubleshooting a machine built in the UK before the late 1990s, expect to see these legacy symbols, which often look like a hybrid of ANSI and IEC styles.
'Rows People Get Wrong' and Faded Marking Troubleshooting
Never attempt to test continuity or probe switch terminals on a live circuit. Always de-energize the circuit at the breaker, apply lockout/tagout procedures, and verify the circuit is dead with a non-contact voltage tester and a multimeter before removing any switch covers. Local electrical codes (such as the National Electrical Code) require strict adherence to de-energization protocols.
The Rows People Get Wrong
When reading the reference table above, makers and junior electricians frequently misinterpret two specific symbols:
- The Combined “I/O” Symbol (IEC 60417-5010): People often assume this means “push the top for I, push the bottom for O.” In reality, this symbol designates a single button that toggles power states (like the power button on a PC case or a standby button on a monitor). It is a state-toggle indicator, not a directional rocker guide.
- Momentary vs. Maintained: A standard “I” implies a maintained switch (it stays ON when you let go). If you see an “I” with an asterisk (*) or a dot inside the circle, it indicates a momentary action. The switch will only remain closed while physical pressure is applied, springing back to OFF when released (common in doorbells and industrial jog controls).
Safe Interpretation When Markings are Faded or Missing
In harsh environments (marine, outdoor enclosures, or heavy industrial panels), UV exposure and abrasion will wear the “I” and “O” right off the plastic actuator. Here is how to safely determine the switch state:
- The UP=ON Rule (US/Canada): For standard residential and commercial vertical toggle switches (like a standard 15A or 20A single-pole wall switch), NEC 240.81 and standard UL listing requirements dictate that the UP position must be ON. If you are looking at a blank vertical toggle in a US panel, flipping it up closes the circuit.
- Continuity Testing: For rockers and industrial toggles where orientation doesn't guarantee state, use a multimeter set to the continuity (diode/beep) setting. With the power verified dead, place one probe on the common (C) or line terminal, and the other on the load terminal. Flip the actuator. The position that yields a reading of < 1 ohm (and an audible beep) is the ON (closed) state. Mark it immediately with a silver paint pen or engraved label.
- Internal Actuator Mechanics: If you cannot test continuity, look closely at the pivot point of the rocker. The side of the rocker that physically pushes down into the housing deeper is almost always the ON position, as it is depressing the internal copper seesaw to bridge the contacts.
Frequently Asked Questions
What does the I and O on a switch actually stand for?
They do not stand for the letters “I” and “O”, nor are they Roman numerals. They represent the binary digits 1 and 0. In binary logic and electrical engineering, a “1” represents a closed circuit (true, voltage present, ON), and a “0” represents an open circuit (false, no voltage, OFF). The IEC committee chose these binary symbols in the 1970s specifically because they transcend language barriers, unlike the English words “ON” and “OFF” or the French “MARCHE/ARRÊT”.
Is the on and off switch symbol the same worldwide?
The physical markings (“I” and “O”) on the hardware are globally standardized under IEC 60417 and are used on UL-listed equipment in North America, CE-marked equipment in Europe, and CCC-marked equipment in Asia. However, the schematic symbols used to draw those switches on electrical blueprints differ. North American engineers use ANSI/IEEE 315 (knife-switch style), while European and international engineers use IEC 60617 (simple line-and-gap style). Always check the title block of a schematic to see which drawing standard the engineer used.
How do I safely identify the ON position if the symbol is completely worn off?
First, de-energize the circuit and verify it is dead with a tested multimeter. If it is a standard US vertical wall toggle, the UP position is universally ON. For horizontal rockers or industrial toggles, use a multimeter in continuity mode across the line and load terminals; the position that completes the circuit (reads near 0 ohms) is ON. If the switch is a 3-way or 4-way traveler switch, it does not have a dedicated ON/OFF state, as the circuit path changes based on the companion switches in the circuit. In that case, continuity testing must be done across all three terminals to map the internal traveler logic.
