The vertical line (I) means ON, and the circle (O) means OFF. These universal markings are defined by the IEC 60417 standard (originally published as IEC 5007 and 5008). When combined into a circle with a line breaking the top (⏻), the symbol indicates a standby or soft-power state that does not physically isolate mains voltage. Below is the complete technical reference for identifying, interpreting, and testing these symbols across different equipment classes and global regions.
The Master On Off Symbol Switch Reference Table
This table maps the physical symbols you see on rockers, toggles, and pushbuttons to their official IEC graphical symbol database numbers, internal contact states, and common hardware implementations. Use this to verify what a switch is actually doing to the circuit.
| Symbol | IEC Standard No. | Name / Description | Internal Contact State (Actuated) | Common Hardware / Application |
|---|---|---|---|---|
| | (Vertical Line) | IEC 60417-5007 | Power On | Closed (Continuity) | Mains rockers (Carling V-Series), heavy machinery disconnects |
| O (Circle) | IEC 60417-5008 | Power Off | Open (No Continuity) | Appliance inlets, power strips, DPST mains breakers |
| ⏻ (Line in broken circle) | IEC 60417-5009 | Standby / Soft Power | Closed to low-voltage logic; Open to mains | PC towers, modern TVs, smart home hubs |
| ⏼ (Line in full circle) | IEC 60417-5266 | On/Off Pushbutton (Toggle) | Toggles state on each press | Momentary smart switches, Bluetooth trackers |
| 1 / 0 (Numbers) | N/A (Legacy) | Numeric On/Off | 1 = Closed, 0 = Open | Older US/UK appliances, HVAC control panels |
Understanding the Standby (⏻) vs. Hard Disconnect (I/O)
The most critical distinction in this table is between the I/O rocker and the ⏻ standby button. A switch marked with I / O is typically a Single-Pole Single-Throw (SPST) or Double-Pole Single-Throw (DPST) mechanical switch. When flipped to 'O', it creates a physical air gap that breaks the hot (and sometimes neutral) conductor, isolating the load from mains voltage.
Conversely, the ⏻ standby symbol almost always denotes a momentary tactile switch connected to a low-voltage logic board. Pressing it sends a 3.3V or 5V signal to a microcontroller or Super I/O chip, which then triggers a solid-state relay or MOSFET to wake the main power supply. Pressing a standby button does not de-energize the internal power supply capacitors. If you are opening a chassis for repair, a standby switch offers zero electrical protection; you must unplug the device from the wall.
Regional Standards and the "Rows People Get Wrong"
While the IEC symbols are globally recognized on appliance rockers, regional electrical codes and legacy practices introduce variations that trip up both hobbyists and junior technicians.
Regional Code Variations
- North America (NEC/UL): The National Electrical Code (NEC) Article 404.8 requires that switches clearly indicate their open and closed positions. For hardwired wall switches, this is achieved via physical orientation (UP is ON, DOWN is OFF) rather than printed symbols. However, UL-listed appliance rockers and power strips heavily utilize the I/O symbols.
- UK / Australia / EU (IEC/BS): IEC symbols are the absolute standard for all equipment. Note that for hardwired wall toggles in the UK and Australia, the physical orientation is often reversed from the US: DOWN is typically ON, and UP is OFF. Always rely on the printed I/O or L1/L2 markings on the switch faceplate rather than physical position.
The "Rows People Get Wrong" Notes
Common Mistake 2: Assuming Rocker Orientation. On a standard IEC-marked rocker switch, the side with the "I" must be depressed for the circuit to close. However, if a panel is mounted sideways or upside down (common in custom CNC builds or marine dashboards), the physical "up/down" feel becomes useless. Always read the symbol facing you, not the physical detent.
Common Mistake 3: Misinterpreting the ⏼ Toggle Symbol. The line inside a complete circle (⏼) indicates a pushbutton that toggles state with every press. People often hold this button down expecting a continuous "ON" state, but it is designed as a momentary switch. Holding it down can trigger a hard-reset or fault state on modern logic boards.
Safe Interpretation When Markings Are Faded or Missing
On older workshop equipment—like a 1990s Delta bandsaw or a well-used soldering station—the painted I and O symbols frequently wear off, leaving a blank black rocker. Never guess the state of a faded switch based on which side "feels" more depressed, as internal plastic actuators warp over time.
The Multimeter Verification Protocol
When markings are missing, you must verify the switch state electrically. This requires a digital multimeter (DMM) and strict adherence to safety protocols.
- De-energize and Isolate: Unplug the equipment or lock out the breaker. Never attempt to test switch continuity on a live circuit using the ohms setting; you will destroy your multimeter's internal fuse and risk an arc flash.
- Verify Dead: Use a Non-Contact Voltage Tester (NCVT) and a CAT III/IV rated voltage tester to confirm zero voltage at the switch terminals.
- Set the DMM: Turn your multimeter to the Continuity (diode/beep) or Ohms (Ω) setting.
- Probe the Terminals: Place one probe on the Line (input) terminal and the other on the Load (output) terminal of the switch.
- Read the State:
- If the meter reads < 0.5 Ω or emits a continuous beep, the internal contacts are closed. The switch is in the ON position.
- If the meter reads OL (Open Loop) or infinite resistance, the contacts are separated. The switch is in the OFF position.
- Mark the Switch: Once verified, use a silver paint pen or an engraved label to permanently mark the I and O positions on the bezel. Do not rely on tape, which will degrade from heat and UV exposure.
Mechanical Inspection for Faded Toggles
If you are dealing with a legacy metal-body toggle switch (common in aviation and older industrial panels) where the ON/OFF engraving is worn smooth, inspect the bat actuator. On standard US-specification Mil-Spec toggle switches (like the Carling 216 series), pushing the bat toward the keyed side of the bezel (the flat side) is universally the OFF position. However, verifying with a multimeter remains the only 100% reliable method before applying mains power.
