The universal power on and off symbol—a circle intersected by a vertical line—represents the binary states 0 (off) and 1 (on). While it appears on everything from ATX motherboards to industrial motor starters, the exact graphical representation dictates whether a device is fully de-energized, in a low-power standby state, or toggling between the two. Below is the definitive reference for identifying, interpreting, and safely testing these symbols across global standards.
The Complete Power Symbol Reference Table
Use this table to identify the exact IEC 60417 reference number and practical electrical state for the markings on your equipment. This data applies to both consumer electronics and industrial control panels.
| Symbol Name | Visual Description | IEC 60417 Ref | IEEE 5009 State | Practical Meaning & Electrical State |
|---|---|---|---|---|
| Power On/Off (Toggle) | Circle with a vertical line breaking through the top | 5009 | Power On/Off | Toggles between fully energized and fully de-energized. Physically breaks the circuit in mechanical switches. |
| Power On | Vertical line only (no circle) | 5007 | On | Device is fully energized. Represents the binary '1'. Often found on one side of a rocker switch. |
| Power Off | Circle only (no line) | 5008 | Off | Device is fully de-energized. Represents the binary '0'. Indicates the circuit is open. |
| Standby / Sleep | Crescent moon or broken circle with line not breaking the top | 5926 / 5010 | Standby | Low-power state. Mains may still be connected to auxiliary circuits (e.g., ATX 5VSB rail). Logic remains active. |
| Wait / Soft Power | Broken circle with a horizontal line inside | 5011 | Sleep/Wait | System is in a suspended state. Main processing is halted, but RAM and wake-on-LAN circuits remain powered. |
Regional Standards: IEC 60417 vs. IEEE 5009
The symbols you see on modern equipment are governed primarily by the IEC 60417 Graphical Symbols database, which serves as the international baseline. However, understanding the historical and regional context is critical when working on legacy equipment or US-spec industrial panels.
In the United States, the IEEE 5009 standard (and its predecessor ANSI/IEEE 315) historically dictated graphical symbols for electrical engineering. While IEEE 5009 aligns closely with IEC 60417 for basic power states, older US military and industrial gear often relied on explicit text ('ON' / 'OFF') or a simple 'I' and 'O' stamped into toggle switches. The 'I' represents the closed circuit (On), and the 'O' represents the open circuit (Off).
Bench Note: The binary origin of the symbols is not a myth. The vertical line is a stylized '1' (closed circuit, high logic state), and the circle is a stylized '0' (open circuit, low logic state). The combined symbol (5009) literally overlays the 1 onto the 0 to indicate a toggle between the two states.
In the UK and older European installations, you may still encounter the old color-coded pushbuttons (Red for Off/Stop, Green for On/Start) without any IEC symbols, governed by legacy IEC 60204-1 machine safety standards. If you are retrofitting a panel, replacing a faded 'I/O' rocker switch with an IEC 60417-5009 illuminated rocker is the current best practice for global compliance and user clarity.
Rows People Get Wrong: Standby, Sleep, and Hard Off
The most dangerous mistake in electronics and electrical troubleshooting is confusing a 'Standby' symbol with a 'Hard Off' symbol. This confusion leads to live-circuit shocks and bricked microcontrollers.
The ATX 5VSB Trap
When you press a button bearing the IEC 60417-5926 (Standby/Moon) or 5009 (Toggle) symbol on a PC case or modern TV, you are not physically disconnecting the mains AC voltage. You are sending a logic-low signal to the power supply. The PSU drops the main 12V, 5V, and 3.3V rails, but it keeps the 5VSB (5 Volt Standby) rail live. This rail powers the infrared receiver, Wake-on-LAN circuitry, and the PSU's own internal monitoring logic. If you probe the DC side of a motherboard in 'Standby', you will still read 5V on the purple wire. To achieve a true 'Power Off' (IEC 60417-5008), you must flip the mechanical rocker switch on the back of the PSU or unplug the device.
The Broken Circle vs. Solid Circle
Look closely at the circle in the symbol.
- Solid Circle with Line Breaking Through (5009): This is a hard toggle. On a mechanical rocker switch, this physically breaks the hot (line) conductor.
- Broken Circle with Line Inside but NOT Breaking the Top (5010): This indicates a soft-power or sleep state. The switch is likely a momentary pushbutton connected to a microcontroller GPIO pin, not a hardwired mains breaker.
Safe Interpretation and Testing When Markings Fade
On workshop equipment, 3D printers, and industrial motor controllers, the silk-screened power symbols on rocker switches frequently wear off due to UV exposure, heat, and friction. Guessing the orientation of an unmarked DPST (Double Pole, Single Throw) switch on a 240V circuit is a severe shock hazard.
Never assume a switch is in the 'Off' position based on physical feel or faded markings. Always treat unverified conductors as energized. De-energize the main breaker, apply lockout/tagout (LOTO), and verify the circuit is dead with a known-working multimeter or non-contact voltage tester before touching any terminals.
How to Verify Switch Orientation with a Multimeter
If the 'I' (Line/On) and 'O' (Circle/Off) markings are completely gone from your rocker switch, use this procedure to map it safely:
- Isolate the Switch: Ensure the device is unplugged or the breaker is locked out. Disconnect the load and line wires from the switch terminals to avoid false continuity readings through the load.
- Set DMM to Continuity: Turn your multimeter to the continuity setting (the diode/soundwave icon). Verify it works by touching the probes together; it should beep and read < 1 ohm.
- Probe the Terminals: Place one probe on the input terminal and the other on the output terminal of one pole of the switch.
- Toggle and Observe: Flip the rocker. The physical orientation that yields a continuous beep (near 0.00 ohms) is the Power On (IEC 5007 / 'I') position. The orientation that reads 'OL' (Open Loop) or infinite resistance is the Power Off (IEC 5008 / 'O') position.
- Mark Permanently: Once verified, use a paint pen or engraver to permanently mark the 'I' and 'O' on the switch housing. Do not rely on tape or sharpie, which will degrade in high-heat environments.
By understanding the exact IEC reference numbers and the electrical reality behind the symbols, you can safely design control panels, troubleshoot standby power drains, and avoid the lethal assumption that a 'soft off' button means the circuit is safe to touch.
