The universal power symbol—a circle with a vertical line breaking the top—is actually a superposition of the binary '1' (On) and '0' (Off). Defined under IEC 60417, the standalone line ('I') means the circuit is closed (On), and the standalone circle ('O') means the circuit is open (Off). If you are looking at a toggle or rocker switch, 'I' is always power flowing, and 'O' is power disconnected.
The Complete On and Off Symbols Reference Table
Before wiring a panel or debugging a PCB, confirm you are reading the switch state correctly. The following table covers the standardized on and off symbols you will encounter on modern electrical equipment, power supplies, and control panels.
| Symbol | Graphic | Name | Circuit State | Standard | Typical Use Case |
|---|---|---|---|---|---|
| I | Vertical Line | Power On | Closed (Energized) | IEC 60417-5007 | Mains rockers, industrial disconnects, SPST toggles |
| O | Circle | Power Off | Open (De-energized) | IEC 60417-5008 | Mains rockers, emergency stops, power strips |
| ⏻ | Circle + Line | Power / Standby | Partial / Soft-Off | IEC 60417-5009 | PC motherboards, ATX supplies, consumer AV gear |
| ⏼ | Circle + Broken Line | Sleep / Hibernate | Low Power State | IEC 60417-5010 | Laptops, smart home hubs, microcontrollers |
| II | Double Vertical Line | Pause / Hold | Maintained State | Media Standard | Media players, motor jog controls, VFD keypads |
Regional and Standard Variants (IEC vs. NEC vs. Legacy)
While the IEC symbols are globally dominant for consumer electronics and PCB-level components, industrial and regional standards introduce critical variations in how on and off states are labeled and color-coded.
IEC (International / EU / Modern Global)
The International Electrotechnical Commission relies heavily on the 1/0 and ⏻ symbols. Under IEC graphical symbol standards, text labels like 'ON' and 'OFF' are discouraged in favor of the universal 'I' and 'O' to bypass language barriers. For machine control panels (IEC 60204-1), green indicates a safe 'ON' or 'Start' condition, while red indicates an 'OFF' or 'Stop' condition.
NEC / NFPA (North America)
In the US, the National Electrical Code (NEC) and NFPA 79 (Electrical Standard for Industrial Machinery) have stricter text and color requirements for safety disconnects. While a PCB toggle will still use I/O, a heavy-duty rotary disconnect switch on a 480V motor starter must often be explicitly stamped with 'ON' and 'OFF'. Furthermore, NFPA 79 dictates a dangerous gotcha: while green pushbuttons mean 'Start/On', a red handle on a rotary mains disconnect actually indicates the 'ON' (energized) position to warn operators of live voltage.
Legacy UK and Older Domestic Wiring
Prior to harmonization with European standards, older UK domestic installations sometimes used red for 'ON' and black for 'OFF' on switch plates and fuse carriers. If you are retrofitting a pre-2004 UK consumer unit or industrial board, do not trust the color of the toggle bat; verify the physical I/O stamping or test with a meter.
Rows People Get Wrong (and How to Avoid Catastrophe)
The most misinterpreted row in the table above is IEC 60417-5009 (the circle with the line at the top). Many hobbyists and technicians assume pressing a standby button fully disconnects the mains. It does not. In an ATX power supply or modern TV, the ⏻ state only switches off the secondary DC rails. The primary AC side, including the bridge rectifier and bulk smoothing capacitors (often holding 300V-400V DC), remains fully energized. Always unplug the physical IEC C13/C14 cable before opening a chassis, regardless of the standby LED state.
Mistake 1: Confusing 'O' with a Zero or Degree Mark
On cheap, injection-molded rocker switches, the 'O' (Off) can look like a zero or a poorly stamped degree symbol. If the 'I' (On) side is distinctly a straight line, the opposing side is Off. Never assume a switch is Off just because the marking looks like a '0'—verify the physical click and the internal contact state.
Mistake 2: Trusting Faded Silk-Screen on PCBs
On development boards and custom PCBs, the white silk-screen paint indicating 'ON' and 'OFF' for slide switches frequently rubs off after a few dozen cycles. When markings are faded, never guess the state based on the physical direction of the actuator. Slide switches vary wildly; some close the circuit when pushed left, others when pushed right.
The Safe Interpretation Protocol:
- De-energize the circuit completely.
- Set your digital multimeter (DMM) to continuity mode (the diode/sound icon).
- Place probes on the Common (C) and Normally Open (NO) terminals of the switch.
- Flip the actuator. The position that yields a reading of less than 1 ohm (and an audible beep) is your true 'I' (On) position. Mark it with a paint pen.
Frequently Asked Questions About On and Off Symbols
What does the circle with a line through the top mean on my power supply?
This is the IEC 60417-5009 'Standby' or 'Soft Power' symbol. It indicates that the switch controls a logic signal or a secondary relay, not the main AC input. Pressing it puts the device into a low-power state where the microcontroller or IR receiver remains active to listen for a 'wake' command. For bench power supplies, it usually disables the output terminals while keeping the cooling fan and internal logic powered.
Why is the '1' symbol for ON and '0' for OFF in binary logic?
The symbols originate from binary mathematics and Boolean logic, which form the foundation of digital electronics. In this system, '1' represents 'True', 'High', or a closed circuit allowing current flow. '0' represents 'False', 'Low', or an open circuit blocking current. The International Electrotechnical Commission adopted these binary states in the 1970s to replace localized text labels (like 'Marche/Arrêt' or 'Ein/Aus') with a single, mathematically universal standard.
How do I safely test a switch with faded on and off symbols?
Never test a faded switch while the circuit is live. Turn off the main breaker or unplug the device. Use a CAT III or CAT IV rated multimeter in continuity mode. For a standard SPST (Single Pole Single Throw) switch, place one probe on each of the two terminals. Toggle the switch. The position that causes the meter to beep (indicating near-zero resistance) is the 'I' (On) position. For a DPDT or 3-way switch, map the Common terminal to the traveler terminals to determine the exact throw direction for the closed state.
Are red and green colors still legally required for on and off switches?
It depends entirely on the application and your local Authority Having Jurisdiction (AHJ). For industrial machine control panels governed by NFPA 79 or IEC 60204-1, color coding is strictly enforced: Green is mandatory for 'Start/On' pushbuttons, and Red is mandatory for 'Stop/Off' or Emergency Stops. However, for standard residential wall switches, branch circuit breakers, and consumer electronics, there is no legal requirement for red/green coloring. Most modern residential breakers simply use black toggles with embossed 'ON' and 'OFF' text, relying on physical position (up for On, down for Off in North America) rather than color.
