The universal symbol for ON is a vertical line (|), representing the binary number 1. The universal symbol for OFF is a circle (O), representing the binary number 0. These markings are not derived from the English alphabet, but from binary logic states defined by the International Electrotechnical Commission (IEC). When you see a line, the circuit is closed (power applied). When you see a circle, the circuit is open (power removed).

The Complete IEC Power Symbol Reference Table

The following table details the standardized graphical symbols for power states as defined in IEC 60417 and ISO 7000. Keep this reference handy when designing control panels or labeling custom enclosures.

Symbol IEC Designation Meaning in Practice Binary / Logic State Common Applications
| (Vertical Line) IEC 60417-5007 Power ON. The switch physically closes the circuit, allowing current to flow to the load. 1 (Closed / High) PC power supplies, mains rocker switches, industrial motor starters.
O (Circle) IEC 60417-5008 Power OFF. The switch physically opens the circuit, isolating the load from the power source. 0 (Open / Low) Appliance disconnects, bench power supplies, AV equipment.
⏻ (Broken Circle + Line) IEC 60417-5009 Standby / Sleep. The device is not fully isolated from mains; low-voltage logic remains active. Standby (Soft-Off) Modern TVs, ATX computer motherboards, smart home hubs.
⏼ (Circle + Line inside) IEC 60417-5010 Push-to-toggle. A single momentary button that alternates between ON and OFF states. Toggle (Flip-Flop) Flashlights, desk lamps, momentary panel pushbuttons.

Why People Get the 'O' and '|' Wrong (And How to Remember)

The most common mistake on the bench or jobsite is assuming O means "On" because it is the first letter of the word, and | means "Off" because it looks like a physical wall or barrier blocking the flow of electricity. This linguistic assumption is dangerously wrong in electrical systems.

Memory Trick: Forget the English alphabet. Think in binary. The line is a 1 (circuit is complete, 100% power). The circle is a 0 (circuit is broken, 0% power). If you are wiring a 3-way toggle, the 1 position sends voltage to the load; the 0 position drops it to zero.

The "Standby" Confusion: Many consumers assume the broken circle with a line through it (⏻) means the device is completely safe to service. It does not. In an ATX power supply or a modern television, the ⏻ state only cuts power to the high-current loads. The 5V standby (5VSB) rail remains energized to listen for remote control signals or Wake-on-LAN packets. Always unplug the device or trip the upstream breaker before opening an enclosure marked with the standby symbol.

Regional Standards: IEC vs. NEMA vs. Legacy Markings

While IEC 60417 is the globally accepted standard for consumer electronics and general equipment, regional industrial standards sometimes dictate different labeling approaches, particularly in heavy machinery and high-voltage panels.

  • IEC 60417 (Global / EU / Modern US Consumer): Relies strictly on the 1/0 binary symbols. This is what you will find on 99% of imported electronics, PC power supplies, and modern rocker switches.
  • NEMA & UL (US Industrial): The National Electrical Manufacturers Association (NEMA) often requires explicit text (START/STOP or ON/OFF) or relies on color-coding (Green for ON/START, Red for OFF/STOP) on heavy industrial contactors and motor starters. If a US industrial panel uses symbols, it will usually pair the IEC 1/0 with the text to satisfy OSHA readability requirements.
  • Legacy UK/AU Rotary Selectors: Older British and Australian equipment (pre-1990s) sometimes used a dot (•) for OFF and a line (|) for ON on rotary cam switches, or simply relied on numbered positions (0-1-2) where 0 was universally the off position. If you encounter unmaintained legacy gear, never assume the symbol orientation without testing.

Safe Interpretation When Markings Are Faded or Missing

On older equipment, UV exposure, heat, and mechanical abrasion frequently wear away the molded 1 and 0 indicators on plastic rocker switches. Never guess the state of a faded switch based on the physical toggle position, as internal mechanical linkages can fail, leaving the actuator in the "Off" position while the internal contacts remain welded closed.

SAFETY WARNING: Before testing any switch suspected of carrying mains voltage (>50V AC / >120V DC), de-energize the circuit at the breaker panel. Use a lockout/tagout (LOTO) procedure and verify the circuit is dead using a Category III or IV rated non-contact voltage tester or multimeter.

The Multimeter Verification Procedure:

  1. Isolate: Remove the switch from the live circuit or ensure the upstream breaker is locked out.
  2. Set the Meter: Turn your digital multimeter (e.g., Fluke 87V) to the Continuity setting (the diode/sound wave icon).
  3. Probe the Terminals: Place one probe on the Common (COM) terminal and the other on the Normally Open (NO) terminal.
  4. Toggle and Read: Flip the switch to the suspected 1 (ON) position. A reading of 0.1 Ω to 0.5 Ω accompanied by an audible beep confirms a closed circuit. Flip to the suspected 0 (OFF) position. The meter must read OL (Over Limit), confirming physical isolation.
  5. Label: Once verified, use a paint pen or engraver to permanently remark the actuator.

Decision Path: Which Switch Symbol Do You Actually Need?

When sourcing components for a custom build, panel retrofit, or replacement, use this decision matrix to select the correct switch type and physical symbol molding. Stop guessing and order the exact part required for your voltage and environmental constraints.

Application Scenario Required Rating & Environment Symbol Requirement Concrete Part Pick
Mains AC Panel Mount
(Bench power supplies, custom 120V/240V enclosures)		 15A @ 125VAC / 10A @ 250VAC. Panel snap-in. Indoor use. Standard I/O (1/0) molded into the rocker actuator. Carling V-Series V1D1B60B
(~$5.50. Industry standard, UL recognized, highly reliable snap-in rocker).
Marine / Outdoor 12V DC
(Boat dashboards, off-grid solar control boxes)		 20A @ 12VDC. IP67 waterproof seal required. UV resistant. I/O symbols with integrated LED backlighting for night visibility. Blue Sea Systems 8245
(~$18.00. Waterproof contour rocker, clear I/O markings, handles DC arcing).
Low Voltage PCB Mount
(Arduino/ESP32 custom shields, 3.3V/5V logic control)		 50mA @ 12VDC. Surface mount (SMD) or through-hole. Momentary action. No physical symbol on part; use ⏻ or ⏼ silkscreen on the PCB/enclosure. C&K PTS645SM43SMTR92
(~$0.15. 6x6mm tactile SMD switch. Use microcontroller logic to toggle state).
Industrial Motor Control
(3-phase 480VAC contactor control circuits)		 NEMA 4X enclosure. 10A @ 600VAC. Maintained contact. Explicit START/STOP text + Green/Red color coding (NEMA/OSHA). Allen-Bradley 800FP-F2
(~$35.00. Heavy-duty flush pushbutton, relies on color/text over binary symbols).

For 90% of hobbyist and commercial DC/AC maker projects operating under 250V, the Carling V-Series with standard I/O molding is the default, most cost-effective choice. If your project involves logic-level standby states controlled by an ESP32 or Raspberry Pi, use a momentary tactile switch (like the C&K PTS645) and print the IEC 5009 Standby symbol (⏻) on your 3D-printed enclosure directly above the button.