The universal electrical on and off symbols are the vertical line "I" (representing a closed circuit or binary 1 for ON) and the circle "O" (representing an open circuit or binary 0 for OFF). These are codified primarily in the IEC 60417 standard. If you are designing a control panel, replacing a mains switch, or troubleshooting faded equipment, use "I" for energize and "O" for de-energize as your global default. Never rely on the English words "ON" and "OFF" for international or industrial equipment.

The Complete Electrical On and Off Symbols Reference Table

The following table maps the physical markings you will encounter on switchgear, PCB silkscreens, and consumer electronics to their exact circuit states and governing standards. Reference this before ordering panel components like Carling V-series rockers or Schurter piezo switches.

Symbol IEC 60417 ID Circuit State / Meaning Primary Application
I (Vertical Line) 5007 ON (Closed Circuit / Energized) Mains disconnects, physical toggle/rocker switches, emergency stop resets.
O (Circle) 5008 OFF (Open Circuit / De-energized) Mains disconnects, physical toggle/rocker switches, safe working state.
⏻ (Circle with Line) 5009 Power / Soft Standby Consumer electronics, PC power supplies, microcontroller soft-latching circuits.
⏼ (Crescent Moon) 5011 Sleep / Deep Standby IT equipment, modern HVAC thermostats, low-power logic states.
⏽ (Circle with Full Line) 5010 Push On/Off (Toggle Action) Momentary pushbuttons that toggle state, solid-state relays (SSRs).
1 / 0 (Digits) N/A (IEEE/Logic) Logic HIGH (1) / Logic LOW (0) PCB jumper blocks, DIP switches, logic-level enable pins, Arduino/ESP32 silkscreen.

Regional Standards: IEC vs. IEEE vs. Legacy Markings

While the IEC standard dominates physical hardware, schematic and regional differences still cause confusion on the bench and in the field.

IEC 60417 (Global / EU / Modern US Physical Hardware)

The International Electrotechnical Commission (IEC) standard 60417 is the undisputed authority for physical switch markings worldwide. In the EU, it is mandated by the Low Voltage Directive. In the US, while IEEE 315 and NEMA govern schematic symbols (using specific contact drawings for SPST/DPDT), physical panel hardware sold by major US distributors (like Allen-Bradley or Eaton) strictly uses IEC 60417 "I" and "O" engravings for global supply chain compatibility.

Legacy UK and Older US Installations

Before IEC harmonization, physical switches often relied on positional semantics rather than printed symbols. In older US and UK installations, a standard toggle switch pointed UP for ON and DOWN for OFF. If you are retrofitting a 1970s control panel, you may find literal "ON/OFF" text or no markings at all, relying entirely on the toggle lever's gravity position. Note: Modern NEC and IEC requirements now mandate that vertical operating handles must be UP for ON to prevent gravity from accidentally energizing a circuit if the mechanism fails.

The "Rows People Get Wrong" Notes

Critical Safety Misinterpretations: Confusing standby symbols with hard disconnects is a leading cause of shock incidents during bench repairs. Always verify dead with a meter.
  • "O" does not mean ON: The most common beginner mistake is assuming "O" stands for "On" because it is the first letter of the English word. Remember the binary origin: 1 (I) = closed path, 0 (O) = open path. The "I" is a 1, and the "O" is a 0.
  • Standby (⏻) is NOT a safe working state: IEC 60417-5009 (the circle with the line breaking the top) indicates "Soft Power" or "Standby." On a PC power supply or a modern inverter, this symbol means the low-voltage logic is off, but the primary mains bus and bulk capacitors are still fully energized. Never use a standby switch as a Lockout/Tagout (LOTO) point.
  • PCB "1/0" vs. Physical "I/O": On an ESP32 or Arduino carrier board, silkscreen "1" and "0" refer to logic voltage levels (e.g., 3.3V HIGH and 0V LOW), not physical mains switching. Do not use 1/0 digits on a 120V/240V AC panel faceplate; it violates industrial safety norms and confuses operators.
  • The Moon Symbol (⏼) vs. Power (⏻): The crescent moon indicates a low-power sleep state where RAM is retained but clocks are halted. The ⏻ symbol usually cuts power to the main processing unit but leaves the power supply's 5V standby rail active. They are not interchangeable in firmware-driven hardware design.

Safe Interpretation When Markings Are Faded or Missing

In industrial environments, UV exposure, chemical cleaners, and physical abrasion frequently destroy switch markings. According to OSHA's hazardous energy control guidelines, you must never guess the state of an unmarked disconnect. If the "I" or "O" is illegible, follow this verification sequence:

  1. Assume Energized: Treat the equipment as live. Don appropriate PPE (arc flash suit/gloves if dealing with >240V industrial panels).
  2. Non-Contact Voltage (NCV) Test: Use a CAT III or CAT IV rated NCV tester (like a Fluke 1AC-II) on the load side of the switch. If it beeps, the switch is either ON or failed-closed.
  3. Live-Dead-Live Meter Test: Using a CAT-rated multimeter, test a known live source to verify the meter works. Then test the load terminals. Finally, test the known live source again to ensure the meter didn't blow its internal fuse during the test.
  4. Continuity Check (De-energized Only): Once upstream power is physically locked out, set your multimeter to continuity mode. Probe across the switch terminals. A reading of < 1 ohm means the switch is in the "I" (ON/Closed) position. An "OL" (Open Loop) reading means it is in the "O" (OFF/Open) position.
Pro-Tip for Panel Builders: If you are building a custom panel and want to prevent fading, avoid cheap ink-printed labels. Use laser-engraved stainless steel or molded thermoplastic rocker switches (like the Eaton 216 series) where the symbol is physically cut into the material and backlit by an LED.

Decision Path: Selecting the Right Symbol for Your Panel or PCB

Use this decision matrix to terminate your design choices with a concrete, standard-compliant pick. Do not mix these conventions on the same faceplate.

Your Application Scenario If-Then Condition Concrete Pick / Part Standard
Mains AC Disconnect (120V-600V) If the switch physically breaks the hot/live conductor to de-energize a load... Use IEC 60417-5007 (I) and 5008 (O). Buy rockers with molded I/O markings.
Microcontroller / Logic Soft Power If a GPIO pin triggers a MOSFET to latch power, or puts an MCU to sleep... Use IEC 60417-5009 (⏻). This tells the user the button triggers a logic sequence, not a physical air-gap disconnect.
PCB Jumper / Configuration Block If setting a hardware address, pulling a pin high/low, or enabling a feature... Use Binary 1 and 0. Silkscreen "1" for VCC/High and "0" for GND/Low. Add a dot indicating pin 1.
Momentary Pushbutton (Toggle Action) If pressing the button once turns it on, and pressing again turns it off... Use IEC 60417-5010 (⏽). Common on modern solid-state lighting dimmers and piezo switch panels.

Default Recommendation: For 95% of physical hardware designs involving human interaction with power, default strictly to IEC 60417-5007 (I) and 5008 (O). Reserve the standby (⏻) symbol exclusively for devices with internal power supplies that maintain a standby rail, and reserve binary 1/0 strictly for bare-PCB logic configuration. When in doubt, the physical "I" and "O" are universally recognized by both licensed electricians and hobbyist makers.