The universal on and off symbols on a switch are the vertical line ("I" or "1") for the ON (closed circuit) state and the circle ("O" or "0") for the OFF (open circuit) state, as defined by the IEC 60417 standard. However, in North American mains wiring, UL and NEC guidelines mandate literal "ON" and "OFF" text for disconnects to eliminate binary ambiguity. If you are looking at a faded rocker, designing a control panel, or replacing a residential toggle, your region and voltage level dictate which marking standard is legally and practically required.

The Complete Switch Symbol Reference Chart

Before wiring or replacing any component, verify the markings against this reference. This table covers the physical embossing, printed ink, and illuminated indicators you will encounter on modern and legacy switches.

Symbol / Marking Meaning Standard Origin Binary / Logic Equivalent Common Application
| or I (Vertical Line) ON (Closed Circuit) IEC 60417-5007 Binary 1 / Logic HIGH Global electronics, DC panels, marine rockers
O (Circle) OFF (Open Circuit) IEC 60417-5008 Binary 0 / Logic LOW Global electronics, DC panels, marine rockers
ON / OFF (Text) Closed / Open Circuit UL 62 / NEC 404.8 N/A (Explicit Text) US/Canada residential mains, industrial disconnects
• (Solid Dot) Indicator ON (Illuminated) IEC / Manufacturer Spec Active State Illuminated rockers, push-button indicators
— (Horizontal Dash) Indicator OFF IEC / Manufacturer Spec Inactive State Illuminated rockers, push-button indicators
UP (Physical Position) ON (Mains Toggle) NEC 404.6 N/A (Mechanical) US residential wall switches, breaker handles

Regional Standards: IEC 60417 vs. UL/NEC Markings

The divide between symbol-based and text-based switch markings is fundamentally a regional and safety-code issue. Understanding which standard applies to your workspace prevents dangerous misinterpretations.

The International Standard (IEC 60417)

Globally, the International Electrotechnical Commission (IEC) governs graphical symbols. Standard 60417-5007 defines the vertical line for "ON" and 50417-5008 defines the circle for "OFF". This binary system (1 and 0) was adopted to transcend language barriers. You will find these on almost all consumer electronics, computer power supplies, and low-voltage DC marine panels worldwide.

The North American Standard (NEC / UL)

In the United States and Canada, the National Electrical Code (NEC) takes a stricter view on mains voltage disconnects. NEC Article 404.8 requires that switches clearly indicate the open and closed positions. For standard residential and commercial AC mains switches, UL listing and NEC enforcement heavily favor or mandate the literal words "ON" and "OFF". Furthermore, NEC 404.6 dictates that for vertically operated toggle switches, the UP position must be ON, and DOWN must be OFF. You cannot install a switch upside down in a US junction box.

The Old UK / Industrial Color Anomaly

When working on older industrial equipment or legacy UK installations, beware of illuminated switch colors. In modern US/IEC consumer contexts, Green means ON (safe/running) and Red means OFF (stop). However, in older British and some heavy industrial paradigms, a Red illuminated indicator meant the circuit was energized (ON/Danger), while Green meant the circuit was de-energized (OFF/Safe). Always verify with a meter rather than trusting legacy indicator colors.

The "Rows People Get Wrong" Notes

Even experienced makers and electricians misinterpret specific rows from the reference chart above. Here are the most common field errors:

Mistake 1: Treating the "I" as a Letter or Roman Numeral
Novices often look at the IEC "I" symbol and assume it stands for "Input," "Interrupt," or the Roman numeral 1, leading to confusion on multi-pole switches. It strictly represents the binary digit 1 (closed circuit, current flowing).
Mistake 2: Confusing the Indicator Dot (•) with the Circle (O)
On illuminated rocker switches, you will often see a solid dot (•) and a horizontal dash (—). Makers frequently confuse the dot with the IEC "O" (OFF) circle. The dot actually indicates the illuminated/active state. If the dot is pressed in, the switch is ON and the internal neon or LED is lit.
Mistake 3: Misreading Center-Off (ON-OFF-ON) Toggles
On a 3-position DPDT toggle switch, the center position is OFF. Makers sometimes wire the center position as a common feed, assuming it is a transitional state. In a standard (ON)-OFF-(ON) momentary switch, the center is the only stable open-circuit state; the outer positions are spring-return.

Safe Interpretation When Markings Are Faded or Missing

UV exposure, chemical cleaners, and mechanical wear frequently erase printed "ON/OFF" text or wear down embossed I/O symbols on older panels. Never guess the state of a faded mains switch. Follow this verification protocol:

  1. De-energize and Lockout: If the switch controls a hardwired appliance or subpanel, turn off the upstream breaker. Apply a lockout/tagout (LOTO) device if you are in a shared workspace.
  2. Verify Dead: Use a non-contact voltage tester (NCVT) and a CAT III or CAT IV multimeter to confirm zero voltage at the load side of the switch.
  3. Continuity Test: Set your multimeter to the continuity (diode/beep) setting. Place one probe on the line terminal and one on the load terminal. Toggle the switch. The position that yields a reading of < 1 ohm (and a continuous beep) is the ON (closed) state.
  4. Mechanical Default Check: If it is a standard US residential toggle switch and the markings are entirely gone, NEC 404.6 guarantees that the physical UP position is ON, provided the switch was installed legally and not upside down.
Pro-Tip for Smart Switches: If you are dealing with a faded smart switch (e.g., an older Lutron Caseta or Leviton Decora Smart), the physical paddle may lack any markings. These switches often use a momentary "tap" anywhere on the paddle to toggle state, while a small physical rocker at the bottom serves as the hardwired air-gap disconnect. The bottom rocker will always have a tiny embossed "O" or "I" on the side edges—use a flashlight at an oblique angle to read the shadow casting.

Decision Path: Which Switch Symbol Standard to Specify

When sourcing switches for a new build, repair, or panel design, use this decision matrix to select the exact standard and a concrete, reliable part number. Do not mix IEC binary symbols with US mains disconnects.

Your Scenario / Application Required Standard Symbol Type Needed Concrete Part Pick (2026)
US Residential Mains (120V/240V AC)
Replacing a wall switch or installing a dedicated appliance disconnect.		 NEC 404.8 / UL 20 Literal "ON" and "OFF" text. Vertical UP = ON. Leviton 5601-2W (15A/120V AC Quiet Toggle, White, UL Listed with clear text markings).
Global DC Panel / Marine / Consumer Electronics
Building a 12V/24V LiFePO4 battery bank control panel or export electronics.		 IEC 60417 / UL 1054 (for component) IEC "I" and "O" embossed or printed. Carling (Littelfuse) V851-211 (V-Series Contura II Rocker, 20A 12VDC, I/O illuminated).
US Industrial Motor Control / Machinery
Wiring a 3-phase motor starter or CNC spindle control box.		 NFPA 79 / NEMA ICS Literal "START" / "STOP" or "ON" / "OFF". Avoid I/O to prevent binary confusion in high-stress environments. Eaton E22KPB11 (E22 Series 30mm Pushbutton, Green Flush START / Red Mushroom STOP).
PCB Mount / Low Voltage Logic
Designing a custom PCB with an Arduino/ESP32 power cut-off.		 Manufacturer Datasheet Slide switch with schematic pinout (usually no physical markings, rely on silkscreen). C&K OS102011MA1QN1 (SPDT Miniature Slide Switch, right-angle PCB mount).

By matching the physical symbol to the governing regional code and the specific voltage domain, you ensure that anyone interacting with your panel or device—whether a homeowner, a marine mechanic, or an industrial inspector—can instantly and safely identify the circuit state.