What Are the Symbols for On and Off? IEC & NEMA Switch Guide

The universal symbol for On is a vertical line (I or |), and the universal symbol for Off is a circle (O or ○). These markings are defined by the IEC 60417 standard and represent the binary states of a closed circuit (1) and an open circuit (0). However, if you are wiring industrial panels in North America, you will frequently encounter literal ON / OFF text markings governed by NEMA and NEC-style practices instead of symbols.

Power Switch Symbol Reference Table

Use this chart to identify the markings on your rocker, toggle, and rotary switches. Always verify the standard applied to your specific equipment before assuming the switch state.

Regional and Standard Variants (IEC vs. NEMA vs. Legacy)

Assuming a single global standard for switch markings is a fast track to a miswired panel or an accidental startup. The symbols for on and off change depending on the region, the era of the equipment, and the governing safety body.

The IEC 60417 Standard (Global & Consumer Electronics)

The International Electrotechnical Commission (IEC) standardized the line and circle symbols to overcome language barriers. According to the IEC 60417 graphical symbols database, the vertical line represents the binary '1' (circuit closed, power flowing), and the circle represents the binary '0' (circuit open, power halted). You will find these on almost all modern IT equipment, consumer appliances, and imported test gear. If you are building a custom enclosure for an ESP32 or Arduino project using off-the-shelf Carling V-series rocker switches, you will almost exclusively see the IEC symbols.

NEMA and NEC Practices (North American Industrial)

In North American industrial environments, the National Electrical Manufacturers Association (NEMA) and the National Electrical Code (NEC) heavily favor unambiguous, literal text. While IEC symbols are permitted on consumer goods, industrial motor starters, heavy disconnects, and main breaker panels typically use stamped or printed ON and OFF text. This is largely driven by Lockout/Tagout (LOTO) safety protocols. As outlined in OSHA 1910.147 LOTO standards, energy isolation devices must be clearly identifiable. In high-stress, low-visibility industrial settings, literal text reduces the cognitive load and misinterpretation risk compared to a small stamped 'I' or 'O'.

The Danger of Legacy Color-Only Markings

Older British Standard (BS) and legacy European machinery sometimes relied purely on color coding: Red for Stop/Off, Green for Start/On. Never rely on color alone. This practice is now widely deprecated because it fails colorblind users (specifically those with red-green color vision deficiency) and becomes useless when the paint fades, gets covered in grease, or is obscured by shop lighting. If you are retrofitting an old machine with color-only buttons, overlay them with IEC symbols or NEMA text labels immediately.

The 'Rows People Get Wrong' and Faded Markings Guide

Even with standardized symbols, bench and jobsite realities introduce ambiguity. Here are the most common interpretation errors and how to safely resolve them.

Common Symbol Confusions

• Confusing 'I' with a minus sign (-) or dash: On some cheap, molded rocker switches, the vertical line for 'On' is poorly stamped and looks like a horizontal dash if viewed at an angle. Remember: a dash is not a standard power symbol. If it looks like a minus sign, it is likely a malformed 'I' (On).
• Confusing 'O' with the number '0' (Zero): While they mean the same thing in binary logic, some legacy rotary cam switches use '0' for the off position and '1', '2', '3' for varying speed or voltage taps. If you see a '0' next to a '1' on a rotary switch, '0' is Off. If you see an 'O' next to an 'I', it is the IEC standard.
• The 'Standby' Symbol (Line inside a circle): Do not confuse the IEC On/Off symbols with the IEEE 1621 standby symbol (a vertical line breaking the top of a circle). Pressing a standby button does not fully isolate the circuit from mains voltage; it only drops the device into a low-power state. For physical safety and LOTO, a standby button is not an 'Off' switch.

How to Safely Verify a Faded or Unmarked Switch

If you are troubleshooting a piece of equipment and the symbols for on and off are completely illegible, use a digital multimeter (DMM) to determine the switch state safely.

1. De-energize the circuit: Turn off the main breaker or unplug the device. Verify it is dead using a non-contact voltage tester or by measuring across known live terminals with your DMM.
2. Set your DMM to Continuity (Ω): Use a reliable meter like a Fluke 117 or Klein MM400. Touch the probes together to ensure it beeps and reads < 1 ohm.
3. Probe the switch terminals: Place one probe on the input (line) terminal and the other on the output (load) terminal of the switch.
4. Interpret the reading: If the meter beeps and reads < 1 ohm, the switch is internally closed (This is the ON / I position). If the meter reads 'OL' (Open Loop) or infinite resistance, the switch is internally open (This is the OFF / O position).
5. Mark the panel: Once verified, use a paint pen or adhesive label to permanently re-mark the switch with the correct IEC or NEMA symbol.

Frequently Asked Questions

Why is the 'On' symbol a line and the 'Off' symbol a circle?

The symbols are derived from binary logic and Boolean algebra, which form the foundation of digital electronics and early computing. In binary, a '1' represents a closed circuit, high voltage, or a 'true' state (On). A '0' represents an open circuit, low voltage, or a 'false' state (Off). When the IEC needed a language-agnostic way to label physical power switches in the 1970s, they adopted the binary '1' (stylized as a vertical line 'I') and '0' (stylized as a circle 'O') to represent the physical make-and-break states of the internal copper contacts.

What do the symbols for on and off mean on a 3-way toggle switch?

Standard residential 3-way and 4-way light switches (like the Leviton 5603 or Eaton 705S) do not use 'I' and 'O' symbols, nor do they use 'ON' and 'OFF' text. This is because a 3-way switch is a single-pole, double-throw (SPDT) device. It doesn't break the circuit entirely; it merely diverts the current path to one of two traveler wires. Therefore, the physical toggle position (up or down) does not universally correlate to 'On' or 'Off'—it depends entirely on the state of the other 3-way switch in the circuit. If you see 'ON/OFF' stamped on a toggle, it is a standard single-pole switch, not a 3-way.

Are the on and off symbols ever reversed on imported power supplies?

While the IEC 60417 standard is strictly enforced for certification marks (like CE, UL, or RoHS), cheap, unregulated components from overseas marketplaces sometimes feature malformed tooling where the 'I' and 'O' are swapped on the plastic rocker cap. This is rare but highly dangerous. Never trust the symbol on an unbranded, uncertified inline power cord or bench supply. Always verify the output state with a multimeter or a plug-in receptacle tester before connecting sensitive embedded electronics like a Raspberry Pi or an expensive 3D printer control board.

How do I wire an SPST rocker switch if it has no symbols?

If you are wiring a basic Single-Pole, Single-Throw (SPST) rocker switch with two spade terminals and no markings, the switch is non-polarized. It acts as a simple mechanical bridge. You wire the Line (Hot) from your power source to one terminal, and the Load (going to your device) to the other. The physical orientation of the switch in the panel dictates which way the rocker toggles for 'On'. Use your multimeter's continuity function to identify which physical rocker position closes the circuit, and install it into your panel enclosure accordingly so that pressing the top (or the side marked with your own label) completes the connection.