The universal "on" symbol on a switch is a vertical line (I), representing a closed circuit (binary 1), while the "off" symbol is a circle (O), representing an open circuit (binary 0). This physical marking is mandated globally by the IEC 60417 standard. If you are reading a schematic rather than looking at a physical rocker, the "on" state is depicted by the movable contact arm resting against the fixed contact point. Below is the complete reference for identifying, interpreting, and wiring these symbols across different regions and switch types.
Switch "On/Off" Symbol Reference Table
Use this table to identify the physical markings on your switch actuator and their corresponding schematic states. This applies to rocker, toggle, and rotary disconnects.
| Physical Symbol | Schematic Equivalent | Meaning / State | Standard Designation | Typical Application |
|---|---|---|---|---|
| I (Vertical Line) | Closed contact (lever touching dot) | ON (Circuit Closed) | IEC 60417-5007 | DC rockers, PC power supplies, industrial panels |
| O (Circle) | Open contact (lever away from dot) | OFF (Circuit Open) | IEC 60417-5008 | DC rockers, PC power supplies, industrial panels |
| | (Single Line / UP) | SPST closed | ON (Single Pole) | NEC 404.6 / NEMA | US residential AC wall toggles (e.g., Leviton) |
| O with vertical line | SPDT center open | OFF (Center position) | IEC 60417 | 3-position toggle/rocker (ON-OFF-ON) |
| II (Double Line) | DPST closed | ON (Double Pole) | IEC / ISO | Heavy machinery, 240V AC main disconnects |
Regional & Standard Variants
While the I/O marking is globally dominant in electronics and DC applications, AC mains wiring and regional codes dictate different physical implementations.
- IEC (Global / EU / Modern DC): Strictly uses I and O. You will find this on almost all modern electronics, marine panels (Blue Sea Systems), and automotive rockers (Carling V-Series). The standard explicitly forbids using "1" and "0" to prevent confusion with numerical values.
- NEC / NEMA (US Residential AC): Standard single-pole wall switches (like the Leviton 5601) do not use I/O symbols. Instead, they rely on physical orientation and text. NEC Article 404.6 requires that switches clearly indicate their on/off position. By universal US convention, the UP position is ON, and the DOWN position is OFF. If a switch is mounted sideways or upside down, it must be replaced or reoriented.
- Old UK / Pre-1970s Industrial: Older rotary isolators and heavy-duty toggles often used a Red Dot for ON and a Black or Green Dot for OFF. Some legacy European panels used 1 (Ein/On) and 0 (Aus/Off), which is now deprecated in favor of the IEC line/circle to avoid language and numeric confusion.
Rows People Get Wrong (and Faded Marking Protocols)
Misinterpreting switch symbols leads to reversed logic or, worse, short circuits. Here are the most common pitfalls based on the reference table above.
The most dangerous mistake is assuming the "O" symbol represents the Neutral wire (0V). In AC wiring, Neutral is never switched on a standard single-pole branch circuit. The "O" strictly means Open Circuit. The switch only interrupts the Line (Hot) conductor. Never wire a neutral to a switch terminal based on the "O" marking.
On an ON-OFF-ON rocker (like a Carling VLD2), the center circle does not mean "ground" or "common". It simply designates the mechanical detent where the internal wiper connects to neither output terminal.
Safe Interpretation When Markings are Faded or Missing
On marine, automotive, and industrial panels, the paint on IEC rockers frequently rubs off. Do not guess the "on" direction based on the actuator's physical tilt, as internal mechanisms can be installed upside down in the panel cutout.
- De-energize the circuit. Turn off the breaker or disconnect the battery.
- Set your multimeter to Continuity (or Resistance).
- Probe the Common (C) and Output terminals. On a standard SPST rocker, probe the two metal spades. Flip the switch to the suspected "ON" position.
- Read the meter. If the meter reads < 1 ohm (or beeps), that physical position is your ON state. If it reads OL (Open Loop), that position is OFF.
- Mark it permanently. Use a silver or white industrial paint pen (like a Sanford Sharpie Metallic) to redraw the I on the verified ON side.
Schematic Symbols for the "On" State
When reading a wiring diagram or schematic, the "on" symbol on a switch is not a letter, but a mechanical representation. According to standard electronic schematic conventions, switches are drawn in their normal (unactuated) state.
- SPST (Single Pole Single Throw): Drawn as a hinged lever. If it is a "Normally Open" (NO) switch, the lever is drawn away from the contact dot. The "on" state occurs when you mentally (or physically) close the lever to touch the dot.
- SPDT (Single Pole Double Throw): The common terminal is the hinge. The lever is drawn touching one of the two output dots. The "on" state is relative: if the lever touches Terminal A, Circuit A is ON and Circuit B is OFF.
- Pushbutton (Momentary): A NO pushbutton is drawn with a gap between the actuator and the contacts. The "on" state is transient, lasting only while physical pressure is applied.
Decision Path: Identifying and Wiring Your Switch
Use this decision tree to determine your exact wiring procedure and select the correct replacement part if your current switch is damaged or unmarked.
| If your switch looks like... | Then the wiring logic is... | Concrete Action / Part Pick |
|---|---|---|
| IEC Rocker (I / O) with 2 spade terminals. | Line (Hot/Positive) goes to the terminal nearest the pivot point or marked "1". Load goes to terminal "2". (Check datasheet, as Carling and Blue Sea differ slightly on pin numbering). | Replace with Carling M-Series (M2020) for 12V/24V DC, or Carling L-Series for 120V AC. The molded actuators resist fading. |
| US Residential Toggle (No I/O, just UP/DOWN, brass/green screws). | Line and Load go to the two brass screws (interchangeable on standard SPST). Green screw is Ground. UP must be ON. | Replace with Leviton 5601-2W (15A) or Leviton 5603-2W (20A). Ensure the ground screw faces the bottom of the box. |
| 3-Terminal Rocker/Toggle marked 1, 2, 3 or COM, L1, L2. | This is an SPDT or 3-Way switch. "1" or "COM" is the common feed. "2" and "3" are travelers or separate loads. | For US 3-way AC circuits, use Leviton 5604. For DC ON-ON selection, use Blue Sea Systems 8066. |
| Faded / Unmarked Rocker on a custom DC panel. | Do not trust the physical tilt. Perform the continuity test outlined above to identify the closed circuit before applying power. | Swap to a Blue Sea Systems Contura V rocker. They feature raised, molded I/O symbols that cannot rub off, and integrated LED indicator windows. |
Default Recommendation: If you are building or repairing a panel and need a reliable, unambiguous "on" symbol on a switch that will survive years of use without fading, default to the Blue Sea Systems Contura V-Series for DC applications (rated up to 20A at 12V) and the Leviton Decora 5611 for AC mains. Avoid cheap, unbranded import rockers where the I/O paint flakes off within months, creating a severe safety and operational hazard.
