The universal physical symbols for power are "I" (ON) and "O" (OFF), defined by IEC 60417 as binary 1 and 0. In schematic diagrams, IEEE 315 (US) and IEC 60617 (Global) use specific line-and-gap graphics to denote Single Pole Single Throw (SPST), Double Throw (SPDT), and momentary states. If you are wiring a panel, reading a datasheet, or troubleshooting a control board, confusing a Normally Open (NO) symbol with a Normally Closed (NC) symbol can result in a dead short or a failed safety interlock. Below is the exact reference data you need to identify, draw, and test these switches safely.

The Master Reference: On and Off Switch Symbols

Before pulling wire or soldering a PCB, verify your switch type against this table. This covers both the physical markings you see on the device housing and the schematic symbols you will find in wiring diagrams. The data below assumes standard maintained (latching) states unless marked as momentary.

Symbol / GraphicStandardSwitch TypeState (On/Off)Typical Application
I / O (Physical Marking)IEC 60417-5007/5008Power Rocker / ToggleI = ON (Closed)
O = OFF (Open)		PC power supplies, appliance mains, bench power supplies.
Single line with gap (Schematic)IEEE 315 / IEC 60617SPST (Single Pole Single Throw)ON when gap is bridged
OFF when gap is open		Basic lighting circuits, simple disconnects, inline fuse holders.
Line with two gaps (Schematic)IEEE 315 / IEC 60617SPDT (Single Pole Double Throw)ON1 / ON2 (Break-before-make)3-way stairway switches, motor forward/reverse selectors.
Pushbutton with open gap (Schematic)IEC 60617Momentary Normally Open (NO)OFF at rest
ON when pressed		Doorbells, motor start buttons, PLC input triggers.
Pushbutton with closed gap (Schematic)IEC 60617Momentary Normally Closed (NC)ON at rest
OFF when pressed		Emergency stops (E-stops), motor stop buttons, safety interlocks.

Note: For comprehensive schematic drafting rules, refer to the All About Circuits switch reference guide, which breaks down the mechanical linkage representations for multi-pole switches.

Regional Variants: IEC, IEEE, and Legacy Markings

Switch symbols and physical toggle orientations are not universally identical. Your region and the origin of the equipment dictate which standard applies. Misinterpreting a legacy UK switch or a US-spec toggle in an international control panel is a common source of wiring errors.

Physical Device Markings (The I/O Rule)

A widespread myth is that the "I" stands for the Roman numeral one, or "Input". In reality, the IEC 60417 standard explicitly defines these as binary digits: 1 (closed circuit/ON) and 0 (open circuit/OFF). This is globally harmonized for physical equipment markings. If you see a switch marked with "ON/OFF" text instead of I/O, it is likely an older or region-specific consumer device, but the electrical logic remains identical.

Schematic Standards: IEEE 315 vs. IEC 60617

  • IEEE 315 (North America): Uses a "hockey stick" or angled lever to represent the movable contact of a switch. A dot indicates a fixed pivot point. This is the standard you will see in most US-based NEC-compliant wiring diagrams and older military schematics.
  • IEC 60617 (Europe/Global): Uses a simple, straight angled line for the movable contact, without the distinct "hockey stick" bend. It relies heavily on standardized alphanumeric designators (e.g., S1 for switch 1) rather than purely graphical cues.

Toggle Switch Orientation (Up vs. Down)

In the US, the National Electrical Code (NEC) and general NEMA practices dictate that for standard vertical toggle switches (like a residential light switch), UP is ONDOWN is OFF. However, in older UK, Australian, and some European installations, you may encounter legacy panels where DOWN is ON

Rows People Get Wrong and Faded Marking Protocols

Even experienced makers and electricians stumble on specific switch definitions. Here are the most common misinterpretations and how to resolve them on the bench.

The "Open = On" Fallacy

In everyday language, an "open" door means you can walk through it. In electrical theory, an open circuit means the path is broken (OFF). When reading schematic rows for NO (Normally Open) and NC (Normally Closed) contacts:

  • Normally Open (NO): The switch is physically OFF (open gap) when at rest. It only turns ON when actuated. (Think: Doorbell).
  • Normally Closed (NC): The switch is physically ON (closed gap) when at rest. It turns OFF when actuated. (Think: E-Stop button).

Getting this backward on a 240V motor contactor control circuit will cause the motor to run the second the control power is energized, bypassing your start button entirely.

Momentary vs. Maintained (Latching)

Schematics will often show a small spring symbol or a dashed line next to the switch actuator. If you see this, the switch is momentary—it will return to its normal state the second you remove your finger. If there is no spring symbol, it is maintained (latching), meaning it stays in the ON or OFF position until you physically flip it back. Never substitute a maintained switch for a momentary requirement in a PLC jog circuit, or you will lose positional control.

Safe Interpretation When Markings Are Faded or Missing

WARNING: Never guess the state of a switch on a live mains circuit (>50V AC / >120V DC). If a switch's I/O molding is worn away, or a schematic is missing, you must de-energize the circuit, apply Lockout/Tagout (LOTO), and verify dead with a known-working multimeter before probing.

When you are staring at an unmarked 3-terminal toggle switch on a piece of salvage equipment and need to know which pin is Common (C), Normally Open (NO), and Normally Closed (NC), follow this exact bench procedure:

  1. Set your DMM to Continuity: Select the mode that beeps when resistance is below 10 ohms.
  2. Probe the Center Terminal: Place one probe on the middle terminal (usually the Common pole on standard SPDT toggles).
  3. Probe Terminal A: Place the second probe on one of the outer terminals. Flip the switch. If it beeps in one position and is silent in the other, you have found your Common and your first throw.
  4. Probe Terminal B: Move the second probe to the remaining outer terminal. Flip the switch. It should beep in the opposite position from Terminal A.
  5. Map it out: Mark the Common (C) with a silver sharpie. Mark the terminal that connects when the toggle is thrown "forward" as NO, and the terminal that connects when thrown "backward" as NC (relative to your desired logic).

For deeper code compliance regarding switch placement and disconnect requirements, always cross-reference your local AHJ guidelines and the NFPA 70 (National Electrical Code) Article 404, which governs the installation of switches in residential and commercial environments.