The universal symbol for off and on is governed by the IEC 60417 standard. A vertical line (I) denotes ON (closed circuit), a circle (O) denotes OFF (open circuit), and a circle broken by a vertical line denotes Power or Standby. When designing control panels, wiring enclosures, or restoring legacy equipment, using the correct symbol and understanding its regional variations is critical for safety and compliance.
The Master Reference Table: Power, Standby, and Switch Symbols
Before wiring any panel, confirm your actuator markings match the intended circuit state. The table below maps the standard graphical symbols to their IEC registry numbers, physical meanings, and internal switch states.
| Symbol Graphic | IEC 60417 ID | Meaning | Practical Application | Internal Switch State |
|---|---|---|---|---|
| I (Vertical Line) | 5007 | ON | Equipment is fully powered and operational. | Closed (Continuity) |
| O (Circle) | 5008 | OFF | Equipment is completely disconnected from power. | Open (No Continuity) |
| ⏻ (Circle + Line) | 5009 | Standby | Main load is off, but low-voltage logic/control circuits remain energized. | Partial (Auxiliary closed, Main open) |
| ⏼ (Circle + Line breaking top) | 5010 | Power | Connects equipment to the mains grid; device may enter ON or Standby state. | Closed (Mains Continuity) |
| I / O (Combined) | N/A (Common) | ON / OFF | Standard SPST/DPST rocker and toggle switches. | Binary (Closed / Open) |
Regional and Standard Variants (IEC vs. NFPA vs. Legacy)
While the I/O glyphs are globally recognized, the color coding and physical implementation of these symbols vary wildly by region and standard. Assuming a single global standard will lead to dangerous panel wiring.
IEC 60204-1 (Global / EU Machinery)
Under IEC machinery standards, Green is strictly reserved for ON/Start, and Red is strictly reserved for Emergency Stop or OFF/Stop. However, for standard power switches (not E-Stops), Black or Grey actuators with white I/O legends are preferred to prevent confusion with safety-critical E-Stop circuits.
NFPA 79 & NEMA (US Industrial)
In the US, NFPA 79 aligns closely with IEC for colors (Green = ON, Red = OFF/Stop). However, US industrial panels frequently use illuminated pushbuttons where a Red light indicates a fault or running state (depending on the specific HMI standard), making the physical I/O stamp on the actuator more reliable than the LED color alone.
Old UK (Pre-1970s BS 3939)
Rows People Get Wrong (And How to Fix Them)
When ordering switch caps or designing PCB silkscreens, makers and junior engineers frequently confuse the nuances of the IEC 60417 registry. Here are the most common errors:
- Confusing Standby (5009) with Power (5010): The Standby symbol (⏻) has the vertical line inside the circle, meaning the circuit is partially active (logic is alive, main load is off). The Power symbol (⏼) has the line breaking the top of the circle, indicating a hard mains connection. Use 5010 for your main AC inlet rockers, and 5009 for microcontroller sleep/wake buttons.
- Using 'O' for Earth/Ground: The circle (O) means OFF. It is not a ground symbol. Using an O near a grounding terminal will confuse technicians into thinking the ground can be switched off. Always use the standard IEC 60417-5019 earth symbol (a vertical line with three descending horizontal lines).
- Red Rockers for Mains Power: Using a red rocker switch with an I/O legend for a standard 120V AC power supply implies an emergency stop. If the user needs to cut power in a panic, they will slam the red switch. If that switch is just your main power and not an E-Stop, use a black or white rocker to avoid panic-induced hardware resets.
Safe Interpretation of Faded, Missing, or Non-Standard Markings
On jobsites and in repair shops, you will frequently encounter switches where the I/O paint has worn off, or where a previous technician replaced a rocker with an unmarked blank. Never guess the state based on the physical toggle position (up/down or in/out), as panel mounting orientations vary.
1. De-energize the circuit completely. Lock out the breaker or unplug the mains.
2. Set your multimeter to Continuity (or lowest Ohms range).
3. Place probes on the two main load terminals (usually the center and one outer pin on a standard SPST rocker).
4. Flip the actuator. If the meter reads < 1 ohm (or beeps), that physical position is your ON (Closed) state. If it reads OL (Over Limit), that is your OFF (Open) state.
5. Mark the panel with a silver paint pen or adhesive vinyl once verified.
If you are testing a live circuit (e.g., a 3-way residential switch or a 24V DC control circuit where de-energizing is impractical), switch your meter to AC or DC Voltage. Measure across the switch terminals. If you read the full source voltage (e.g., 120V AC or 24V DC) across the terminals, the switch is OPEN (OFF). If you read 0V across the terminals, the switch is CLOSED (ON) and the voltage is passing through to the load.
Decision Tree: Choosing the Right Switch and Symbol for Your Panel
Stop guessing which component to add to your BOM. Use this decision path to select the exact switch type and legend for your specific application.
| Application Scenario | Required Symbol / Legend | Concrete Component Pick |
|---|---|---|
| 120V/240V AC Mains Inlet (Desktop power supplies, amplifiers, motor controllers) | I / O (White legend on Black actuator) | Carling V-Series (e.g., V1D1B60B-ARC00-000) - Rated 15A 125VAC, IP65 sealed. |
| 24V DC Industrial Control (PLC panels, CNC limit overrides) | I / O (Green actuator for ON, Red for OFF/Stop) | Schneider Electric Harmony XB5 (XB5AA42 for Red OFF, XB5AA31 for Green ON) - 22mm flush mount. |
| 5V/3.3V DC Logic / PCB Mount (Arduino enclosures, ESP32 custom boards) | Standby (⏻) silkscreen next to tactile button | C&K PTS645 Series (e.g., PTS645SM43SMTR92 LFS) - 12mm tactile, 50mA 12VDC rated. |
| Battery Disconnect / High Current DC (Solar banks, LiFePO4 48V systems) | I / O (Engraved, no paint to wear off) | Blue Sea Systems 6006 (e-Series) - Rated 300A continuous, uses physical I/O mold marks. |
For 90% of DIY maker projects involving 3D-printed enclosures and 120V AC mains, the Carling V-Series rocker with a black actuator and white I/O legend is the default, safest, and most code-compliant choice. It fits standard 0.550" x 1.125" rectangular cutouts, handles up to 15A resistive loads, and the I/O legend is molded into the nylon, meaning it will never fade or rub off during years of bench use.
