The Universal Switch On and Off Symbol Reference Table
The following table covers the physical markings found on hardware actuators (rockers, toggles, pushbuttons) as well as the schematic symbols used in ladder logic and wiring diagrams. These are governed primarily by IEC 60417 for graphical symbols on equipment.
| Symbol / Marking | Standard Designation | Meaning in Practice | Common Application |
|---|---|---|---|
| | (Vertical Line) or 1 | IEC 60417-5007 | "On" (Power closed / circuit complete) | Rocker switches, toggle switches, mains disconnects |
| O (Circle) or 0 | IEC 60417-5008 | "Off" (Power open / circuit broken) | Rocker switches, pushbuttons, E-stop collars |
| ⏻ (Circle with line breaking top) | IEC 60417-5009 | "Standby" or "Soft Power" (Auxiliary circuit remains live) | PC motherboards (ATX 5VSB), consumer electronics, TVs |
| ⏼ (Circle with line inside, fully enclosed) | IEC 60417-5010 | "On/Off" (Toggle power state / Sleep) | Smart home relays, laptop sleep buttons, lighting controllers |
| ⭘ (Circle with dot / NO contact) | IEC 60617 / NEMA | Normally Open (NO) contact (Schematic only) | Relay ladder logic, PLC diagrams, control schematics |
| ⦸ (Circle with slash / NC contact) | IEC 60617 / NEMA | Normally Closed (NC) contact (Schematic only) | Emergency stops, safety interlocks, limit switches |
Regional Standards and the "Rows People Get Wrong"
While the I/O markings are globally recognized, regional interpretations of color codes and specific symbol nuances vary between IEC (International/Europe), NEMA/UL (North America), and older legacy systems.
The Rows People Get Wrong
- The Standby Symbol (⏻) is NOT a Hard Off: This is the most dangerous misconception in electronics repair. Pressing a button with the ⏻ symbol on an ATX computer power supply or a modern microwave does not disconnect the mains. It switches off the high-current rails but leaves a low-voltage auxiliary circuit live (typically the 5VSB rail at 5V DC) to listen for a wake signal. Always unplug the device or lock out the breaker before touching internal components, even if the standby light is off.
- "I" and "O" are Numbers, Not Letters: The markings are not the letters "I" (for Input/Ignition) and "O" (for Output/Off). They are the binary digits
1(closed circuit/true) and0(open circuit/false). This distinction matters when reading faded labels; if you see a marking that looks like a rectangle or a slashed zero, it is the binary zero (Off). - Pushbutton Color Codes (IEC 60204-1 vs. Old UK/US): Under modern NEMA and IEC 60204-1 machine safety standards, a Green pushbutton means "Start/On" (safe condition) and Red means "Stop/Off" (danger/remove power). However, on older UK and US military/industrial gear, you will frequently find Red for "On/Power Applied" (danger) and Green for "Off/Safe". Never rely solely on color without verifying the mechanical state or consulting the specific machine schematic.
Safe Interpretation When Markings Are Faded or Missing
On industrial equipment, marine panels, or vintage audio gear, the physical I/O markings on toggle and rocker switches often wear off due to UV exposure, chemical cleaners, or mechanical friction. When you cannot visually confirm the switch on and off symbol, you must determine the state electrically.
Step-by-Step Continuity Testing
- De-energize and Isolate: Turn off the main breaker or disconnect the battery. Apply LOTO procedures. Verify the circuit is dead using the "Live-Dead-Live" test method on your meter.
- Set Meter to Continuity/Ohms: Use a multimeter set to the Ω (Ohms) or continuity (diode/sound) setting. For a standard SPST (Single Pole Single Throw) switch, you only need to test two terminals.
- Probe the Terminals: Place one probe on the common (C) or line terminal, and the other on the normally open (NO) or load terminal.
- Read the Actuator State:
- If the meter reads < 1.0 Ω (or beeps), the switch is mechanically in the ON (1) position.
- If the meter reads OL (Open Loop) or infinite resistance, the switch is in the OFF (0) position.
- Mark the Panel: Once verified, use a silver paint pen or industrial label maker to re-mark the switch with the correct IEC 5007/5008 symbols to prevent future confusion.
Dealing with Illuminated Rocker Switches
If the switch contains a built-in neon or LED indicator, continuity testing can yield false readings. The internal current-limiting resistor and diode will cause the multimeter to read a high resistance (often 100kΩ to 220kΩ) in the "On" position instead of a dead short. If you see a high, stable resistance value instead of OL, the switch is ON. If you see OL, it is OFF. For absolute certainty, remove the switch from the circuit entirely to eliminate parallel path interference.
Frequently Asked Questions
What does the circle and line mean on a power switch?
The circle with a vertical line breaking through the top (⏻) is the IEC 60417-5009 "Standby" symbol. It indicates that the primary high-power load is disconnected, but a secondary low-power circuit remains active to monitor for a remote wake-up signal or maintain real-time clock memory. The circle with the line fully enclosed inside it (⏼) represents a soft-toggle or sleep state, commonly found on modern solid-state relays and smart home devices where a single press cycles the power state.
Why do some switches have I and O instead of On and Off?
The "I" and "O" markings originated from the binary number system (1 and 0) adopted by international engineering committees post-WWII to create a language-neutral standard. "On" and "Off" are English-specific, which created safety hazards on exported machinery. By using 1 (closed circuit/true) and 0 (open circuit/false), operators globally could understand the switch state regardless of their native language. Over time, the numbers were stylized into the vertical bar (|) and circle (O) we see on modern rockers.
Is the "I" side of a toggle switch always the power source?
No. The "I" and "O" markings indicate the state of the circuit (closed or open), not the direction of current flow or which terminal is Line vs. Load. On a standard SPST toggle switch, power (Line) should generally be wired to the common terminal, and the load wired to the switched terminal, but the physical orientation of the "I" marking on the bat or rocker depends entirely on the manufacturer's mechanical design. Always check the manufacturer's datasheet or test with a multimeter to confirm which terminal is energized when the actuator is in the "I" position.
How do I read a faded switch symbol on old industrial equipment?
When markings are completely gone, do not guess based on the physical position of the toggle bat (e.g., assuming "up" is always "On", as inverted panels or specific DPDT center-off switches break this rule). De-energize the panel, isolate the specific switch, and use a multimeter in continuity mode to map the terminals. Cross-reference the physical switch state with the machine's original electrical schematic (usually found inside the control cabinet door) to verify if the switch is a maintained contact, momentary contact, or a safety interlock.
