The universal 'on/off symbol' (⏻) that you see on monitors, PCs, and consumer electronics is technically IEC 60417-5009, and it does not mean hard power disconnect. It means standby. True power isolation requires the line (⏽) and circle (⭘) symbols, or explicit text. Misinterpreting these symbols on a workbench or in an industrial panel can leave primary-side AC voltage energized, creating a severe shock hazard during maintenance.
The Complete On/Off Symbol Reference Table
The following table covers the definitive power state symbols registered in the IEC/ISO graphical symbols database. Reference this before silkscreening a custom PCB, designing a front panel, or troubleshooting an unmarked legacy device.
| Symbol | IEC Number | Official Name | Meaning in Practice | Typical Application |
|---|---|---|---|---|
| ⏽ | 60417-5007 | Power On | Hard connection to power source. Equipment is fully energized. | Industrial control panels, heavy machinery rockers. |
| ⭘ | 60417-5008 | Power Off | Hard disconnect. Equipment is fully de-energized from the source. | Industrial control panels, heavy machinery rockers. |
| ⏻ | 60417-5009 | Standby / Power | Soft switch. Removes main power but leaves logic/standby rails energized. | PC motherboards, consumer electronics, SMPS front panels. |
| ⏼ | 60417-5010 | Power On/Off | A single button that toggles between hard on and hard off (rare). | Specialized medical or lab equipment. |
| I / O | N/A (Binary) | Closed / Open | Physical circuit state. 'I' is closed (On), 'O' is open (Off). | Physical rocker switches, toggle switches, breaker handles. |
Rows People Get Wrong (And Why It Matters)
The most dangerous mistake on the bench is assuming the ⏻ (5009) symbol means the device is safe to open. In an ATX power supply, pressing a 5009 button shuts down the main 12V/5V/3.3V rails, but the 5VSB (5V Standby) rail remains active. The primary side of the switch-mode power supply (SMPS) is still connected to mains AC. If you are probing the primary side, a 5009 symbol means you must still unplug the physical AC cord or lock out the upstream breaker.
The 'I' and 'O' Rocker Confusion: Many hobbyists assume the 'O' on a rocker switch stands for 'On' because it matches the first letter of the word. In electrical engineering, 'I' and 'O' represent binary states or physical contact states. 'I' represents a closed circuit (current flows, device is On). 'O' represents an open circuit (current is interrupted, device is Off). If you wire a DPST rocker switch backward based on the letter 'O', your equipment will power on when the user expects it to be off.
Faded or Missing Markings: If you encounter a legacy industrial panel where the distinction between 5009 (Standby) and 5007/5008 (Hard Disconnect) is faded or painted over, always assume it is 5009 (Standby). Do not rely on the switch for Lockout/Tagout (LOTO). Trace the circuit back to a verified upstream disconnect or pull the physical plug.
Regional Standards: IEC vs. UL/NEC Requirements
While IEC 60417 is the global standard for consumer and light commercial electronics, North American industrial and mains-wiring codes have stricter requirements for main disconnects.
- IEC 60417 (Global/EU): Permits the use of 5007, 5008, and 5009 symbols on consumer electronics, IT equipment, and light commercial appliances without supplementary text.
- UL 508A & NEC (North America): For industrial control panels, UL 508A standards and NEC Article 110.22 require that the main disconnect be clearly identified. A simple ⭘ (5008) or 'O' symbol on a 480V 3-phase main breaker is insufficient. It must be explicitly marked with words like 'MAIN', 'DISCONNECT', or 'ON/OFF' to ensure first responders and maintenance personnel can identify it instantly under stress.
- Legacy UK (BS 3936): Older British equipment sometimes used color-coding alongside symbols (Red for Off/Open, Green for On/Closed). Note that this is the opposite of modern North American indicator light conventions (where Red often means energized/running and Green means safe/stopped). Always verify the schematic rather than trusting legacy color codes.
Decision Tree: Which Symbol to Specify on Your Panel
Use this decision path to select the correct marking for your next PCB silkscreen, front panel overlay, or wiring project. Do not default to the ⏻ symbol out of habit; match the symbol to the actual circuit topology.
| Condition / Circuit Topology | Action / Symbol to Use | Concrete Implementation |
|---|---|---|
| Switching mains AC directly via a mechanical relay or contactor. | Use I / O or 5007/5008 | Use a Carling V-Series rocker switch with physical I/O moldings. Add 'ON/OFF' text if >50V AC. |
| Switching low-voltage DC logic (e.g., microcontroller GPIO triggering a MOSFET). | Use ⏻ (5009) | Silkscreen the IEC 60417-5009 icon next to a tactile switch or capacitive touch pad on the PCB. |
| Designing a software UI or touchscreen interface. | Use ⏻ (5009) | Use the standard Unicode U+23FB (⏻) character. It is universally recognized in software contexts. |
| Designing a main disconnect for an industrial panel (>50V). | Use Explicit Text | Engrave 'MAIN DISCONNECT' on the handle. Use Red/Yellow handle per NFPA 70 (NEC) guidelines. |
Wiring and Implementation Best Practices
When wiring a physical switch to correspond to these symbols, the topology must match the user's expectation set by the icon.
Wiring the 5009 (Standby) Symbol: This is typically a momentary pushbutton wired to a microcontroller or a dedicated power management IC (PMIC). The switch does not carry the main load current. Instead, it pulls a logic pin low (via an internal or external pull-up resistor) to signal the PMIC to disable the main buck/boost converters while keeping the LDO (Low Dropout Regulator) for the logic rail active. Use an optocoupler (like the PC817) if the logic side needs to be galvanically isolated from a higher-voltage control circuit.
Wiring the 5007/5008 (Hard On/Off) Symbols: If you are using a DPST (Double Pole, Single Throw) rocker switch with I/O markings to hard-switch a 120V AC device, you must break both the Line (Hot) and the Neutral conductors. Breaking only the Line leaves the internal circuitry referenced to Neutral, which can still present a shock hazard if the building's wiring has a reversed polarity fault. Always verify your switch ratings: a switch rated for '10A 250V AC' may only be rated for '1A 24V DC' due to the lack of a natural zero-crossing to extinguish DC arcs.
