Whether you are wiring a custom ATX power supply, debugging an industrial control panel, or just trying to figure out if a European appliance is actually disconnected from the mains, understanding on off symbols is a baseline safety requirement. These markings are not arbitrary; they are strictly governed by international standards to prevent catastrophic operator error.
Below is the master reference table for physical panel symbols and schematic switch representations. Use this to identify exactly what state your circuit is in before you touch a terminal.
The Master Reference: Physical & Schematic On-Off Symbols
The following table covers the physical markings you will find engraved or printed on switches, rockers, and pushbuttons, alongside the schematic symbols used in wiring diagrams. The physical symbols are governed by the IEC 60417 standard, while schematic representations follow IEEE 315 and IEC 60617.
| Symbol Shape | Standard Ref | Official Name | Practical Meaning | Typical Application |
|---|---|---|---|---|
| Vertical Line (|) | IEC 60417-5007 | Power On | Circuit closed, main power active | Rear PSU rockers, industrial isolators |
| Circle (O) | IEC 60417-5008 | Power Off | Circuit open, main power disconnected | Rear PSU rockers, mains isolators |
| Line overlapping Circle (⏻) | IEC 60417-5009 | Power Toggle | Soft power, toggles system state | PC ATX power buttons, modern appliances |
| Line breaking Circle (⏾) | IEC 60417-5010 | Standby | Sleep mode, logic powered, main load off | TVs, monitors, smart home hubs |
| SPST Schematic | IEEE 315 / IEC 60617 | Single Pole Single Throw | Basic mechanical on/off interrupt | Wiring diagrams, control schematics |
| Text '1' and '0' | Legacy / Non-Std | Binary State Indicator | 1 = Closed (On), 0 = Open (Off) | Older PLCs, legacy industrial panels |
Rows People Get Wrong (And the Standby Hazard)
When reading the table above, hobbyists and junior technicians frequently misinterpret two specific rows, leading to either damaged components or electrical shock.
The Standby (⏾) vs. Hard Off (O) Confusion
The most dangerous mistake on the bench is treating the Standby symbol (Row 4) as a Hard Off (Row 2). When you press a button marked with the ⏾ symbol on a PC power supply or a modern inverter, you are not breaking the main AC input. You are signaling the internal logic to shut down the high-power rails, but the 5V Standby (5VSB) rail remains live.
If you are probing a 24-pin ATX connector and assume the PSU is dead because the fan stopped and the standby light is on, you can easily short the 5VSB line to ground or touch a primary-side capacitor that is still charged. Hard Off (the 'O' symbol on the rear rocker) physically breaks the AC line connection via a mechanical switch. Always flip the physical rocker to 'O' before opening a chassis.
The '1 and 0' Myth
Look at Row 1 and Row 2. It is a widespread misconception that the '|' and 'O' symbols are the literal numbers '1' and '0'. While they map conceptually to binary logic (1 for closed circuit, 0 for open circuit), the IEC 60417 standard defines them strictly as geometric shapes: a line and a circle. This distinction matters when you are ordering replacement rocker switches from suppliers like Carling or Cherry. If you ask for '1 and 0' printed rockers, you may receive legacy text-labeled switches that do not meet modern IEC compliance for international equipment export.
Regional Variants: IEC vs. IEEE vs. Legacy Mains
While the IEC 60417 symbols are the global standard for physical equipment markings, regional practices and schematic standards introduce variations you must recognize.
Schematic Symbols (IEEE 315 vs. IEC 60617)
If you are reading a wiring diagram rather than looking at a physical switch, the symbols change entirely. The IEEE 315 standard (dominant in North America) represents a basic SPST on-off switch as a line with a hinged lever breaking contact. The IEC 60617 standard (dominant in Europe) represents the same switch as a simple line with a diagonal slash breaking the circuit. If you are troubleshooting a control panel, ensure you know which schematic standard the original equipment manufacturer (OEM) used, or you might misinterpret a normally-open (NO) contact as a normally-closed (NC) one.
Legacy UK and North American Mains Indicators
Before the widespread adoption of IEC binary symbols, physical switches relied on color and text. In older UK installations, you will frequently find Red/Green illuminated rockers where Red indicates ON (closed) and Green indicates OFF (open). This directly contradicts modern safety logic where green usually implies 'safe to proceed' or 'active'. In North America, older industrial panels often used the literal text 'START' and 'STOP' or 'I' and 'O' stamped into steel. When retrofitting these panels, NEC-style guidance dictates that you must maintain the original operational logic or completely re-label the panel to avoid operator reflex errors during an emergency.
Safe Interpretation When Markings Fade or Fail
In industrial environments, UV exposure, chemical solvents, and physical abrasion will eventually wipe the IEC symbols off a rocker switch or disconnect handle. Never guess the state of a circuit based on the physical position of the actuator alone.
The Verification Protocol
When the on off symbols are faded, missing, or obscured by grime, follow this exact verification sequence before touching any conductors:
- Actuate to the presumed 'Off' position: Flip the switch to the circle (O) position, or rotate the disconnect handle to the marked or presumed open position.
- Test your meter: Set a CAT III or CAT IV rated multimeter (like a Fluke 117 or 87V) to AC voltage. Test the meter on a known live source (like a proven outlet or voltage proving unit) to verify the leads and internal fuse are intact.
- Test Line-to-Line and Line-to-Ground: Measure across all phase combinations (L1-L2, L2-L3, L1-L3 on 3-phase; L-N on single-phase) and from every line to the equipment grounding conductor. You must read 0V (or negligible induced millivolt ghost readings) across all combinations.
- Verify Continuity (De-energized only): If you are testing a low-voltage DC switch or a disconnected component, switch your meter to continuity mode. Place probes across the switch terminals. A reading of '< 1 ohm' means the switch is closed (ON). An 'OL' (Over Limit) reading confirms the circuit is open (OFF).
If you are working on mains voltage equipment and the physical markings are entirely gone, OSHA Lockout/Tagout (LOTO) procedures require you to apply a physical lock to the disconnect and label it clearly before beginning work. Do not rely on faded paint or memory when 120V to 480V is involved; let the multimeter dictate the truth.
