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On a PSU rocker switch marked I and O, O means off and I means on. The marks are standardized graphical symbols, not arbitrary labels, and they describe whether the unit is connected to mains power.
What the I and O symbols mean
The two marks come from the IEC 60417 symbol set. Manufacturer symbol glossaries that reproduce the standard list them as follows:
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| Mark | IEC 60417 number | Meaning |
|---|---|---|
| I (vertical line) | 5007 | On: connection to mains |
| O (circle) | 5008 | Off: disconnection from mains |
| Broken circle with a line | 5009 | Standby: brings part of the equipment into standby |
Leica Biosystems’ symbol glossary lists 5007 as On, 5008 as Off and 5009 as Stand-by. Boston Scientific’s symbols glossary gives the mains connection and disconnection wording for 5007 and 5008. This is the usual layout on a PSU back panel, as shown in the I/O rocker in the Siemens SIMATIC IPC1047 operating instructions.
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The broken-circle power symbol (5009) is not the same as O. It marks a standby switch or position in which part of the equipment stays powered. The O on a PSU switch means the supply is disconnected from mains. That is why the front power button on a PC, which often carries the standby-style symbol, is not a substitute for the PSU’s rocker switch.
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What the symbol does not tell you
The symbol defines a mains-switch state only. It does not show that internal capacitors have discharged or that a particular PSU is safe to open. For that reason, nothing here should be read as repair guidance. If you need to know how a specific unit behaves, check its manual.
Where the standard comes from
The IEC describes IEC 60417 as its database of graphical symbols for use on equipment. The current listing, IEC 60417:2026 DB, is dated March 4, 2026. The symbol wording above comes from the manufacturer glossaries cited, not from the paid database itself.
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