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Any idea of the symbol marked in red in the picture below?

This is on a power supply of an "electric height adjustable desk frame".

Is this "use below or equal 2000m altitude" due to smaller dielectric strength of air at higher altitudes, as explained in How does altitude affect AC-DC power supplies?

Any recommended website to look for such symbols?

Thank you

enter image description here

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It's an altitude restriction, saying that it's only certified for use at altitudes less than 2000 meters. The symbols aren't very standard, so each manufacturer is free to decide how to convey that information.

There is some component inside that is not tested above that altitude, I'm not sure if that is due to the capacitors, or some other component, or due to spacing of the traces on the PCB and the current carrying abilities, etc.

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The main issue, as explained in the link you included, is creepage and clearance distances from primary to secondary. This needs to be increased at higher altitudes due to the reduced dielectric strength of the air.

They apparently only designed to the basic spec, which is 2000m, and didn't use a multiplier.

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