This is most often a warning on a UPS that indicates that there is something wrong with the electrical ground. (In the US, that would be the round pin on the bottom of the plug) The most common problem would be that the ground in your wall outlet isn't actually connected to ground and has a non-zero electrical charge to it. This could also be caused by a short-circuit or a faulty Uninterruptable Power Supply.

There are also electrical test devices that can detect this problem.

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