Ungrounded outlet indicates grounded when an UPS is plugged in

Question

I moved into an old apartment that has a mix of grounded, ungrounded, and GFCI ungrounded outlets. My computer is plugged into an ungrounded GFCI outlet. Plugging in a tester indicates open ground. But when i plug in my uninterruptible power supply (opti ups 1000c) the tester indicates "normal". I'm wondering what's causing the tester to indicate ground. Is this safe the way it is? Is the setup still capable of protecting my equipment without real ground?

Edit: to clarify, the tester alone plugged into the outlet indicates open ground. When I plug the UPS into the remaining outlet, without touching the tester, the tester indicates normal.

Answer 1

The cheapie 3-light testers work by connecting 3 lamps in a triangle fashion across the 3 outlet pins. The hot-neutral lamp works in the obvious way. The hot-ground lamp works if the ground pin can find a path back to neutral, presumably via a ground wire correctly bonded to neutral in the panel. But not necessarily.

It sounds like the UPS is connecting neutral to ground in some way. Perhaps with its own onboard ground tester.

All of this is idle amusement. If you know there's no ground wire, then that's your answer - the tester's output is irrelevant. These testers are a little bit hokey and are not the last word on the subject.

Answer 2

I'm going to guess you has cable TV or a coax antenna plugged into that UPS. Most connect the ground on the cable to the ground bus - that is probably what grounded it.