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.