On a GFCI outlet, is the imbalance of electrical current coming from the ground conductor and the neutral conductor, or is it read from the hot and neutral conductor? Will a GFCI outlet work on a two-wire only electrical system in a house?
A ground fault circuit interrupter (GFCI) detects an imbalance in electrical flow between the hot and neutral wire. A slight leakage will trip the GFCI and protect you from a shock. The electrical leak may be flowing through your body!
A GFCI outlet will work on a two-wire hot and neutral system with no ground. In operation, it measures between the hot and neutral wire. Without a ground, a GFCI tester will tell you it has no ground but the GFCI will work. This type of installation should have a sticker that says no equipment ground.
A GFCI tester uses a current test that leaks current to the ground wire and causes the device to trip. When there is no ground, the tester will read no ground and will not work. This may be the confusing part of a GFCI and GFCI tester.