"A GFCI is not dependent of a ground to function. It does not measure shorts to the ground, it measures the current difference between the hot and neutral wires. A sudden difference of 5 ma. or more, indicating that there is another path for the electricity to flow through will trip this device."
I have heard that a GFI can be wired so that it still works without a ground. Is this true? Every time I install one without a ground, it acts like a ordinary receptacle and the GFI does not trip when shorted to ground.
Yes, it's true. The GFI breaker in a panel doesn't have a ground connection run to it. A GFI receptacle's ground terminal connects only to the yoke and the grounding contact, not to the GFI electronics.
If you fit a GFCI on an old 2-wire ungrounded circuit, then the reason the GFI doesn't trip when you use a plug-in tester is that the ground pin on the receptacle isn't actually grounded. If you ran a circuit from the receptacle hot to a grounded object, then it will trip.
rowd, A plug in tester will not test a GFCI that is installed without an EGC. That tester works by placing a resistor between the hot and ground. If there is no ground, then there will be no current flow and the GFCI will not trip. Just use the test button on the GFCI. It works by placing a resistor between the line side hot and the load side neutral. It doesn't need the EGC to function. There is no need to use any type of external tester to test GFCI receptacles. Don
The test button on the GFI itself tests the electronics and trip circuitry just as effectively as applying an external hot to ground connection.
All the test button does is unbalanced the hot & neutral currents through the sensing xfmr by connecting load-side hot to line-side neutral via a suitable resistance.
About the only way that you could have the test button trip the GFI while a 6mA external fault would not, would be if the test resistor went low in value and at the same time the GFI lost sensitivity. Not likely!