Confusion while diagnosing an open ground

Question

I have an outlet tester and it says the outlet in my bathroom has an open ground and I'm trying to see if I can fix this by replacing the receptacle or if the problem is elsewhere.

With a voltage meter I tested voltage between hot and neutral and got 120 (good), I tested between hot and ground and got 50ish (bad, should be 120), and I tested between neutral and hot and got 40ish (bad, should be 0).

Next I shut off power, pulled out the receptacle, and then turned the power back on. With the outlet sticking out of the wall I can test with the volt meter by touching the screws where the wires connect and the bare copper ground wire. I still find 120 volts between hot and neutral but now I read no voltage between ground and either hot or neutral... huh?

Questions:

1) Why would the voltage difference disappear on the other side of the receptacle? For the volt meter to read positive volts I had to have a completed circuit right? So where did it go?

2) The outlet itself is not a GFCI outlet. Instead it looks like the breaker it's connected to in the breaker box is where the GFCI test button is located. Is that screwing up my voltage readings?

3) My volt meter doesn't have probes that are long enough to poke into a receptical, so what I do is plug in the outlet tester but only halfway and touch the volt meter probes to the exposed prongs of the outlet tester. But the outlet tester has some kind of circuitry in it, will that screw up my volt readings?

4) And most importantly: Is replacing the receptacle likely to fix this issue, or is it time to call a professional?

Answer 1

The funny voltages you are seeing are due to the lamps in the 3-lamp tester. There is a hot-ground lamp (yellow) and a neutral-ground lamp. They are in a 3-way tug of war with the voltmeter. So wacky numbers like that are not unusual. But it means the ground wire is floating (not attached to anything) so it is easily influenced by whatever it's attached to.

When you removed the socket, you no longer needed the 3-lamp tester so the voltmeter won the tug of war, pulling the floating ground to whatever voltage you were testing to.

The ground wire is simply not working. This would be of serious concern if the circuit was not GFCI protected. But it is. So, shrug, there you go.

Answer 2

If your tester shows an open ground and if you want a genuine ground then you need to bring a ground wire to that location. No device change will help.

Answer 3

Your ground wire seems not to be connected on the other end or broken in the middle. The voltage you're seeing in the first case is phantom voltage. You're presumably testing with a digital volt meter which has a very high input impedance. If you do what you last propose (pull test tester out half way and probe the pins) you should see the voltage on ground disappear. (And no, that won't screw up anything.)

You say that this outlet is on a GFCI circuit breaker. If so, this will protect you from electrocution with the non-grounded outlet. However, this is against the national electrical code. The code allows a GFCI outlet or GFCI breaker feeding a non-grounded 3-prong outlet only where there is no existing ground wire. Since you have a ground wire, just one that doesn't work, the code doesn't allow this.

First thing to check, if you're comfortable working on your electrical panel with the cover off (turn of the main breaker first, please!), find the cable coming to that breaker and check that the ground wire is properly connected. Also check any other outlet boxes on that circuit.