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I just moved to a house which was built in 1958 (almost 60 years ago).

One GFCI outlet has the indicator light on but no power when plug in appliance. So I opened it and measured the voltage with a multimeter. It is very strange. See below diagram:

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I guess A and B is input line whilst C and D is output line to other outlets. When I measure it, I have removed the GFCI receptacle. Why A and D still has 5.5v?

And I didn't find ground line from the outlet.

If I connect A,B to LOAD and C,D to LINE of the GFCI receptacle my receptacle indicator light is on but recepacle itself is always off. When I press the reset/test button there is no effect.

I still cannot figure out what's wrong.

  • What color is the light? Do you know the make and model of the device? – Tester101 Sep 17 '15 at 13:07
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If A and B are the wires coming from the breaker, they should be connected to the LINE terminals of the GFCI. C and D; that go off to serve other loads, should be connected to the LOAD terminals. If when connected properly, the GFCI will not reset. It means that the GFCI device is dead, and needs to be replaced.

Since C & D are "floating", the 5.5 volts could be phantom voltage induced by other electrical lines in close proximity to these lines, or any other magnetic field. A low impedance voltmeter could be used to rule out phantom (ghost) voltage. However, since lines C & D have no reference to the electrical system (floating), don't expect a meaningful reading.

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It sounds like the GFCI outlet itself is bad. Replace it. It's certainly the easiest thing to do. If that doesn't work, then you probably have a problem with the wire between the outlet and the breaker panel. Look for breaks, scorches, corrosion, etc.

I guess A and B is input line whilst C and D is output line to other outlets

You are likely correct. A GFCI outlet can protect itself and any devices wired to the LOAD terminals on the back.

The measurement from A to D is meaningless and not indicative of a problem. If you test any two ungrounded metal items anywhere in your house, you will certainly find similar voltages all over the place. For example, a fork on your kitchen counter and a fork on the floor may read a voltage between them.

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That you have voltage between A and D would indicate that you have, what we used to call in the phone business, a high resistance open. One of the outlets downstream from your GFCI outlet has (probably) corrosion (high resistance) between neutral (white) and ground. This will cause a ground fault and trip the GFCI. The light being on on the GFCI indicates a ground fault, which is why the outlet has no power. (Insects can also cause a high resistance ground.)

I suggest opening all the downstream outlets and checking for corrosion, including in the outlet itself. If there is none, then it's in the wall, and you should probably hire an electrician.

  • it could be the bond in the main panel – Skaperen Sep 17 '15 at 11:06
  • Also check for nicks and loose wires. They can act like a corroded (high resistance). They're the typical culprit in a lot of shorts too. – BrownRedHawk Sep 17 '15 at 11:34
  • C & D are floating, the 5.5 volts could be coming from anywhere. – Tester101 Sep 17 '15 at 13:06

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