Weird grounding issue

Here is a weird one. I recently replaced my two-prong receptacles with three prong self-grounding receptacles. I have armored cable with the strip so there is a good ground through the wall box.

Nevertheless, I tested all the receptacles for a ground using a three light thing which I know most of you detest. Most of the receptacles showed a ground. Two did not, on the same circuit. On this cicruit I installed a GFCI receptacle protecting the second receptacle as well. Here is the weird bit. Now the three light tester shows a ground.

What could I have got wrong with the original installation? If I installed the load and line the wrong way round on the original receptacles would this have caused the grounding problem (surely not since neither was a GFCI)? Or did I attach the wrong neutral to the live on the wall box where there were two sets of wires (but why would this matter on non-GFCI receptacles)?

Or is the GFCI showing a ground? Now that would be silly.

Most likely oxidation on a ground connection somewhere.
When you installed the GFCI and tested it with the tester you sent enough current through the ground to cause the resistance to change enough to show a light on the tester.

Have you noticed the “ground” light on the tester is dimmer than it should be?

Thanks Bruce.

How did you bond the three-prong receptacles?


…sang “Kum Bai Ya” around the camp fire with them. :smiley:

:smiley: :smiley: :smiley: :smiley: