I am a non-professional with limited electrical experience. I recently bought a house built in the mid-fifties that has some two-prong receptacles (i.e., no ground hole). I planned to replace the first outlet in the circuit with a gfci outlet, but when I turned off the power and removed the outlet from the box, I was surprised to find that the cable feeding it and the cable continuing to the next outlet are three-wire. So my question is: why would the person who did the original wiring not use a three-prong receptacle? How do I test to ensure the ground wire at the outlet box does in fact go to ground?
How to verify that the ground wires in the outlet boxes are grounded properly
receptacle
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Best Answer
Connect the new receptacle with hot going to small slot, neutral to larger slot, "ground" wire to ground screw on receptacle. Turn power back on, insert a 3-prong circuit tester into the receptacle and see if the tester says you have a good ground.
Even if you don't have a good ground you can install a GFCI receptacle and get protection from any shock that a GFCI protects from (or at least some types of shocks).
If you find that the ground is not good then unless and until you fix this, just leave the new GFCI receptacle in place and put a sticker on the receptacle stating it is ungrounded. You can legally use it that way.