Originally Posted by jodywoj
In Chicago, there normally is no ground wire. The emt connected to the panel carries the ground to the box which carries the ground to the outlet. This outer extension box connects to the mudring of the metal box in the wall via the two screws normally used to attach the outlet.
that is only acceptable if you use self grounding receps and follow the directions for installing them so they will be self grounding.
So, have you installed a mud ring on the front of that extension for the recep to fit onto? Are you using a raised cover?
the screws connecting everything together are rated as grounds but obviously, if you removed the recep and did not have a tail attached that was attached to the box, you would lose your ground until it was remounted.
and that is just one more reason why I still pull an EGC when using metal conduit.
One thing that came to mind is the extension ring must mount snugly to the plaster ring and the plaster ring needs to be clean. The screws are acceptable as a ground but only if the two pieces are joined tightly so the screw threads will be snug in the mating threads.