Upon inspecting my light bulb socket in the garage of my new construction home (2018), I found a random grounding screw which wasn't wired to anything but attached to some bar. Does anyone know what the grounding screw and the bar it's attached to is for? In fact, the ground wire wasn't connected to anything.

unknown grounding screw

  • it doesn't really need a ground, so it's not hooked up to anything. a fixture you touched or that had exposed metal could make use of the wire/bar later. – dandavis Jun 18 '18 at 19:38

The bare ground wire in the electrical box is supposed to connect to the green screw on that short metal bar. The body of the light fixture is meant to be grounded through the mounting screw that goes into that metal bar. Do be aware that this technique of grounding the light fixture would have been the technique used in older installations. These days a light fixture will have a separate green or bare grounding wire that can be connected directly to the bare copper wire in the electrical box.

You may want to inspect that old light fixture and see if there is an existing screw or mounting hole to which you can add a separate grounding wire. You would then connect this added wire to the bare ground wire in the electrical box.

  • Many simple porcelain and plastic ceiling fixtures, that have no metal cases, have no grounding screws. – bib Jun 18 '18 at 10:27
  • @bib - I know. There is nothing in what I said: "You may want to inspect that old light fixture and see if there is an existing screw or mounting hole to which you can add a separate grounding wire" that says they have to add a grounding wire at all. – Michael Karas Jun 18 '18 at 10:30
  • +1 Agreed, just adding an observation. – bib Jun 18 '18 at 10:35
  • This is a new construction house, I'm curious if that changes anything since you only reference older installations. The porcelain fixture has no grounding mounting points, so I guess it's grounded via that screw that secures the fixture to the box? – Tom S Jun 18 '18 at 15:21
  • @TomS I wouldn't count on that. "Via the screws" is only a valid grounding path if (by cause of tightening the screw) two metal surfaces are clamped together flush, hard and without contamination (rust, paint, those little plastic squares that capture the screws etc.) Grounding via the threads of a screw (or to be more precise, the screw-head) is not allowed. – Harper Jun 18 '18 at 16:28

In the case where you're daisy-chaining multiple boxes together, that terminal block would be used to bind the grounding conductors of all the cables together, along with the grounding wire of the lighting fixture, if it had one.

  • I see no terminal block/grounding bar in there... – ThreePhaseEel Jul 11 '18 at 3:20
  • The metal strip on the left appears to have room for a second grounding screw to be installed. – Paul Belanger Jul 11 '18 at 12:01

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