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lack of equipment ground


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I inspected a 1947 home that I believe (no access to verify) has prefab steel wall structure built on a slab. It had metal boxes with rag wrap cables (no equipment ground). The receptacles are three prong and tested as being grounded even though there is no grounding conductor at the boxes. I suspect that the metal box is attached to the metal wall stud which is attached to the concrete slab and my cheap three prong tester says everything is fine.

I think a proper ground would need to be established by the use of an actual equipment ground conductor that goes back to the main panel. Is grounding through structure to slab acceptable, or, to consider it properly grounded, does it need an actual grounding conductor?

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Interesting, John. I would never expect to see steel studs in a 1947 residence here - too many sawmills and cheap lumber. You however are closer to the steel mills and production was high after the war.

Would the plaster be on gypsum lath? Or would they use metal lath?

Re: Wiring, the equipment grounding conductor is missing and there is no substitute. If grounding by studs to the slab was sufficient, we would have seen it more when copper prices jumped in the 60's, JMO.

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OK I remember reading about those projects.

The PS article mentions lathers and plasterers, but with the speed they built those places, I will bet they used gypsum, of which the narrow sheets were called lath I believe.

The steel stud walls are welded, so the integrity of the grounding would not be a problem. What about resistance, though? Steel is not the best conductor, and the furthest points from the panel would have a higher R. I think an electrician could measure the resistance and see if it meets minimum standards for equipment grounding.

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OK I remember reading about those projects.

The PS article mentions lathers and plasterers, but with the speed they built those places, I will bet they used gypsum, of which the narrow sheets were called lath I believe.

The steel stud walls are welded, so the integrity of the grounding would not be a problem. What about resistance, though? Steel is not the best conductor, and the furthest points from the panel would have a higher R. I think an electrician could measure the resistance and see if it meets minimum standards for equipment grounding.

What is that standard?

Marc

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OK I remember reading about those projects.

The PS article mentions lathers and plasterers, but with the speed they built those places, I will bet they used gypsum, of which the narrow sheets were called lath I believe.

The steel stud walls are welded, so the integrity of the grounding would not be a problem. What about resistance, though? Steel is not the best conductor, and the furthest points from the panel would have a higher R. I think an electrician could measure the resistance and see if it meets minimum standards for equipment grounding.

What is that standard?

Marc

When an electrician installs a ground rod, he often installs two because one does not provide a low enough resistance to earth. You will correct me if I got that twisted. [:)]
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OK I remember reading about those projects.

The PS article mentions lathers and plasterers, but with the speed they built those places, I will bet they used gypsum, of which the narrow sheets were called lath I believe.

The steel stud walls are welded, so the integrity of the grounding would not be a problem. What about resistance, though? Steel is not the best conductor, and the furthest points from the panel would have a higher R. I think an electrician could measure the resistance and see if it meets minimum standards for equipment grounding.

What is that standard?

Marc

When an electrician installs a ground rod, he often installs two because one does not provide a low enough resistance to earth. You will correct me if I got that twisted. [:)]

That wasn't me question but yes, in my area beginning a few years ago, the local AHJ started requiring two rods at least 6' apart instead of one I guess because megger reading were dropping too close to the minimum.

I don't have an answer for John D but I do know that local commercial wiring can't depend on metal framing for bonding requirements. Still need to have that extra wire in branch/feeder circuits for that.

Marc

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I haven't chimed in because I really don't know the answer.

In 1947, the receptacles would not have been grounded, so I doubt that anything about this system was designed or intended to provide equipment grounding.

When the three-slot receptacles were installed, there would have been rules about what was and was not considered a proper equipment grounding conductor. Whether or not the rules at the time allowed the steel framing to function that way is an interesting question, but it would require some time to look up. And even then, it really wouldn't be meaningful to anyone but a code historian.

The real question is whether or not the steel provides an adequate fault path and the answer is, "no one knows." I think that I'd advise the buyers to have new cables installed at those locations where they want to have nice, reliable grounding. Otherwise, the grounding is a crapshoot.

I suppose that someone could pay to test the resistance of the ground path at each receptacle, but it seems like overkill.

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