ECN Forum Forums General Topics of the Trade General Discussion Area Ground in EMT

George, Kudos to you.

That was a great explanation.

I love it when guys take the time to explain things to some of us who are no so versed on a topic, rather than posting "see section xxx.@@".

Thanks again for taking the time and making Chimo's question make sense to some of us.

Dnk....

George, I second Dnk's comment, and I appreciate the amount of time you took to compose that post. I think I see the light.

Even if we completely disregarded the inductive effects, we'd still need to bond at both ends. A fault current of, for example, 100 A through an awg-6 wire just 10 feet long produces a purely resistive voltage drop of some 1/2 volt. Failing to bond to the raceway at one end or the other results in an "arc welder" effect at that end where the conductor touches the raceway, causing erosion of the conductor. This effect alone is probably why 250-92 uses the term "metallic" rather than "ferrous metallic." From Simmons' statement you quoted, the reactance added by the inductive effects of the wire in a steel raceway apparently double that voltage drop.

That leads to another question: Wouldn't the same sort of arc-induced erosion occur where a bare GEC is secured at intervals to a metal structure? I don't recall that this is prohibited by NEC.

Thank you again for the detailed explanation.

Thanks to all who responded to poorboy/chimo's question so eloquently. Goes to show, you can learn something new every day here on ECN!
It's better than tv!

Alan



[This message has been edited by Alan Belson (edited 07-13-2005).]

John,
Sore subject. My list of deficiencies in the NEC is a long one. I've forwarded many of them to the committees, but most often get the reply that "The committee agrees in principal with your analysis". That means that the official NFPA interpretation agrees but they won't change the code language.

We would certainly benefit from a covering on a GEC in a metallic enclosure. It's such a minor item, I doubt they would consider the change, and you're likely to get the reply that it's been that way a long time with no PROVEN detrimental effect.

That's the language they used to send back on the ranges and dryers when it was legal (since the beginning of WW2) to ground the frame with the neutral. It took a LOT of PROOF of death and injury for the code committee to make the change.

The code, to say the least, is far from perfect. I cannot imagine the document that would be perfect in a California suburb being the same perfect document in New York or Washington D.C.

That said, now that you're thinking about the inductive problems with the ground, consider it in a conduit full of branch circuits.

When I came in the trade, EMT was a new and unacceptable wiring method, and PVC was some guys nightmare, IOW we would NOT have accepted it. Even ductbanks were either Rigid or transite-fiber (called Core duct).

After studying the inductive effect for some years (hot pipes, phantom circuits, wasted energy) I've come full circle. I dream of a day when we invent a low smoke PVC that's stiff enough to look good without the characteristic "drip".

If you think about it, most of our 'stuff' is really NOT subject to damage, so we could eliminate a lot of wasted energy, no one would ever be harmed again from a bad conductor inside the pipe, no sparks to start fires. Oh well, that's just me thinking, it AIN'T gonna happen.

Hey, eventually, MAYBE China will sell us back some of the steel.

George, the problem is that just as we finally develop that smoke-free, rigid PVC conduit...

We'll run out of oil to make it from!

BTW, after realizing my complete ignorance of the subject, I ordered a copy of Soares from an Amazon seller yesterday. Thanks for the pointer.