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#221420 05/21/21 07:00 PM
Joined: Jul 2004
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I finally got around to reading Mike Holt's article on ground electrodes in EC&M magazine.
I am left with the same question I always have with these articles. He says the swimming pool steel can not be used as a ground electrode and he also says only one concrete encased electrode can be used.
My question is how do you avoid those pesky electrons from going there?
For now let's just talk about my pool.
The equipotential grounding grid picks up the pool shell, the deck and the screen cage, that is also connected to poured concrete piers with rebar in them. All of that is bonded. That bonding also connects to pumps, heaters, underwater lights and a spa air pump. Those are all connected to the grounding bus via EGCs that are at least 12ga copper. Didn't I just bond the pool shell and deck to the grounding electrode, creating a much larger electrode system? In my case that also picks up Ufers in the structures around the pool deck since all of that steel is bonded to the deck steel, connected to the pool shell steel.

This all might be a nice theory but that pool structure and deck is still the biggest electrode in the system unless you violate article 680


Greg Fretwell
2017 / 2014 NEC & Related Books and Study Guides
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Originally Posted by gfretwell
I finally got around to reading Mike Holt's article on ground electrodes in EC&M magazine.
I am left with the same question I always have with these articles. He says the swimming pool steel can not be used as a ground electrode and he also says only one concrete encased electrode can be used.
My question is how do you avoid those pesky electrons from going there?
For now let's just talk about my pool.
The equipotential grounding grid picks up the pool shell, the deck and the screen cage, that is also connected to poured concrete piers with rebar in them. All of that is bonded. That bonding also connects to pumps, heaters, underwater lights and a spa air pump. Those are all connected to the grounding bus via EGCs that are at least 12ga copper. Didn't I just bond the pool shell and deck to the grounding electrode, creating a much larger electrode system? In my case that also picks up Ufers in the structures around the pool deck since all of that steel is bonded to the deck steel, connected to the pool shell steel.

This all might be a nice theory but that pool structure and deck is still the biggest electrode in the system unless you violate article 680
At the risk of being pedantic I will point out that a Concrete Encased Electrode (CEE) is not an Ufer grounding mat even though an Ufer Grounding Mat is a concrete encased electrode. Why I make the distinction is that I never see the work put in to follow Herbert Ufer's specifications for a grounding mat except were the structure built to house radio equipment or a large number of computer servers. There is a reason for maintaining the distinction between the 2 electrodes. At any given location the effectiveness of the two grounding electrodes is very likely to be markedly different. It is common for an Ufer Grounding System to cause electricians and inspectors to momentarily doubt that the instrument they are using to measure the impedance to ground is working. Ufer Grounding Systems will often present a measured impedance which is less than 5 Ohms. Although minimally compliant CEEs often present a much lower impedance than the other electrodes available they will seldom have a measured impedance that low.

I write all that to support you in your statement of the practical effects of the code as written as well as to offer a possible explanation for the distinction. If equal potential bonding mats were used as a service's only electrode; which would certainly be permissible if it were a concrete encased electrode in your homes foundation; then you would be likely to use it as the service's grounding electrode and not install another permissible grounding electrode. Forbidding the use of the equipotential bonding mat as a Grounding Electrode forces us to install another electrode to comply with the US National Electric Code (NEC). If the pool shell and perimeter area bonding system were the service's only electrode then during any open neutral condition there may well be enough current flowing through the pools equipotential bonding conductors to cause a voltage gradient through the water in the pool. That is a condition that is never desirable. I suspect that is the reason that the code making panel excluded the use of the pool's equipotential bonding mat as a grounding electrode.

Like you I do see a big gap between that possible reasoning and the rules as written. Since pools, spas, and fountains are often added to existing homes, and the home's code compliant Grounding Electrode System may well be of much higher impedance to ground than the pool's equipotential bonding system, the distinction becomes worse than meaningless. [How much can we expect from 2, 8 foot long, driven rods spaced 6 feet apart?] When that is true the very heart of your concern becomes a lot more significant. There really is no way to, as you put it, "avoid those pesky electrons from going there." What I think is needed is some comprehensive peer reviewed research on what the actual effect of current flowing through the bonding mat is on the safety of swimmers in the pool as well as the somewhat less vulnerable persons who are on a pool's conductive perimeter area.

Tom Horne

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I guess I live in the real world where electrons tale all paths wink

I do have a significant GES with 5 rods and a ufer but I can't ignore the other paths to earth when I try to explain being the ground for the neighborhood. I typically have a few amps on my service neutral with the main breaker tripped. Some of the pole/transformer grounding wires have the same kind of current readings. I understand some of this is unavoidable with wye distribution since that little 2ga neutral on the pole is in series with the earth but I also suspect barely legal grounding electrodes at the houses contributes and explains the different readings at different poles.
This is the pole at my house. 0.86a

The pole up near the main primary tap coming down my road.
Almost 3a

Attached Images
Pole at my house.jpg First xfmr.jpg

Greg Fretwell

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