Grounding Thought Experiment

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bphgravity

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I'd like to offer a thought experiment on grounding requirements in order to hopefully understand a little better what we are hoping to accomplish with an electrode system.

I have a stucture with a concrete encased electrode, metal water pipe, and structural steel. For whatever reason, two ground rods are also installed. Next door is another structure with no concrete encased electrode, no metal water pipe and no structural steel. It only has two driven rods.

At the first structure, someone accidently comes along and cuts the conductors going to all the electrodes except the ground rods. So effectively, the first building is now basically indentical to the second one. The question. IS my first building any less safe now or less protected from transients and surges?

If yes, I will conclude that it is imperative to require all present electrodes to be used when designing the electrdoe system. If no, I conclude that one or more of the electrodes could be permitted to be used if desired, however only one would be absolutely required.

Second question. Lets say the first building has approx. 11A flowing on the electrode system before the conductors were cut off to the extra electrodes. After, the current drops to .75A. Would this not be considered objectionable current and therefore the removal of the other electrodes may be acceptable?

The point that I am trying to get across, is that if a structure with only grounds rods satisfies the minimum requirement of the code, why would the code require more and more electrodes for another structure with slighty different conditions? This sounds like design to me.
 
Re: Grounding Thought Experiment

Bryan,
I think that I agree. The code has way too much emphasis on grounding when it really should be on bonding. As far as eliminating grounding electrodes because of "objectionable current", CMP 5 does not agree. I submitted a proposal for an exception to the use of the metal underground water pipe as a grounding electrode because of the "objectionable current" on the water piping system. Their rejection comment was that "that is not a reason to discontinue the use of the water pipe as a grounding electrode".
Don
 
Re: Grounding Thought Experiment

I would say "yes, it is less safe than it was before." I will also concede that the difference is most likely to be immeasurably small.

I agree with Don, in that the issue is bonding and not "earthing." It does not matter so much that the steel frame (which is bolted to the steel wall that you are leaning against) has a low resistance connection to planet Earth. It is more important that the steel frame be at the same potential (or as nearly so as you can reasonably achieve) as the panel enclosures, conduits, and non-current-carrying parts of all electrical equipment. Including building steel as part of the Grounding Electrode System is the easiest way to make sure this happens. Anyone can look at the connections between the distribution system and all the grounding electrodes, and can be assured that they are all at the same potential. But if you don't have a GEC that connects the GES to building steel, then you cannot be sure that there is no elevated potential somewhere along the steel frame. There probably won't be, even if some component experiences a "high impedance ground fault" (i.e., one that does not trip the breaker). But you are more confident that there won't be, if all available electrodes are included in the GES.
 
Re: Grounding Thought Experiment

Now correct me if I'm wrong but as my understanding goes, only two forms of grounding are required out the seven mentioned in 250.52. The code reference I'm still looking, or could this vary from city to city, in the city I contract in that is what is required. Now I understand where in a metal building with water lines you would go to cold water, ground rod, and then bond the metal frame with the other two, but in a wood structure with water lines, cold water and ground rod would be ok, but if it was a case where there is a wood structure no water lines, then the case would be for example, concrete encased electrode or any of the 7 in 250.52 and ground rod. I had a situation where this was the only option which was ok by the AHJ. Your thoughts.
 
Re: Grounding Thought Experiment

Originally posted by Jhr:
Now correct me if I'm wrong but as my understanding goes, only two forms of grounding are required out the seven mentioned in 250.52. The code reference I'm still looking, or could this vary from city to city, in the city I contract in that is what is required. Now I understand where in a metal building with water lines you would go to cold water, ground rod, and then bond the metal frame with the other two, but in a wood structure with water lines, cold water and ground rod would be ok, but if it was a case where there is a wood structure no water lines, then the case would be for example, concrete encased electrode or any of the 7 in 250.52 and ground rod. I had a situation where this was the only option which was ok by the AHJ. Your thoughts.
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You must use all that are available. See 250.50.

For example, if you have a building that has all seven, you will have to use all seven. If you have a building that only has one, you use one. The exception to this rule is the water pipe, which can never be the only electrode.
 
Re: Grounding Thought Experiment

The problem with not using all of the grounding electrodes that are present is that they will not be bonded together as part of the "grounding electrode system". The concrete encased electrode will still be grounded (as in connected to the earth) but now it is not connected to your electrical ground. There may now be (probably will be) a difference in potential between this "unused ground" and the ground rods. Where there is a difference in potential there is the possibility of current flow. This current will take all available paths, through the earth, through building steel, copper water lines, electrical grounding conductors, metal conduit, whatever is available. You are very likely to end up with current on systems that were never intended to carry current. That is why we need to stop thinking of grounding as a "ground rod" and start thinking about grounding electrode systems.
I once was working on a mlitary installation where they were seeing a sign wave on the video surveillance equipment. We found a loose neutral at an installation about a quarter mile away. The only ground return path was the shield on the video cable that was run to both buildings. It would not carry enough current to trip a breaker but sure caused the video screens to be a mess!
 
Re: Grounding Thought Experiment

Groundaholics.

I think they make a cream for that now. :D

[ July 27, 2005, 04:33 PM: Message edited by: physis ]
 
Re: Grounding Thought Experiment

Originally posted by physis:
Groundaholics.
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Indeed! I'm considering this tatoo on my forehead. ;)


images
 
Re: Grounding Thought Experiment

Originally posted by physis:
Are the extra lines what I think they are? :D
Click to expand...
(In my best Sigmund Freud accent...) "Und vat exactly do you zink zey are, hmmm?"
 
Re: Grounding Thought Experiment

This is the one I'm used to. I'm thinking Bryan has installed additional electrodes in his. :)
 
Re: Grounding Thought Experiment

I just received a report from the IEEE, the ground symbol has been changed to better indicate its purpose! :p

images
 
Re: Grounding Thought Experiment

By the way, if a water piping system doesn't qualify for a grounding electrode or isn't present, you are only required to use a single grounding electrode. :D
 
Re: Grounding Thought Experiment

Charlie,
I think I understand what you are saying, but remember that in the 2005 code 250.50 says that all of the grounding electrodes that are PRESENT, must be bonded together to form the grounding electrode SYSTEM. You do not have a choice, if it is there it must be used.
 
Re: Grounding Thought Experiment

I agree but if none are present, you are permitted to use a single electrode and it does not have to be supplemented like the water does. :D
 
Re: Grounding Thought Experiment

So I could get by on just a ground rod if that was the only thing present, but then again the ground rod would not be present until I trucked it in ;) .
 
Re: Grounding Thought Experiment

Hi guys. Just using this thread to let you know I have been off the forum due to having to deal with Lyme disease, our favorite Island disease. the antibiotic is working, but I'm not yet up to responding to the Forum.
Karl
 
Re: Grounding Thought Experiment

Karl, I hope the rest of your recovery is speedy.

Roger
 
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