Measuring for 25 Ohms or Less - 250.56

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If you are looking for an inexpensive way to test the 25 ohms to ground there is a method used for approving the grounding plate of a generator setup at a portable pit for road construction. I cannot find the info so if anyone has it would be good to know. It uses a battery and two rods and some other stuff. The downfall is it takes about an hour or so to do. The best way is a ground resistance tester takes minutes to do. But I find the discussion about whether one or two rods will make a difference interesting. When the IEEE has determined that a rod less than 10ft in length will not provide 25 ohms or less to ground on a continual basis due to a variety of weather condition and soil conditions. So if you are actually looking for effective grounding you need to use screw together rods and go deeper into the earth. This is a link to just one of the papers produced in this debate. http://www.cpccorp.com/deep.htm The findings of the IEEE is why the NEC has required us to use the rebar in the cement foundation. So I don?t really believe in the 2 rod issue yet every inspector requires it if you do not have a foundation (grid) ground. I believe that rebar in a foundation equals a grid. So if 25 ohms to ground is an issue use screw together rods and go deeper.
 
Welcome to the forum cschmid, grounding is always a hot topic here.

cschmid said:
So if you are actually looking for effective grounding you need to use screw together rods and go deeper into the earth.
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What will going deeper do for us, what is the significance of 25 ohms?

Does 20 ohms make a premise wiring system 'safer' then 30 ohms?

What are we expecting the GES to protect the premise system from?

There are a lot of conflicting opinions about the necessity of a 'good ground' (connection to earth) for typical premise wiring systems.
 
The best way is a ground resistance tester takes minutes to do.
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A properly conducted 3-point test takes more than a few minutes, a clamp on tester while faster has issues with downstream grounds on the neutral conductor, and in my expierence about 80%-90% of large commercial electrical distribution systems, have neutral ground issues.
 
cschmid,
The grounding paper you linked to is very suspect to me because it starts out with the following statement.
[FONT=Verdana, Arial, Helvetica]Low resistance earth grounding is essential for safety and protection of sensitive electronic equipment. It is the basis for any facility's power quality assurance program.[/FONT]
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As far as I know there is no factual basis for that statement.
Don
 
DSamson said:
And a second question:

I can't supplement the one driven rod with the underground water service. I'm allowed to supplement a single electrode with one listed in 250.52(A)(2) - (7), but NOT 250.52(A)(1). Correct?
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No but as per 250-50 ?If available on the premises?each item in 250-52(A)(1) through (A)(6) SHALL be bonded together to form the grounding electrode system.? In other words if these exist you?ve got to bond them all together. Notice 250-53(D)(2) ?a metal underground water pipe shall be supplemented by an additional electrode of a type specified in 250-52(A)(2) through (A)(7).? I believe this is the only electrode requiring a supplemental electrode; this is due to the face that another trade can destroy the water electrode.
It is my opinion the best electrode is the concrete encased ?ufer?. Notice the purpose of the electrode in 250-4(A)(1), it?s really a drain for over-voltages like lightning and with that you certainly want a short clear path to help control these unknown voltages.
 
As far as I know there is no factual basis for that statement.
Don
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Sure there is, ask any factory electronic field tech, he'll have you driving rods all over the place to solve what ever the issue is, unless it's harmonics
 
In the grand scheme of it all, the 25 Ohms to ground is just that. It is the resistance of the rod to the earth. Now lets take it in context, the effective ground fault current path (EGFCP)back to the source. Does the 25 Ohms or less make the EGFCP from our premises wiring any more effective?

I think there are so many variables, that in the "grand scheme" it does not reduce the EGFCP enough to make any difference in the operating of an OCPD, so it is still relegated to 250.4(A)(1)&(2). And for the general usuage, does it really make much of a difference if it is 25, 30, 50 Ohms?
 
what about building steel

what about building steel

I would think that the connection to building steel and all of the parallel paths through the concrete which is in direct contact with the earth and thousands of tons of weight holding that contact would be a pretty good ground. I have not tested this with a ground resistance tester. The last time I actualy had to use a tester was at a federal laboratory around 1990. I usually use building steel where it is available at xformer connections for sds connections. This seems to work to clear plenty of breakers
 
quogueelectric said:
I usually use building steel where it is available at xformer connections for sds connections. This seems to work to clear plenty of breakers
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Your connection to the building steel is not installed for the purpose of opening the OCPD. The flow of fault current will return to the source (transformer windings) via the fault current path and back through the transformer windings to the breaker on the phase of the ground fault.

The connection to steel is generally for the purposes of 250.4(A)(1) & (2)
 
I could see how a grounding discussion would lead into a very in-depth conversation. I believe this discussion is on the reference point to an electrical service. The 25 ohms to ground is to ensure that the rod is the common reference point and not the secondary point. the 25 ohms is the figured used to ensure the rod is the common point. So many contractors install rods in such a common place matter and inspectors take them for granted as well. This has lead to so many secondary point becoming reference points that we now have new issues. Like if a ungrounded wire is emitting flow to a ground wire and the circuit breaker does not activate than do we have actual flow to the earth? The original question is how do we measure the rod to ensure it is the main reference point with out spending several thousand dollars on high tech equipment.
 
cschmid said:
I could see how a grounding discussion would lead into a very in-depth conversation. I believe this discussion is on the reference point to an electrical service. The 25 ohms to ground is to ensure that the rod is the common reference point and not the secondary point. the 25 ohms is the figured used to ensure the rod is the common point. So many contractors install rods in such a common place matter and inspectors take them for granted as well. This has lead to so many secondary point becoming reference points that we now have new issues. Like if a ungrounded wire is emitting flow to a ground wire and the circuit breaker does not activate than do we have actual flow to the earth? The original question is how do we measure the rod to ensure it is the main reference point with out spending several thousand dollars on high tech equipment.
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If what you are saying is fact, then why does the NEC permit a second ground rod installed without testing for the Ohm value?

How about the use of other electrodes and not installing a ground rod at all?

I think you need to think through your process a little more. Try reading in Soares book, it has some excellent information.
 
cschmid,
The 25 ohms to ground is to ensure that the rod is the common reference point and not the secondary point.
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What is a common or secondary reference point?
This has lead to so many secondary point becoming reference points that we now have new issues. Like if a ungrounded wire is emitting flow to a ground wire and the circuit breaker does not activate than do we have actual flow to the earth?
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Why do we want or need flow to the earth?. Electricity does not try to get to earth, it only tries to get back to its source. The fault clearing path does not depend on the earth...the most important parts are the EGCs and the main bonding jumper.
The original question is how do we measure the rod to ensure it is the main reference point with out spending several thousand dollars on high tech equipment.
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There is no real reason to measure the rod. The code does not require that we have a grounding electrode with a resistance of less than 25 ohms. It only says that if you have a single rod with more than 25 ohms, you add a second rod and go home.
Don
 
So many contractors install rods in such a common place matter and inspectors take them for granted as well. This has lead to so many secondary point becoming reference points that we now have new issues. Like if a ungrounded wire is emitting flow to a ground wire and the circuit breaker does not activate than do we have actual flow to the earth?
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I think you are missing the point or a complete understanding of grounding.
 
I knew this would turn into a feeding frenzy. I will again point us to the original question dsamson wanted to know an inexpensive way to test the resistance of the ground rod.

Yes I own a soars book on grounding and yes I do know how to read. If I remember correctly we are talking ground rod resistance only. Reference is the earth at a potential of zero. one of the functions of a ground rod is to maintain the electrical equipment connected to it at the earths potential; another purpose of a ground rod is to dissipate over voltages to the earth.

I shall say this has been interesting, when I find the procedure I mentioned earlier I will post it for you David or I will email it to you. Thank you and have a good night I hope my pain meds kick in soon broken ribs hurt.
 
For those who don't like to be teased, here is the thread he started. :)

one of the functions of a ground rod is to maintain the electrical equipment connected to it at the earths potential;
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Earth's potential where? There are many 'tenses' of earth. The potential of the earth at the service could be different from the voltage at a grounded piece of equipment several hundred feet away.

another purpose of a ground rod is to dissipate over voltages to the earth.
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Can you elaborate on this concept?
 
HI George
I must ask are you referring to an earthing question that is going to lead to a discussion between a protective earth and a functional earth? If this is where were headed than I must get out my books and brush up on stuff.
 
I guess I am not sure what you are after here. I am assuming you are not sure of my wording of my comment. When I talk common place I mean we just take installing a ground rod at face value, we install it and move on never even question whether it is 25 ohms or less we just assume it is good and get on with the installation. Then when I used the term secondary which is bonded like your water lines and the foundation rebar. Now if we look at a ground rod in dry sand and a sweaty water line in the ground which one has the least resistance to the earth/ground? so if we have both the ground rod and the water line to the same point the water line is most likely the fault point. Now that would have the potential to energize the entire line during the fault. So then if that is the case than how important is 25 ohms to ground or less? I hope you this does not confuse the issue. I know I am no expert on this issue and pray I do not come accross as one. I am always open to learning; a day with out learning is a day wasted.
 
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