Resistance of ground

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  #1  
Old 06-05-01, 08:57 PM
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What's the resistance of dirt? I know you guys like these quirky theory questions.

If I drive a grounding rod in out in the middle of my back yard, and then I connect a wire from that rod directly to the main power feed, how many amps will flow?

I know the answer depends on a lot of factors, such as how much moisture is in the dirt. But under very dry conditions, it's conceivable that very little current would flow -- right??? And even under damp conditions, will a lot of current flow??

I'm not really too eager to try this hypothetical experiment I just proposed.
 
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  #2  
Old 06-06-01, 06:03 AM
resqcapt19
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John,
I've done that experiment a number of times as part of a grounding class. (DON'T TRY THIS AT HOME) I've never had enough current flowing to trip a 15 amp breaker. One time this was done after 3 or 4 days of heavy rain. I had 9 amps of current that day. Most of the time the flow was less than 4 amps.
Don(resqcapt19)
 
  #3  
Old 06-06-01, 07:04 AM
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Don,

Thanks. Interesting answer.

I appreciate all your contributions to this forum. Your answers always show a great deal of expertise and tact and clarity.
 
  #4  
Old 06-06-01, 03:30 PM
RickM
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Cool

John, check out a copy of Mike Holt's grounding video. He does just what you suggest. He measures using different devices, and does so at different depths (up to 50'). It is a very informative video. You can get it from his web site.

http://www.mikeholt.com


Rick Miell
 
  #5  
Old 06-06-01, 05:53 PM
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Question

I have heard the IAEI is doing a study on soil conditions pertinent to grounding, but i have had no time to read their magazine lately.
 
  #6  
Old 06-06-01, 09:06 PM
Wgoodrich
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As I understand it the ability of a ground rod to perform as a grounding source is more dependant on soil make up than water content.

Concerning the NEC, the earth is forbidden to be used as a conductor of electricity.

Also concerning the NEC the earth involving a ground rod must be tested by what is called megging a ground. No made electrode may be used if that grounding source megs less that 25 ohms to ground.

The proper way to judge what a ground rod can do and how good of a grounding source that ground rod is would be to meg that rod. Try a search engine and type in the phrase megging a grounding source.

The proceedures in megging a ground rod is very detailed, time consuming and takes special equipment. The proceedures for megging a grounding source usually are explained in about a ten page chapter of instructions using the special equipment required to perform that proceedure.

Hope this helps

Wg
 
  #7  
Old 06-07-01, 09:00 AM
resqcapt19
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wg,
Also concerning the NEC the earth involving a ground rod must be tested by what is called megging a ground. No made electrode may be used if that grounding source megs less that 25 ohms to ground.
The NEC does not really require that the "made" grounding electrode be tested. Most contractors comply with the rule in 250-56 by routinely driving 2 rods. This is more cost effective than testing the ground resistance. The code says if a "made" electrode has more than 25 ohms you must add a second rod. There is no requirement to continue installing rods until you get to less than 25 ohms.
Don(resqcapt19)
 
  #8  
Old 06-07-01, 10:04 AM
Wgoodrich
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resqcapt19, I agree with what you said. However in my opinion, if you had an electrical contractor install only one ground rod then you would either have to accept that one rod or prove that rod does not meet the 25 ohms to ground. Some inspectors put the burden of proof on the electrical contractor. If your ruling is challenged when you required that second rod then an appeals board or civil court will require you to show proof that that rod did not measure less than the 25 ohms to ground. If challenged the burdon of proof would be on the Authority Having Jurisdiction in a hearing.

Be careful to avoid mud in your face if you require that second rod.

In my area they would challenge. Maybe knowledge is too high due to availability of formal education in my area if that is possible, probably not possible to have too much knowledge in the industry, huh.

Wg
 
  #9  
Old 06-07-01, 01:31 PM
resqcapt19
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wg
The burden is on the contractor not the inspector. If the contractor wants to use a single rod, he must prove to the inspector that the rod meets the 25 ohm rule. The only way that the installer can comply with 250-56 is to do the testing. If he doesn't test, he is in violation because he doesn't know that his single rod meets the rule.
Don(resqcapt19)
 
  #10  
Old 06-08-01, 06:43 PM
Wgoodrich
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I have heard your argument placing burden of proof on the Contractor by many inspectors. Do not agree with ordering something that I can not prove myself.

In my opinion an inspector should make no ruling he assumes without knowing he can back it up with law or fact. An inspector should not enforce his feelings, just his best knowledge as to the minimum safety standards accepted in his area. I am afraid that if you hold a stance requiring two rods without performing an accepted test proving that first made electrode does not meet minimum safety standards, you open yourself up for mud in your face sometime by someone, if challenged.

I wouldn't want to invite that mud. I get mud in my face every time I learn, this is good. To get mud in my face because I arbitrarily ordered something that I could not back up, is not good, in my opinion. I just try to believe I am right and can prove it before I make a ruling ordering a contractor to add more grounding sources that are not available at the time of wiring the project as said in 250-50.

Wg
 
  #11  
Old 06-08-01, 07:02 PM
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Question

I am a contractor. If i were to prove ( or rather be very lucky) that one rod is less than 25 ohms, would a one-time test be sufficent?

Also, it must be noted that the earth is in violation to a certain degree in many installations.

 
  #12  
Old 06-08-01, 08:13 PM
resqcapt19
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wirenuts,
My point is that it will cost the contractor more to test the resistance of a single rod than it will to install the second rod. If you don't KNOW that the single rod is less than 25 ohms, you must install the second rod.
Don(resqcapt19)
 
  #13  
Old 06-08-01, 10:57 PM
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The electrician should be ethically bound to adhere to the NEC. The NEC requires 25 ohms or less with one driven rod or the installation of a 2nd rod. The electrician must either prove less thn 25 ohms with one rod or drive a second rod. My licenses state I will comply with all provisions of the adopted NEC as well as any local requirements in my electrical installations. That puts the "burden of proof" squarely on me. It would take a horse's rear of gigantic proportions to try to dodge his/her responsibility over a $6.00 ground rod. If you argue with an inspector for 15 minutes over this point your ego already cost you more than the rod and clamp.
 
  #14  
Old 06-09-01, 03:46 AM
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Your both right, of course. economically, it would be far easier to simply pound rods the rest of my carear. the only aurgement i was trying to pose was pertinent to the original Q. would a complaint 'earth' resistance one day be the same all seasons?

The NEC allows us, via second rod, to total ingnore the entire matter of verification as to earth resistance.

to further the latter statement, utilities may ground an X-former right next to the service equipments GEC and electrodes.

250-32(b)(2) allows me the same scenario in a 3-wire feeder with a close outbuildings GEC and electrode in close proximity to the mother buildings service GEC and electrode.

The NEC does not allow us to quantify, calculate, or reason this in any manner, the only written offering i can find is 250-30(a)(1) EX, last sentence;

" for purposes of this exception, connection through the earth is not considered as providing a parrallel path"

So how can the NEC require low ohms in one section, and make claim against it in another?????

I refer to my last post,

"the Earth is in violation!"

Sirs,
inasmuch as i do try to be an ethical electrician, i do not beleive in blindly stumbling along behind the NEC, there HAS TO BE some sort of electrical theory involved.

i appologize if in fact, the theory escapes me here.



[Edited by wirenuts on 06-09-01 at 07:04]
 
  #15  
Old 06-09-01, 05:41 AM
Wgoodrich
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The ability of the earth to conduct electricity can vary by the minute, hour, day, etc. depending on the conditions of that earth at the time. Did it rain, has it been a drought, it the earth hot, cold, neutral with air? All these and many more factors change readily.

I believe that John is grinning thinking I just asked a little old question and look at everyone's reaction. John you have an impish nature at times if my thoughts are right.

The Wg as an electrician would like to add all the grounding sources that I can to make the best grounding source available. Wg as the electrician has seen ground rods melted into molten puddles by lightening strikes. Wg as the electrician truly believes that a ground rod being used as a made electrode and serving as a grounding source is the worst excuse of a grounding source. However many times the made electrode is our only choice.

Wg as the electrician believes a plate electrode to be much better than a ground rod. Do the math the minimum size plate electrode has much more surface contact with earth than a 1/2 or 5/8 ground rod that is 10' long. Yet all that seems to be looked at or talked about is a ground rod.

If those speaking replies on this subject is condemning my stance about the inspector having to back up his rulings becuase he did not require two ground rods should think to themselves. Should you not be condemned because you installed a ground rod instead of a 10' x 10' plate electrode, or dig up the plastic water line and install a metal water line. Both would be a better grounding source than those two ground rods you feel must be installed rather than do the required testing?

Wg as the electrician has in the last thirty years as an electrician experienced Authority having jurisdictions that were mayor's sons, son inlaws etc. that were proviously taxi drivers order me to do what you suggest, "you as the electrician prove it or do it"

Wg as the inspector have also experienced mayors and commissioners and etc. that have the power of appointment of the AHJ, order that inspector to enforce what they thought was best. Wg as the inspector has a real problem allowing beginner level people in the electrical industry to be an inspector, going on a qualified electrician's job site and deciding whether that qualified electrician did a good job or not, then ordering him to install electrical equipment just because he said so. Big let down to me considering big brother as government.

Wg as the inspector has a real problem in my mind enforcing my thoughts influencing other qualified electricians to do as I think is best.

Wg as the inspector strongly believes that if an inspector makes a ruling, then the burden of proof must be on that inspector.

Agreed more grounding sources are definitly better. However an inspector requiring electrical designs to be changed because he thinks it might not measure up and without being able to prove it is deffinitly not better for the industry.

If you are an inspector and you want to enforce to ground rods the buy a meggar and prove that rod does not meet the 25 ohms to ground. Please don't enforce other to wire your way prove it or let that qualified electrician's wiring design stand, or let someone else be the inspector.

I am not saying more ground rods is not better, I am saying an inspector enforcing others to wire the way he would without a law and proof to back it up is definitly not better.

Wg
 
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Old 06-09-01, 01:54 PM
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I would like to clarify a point from a legal standpoint. An appointed inspector is not the AHJ. The inspector is a code enforcement officer. The inspector can only enforce the code or ordinances which have been legally adopted by the local government such as the elected city council or county council or board of commissioners. The council or commissioners who adopt the NEC as is or ammend it is the AHJ. This has been to court many times nationally and recently in my area, and it has been established as fact. We had an inspector in my town and another one in the next county who attempted to impose their own make-em-up-as-you-go rules. Both got sued and both lost. The judges in both cases ruled that an inspectors rules or requirements beyond the adopted version of the building or electrical codes were not enforceable unless those rules had been voted into law via legal ordinances. The rulings made professionals out of the inspectors around here. Courteous ones at that!
As for the efficency of ground rods, I have to agree with WG. The driven rod(s) are not the most effective but they are the cheapest and handiest. On new construction I prefer to install concrete-encased electrodes.
 
  #17  
Old 06-09-01, 06:08 PM
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Grounding

I have read with interest the entire discussion on Ground Rods and grounding electrodes. Here is my input for what it is worth.

I worked for a power company for many years, and they always installed a ground grid of 4/0 copper over the entire area of a substation facility, as I recall the 4/0 was on 8" centers about 12" - 18" below grade. Everything was grounded to this grid.

My home was built in 1956 and was grounded to the water system, however the distribution system feeding my home and many others is a single phase tap, with no ground. ie Two phases of a three phase circuit with no apparent ground.

Last year I built a barn behind my house and installed from my main box a 40 amp circuit (4 #8 wires) to feed the Barn. At the Barn I was required by the inspector to install a ground rod or tie a 20' section of 1/2 in rebar to the foundation steel to use as a ground. This is connected to the main box with a #8 wire (not too adequate) and to the sub panel with a #8 wire.

Some time in the past my water main was replaced with plastic water line so the original ground to the water system was cut. As I see it my choices are to install a second ground rod at the house to help protect my home from excess damage from a lightning strike, or dig up my water line and either replace it with a metal line or connect a 2/0 or 4/0 copper cable to the meter and my metal water lines in the house.

Roger
 
  #18  
Old 06-09-01, 08:21 PM
resqcapt19
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The grid and bonding in a substation is to eliminate step and touch potential in the event of a ground fault in the system. In high power stations it is very possible for the voltage on that bonding grid to rise 5000 or more volts above the earth a few hundred feet away. With everything bonded any linemen working in the area are like birds on a wire. There is no large potential beween any two points that they can touch with their hands or feet.
Don(resqcapt19)
 
  #19  
Old 06-09-01, 09:32 PM
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Grounding

That is true but the Ground grid is also connected to the fourth wire (ground) on a distribution circut, and any ground fault in the line is carried back to it's source by ground and the station grid. I have seen a 600 volt differential between probes in the ground placed three feet apart in a substation because of ground return from a three wire distribution circuit. Three wire Distribution circuits will follow the path of least resistance to return a ground to it's source, a heavy ground fault will destroy water lines gas lines and phone lines.

 
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