NEC grounding

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apauling

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Charlie closed a post with a question about the 25 ohm requirement and I followed his thread to the "duplicate" post, but it was actually not the same question. To me this question is "is there a NEC defined method, specified in the code, for determining where and how this test is to take place".

This is one of those more than gray areas of the code to me. It seems to be left up to the jurisdiction. No conditions are defined. In dry climates the tester might test at the driest time of the year or the wettest. The contractor can empty the well wetting the soil around the rod to promote conduction, (maybe gatorade works better in Florida). There are tests but since they are not mentioned in the code, how can that number be actually applied?

maybe there is an actual duplicate post that actually answers this question, but I have never seen it answered.

paul :cool:
 
Re: NEC grounding

The NEC does not give a method for determining the resistance to ground of any grounding electrode. It is left up to the installer to achieve the 25 ohms, or to install the second ground rod.

But are you (or is anyone else) having problems with the linking that I created in the post that I closed? Or are you talking about a post that was closed by ?the other Charlie??

The one I closed was in the NEC topic area; its title was ?NEC Grounding.? It can still be read. It has a link that should take you to a thread in the Grounding versus Bonding topic area, called ?NEC Multiple electrodes.? Try looking for it without using the link. The two are exact (i.e., word for word, letter for letter) duplicates.
 
Re: NEC grounding

Charlie: i followed the link but the post seems to go in a different direction. Are you saying that we should stop this aspect of the discussion, and move it to that thread?

I for one would be interested in how this aspect of this discussion pans out in actual application. Since the NEC doesn't define the testing, or limitations on the testing, how can it be enforced without jurisdictional amendment by local ordinance or adopted modified code, not just by inspector discretion. inspector.

paul :cool:
 
Re: NEC grounding

i understand the rules, or so i think. I am left with the impression that any electrician could make any reasonable type test and it should suffice as long as it reads 25 ohms.

Any takers?

paul
 
Re: NEC grounding

Sure apply a 120 volts to the rod (without the GEC connected of course) and mesure the current, then use ohms law and you have it. well unless you touch the rod while doing this then
speechless-smiley-039.gif
 
Re: NEC grounding

Originally posted by apauling:
"is there a NEC defined method, specified in the code, for determining where and how this test is to take place." ..... No conditions are defined. In dry climates the tester might test at the driest time of the year or the wettest. The contractor can empty the well wetting the soil around the rod to promote conduction, (maybe gatorade works better in Florida). There are tests but since they are not mentioned in the code, how can that number be actually applied?
Click to expand...
Paul, here are some articles I found that may answer part of your question:
Why 25 Ohms?
Why Test Grounding?

The last section of this article (Section Title: "High Performance Grounding and the NEC") may make sense too:
High Performance Grounding

Hope this helps!

Vaughn

[ March 24, 2005, 12:51 AM: Message edited by: bhsrnd ]
 
Re: NEC grounding

okay: i'll play with sarcasm, define the conditions that you use to measure the resistance. The method you actually use. If no one actually does this, it tells me that all the complainers about inspectors are paper tigers. Talk a big game. here is a requirement for a second ground rod, without defined parameters for testing, and no one argues, tests, asks about the tests. paper tigers.

If a jurisdiction has no actual ordinance, how can a second rod be required. There is no method inplace for the test. It's a real live lawsuit for those so inclined.

paul :)
 
Re: NEC grounding

Here's a good document (PDF file) that talks about the testing procedure and other aspects of Ground Resistance Testing:
Understanding Ground Resistance Testing

A quote from this document (pg. 9):
The National Electrical Code? (NEC) states that the resistance to ground shall not exceed 25 ohms. This is an upper limit and guideline, since much lower resistance is required in many instances.

"How low in resistance should a ground be?" An arbitrary answer to this in ohms is difficult. The lower the ground resistance, the safer; and for positive protection of personnel and equipment, it is worth the effort to aim for less than one ohm. It is generally impractical to reach such a low resistance along a distribution system or a transmission line or in small substations. In some regions, resistances of 5 ohms or less may be obtained without much trouble. In other regions, it may be difficult to bring resistance of driven grounds below 100 ohms.

[ March 24, 2005, 01:20 AM: Message edited by: bhsrnd ]
 
Re: NEC grounding

Hi hurk27,

The method that you described is in some editions of Practical Electrical Wiring. It would work on systems that have multiple grounds that are tied together using the neutral or an equipment grounding conductor. The method described in Practical Electrical Wiring recommends using a fuse to protect the ammeter.

This method may not work on corner grounded 3-phase systems because the grounded conductor carries full load current which means that there will be a voltage drop that will mess up the test.

Any kind of fancy ground resistance machine essentially uses 2 or 3 reference ground rods using the fall of potential method. If you have multiple other grounds then you already reference grounds anyways.
 
Re: NEC grounding

So after all that reading i am back to the same question, as to code compliance, not technical proficiency. One of the articles points out that false results are easy to obtain, and correct results can only be obtained by a rigamaroll of greater trouble than driving the 2nd rod.

Since the false test, by their standards, is relatively easy to do, and the code does not specify a method or any testing parameters, how can the more accurate test be required? (fall of potential method)

The article pointed out the fact that almost no one actually does the tests, just installs the ground rods. The article also points out most electricians do not know the correct method of testing (am, is, was in that category myself). What inspectors could actually verify the shown results? Would they even be able to verify test parameters?

Anyway, I am clear that I am not alone in this lack of ability to verify test results, and that in reality, most jurisdictions aren't even aware of what a test looks like.

paul :cool:
 
Re: NEC grounding

MC5W to even think about doing the test like that would be very dangerous. But to have any conductor from the service connected to the electrode would not give you the true resistance of the electrode as you would be reading any other grounding that is also connected to this wire. And for that matter in a 120/240 3 wire service the MBJ would just short out the voltage that you applied to it so if you test the grounding electrode it has to be "Un-connected" to any wire.
 
Re: NEC grounding

Paul:

Here is how I see it:

Follow the instructions included with the tester.

Pouring Gatorade or water around the electrode would not be fair. That would not give a fair reading since the water or gatoraide would not be there under normal operation conditions.

I don't see wetting the electrode as any different than installing additional ground electrodes for the test, and then taking them out after you get the reading you want.

On the other hand, if you are lucky enough to preform the test on a rainy day, so be it.

P.S. The gatoraide probably would work better since it would have salts and minerals in it!

Steve
 
Re: NEC grounding

Why go to all the trouble of wasting good gatorade? Just good old pencil whipping the numbers to come out fine, but make sure the numbers are believeable! I once saw some resistance testing results where nobody was there to see the test being done and they were coming back with 0.1 ohms and sometimes even less! And all this from a single 3/4" x 10' ground rod. Amazing! I was thinking, do you really expect me to believe this c**p! By the way, in case nobody noticed, I am being sarcastic and do not condone fudging any ground resistance test results. :D
 
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