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Weird ground testing experiment.

B

Beniah Naylor

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Joined
Sep 10, 2020
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Location
Manhattan, Kansas
Wondering if anyone can explain what is going on here.

I have been trying to figure out a way to test a grounding electrode's resistance to ground in a cheap and safe way, without buying specialized equipment or energizing the electrode and measuring the amperage drawn.

The NEC does not require a properly installed Concrete Encased Electrode (Ufer) to meet any kind of resistance to ground criteria, I am doing this more in the interest of science.

So, what I did was plug an extension cord into the temp power at at the site and run the extension cord over next to the Ufer connection point. I attached one lead of my meter to the neutral conductor of the extension cord, which is bonded to the utility neutral, which is bonded to the utility grounding system. I attached the other lead directly to the Ufer I was trying to test. I used lever style wagos to connect my leads to the wires to insure a solid connection. I then selected the "ohms" setting to measure the resistance between the Ufer and the utility grounding electrode system with the DC pulse generated by the meter passing through the earth to get from the Ufer to the utility grounding electrode system.

Going in, I was assuming that since the utility has a grounding electrode at every ground level transformer and utility pole, and that they are all bonded together by the utility's neutral wire, I was assuming that the utility's grounding electrode system is at either 0 ohms of resistance to ground or as close to 0 ohms as is feasibly possible. Therefore, any resistance on this circuit would come from the Ufer.

When I did the above experiment, my meter said that there was 0 ohms of resistance between the Ufer and the utility neutral. I left and came back the next day, still 0 ohms. There is no connection from any part of the slab to the temp power pedestal. When I unplug the extension cord, the meter reads "OL", which means no connection whatsoever. Plug it back in, 0 ohms. I calibrated my meter on a 10 ohm resister, and the meter read 10.1 ohms, which is within tolerance for that meter. When I switched to testing continuity, the meter rang like a bell. When I shoved the temp power end of the extension cord into the dirt, it had a fluctuating reading of around 500 M ohms. When I touched the concrete with the end of the cord, it measured a fluctuating 350 M ohms. It had rained 4 days before the first experiment, 5 days before the second experiment.

I did not even know that a 0 ohm resistance to ground was possible for a Ufer only the size of a house. No one else in my department can explain it either. Can anyone here explain to me:
(1) How a 0 ohm resistance through the earth is possible?
(2) Is there is a flaw in the method I am using?

I have never heard of anyone else testing a ground by measuring ohms between a known good electrode and an unknown electrode.

Any input would be appreciated. Thanks!

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Thanks for the replies.

My understanding is that if you can theoretically make a good connection to the earth, electricity will travel for many miles through the earth to another good connection basically without impedance.

Any conductor will have less resistance when you increase its diameter, for example a 14 AWG wire has more resistance than a 1/0 wire over a long distance. The Earth is not composed entirely of material that is usually considered conductive by itself, but the earth is 7,917.5 miles in diameter. Theoretically, you should have a negligible amount of actual earth resistance, just by virtue of the Earths massive diameter, for miles.

Because of this, and because all of the utility grounding electrodes are bonded together, the utility's grounding electrode system has an extremely low resistance to ground if you are trying to send electricity from a grounding electrode to the utility's grounding electrode system. There are probably several hundred utility grounding electrodes within a mile radius of the Ufer I am testing, and they are all bonded together by the utility neutral. Additionally, every house that was built with a Ufer in that mile radius are also bonded to the utility grounding system through the utility neutral, which adds surface area to the aggregate. I think assuming that the utility grounding electrode system is 0 ohms to ground or close to it is reasonable if you consider the aggregate of all of the electrodes present.

I'm really struggling to understand this grounding stuff - every time I think I have something figured out, I get a curveball thrown at me. I am fully prepared to change my hypothesis if someone has better info.

Here is a somewhat relevant pdf I got from an old Mike Holt site, I thought it was very interesting.
 

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Went out this morning and tested again. 0 ohms Ufer to neutral again. I measured AC and DC voltage between the neutral and the Ufer as suggested by others, and I had 49.2 mV DC and .066 V AC. The presence of a load made no difference.

I walked the site and can find no metallic bond between the two electrodes. The water pipe (including from the water service) is PEX, the drain pipe is PVC. There is just a slab foundation on dirt at this site, no telecom connection or gas pipe to the slab yet. No conduit from the transformer to house either.

Obviously the presence of voltage means that the resistance is not precisely 0 ohms, but it could easily be close enough for my meter not to know the difference.

Alternately, the 49.2 mV of DC current could be making my meter think there are 0 ohms of resistance regardless of reality. But wouldn't interference cause higher resistance rather than lower resistance? And why would I get the same reading 3 days in a row? And how good a connection would a Ufer have to have to the earth for the interference to be high enough to fool my meter?

I am open to any theories. If any of the assumptions I made in the beginning were wrong, please let me know.
 
Copied from my reply on "that other forum" but I would be interested to see if anyone here thinks I might be on the right track with this response.

I have read that Ufer grounds (CEE's) are far superior to ground rods because of the larger amount of surface contact that the rebar has to earth, and something about being horizontal instead of vertical too. If you want to take this experiment a step further drive a ground rod and repeat the test on the single ground rod. It's possible that the Ufer ground is that much better, with a rod instead you might see measurable resistance.
 
Individual on "that other forum" posted this suggestion:
winnie said:
Try swapping the lead polarity when you measure resistance. It is possible that the external imposed voltage is swamping the measurement, trying to produce a negative value which the meter is converting to zero. The opposite polarity will read a very high resistance.

You are dealing with a measurement of the sort where the nuances of way the meter makes measurements affects the results.

Jon
Click to expand...

I went out and tried it this morning, I got 0 ohms again the first time, but when I swapped the leads I got .633 M ohms (633,000 ohms). Which basically proves that this method will not work at all for measuring a grounding electrode's resistance to earth, probably due to interference in the earth.

Thanks all!
 
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