Weird grounding electrode testing experiment.

[NEC Inspector]

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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[synchro]

Did you measure the voltage between the neutral and the Ufer, using both AC and DC settings on the meter? The presence of another source of voltage and current other than the meter itself might interfere with a resistance measurement.
You can also see how the measurements are affected by loads placed on the temporary power.

 

[winnie]

As you've described things, you are not measuring the resistance to earth, but rather the resistance between two electrodes.

If the utility earthed neutral is connected to something like a water pipe which penetrates the slab, then you might have a metallic bond between the two electrodes.

If you actually have two separate electrodes (the slab and the utility aggregate) then you can think of them as 'antennas in the soil', picking up any stray voltage or current flowing in the ground past them.

As synchro describes, these external currents can greatly influence the measurement.

Jon

 

[NEC Inspector]

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, 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.

You are correct that I am trying to measure the resistance between two electrodes - but the only connection between them is through the earth, so any resistance to earth between the electrodes should affect the measurement and thus be able to be approximately measured.

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.

 

[Joe.B]

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.

 

[winnie]

2 measurements you might consider:

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.

Try putting the meter into current mode and measuring any current flow between the two electrodes.

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

Jon

 

[drcampbell]

I have read that Ufer grounds (CEE's) are far superior to ground rods ...

If the concrete has been recently placed, it might temporarily (a few weeks) have a higher moisture content and lower resistivity than it will ultimately have over the long term.

For a more confidence-inspiring measurement, you might apply higher, isolated voltages and measure current. Maybe a 12-volt battery and a 24-volt control transformer.

 

[electrofelon]

Just remember that the utility does not always have an MGN distribution system. Mostly Delta distribution in my neck of the woods. (Not that that explains the zero reading but just saying....)

 

[synchro]

...
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 resistor, and the meter read 10.1 ohms, which is within tolerance for that meter.

When you measured the 0 ohms of resistance, was there a digit to the right of the decimal point. If you're on the lowest resistance range, it appears that from the following table in the Klein CL600 manual you should see a 0.1 ohm resolution displayed. If the default auto-ranging doesn't get you there, then try hitting the Range button until it does.

 

[NEC Inspector]

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

You nailed it. I went out and measured again, and I got 0 ohms with the lead configuration I had, then I swapped the leads and got .633 M ohms. I guess that proves this method is not going to work to measure the resistance of electrodes like I was hoping.

For a more confidence-inspiring measurement, you might apply higher, isolated voltages and measure current. Maybe a 12-volt battery and a 24-volt control transformer.

I think I'm going to try this - I have a doorbell transformer I could use...

Thanks all!

 

[jim dungar]

I guess that proves this method is not going to work to measure the resistance of electrodes like I was hoping.

Yeah.
This is why 'ground resistance' testing is expensive and time consuming. Special equipment and procedures are needed.

Hopefully you had fun experimenting.

 

[W@ttson]

in this video of Mike Holt he energizes a ground rod and measures the current that flows. He then uses a clamp on ground resistance tester and a 3 point tester to confirm the resistance. The current readings were pretty applicable.