Where to measure 25 OHMS

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Dennis Alwon said:
Mike how are you testing? Do you want to use the water pipe as one of the three points of the testing parameters?
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If he wants to see if the water pipe has less than 25 ohms earth resistance, the pipe must be disconnected from the service.

I have a very good underground water pipe system. 30 feet of pipe to the garage and at least a 30 foot deep metal well pipe.

I get 'less than 0' ohms when tested with the pipe connected to the service. That's because there is actually a small amount of voltage on the pipe when it is connected to the service. (1.6 to 1.9 VAC) This is common. Up to 3 volts is considered safe. The voltage is there due to the parallel path that exists when connected to the service neutral which is also grounded at the transformer pole.

It's interesting to note that the voltage is the same an inch from the pipe as it is 35 feet from it.

Rod to rod, no AC mains connection, shows no voltage, regardless of distances (up to the 50 feet I measured).

Mike, Google 'fall of potential'. Some reading up on that will help you understand what you are trying to measure.
 
I really see no reason to measure and verify 25 ohms since it isn't a requirement. supply the one, and the supplemental (if required) and be done with it.
I also agree with K8MHZ about the clamp on. We have them, and the guys think we check resistance of the earth. All we are checking is resistance in ground loops with these.
Resistance of the earth and resistance of rods in earth are two different things. Fall of potential for one, clamp on for the other.


Here is a pretty good link for anyone interested.
http://www.rainbirdservicescorporat...rstanding Earth Ground Resistance by AEMC.pdf
 
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Hv&Lv said:
I really see no reason to measure and verify 25 ohms since it isn't a requirement. supply the one, and the supplemental (if required) and be done with it.
I also agree with K8MHZ about the clamp on. We have them, and the guys think we check resistance of the earth. All we are checking is resistance in ground loops with these.
Resistance of the earth and resistance of rods in earth are two different things. Fall of potential for one, clamp on for the other.


Here is a pretty good link for anyone interested.
http://www.rainbirdservicescorporat...rstanding Earth Ground Resistance by AEMC.pdf
Click to expand...

Intriguing.

Induce a voltage at 2.4 kHz and filter out the 60 Hz voltages then measure the current to extrapolate the resistance.

Kind of cool, but I wonder how it compares (in readings) to FOP tests.
 
K8MHZ said:
Intriguing.

Induce a voltage at 2.4 kHz and filter out the 60 Hz voltages then measure the current to extrapolate the resistance.

Kind of cool, but I wonder how it compares (in readings) to FOP tests.
Click to expand...

Quite honestly, I haven't thought about doing a FOP test and comparing the two...
 
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taylorp said:
If you carefully read 250.53(A) you will see that they are talking about rod, pipe, and plate electrodes as the only electrode present, now we need a supplemental electrode unless.

Only if you have a rod, pipe, or plate and have no other supplemental electrodes, do you need to prove the 25 ohm limit.

When single rods, pipes, or plates are combined with other grounding electrodes, no ohm measurements are needed or required.

At least this is the way I read 250.53 as a whole.
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Where in that section does it mention the rod, pipe or plate electrode as the only electrode? One other note, the pipe mentioned in this section is not the water pipe.
 
infinity said:
Where in that section does it mention the rod, pipe or plate electrode as the only electrode? One other note, the pipe mentioned in this section is not the water pipe.
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250.53(A)(2). "A single rod, pipe, or plate ....

And yes, I know what kind of pipe 250.53(A) is talking about.
 
jxofaltrds said:
Good question since I have not the proper tools to do so.

http://forums.mikeholt.com/showthread.php?t=88785

Now back to 'my' question. Where must the 25 ohm reading be taken?
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With a 3 point test you usually have two earth rods pushed into the ground separated by a large distance then you connect to the electrode you want to measure. Of course the electrode cannot be connected to the system.

06-3-point-test-2.jpg
 
taylorp said:
250.53(A)(2). "A single rod, pipe, or plate ....

And yes, I know what kind of pipe 250.53(A) is talking about.
Click to expand...

That means a single electrode of that type not the only electrode. In order for a ground rod to qualify as an electrode it would need supplementation or it has to meet the exception of 25 ohms or less even if there was a water pipe electrode.
 
Hv&Lv said:
Quite honestly, I haven't thought about doing a FOP test and comparing the two...
Click to expand...

Mike Holt did. He videoed a comparison between Ohm's Law using an ammeter on an energized ground rod, a three point FOP and a clamp. They were comparable. In fact, IIRC the clamp was surprisingly accurate.
 
Dennis Alwon said:
With a 3 point test you usually have two earth rods pushed into the ground separated by a large distance then you connect to the electrode you want to measure. Of course the electrode cannot be connected to the system.

06-3-point-test-2.jpg
Click to expand...

So the rod is not part of the 'system' until it measures 25 ohms or is supplemented by a 2nd:

(2) Supplemental Electrode Required. A single rod, pipe,
or plate electrode shall be supplemented by an additional
electrode of a type specified in 250.52(A)(2) through
(A)(8).
 
George Stolz said:
Mike Holt did. He videoed a comparison between Ohm's Law using an ammeter on an energized ground rod, a three point FOP and a clamp. They were comparable. In fact, IIRC the clamp was surprisingly accurate.
Click to expand...

When performed properly about 90% of the test reports completed by "other firms" that I have seen the test was not properly performed.
 
For a clamp on tester the clamp is measuring the "loop" through the earth so in effect is giving you the resistance of the loop which would be the total for two earth grounding points. In other words you have to have more than one grounding electrode for it to work. Your basic house service will be grounded by utility at the transformer and then again at the service by the Contractor providing you with two points. As for earth testing there are several methods the threee point being one of the simplest. Here is a really good document on the subject:

www.biddlemegger.com/biddle-ug/GettingDownToEarth-MC.pdf

Gives a good overall view and page 48-49 gives a quick explanation of clamp-on testers.
 
Also for the 25 ohm value.... I know that it has to do with step voltage and think I remember reading somewhere that value would limit the ground station to a rise of no more than 5000V. I will see if I can find the reference (IEEE I think). And the IEEE and NETA Standards for Industrial locations in 5 ohms and the value for substations in 1 ohm. These again are to limit step potentials.
 
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