25 ohms resistance measurement

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How does one measure the 25 ohms (or less) of resistance required for driven ground rods? Should I isolate the service neutral, and place my ohmmeter leads from there to the driven rod, or is there something else that a field hand wouldn't know? Please make it simple so that all reading can understand.
 

Dennis Alwon

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How does one measure the 25 ohms (or less) of resistance required for driven ground rods? Should I isolate the service neutral, and place my ohmmeter leads from there to the driven rod, or is there something else that a field hand wouldn't know? Please make it simple so that all reading can understand.


An ohm meter will not do it. You will need a 3 point tester to an earth to ground resistance tester. Not worth it because in most areas of the country you will never get 25 ohms or less. Best to do a concrete encased electrode if it is available or drive the rod
 

GoldDigger

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And in a large number of applications you will not be able to do a valid three point Fall Of Potential test because you will not have undisturbed earth far enough away to place the other two electrodes.
A nearby UFER or conductive pipe in the earth will distort the voltage field, screwing up the measurement.
A clamp type ground resistance meter will be more useful (assuming that an additional ground electrode(s) such as POCO MGN are available.)
 

iwire

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I have done a few of these tests and do not recall any requirement not for up disturbed soil, only adequate distances.
 

GoldDigger

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I chose my words poorly.
The soil must be free from disturbing influences such as concrete slabs, buried metallic conduits and water or sewer pipes and even nearby building ground electrodes and POCO ground rods.
The three point method also assumes that the resistivity of the earth (to a depth comparable to that of the electrode being tested) is uniform all the way from the electrode under test to the remote reference electrode.
Digging and compacting the native soil as well as adding sand or moist topsoil will all change the soil resistivity.

These often unstated assumptions account for a large part of the lack of reproducibility and accuracy of FOP measurements. (Take a look at the differences in readings between methods and lack of scalability in Mike's long video on driven ground electrodes at his house.)
 

don_resqcapt19

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I chose my words poorly.
The soil must be free from disturbing influences such as concrete slabs, buried metallic conduits and water or sewer pipes and even nearby building ground electrodes and POCO ground rods.
The three point method also assumes that the resistivity of the earth (to a depth comparable to that of the electrode being tested) is uniform all the way from the electrode under test to the remote reference electrode.
Digging and compacting the native soil as well as adding sand or moist topsoil will all change the soil resistivity.

These often unstated assumptions account for a large part of the lack of reproducibility and accuracy of FOP measurements. (Take a look at the differences in readings between methods and lack of scalability in Mike's long video on driven ground electrodes at his house.)
But you have many of the same issue for a clamp on meter.
 

GoldDigger

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But you have many of the same issue for a clamp on meter.
True, but those issues are less critical as applied to the remote ground conditions. Any disturbances in the area of influence of the electrode being tested will just be correctly detected as affecting the real electrode to remote, infinite, earth resistance instead of messing up a very sensitive measurement that assumes uniform conditions over a wide area.

It is true that if part of the reference electrode system is within the sphere of influence of the electrode being tested the clamp method will read too low a resistance.

The three point method, by its nature, will be harder to apply for a distributed electrode like a UFER or a ring than for a simple rod. The clamp method is less sensitive to the geometry of the tested electrode. (IMHO)
 
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iwire

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True, but those issues are less critical as applied to the remote ground conditions. Any disturbances in the area of influence of the electrode being tested will just be correctly detected as affecting the real electrode to remote, infinite, earth resistance instead of messing up a very sensitive measurement that assumes uniform conditions over a wide area.

It is true that if part of the reference electrode system is within the sphere of influence of the electrode being tested the clamp method will read too low a resistance.

The three point method, by its nature, will be harder to apply for a distributed electrode like a UFER or a ring than for a simple rod. The clamp method is less sensitive to the geometry of the tested electrode. (IMHO)

If someone could explain to me why 25 ohms is so critical* I might be more concerned about all that. :)



*Unless there is more than one rod, than it does not mater at all. :dunce:
 

don_resqcapt19

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...
It is true that if part of the reference electrode system is within the sphere of influence of the electrode being tested the clamp method will read too low a resistance. ...
...
Which is almost impossible to avoid in an industrial or communications grounding electrode system.
 
If there is such an issue with the soil, then why not mandate the supplemental electrode. Here in Houston we have gumbo clay, sand and dirt (nothing like Austin or maybe San Antonio), so really how many places would have the ideal situations for only one rod. Seems easier to just require 2 and be done with it. Thanks for the replies, we continually look for simple solutions for difficult problems
 

Strathead

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If there is such an issue with the soil, then why not mandate the supplemental electrode. Here in Houston we have gumbo clay, sand and dirt (nothing like Austin or maybe San Antonio), so really how many places would have the ideal situations for only one rod. Seems easier to just require 2 and be done with it. Thanks for the replies, we continually look for simple solutions for difficult problems

Remember that there is politics as well as practicality in the NEC. Most of us feel the AFCI issue is a huge example of this. The real desire for grounding is to get a low enough resistance to trip a breaker during a ground fault. Some places this is not difficult depending on location and season. Other places it is virtually impossible. In my opinion, housing couldn't absorb the cost of installing ground rings around very house that had high resistance soil, and there isn't a manufacturer who would make enough money to lobby for it. So, they now require the UFER (concrete encased electrode) for new installations (which I think is a really good thing) and give the installer the out of two ground rods maximum in an existing installation.
 

kwired

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If there is such an issue with the soil, then why not mandate the supplemental electrode. Here in Houston we have gumbo clay, sand and dirt (nothing like Austin or maybe San Antonio), so really how many places would have the ideal situations for only one rod. Seems easier to just require 2 and be done with it. Thanks for the replies, we continually look for simple solutions for difficult problems
Which NEC has made use 2 rods and be done with it the most practical method for most installations. If you have a need for a specific resistance then that is a design requirement not a code requirement.

NEC also doesn't even require the use of a rod if you have building steel or concrete encased electrodes, and water pipe electrodes only need supplemented with another electrode which does not have to be a rod.
 

charlie b

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The real desire for grounding is to get a low enough resistance to trip a breaker during a ground fault.
I think not. Unless you are dealing with medium voltage systems, planet Earth is never going to play a role in getting a breaker to trip. (What, never? No never! What, never? Well, hardly ever!) Indeed, the last sentence in 250.4(B)(4) tells us never to count on it doing so.

 

zog

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I would say if I follow the meters instructions the results are valid enough for this arbitrary nonsense. :D

Exactly, it's not just the measurement but taking multiple measurements and plotting them to prove you are outside any spheres of influence. Clamp on has no way to prove this which is why it is generally not deemed a suitable test.
 

Strathead

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I think not. Unless you are dealing with medium voltage systems, planet Earth is never going to play a role in getting a breaker to trip. (What, never? No never! What, never? Well, hardly ever!) Indeed, the last sentence in 250.4(B)(4) tells us never to count on it doing so.


I know that, but I assume it is the intent, or they wouldn't even bother.
 
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