# Ground rod resistance



## RIVETER (Sep 26, 2009)

Assuming you mean the resistance after it is driven.Is that the question? There is the "fall of potential" test I've heard about on this forum. I've not used it. Seems like it would take too much time. There is the clamp-on ground tester. I work in a factory and I just drive the rod and then measure from an existing known "Good grounding source" to the new rod. Maybe it's too simple but it's all I got.:thumbsup:


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## oldtimer (Jun 10, 2010)

Grapejuice said:


> How do you measure the resistance of a ground rod and what instrument do you use?


I use a Megger.


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## Zog (Apr 15, 2009)

oldtimer said:


> I use a Megger.


And how exactly does that work?


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## RIVETER (Sep 26, 2009)

Zog said:


> And how exactly does that work?


Great question.:thumbsup:


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## Zog (Apr 15, 2009)

Grapejuice said:


> How do you measure the resistance of a ground rod and what instrument do you use?


A 3 or 4 point FOP test is generally the only method accepted. Clamp on testers were a fad when they came out but testing has shown they won't produce accurate results on most types of systems. Not to mention them being mis-applied by untrained people that think they work like a clamp on ammeter. 


The most common method used to conduct this test is the fall of potential test using the 3 point method. It is important that the ground rod under test is disconnected from the rest of the grounding system prior to conducting this test, otherwise you will be measuring parallel resistances of all rods connected to the system (Other areas of the plant, the facility next door, etc.) 


The test is conducted by driving two ground rods into the earth as shown below in 3.1. The potential difference between rods X and Y is measured by a voltmeter, and the current flow between rods X and Z is measured by an ammeter


The goal in precisely measuring the resistance to ground is to place the auxiliary current electrode (Z) far enough from the ground electrode under test so that the auxiliary potential electrode (Y) will be outside of the effective resistance areas of both the ground electrode and the auxiliary current electrode. The best way to find out if the auxiliary potential rod (Y) is outside the effective resistance areas is to move it between X and Z and to take a reading at each location.


If the auxiliary potential rod Y is in an effective resistance area (photo 3.2) the readings taken will vary noticeably in value. Under these conditions, no exact value for the resistance to ground may be determined.

On the other hand, if the auxiliary potential rod Y is located outside of the effective resistance areas (photo 3.3), as Y is moved back and forth the reading variation is minimal. The readings taken should be relatively close to each other, and are the best values for the resistance to ground of the ground X. The readings should be plotted to ensure that they lie in a “plateau” region, often referred to as the “62% area.”​


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## Greg (Aug 1, 2007)

We use a FOP tester as described in Zog's post. We have a lime mine here in town and we have to certify their grounding (electrodes) and bonding every year in accordance with MSHA standards. The tester we use is made by Ideal and it cost about $1500.00.


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## Zog (Apr 15, 2009)

We do a lot of work in those same mines. Nasty place sometimes.

I am still confused about how one would use a megger, one lead on the rod and jam the other into the dirt? How do you prove your reading? How far away from the rod? I can't grasp that concept.


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## brian john (Mar 11, 2007)

Zog said:


> We do a lot of work in those same mines. Nasty place sometimes.
> 
> I am still confused about how one would use a megger, one lead on the rod and jam the other into the dirt? How do you prove your reading? How far away from the rod? I can't grasp that concept.


Maybe he means an older model Megger brand, I have one that is called an Earth Megger or something close to that. Often times engineers spec an megger test when they mean a earth/ground resistance test.


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## TheRick (Apr 13, 2008)

Greg said:


> We use a FOP tester as described in Zog's post. We have a lime mine here in town and we have to certify their grounding (electrodes) and bonding every year in accordance with MSHA standards. The tester we use is made by Ideal and it cost about $1500.00.


I worked for 5 years in a cement plant with a Limestone quarry and we had to do the same. The FOP tester we had was made by Biddell and was older than me I think!


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## Zog (Apr 15, 2009)

brian john said:


> Maybe he means an older model Megger brand, I have one that is called an Earth Megger or something close to that. Often times engineers spec an megger test when they mean a earth/ground resistance test.


In that case Megger could be the only test you do on anything, they make CB injection sets, relay test sets, IR, TTR, winding resistance, etc.. I don't think the next testing quote you get for transformers, relays, breakers, etc will fly if you say you wil "Megger: everthing in your quote. 

Oldtimer can you please clarify?


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## oldtimer (Jun 10, 2010)

Zog said:


> In that case Megger could be the only test you do on anything, they make CB injection sets, relay test sets, IR, TTR, winding resistance, etc.. I don't think the next testing quote you get for transformers, relays, breakers, etc will fly if you say you wil "Megger: everthing in your quote.
> 
> Oldtimer can you please clarify?


 My meter is one of the older ones. Crank handle. We just always called it a megger. I guess the proper use is as an insulation tester.


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## brian john (Mar 11, 2007)

TheRick said:


> I worked for 5 years in a cement plant with a Limestone quarry and we had to do the same. The FOP tester we had was made by Biddell and was older than me I think!


 
Was this a null balance style meter?


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## McClary’s Electrical (Feb 21, 2009)

oldtimer said:


> My meter is one of the older ones. Crank handle. We just always called it a megger. I guess the proper use is as an insulation tester.


 


How do you use that on a ground rod? Where do you put each lead? What does that prove? Please explain


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## RIVETER (Sep 26, 2009)

oldtimer said:


> My meter is one of the older ones. Crank handle. We just always called it a megger. I guess the proper use is as an insulation tester.


It was a megger. I've used them as well as a type I just pushed a button. And we used it as an insulation tester as well.


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## Skipp (May 23, 2010)

Oldtimer please tell us the procedure of ground rod test using a crank megger? I had to buy an expensive Biddle 250260 Ground tester. 
Are you saying I could have used my Megger to do the same test?


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## brian john (Mar 11, 2007)

Skipp said:


> Oldtimer please tell us the procedure of ground rod test using a crank megger? I had to buy an expensive Biddle 250260 Ground tester.
> Are you saying I could have used my Megger to do the same test?


 
Biddle made a null balance meter that was called amn Earth Megger that was a hand crank, I THINK I still have mine, if so when I get back to DC I will post pics.


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## oldtimer (Jun 10, 2010)

brian john said:


> Biddle made a null balance meter that was called amn Earth Megger that was a hand crank, I THINK I still have mine, if so when I get back to DC I will post pics.


 I hope I can explain this right. With disconnect open, One lead on conductor, other lead on ground rod. Crank your megger, you should have 25 ohms or less.

I'm not sure how to explain it right. Maybe someone can give a better explanation of the procedure.


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## Zog (Apr 15, 2009)

oldtimer said:


> I hope I can explain this right. With disconnect open, One lead on conductor, other lead on ground rod. Crank your megger, you should have 25 ohms or less.


When you test a groud rod (Or grid) the test needs to be done with the rod under test disconnected from the system (To prevent measuring multiple groudn paths). Then you need to measure resistance from the rod to earth (as I explained above). 

What you are doing is more like a ground continuity test, which has nothing to do with the 25 ohm requirement.


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## oldtimer (Jun 10, 2010)

Zog said:


> When you test a groud rod (Or grid) the test needs to be done with the rod under test disconnected from the system (To prevent measuring multiple groudn paths). Then you need to measure resistance from the rod to earth (as I explained above).
> 
> What you are doing is more like a ground continuity test, which has nothing to do with the 25 ohm requirement.


 That explains it better. Thanks.


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