resistance to ground measurement

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vedika.agrawal

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1. Can anyone tell how resistance to ground measurement is performed ideally?

2. And how can it be performed if no specialised instrument is available? If I want to perform it by say current and voltage measurements, then is it possible and how to do? And which voltage measurement should I have? Let's say I want to measure resistance to ground measurement of a conductor surface...

Please pardon me if my questions are not proper. I want to know anything if you can on RTG measurement. I couldn't get anything relevent on the internet.
 
vedika.agrawal said:
1. Can anyone tell how resistance to ground measurement is performed ideally?

2. And how can it be performed if no specialised instrument is available? If I want to perform it by say current and voltage measurements, then is it possible and how to do? And which voltage measurement should I have? Let's say I want to measure resistance to ground measurement of a conductor surface...

Please pardon me if my questions are not proper. I want to know anything if you can on RTG measurement. I couldn't get anything relevent on the internet.
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One could say that if you could get the connection to ground to blow a 5amp fuse it would be less than 25 ohms. Oh, did you say "proper"
 
No it really takes a specialized instrument to do this accurately and safely. There is what is called a "POOR MANS" resistance meter, but it is not safe nor am I going to give details.
 
You must first determine WHY you need to know the grounding resistance and impedance and WHAT that information means. In short, there is no need or reason to know this information for 99% of all installations. If the grounding electrode system is installed per the requirements of the code, the actual resistance to the earth is truly inconsequential.

1. The only tried and tested method is the "Fall of Potential" test method. This is a relatively complicated procedure that uses highly specialized equipment and techniques for determining grounding resistance and impedance. There are "clamp-on" type grounding testers on the market today, however their accuracy is in question.

2. NO. (the method described in the second post is dangerous and shouldn't be attempted)
 
The best procedure to perform this test is CALL ME.

There are companies that specialize in performing ground test, When we perform a ground test I use two methods clamp on and the 3 point method, I like to perform a minimum of 3-3 point taking readings at various points between the electrode under test and the temporary electrode test points.

Then you have to interpret the results and have an understanding of the readings and whether the readings were taken at acceptable test points verifying you were outside the sphere of influence of the made electrode, for a single rod usually a 100' is the standard, for larger electrodes you may have to go out 1,000'-2,000' have done some that required going out 5,000'.
 
brian john said:
The best procedure to perform this test is CALL ME.

There are companies that specialize in performing ground test, When we perform a ground test I use two methods clamp on and the 3 point method, I like to perform a minimum of 3-3 point taking readings at various points between the electrode under test and the temporary electrode test points.

Then you have to interpret the results and have an understanding of the readings and whether the readings were taken at acceptable test points verifying you were outside the sphere of influence of the made electrode, for a single rod usually a 100' is the standard, for larger electrodes you may have to go out 1,000'-2,000' have done some that required going out 5,000'.
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That could be tough in urban locations.
 
Pierre C Belarge said:
That could be tough in urban locations.
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:grin: I was just thinking of being some place like Manhattan Island trying to do this on something like one of the telephone office ground rings, and stringing those two cables east-to-west accross the island stopping all north/south traffic.

I have built two data centers (well part of the design team) in that area, Carterette NJ and Queens area of NYC with ground ring/rod combo. We never specified a target impedance or asked for a measurement from the contractors. I did use a clamp-on AEMC, but did not trust the reading, it was too low.
 
Here is a method for those with out fall of potential Test Sets.

This is from PaulUK posted at ECN

The general method that is often used here in the U.K. is to pass a current through the ground and then measure the potential using a auxiliary electrode.

Call the existing grounding system X. Drive a ground rod, Y, into the earth at some distance from any of the existing ground rods. The reistance areas must not overlap.


Connect an adjustable low-voltage source (we usually use about 25 to 40V AC) between X and Y with an ammeter in series to monitor the current.

Now drive an auxiliary electrode Z at a point roughly mid-way between X and Y, and connect a voltmeter between X and Z. The ground system reistance is then simply E/I by Ohm's Law.

To check that the rod resistance areas are not overlapping, move rod Z about 10% of the X-Y distance closer to X and repeat the test. Do the same with Z moved about 10% closer to Y. If all three readings are approx. the same, then take this as the system earth resistance. If the readings differ substantially, then you will need to move ground rod Y farther away and start over.

To obtain accurate readings, the original ground system X must be disconnected, otherwise any other interconnected paths to ground will affect the reading.

Roger
 
dereckbc said:
:grin: I was just thinking of being some place like Manhattan Island trying to do this on something like one of the telephone office ground rings, and stringing those two cables east-to-west accross the island stopping all north/south traffic.

I have built two data centers (well part of the design team) in that area, Carterette NJ and Queens area of NYC with ground ring/rod combo. We never specified a target impedance or asked for a measurement from the contractors. I did use a clamp-on AEMC, but did not trust the reading, it was too low.
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I remember reading that the clamp-on type of tester was somewhere in the neighborhood of 8% low in the readings?
 
Testing in cities is next to impossible, BUT I try to explain if all the inter-connected systems does not lend itself to a low impedance ground driving a few additionally electrodes in to back fill sure ain't going to be an improvement.

We do two point test and use a clamp on in urban environment.
 
why this test needed for me

why this test needed for me

I think I should have mentioned the reason why I want to perform this test..
I want to detect faults in induction motor. One fault zone is insulation...for which I read there are methods like RTG testing.I want to detect the faults online with some voltage and current readings may be.(which I could take by sensores). So firstly I wanted to know how it is performed ideally with instruments .
 
vedika.agrawal said:
I think I should have mentioned the reason why I want to perform this test..
I want to detect faults in induction motor. One fault zone is insulation...for which I read there are methods like RTG testing.I want to detect the faults online with some voltage and current readings may be.(which I could take by sensores). So firstly I wanted to know how it is performed ideally with instruments .
Click to expand...
Aha! Everyone else is talking about ground resistance and you are talking about insulation resistance. You need an insulation resistance tester, or megohmmeter, commonly known as a 'megger'. See www.aemc.com/techinfo/techworkbooks/megohmmeters/tech_megohm.pdf
 
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