american electric
Member
What is the procedure for testing driven ground rods at a service?
Ground rod testing
- Thread starter american electric
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bphgravity
Senior Member
- Location
- Florida
Re: Ground rod testing
There are several methods for testing ground resistance. The most common are 2, 3, and 4 point testers. These are usually called Fall of Potential testers. The exact process and procedures will be provided by the manufacturer.
A clamp-on meter has recently hit the market and makes the test much more time saving and requires no auxiliary equipment. I got one for around $1,100 that was re-conditioned.
There are several methods for testing ground resistance. The most common are 2, 3, and 4 point testers. These are usually called Fall of Potential testers. The exact process and procedures will be provided by the manufacturer.
A clamp-on meter has recently hit the market and makes the test much more time saving and requires no auxiliary equipment. I got one for around $1,100 that was re-conditioned.
american electric
Member
Re: Ground rod testing
Thanks Dereck. I have the megger. Could you give me a brief instruction on the "fall & potential" method? Thanks ,Glenn @A.E.
Thanks Dereck. I have the megger. Could you give me a brief instruction on the "fall & potential" method? Thanks ,Glenn @A.E.
stew
Senior Member
- Location
- federal way,washington
Re: Ground rod testing
me too derrick
me too derrick
- Location
- Bremerton, Washington
- Occupation
- Master Electrician
Re: Ground rod testing
Glen its a 3 pole or four pole tester, it may be made by megger but its not a "megger".
Glen its a 3 pole or four pole tester, it may be made by megger but its not a "megger".
bphgravity
Senior Member
- Location
- Florida
Re: Ground rod testing
The theory is simply. The operation can be complicated. Ohm's Law tells us that if current is constant, voltage will be inversely proportional to resistance.
Utilizing a ground resistance meter (digital is preferred), two auxiliary electrodes are driven into the soil at predetermined distances, per the testing specifications. During a normal test of ground rod the meter supplies a constant current between the ground rod under test and the most remote auxiliary electrode. A series of measurements of the voltage drops between the ground rod under test and the remote electrode are made by moving the intermediate electrode in steps away from the ground rod under test. Accurate testing is confirmed by testing a second time in an opposite direction.
The results are plotted and an accuracy curve is made. As stated before, this procedure takes time and good training to get accurate results.
The theory is simply. The operation can be complicated. Ohm's Law tells us that if current is constant, voltage will be inversely proportional to resistance.
Utilizing a ground resistance meter (digital is preferred), two auxiliary electrodes are driven into the soil at predetermined distances, per the testing specifications. During a normal test of ground rod the meter supplies a constant current between the ground rod under test and the most remote auxiliary electrode. A series of measurements of the voltage drops between the ground rod under test and the remote electrode are made by moving the intermediate electrode in steps away from the ground rod under test. Accurate testing is confirmed by testing a second time in an opposite direction.
The results are plotted and an accuracy curve is made. As stated before, this procedure takes time and good training to get accurate results.
american electric
Member
Re: Ground rod testing
Thanks Tom. I thought Dereck was reffering to a type of meter ("megohmeter") rather than a manufacturer of the equipment.
Sounds like I won't be able to assure the quality (in ohms) of the driven ground rods without either of these testers that Dereck mentioned.
A local car dealership asked me to verify their grounding. They have lost thier computers a few times after thunder storms. Their computer people are suspicious that the gronding of the service is not good. I have visually inspected and tested the continuity with an ohm meter (Fluke 87) all good to the rods.
The service is 400 amps , 3 phase, 2 cu rods connected with continuous #4 bare and about 10 years old by estimate. The rods are less than 10 ft. away.
Any ideas?
How about manufacturers of these testers? I don't know who AEMC is. Thanks
Thanks Tom. I thought Dereck was reffering to a type of meter ("megohmeter") rather than a manufacturer of the equipment.
Sounds like I won't be able to assure the quality (in ohms) of the driven ground rods without either of these testers that Dereck mentioned.
A local car dealership asked me to verify their grounding. They have lost thier computers a few times after thunder storms. Their computer people are suspicious that the gronding of the service is not good. I have visually inspected and tested the continuity with an ohm meter (Fluke 87) all good to the rods.
The service is 400 amps , 3 phase, 2 cu rods connected with continuous #4 bare and about 10 years old by estimate. The rods are less than 10 ft. away.
Any ideas?
How about manufacturers of these testers? I don't know who AEMC is. Thanks
american electric
Member
Re: Ground rod testing
Thanks again, sounds like the "clamp on" type is the one for me.
BTW, I am brand new to the east coast and have never dealt with the lightning factor. Never really thought about it really. It is confusing in many respects and I notice there are many opinions.
Know of any good reading that would give me a basic understanding?
Thanks again, sounds like the "clamp on" type is the one for me.
BTW, I am brand new to the east coast and have never dealt with the lightning factor. Never really thought about it really. It is confusing in many respects and I notice there are many opinions.
Know of any good reading that would give me a basic understanding?
Re: Ground rod testing
For AEMC loop tester go here: http://www.groundtesterstore.com/index.php?source=EM
Look for model 3731
For soil resistance go here:
http://www.avointl.com/us/products/ProductDetailsBySubGroup.php?PSGC=B105&BS=B
Look on the right for 3-point or 4-point
To be honest I think you are barking up the wrong tree. I would be looking into TVSS and inside wiring distribution.
For AEMC loop tester go here: http://www.groundtesterstore.com/index.php?source=EM
Look for model 3731
For soil resistance go here:
http://www.avointl.com/us/products/ProductDetailsBySubGroup.php?PSGC=B105&BS=B
Look on the right for 3-point or 4-point
To be honest I think you are barking up the wrong tree. I would be looking into TVSS and inside wiring distribution.
american electric
Member
Re: Ground rod testing
Thanks for the lead to the AEMC Dereck.
I usually bark up the tree that the customer points at. In this case the customer does not believe it is the ground either, but the computer vendors for him are insisting that it be verified as "good" . So I will take care of that issue first, as asked.
With the price on those testers , I think I will be looking to rent or borrow rather than own. At least this time. Woof, Glenn@AE
Thanks for the lead to the AEMC Dereck.
I usually bark up the tree that the customer points at. In this case the customer does not believe it is the ground either, but the computer vendors for him are insisting that it be verified as "good" . So I will take care of that issue first, as asked.
With the price on those testers , I think I will be looking to rent or borrow rather than own. At least this time. Woof, Glenn@AE
Re: Ground rod testing
American electric, the meters are expensive. Your best bet is to rent one. It is really a tool of inspectors and engineers. I suppose a large EC firm could justify one. I prefer a 3-point tester as it is more accurate. The loop tester measures the ground under test in series with the utility ground through the service neutral. I have seen results where it measured 54 ohms when ground ring and chemical rods were both used. When the 3-point was used, the GES measured 6-ohms. The remaining 48-ohms was the utility ground.
I work in the telecom sector and deal with equipment vendors and IT people on a daily basis. Ground impedance comes up constantly, and in my experience, ground always gets falsely accused. No matter what impedance is measured, it is never low enough to satisfy them. You mentioned this is a car dealership, so there is no practical reason to have a GES with an impedance of 5-ohms or less to satisfy a IT rep.
Here are a few things I would look into:
Is the GES in good repair, and meets code?
Is the N-G bond in the proper location, and no downstream N-G violations?
Are EGC?s run with all feeders and branch circuits?
Is there a Class ?C?, UL-1499 second edition service entrance rated TVSS installed at the service entrance?
Is there a Class ?B?, UL-1499 second edition TVSS installed in branch panels?
Is there a Class ?A?, UL-1499 second edition, point of use TVSS used on sensitive equipment?
Does sensitive equipment use dedicated AC circuits?
Does sensitive equipment use any ground or grounded signal- interconnect cables such as coax, RS-232, or RS-422?
Do they power their sensitive equipment via a dual conversion UPS?
American electric, the meters are expensive. Your best bet is to rent one. It is really a tool of inspectors and engineers. I suppose a large EC firm could justify one. I prefer a 3-point tester as it is more accurate. The loop tester measures the ground under test in series with the utility ground through the service neutral. I have seen results where it measured 54 ohms when ground ring and chemical rods were both used. When the 3-point was used, the GES measured 6-ohms. The remaining 48-ohms was the utility ground.
I work in the telecom sector and deal with equipment vendors and IT people on a daily basis. Ground impedance comes up constantly, and in my experience, ground always gets falsely accused. No matter what impedance is measured, it is never low enough to satisfy them. You mentioned this is a car dealership, so there is no practical reason to have a GES with an impedance of 5-ohms or less to satisfy a IT rep.
Here are a few things I would look into:
Is the GES in good repair, and meets code?
Is the N-G bond in the proper location, and no downstream N-G violations?
Are EGC?s run with all feeders and branch circuits?
Is there a Class ?C?, UL-1499 second edition service entrance rated TVSS installed at the service entrance?
Is there a Class ?B?, UL-1499 second edition TVSS installed in branch panels?
Is there a Class ?A?, UL-1499 second edition, point of use TVSS used on sensitive equipment?
Does sensitive equipment use dedicated AC circuits?
Does sensitive equipment use any ground or grounded signal- interconnect cables such as coax, RS-232, or RS-422?
Do they power their sensitive equipment via a dual conversion UPS?
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