Fall-of-Potential Test - Apparently not Possible

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charlie b

charlie b

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I have been asked to assist with an issue related to ground resistance testing for a series of small buildings in close proximity to one another. They have been in service for decades, and as-built drawings are not available. It is “believed” that there are two large ground loops, one surrounding half the buildings and the other surrounding the remaining half. Each building has some type of lightning protection system (I have not yet visited the site, so my knowledge is limited). Because of the types of materials stored in the buildings, they are required to undergo a periodic certification process. The process requires confirmation of the resistance of the grounding electrode system. I am told that this cannot be confirmed, and therefore there is an outstanding deficiency in the certification reports. That is because the 3-point fall-of-potential test requires placing probes outside the sphere of influence of the grounding electrode system, and the ground loops are too big.

Two Questions:

1. Is there a viable method of precisely locating the two large ground loops? I don’t think walking around the campus with a metal detector is likely to succeed. Is there a way to impose a signal onto the loop, starting at one of the buildings, and detecting that signal using some type of detector that you can carry around the campus?

2. To what extent can the presence of a grounding electrode system associated with one building adversely impact the results of a fall-of-potential test being performed on the grounding electrode system associated with a nearby building? It has been suggested that we abandon the existing ground loops in place and install an individual GES at each building.
 
A Greenlee 501 tracer might work. I’ve never tried it on ground loops before, but have picked up rebar mats with it before. You may get a wide signal, but should be able to narrow down the approximate area.
 
charlie b said:
I have been asked to assist with an issue related to ground resistance testing for a series of small buildings in close proximity to one another. They have been in service for decades, and as-built drawings are not available. It is “believed” that there are two large ground loops, one surrounding half the buildings and the other surrounding the remaining half. Each building has some type of lightning protection system (I have not yet visited the site, so my knowledge is limited). Because of the types of materials stored in the buildings, they are required to undergo a periodic certification process. The process requires confirmation of the resistance of the grounding electrode system. I am told that this cannot be confirmed, and therefore there is an outstanding deficiency in the certification reports. That is because the 3-point fall-of-potential test requires placing probes outside the sphere of influence of the grounding electrode system, and the ground loops are too big.

Two Questions:

1. Is there a viable method of precisely locating the two large ground loops? I don’t think walking around the campus with a metal detector is likely to succeed. Is there a way to impose a signal onto the loop, starting at one of the buildings, and detecting that signal using some type of detector that you can carry around the campus?

2. To what extent can the presence of a grounding electrode system associated with one building adversely impact the results of a fall-of-potential test being performed on the grounding electrode system associated with a nearby building? It has been suggested that we abandon the existing ground loops in place and install an individual GES at each building.
Click to expand...
I think I would reach out to AEMC. They might be of help. I've only done 3 point fall of potential on ground rods which have a small sphere of influence.
 
Used properly, the current injection clamp-on meter system can give pretty good results as long as collectively better return point or set of points exists for each individual electrode to be tested. Possibly not accepted for certification though.
 
Thanks for the replies.

I started looking at the AEMC web site, and was particularly interested in the clamp-on meter. But I don't think it can work when a ground ring is in place. When it imposes a voltage signal onto the grounding electrode conductor, current will flow around the ring and back to the same GEC. Planet Earth will not be part of the return path, and therefore the resistance of the grounding system cannot be measured.

Or do I incorrectly understand its operation?
 
For a clamp on the the injected signal goes in to the grouding electrode, then to the util ground and back to you via the neutral. AEMC wpuld be a good resource.
 
tom baker said:
For a clamp on the the injected signal goes in to the grouding electrode, then to the util ground and back to you via the neutral. AEMC wpuld be a good resource.
Click to expand...
Yes, that's my understanding. So for the test to work properly you need to apply the clamp over over a single GEC with the bond to neutral (and therefore POCO ground) on one side of the clamp, and the GES under test on the other side. That's to make sure that all of the applied test current flowing into the GES under test returns through the neutral and POCO ground, and not through some parallel conductor or other path that would make the measurement invalid. So you'd want the clamp "upstream" of a connection to a ground ring, and not on the ring itself to avoid what you mentioned Charlie.
 
If there are multiple buildings that each have a GEC to the ground ring then that could create a problem with the method I see being described. A solution could be to temporarily disconnect all but one of the GEC connections to neutral and test on that GEC.
 
If I’m reading this correctly you want to locate the ground loop.
I’ve used a pipe horn and a Dynatel 573 to find ground loops. The dynatel will inject a signal.
the pipe horn will also, but usually it picks up everything in the ground. It’s a little tricky to get exactly on the loop. The horn signals love shielded cables, so it usually jumps all over telephone lines and leads you off ground loops.
 
jaggedben said:
If there are multiple buildings that each have a GEC to the ground ring then that could create a problem with the method I see being described. A solution could be to temporarily disconnect all but one of the GEC connections to neutral and test on that GEC.
Click to expand...
Yes, clarification would be needed if there's one service (and bond to neutral) for the small buildings, separate services, or maybe even no service?
For the 3-point fall-of-potential test the GES needs to be isolated from POCO grounds and so any neutral bond must be disconnected.
 
I’m going to assume induced frequency testing isn’t an option either...
 
charlie b said:
I have been asked to assist with an issue related to ground resistance testing for a series of small buildings in close proximity to one another. They have been in service for decades, and as-built drawings are not available. It is “believed” that there are two large ground loops, one surrounding half the buildings and the other surrounding the remaining half. Each building has some type of lightning protection system (I have not yet visited the site, so my knowledge is limited). Because of the types of materials stored in the buildings, they are required to undergo a periodic certification process. The process requires confirmation of the resistance of the grounding electrode system. I am told that this cannot be confirmed, and therefore there is an outstanding deficiency in the certification reports. That is because the 3-point fall-of-potential test requires placing probes outside the sphere of influence of the grounding electrode system, and the ground loops are too big.

Two Questions:

1. Is there a viable method of precisely locating the two large ground loops? I don’t think walking around the campus with a metal detector is likely to succeed. Is there a way to impose a signal onto the loop, starting at one of the buildings, and detecting that signal using some type of detector that you can carry around the campus?

2. To what extent can the presence of a grounding electrode system associated with one building adversely impact the results of a fall-of-potential test being performed on the grounding electrode system associated with a nearby building? It has been suggested that we abandon the existing ground loops in place and install an individual GES at each building.
Click to expand...

You've been solicited for a 'Herb Ufer'

~RJ~
 
Thanks again. And again I am new to this project, have not seen the site, have no as-built drawings, and am only seeking concepts that I can investigate.
 
charlie b said:
Thanks for the replies.

I started looking at the AEMC web site, and was particularly interested in the clamp-on meter. But I don't think it can work when a ground ring is in place. When it imposes a voltage signal onto the grounding electrode conductor, current will flow around the ring and back to the same GEC. Planet Earth will not be part of the return path, and therefore the resistance of the grounding system cannot be measured.

Or do I incorrectly understand its operation?
Click to expand...

i would imagine that dependent on the 'system'......https://en.wikipedia.org/wiki/Earthing_system

~S~
 
Is JBLM involved in this?

May have done similar at other like sites. Cannot comment on internet.
delete this post if appropriate

re: probes outside the sphere of influence of the grounding electrode system
-- use other remote facility grounding systems for 3rd electrode.
 
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200201-2120 EST

The object of a ground electrode resistance (impedance) measurement is to determine a value to remote earth. This can be estimated by the fall of potential, or impedance method (current transformer).

Because of the two different electrodes in moderately close proximity you would disconnect one while testing the other.

Apparently space does not permeng a fall of potential method. This means the direct impedance methoc needs to be used.

The direct impedance method requires that there be many remote earth connections, typically by many 3 phase ground rods connected to wye neutrals on the primary side, and thus also in turn on the secondary wye neutral.

By the use of this method the ground electrode and its resistance to adjacent earth creates a closed loop. The resistance of this closed loop is essentially that of the ground electrode to remote earth.

A current transformer coupled to this loop provides a means to measure impedance.

.
 
Using a variation of Gar's method and at 120 Vac, a concrete encased electrode over 200 ft in length below the water table in iron/magnesium rich cemented hardpan measured 400 milliohms. Sure beats the 25 ohm requirement.
 
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