Fall of Potential Test

[charlie b]

Consider a site with 50+ small buildings. Each is about 50' x 100'. Typical distance between buildings is around 200 feet. Each has a ground ring bonded to its service panel and to a lightning protection system. The ground ring surrounding each building is bonded to the ground ring of at least one other building, so that all rings are bonded to each other.

Questions:

1. Is it possible to conduct a standard 3-point fall of potential test at each building, and obtain a reliable result, despite the presence of the ground rings associated with the other buildings?
2. Are the buildings too close to one another to allow placement of the probes as far from the building under test as the test process requires, or would the probes have to be so far from the building under test that they end up inside the ground ring of another building?

 

[paulengr]

Consider a site with 50+ small buildings. Each is about 50' x 100'. Typical distance between buildings is around 200 feet. Each has a ground ring bonded to its service panel and to a lightning protection system. The ground ring surrounding each building is bonded to the ground ring of at least one other building, so that all rings are bonded to each other.

Questions:

1. Is it possible to conduct a standard 3-point fall of potential test at each building, and obtain a reliable result, despite the presence of the ground rings associated with the other buildings?
2. Are the buildings too close to one another to allow placement of the probes as far from the building under test as the test process requires, or would the probes have to be so far from the building under test that they end up inside the ground ring of another building?

Effectively it’s all one large ground grid. One test from whatever passes for an “edge” unless you can somehow isolate which it sounds like you can’t.

Alternatively you could use the clamp on method which can test the bonding and what’s going on underground for each building.

 

[gar]

200222-1648

charlie b:

I believe that back on January 21 you asked essentially the same question.

Read the Fluke discussion for some background.

Fall-of-Potential Measurement

The Fall-of-Potential test method is used to measure the ability of an earth ground system or an individual electrode to dissipate energy from a site.

www.fluke.com

Also go to the Stakeless Measurement.

Ground electrode resistance measurements are not some highly accurate type of measurement, but more of an order of magnitude type of thing.

You operate under a rule based system, both NEC and government. This may or may not relate to reality.

I believe you have one very big grounding electrode system. All the ring electrodes of all the buildings tied together. To do the fall of potential method you need to disconnect all these electrodes from the power system, but leave the electrodes connected together.

In the fall of potential method you have three electrodes. The one under test, a voltage probe, and a current injection probe. The injection probe is furthest from the electrode under test. The voltage probe is in between the other two, and has to be far enough away from the electrode under test such that a radial change in position does not produce too much change in its reading.

In your case the current injection might need to be a 1000 ft or more away.

Note this method produces a higher resistance to remote earth than its actual value. This is because there is not uniform current density in the region around the electrode under test.

. .

 

[synchro]

You may be able to get some reduction in the required distance to the stakes for current injection and voltage probes by applying the Tagg slope method to the fall-of-potential measurements (see pages 5 and 8 at link below). This method compensates for a non-zero gradient or "slope" in the voltage vs. distance when there's an insufficient separation of the voltage probe and current injection points away from the ground electrode under test to get a relatively flat slope. Of course the lower the slope the less opportunity for error.

https://dam-assets.fluke.com/s3fs-public/2550440_6115_ENG_C_W.PDF#page=5

 

[gar]

200224-2130

charlie b:

Haven't heard anymore from you.

At this point I don't think you have asked the correct questions, or at least have not provided enough background information.

Has it been mandated that a fall of potential method be used? If so what other conditions are specified? What is the exact wording of the question for which a value must be provided? What is the purpose of the question? Etc.

.

.

 

[charlie b]

Please allow me to decline to offer more background information. The site is a military base. But it's not that there is a security issue at stake. Rather, it is my generally cautious self coming through.

I have read the Dept of Defense regulation that states that the 3-point fall of potential test is the only acceptable method of determining ground resistance for the type of facility under consideration.

The facilities under consideration must undergo periodic safety inspections. For at least the last three inspection cycles, the report included a "critical comment" due to failure to demonstrate the adequacy of the grounding electrode systems. I am told that the on-site electricians believe that the test cannot be performed successfully. I hope to meet with them later this week to learn, first hand, the exact nature of their concerns.

Just now I noticed that my first post did not mention that there is alleged to be a pair of large (over 1/4 mile diameter) ground rings, each of the two surrounding about half the 50+ buildings. I have heard that one reason the on-site electricians don't think the test can be performed is that you can't get instrument leads long enough to reach outside a ground ring that large. I have also heard that another reason is that the proximity of ground rings for other nearby buildings would interfere with, and invalidate, a test of any one of the buildings.

I have never personally witnessed, let alone performed, a 3-point fall of potential test. I am hoping to get some insight from forum members who are more familiar with the test.

 

[gar]

200225-1313 EST

charlie b:

I suspected it was a government project based on rules and specifications. Thus, not necessarily related to whatever might be the real question or concern.

What really is a fall of potential ground resistance measurement? It is a way to get a number for resistance that is an approximation of apparent resistance from a grounding electrode to remote earth.

If the application is a single 8' conductive rod driven into earth that is homogeneous in all directions, then the resistance to remote earth by this measurement test will be somewhat greater than the actual resistance because the test electric field is not uniform in all directions. As a course in our senior year we had to take an A. D. Moore design course that covered manual field mapping, and heat transfer. He was bright, tough, eccentric, and had been president of Tau Beta Pi at 29 (1924).

In your application you have one very big all interconnected grounding electrode. If you are interested in the low frequency resistance to earth, then the entire connected system should be considered the ground rod. If you are concerned with high frequencies, then probably consider isolating to a single building's ground electrode.

If the question, and possible results don't make sense, then do something, and define what you did, and your objections to the question, and the measurement method.

.

 

[FancySheep]

200225-1313 EST

charlie b:

I suspected it was a government project based on rules and specifications. Thus, not necessarily related to whatever might be the real question or concern.

What really is a fall of potential ground resistance measurement? It is a way to get a number for resistance that is an approximation of apparent resistance from a grounding electrode to remote earth.

If the application is a single 8' conductive rod driven into earth that is homogeneous in all directions, then the resistance to remote earth by this measurement test will be somewhat greater than the actual resistance because the test electric field is not uniform in all directions. As a course in our senior year we had to take an A. D. Moore design course that covered manual field mapping, and heat transfer. He was bright, tough, eccentric, and had been president of Tau Beta Pi at 29 (1924).

In your application you have one very big all interconnected grounding electrode. If you are interested in the low frequency resistance to earth, then the entire connected system should be considered the ground rod. If you are concerned with high frequencies, then probably consider isolating to a single building's ground electrode.

If the question, and possible results don't make sense, then do something, and define what you did, and your objections to the question, and the measurement method.

.

I believe you have one very big grounding electrode system. All the ring electrodes of all the buildings tied together. To do the fall of potential method you need to disconnect all these electrodes from the power system, but leave the electrodes connected together.

In the fall of potential method you have three electrodes. The one under test, a voltage probe, and a current injection probe. The injection probe is furthest from the electrode under test. The voltage probe is in between the other two, and has to be far enough away from the electrode under test such that a radial change in position does not produce too much change in its reading.

 

[synchro]

This article describes some advantages of using a Smart Ground Multimeter for fall-of-potential testing:

Conductor Licking and Fall of Potential - Hood Patterson & Dewar

Since the mid-twentieth century, the physical world around us has changed dramatically, so why is a grounding system test method adopted nearly 75 years ago still so widely used today? And what are the inherent problems when using an antiquated approach in the modern world? Read more...

hoodpd.com

 

[FancySheep]

This article describes some advantages of using a Smart Ground Multimeter for fall-of-potential testing:

Conductor Licking and Fall of Potential - Hood Patterson & Dewar

Since the mid-twentieth century, the physical world around us has changed dramatically, so why is a grounding system test method adopted nearly 75 years ago still so widely used today? And what are the inherent problems when using an antiquated approach in the modern world? Read more...

hoodpd.com

krogerfeedback

Thank you so much for helping with this wonderful post.