Ground Testing of System NOT of single Ground Electrode

[W@ttson]

I have a ground system that is composed of 3 ground electrodes. I had a 3 point potential fall test performed on each one and know the resistance of each. I, however, need to know the ground resistance of the whole system not just of each individual one. I understand that the total system resistance will be less than the resistance of least resistive ground electrode, however, I need to make sure, based on a specification, that it is below 5 ohms.

Is there a way to measure the full ground electrode system without detaching a ground electrode and measuring each one individually? I have read about the "clamp on" measurement, this addresses the "not disconnecting" concern but it also goes on to say that it still only measures the individual ground electrode resistance.

I don't believe I can take the resistance values of each electrode and add them in parallel due to differences based on sphere of influence of each acting upon the other.

 

[GoldDigger]

I have a ground system that is composed of 3 ground electrodes. I had a 3 point potential fall test performed on each one and know the resistance of each. I, however, need to know the ground resistance of the whole system not just of each individual one. I understand that the total system resistance will be less than the resistance of least resistive ground electrode, however, I need to make sure, based on a specification, that it is below 5 ohms.

Is there a way to measure the full ground electrode system without detaching a ground electrode and measuring each one individually? I have read about the "clamp on" measurement, this addresses the "not disconnecting" concern but it also goes on to say that it still only measures the individual ground electrode resistance.

I don't believe I can take the resistance values of each electrode and add them in parallel due to differences based on sphere of influence of each acting upon the other.

The only totally foolproof method that I can think of would be to apply the three point fall of potential method with the remote points so far away that you are many times the sphere of influence of each of the electrodes and the distance between the electrodes.

Another possibility would be to measure the resistance to a distant remote earth electrode whose interface resistance is already known. Series circuit rules apply then.

 

[kwired]

I have a ground system that is composed of 3 ground electrodes. I had a 3 point potential fall test performed on each one and know the resistance of each. I, however, need to know the ground resistance of the whole system not just of each individual one. I understand that the total system resistance will be less than the resistance of least resistive ground electrode, however, I need to make sure, based on a specification, that it is below 5 ohms.

Is there a way to measure the full ground electrode system without detaching a ground electrode and measuring each one individually? I have read about the "clamp on" measurement, this addresses the "not disconnecting" concern but it also goes on to say that it still only measures the individual ground electrode resistance.

I don't believe I can take the resistance values of each electrode and add them in parallel due to differences based on sphere of influence of each acting upon the other.

Unless I am not following what you are after, connect all three electrodes together, but make sure they are not connected to the service or anything else you don't want included in the test and run same test you did for an individual electrode.

 

[GoldDigger]

Unless I am not following what you are after, connect all three electrodes together, but make sure they are not connected to the service or anything else you don't want included in the test and run same test you did for an individual electrode.

If you try to do that with a clamp-on tester you have the problem of what to use for a reference/return path. Possibly the POCO MGN?

If you try to do the FOP method that way, you may have to get really far away from the building if the electrodes themselves are spaced far apart.

 

[roger]

What's the reason the specs want less than 5 ohms?

Roger

 

[W@ttson]

If you try to do that with a clamp-on tester you have the problem of what to use for a reference/return path. Possibly the POCO MGN?

If you try to do the FOP method that way, you may have to get really far away from the building if the electrodes themselves are spaced far apart.

I was thinking about that but thought that since all of the literature I could find on the 3 point fall of potential test says that the electrode needs to be disconnected and the test performed on a single electrode that it wouldn't yield a proper result for all of them together. Another piece of literature gave factors to use to reduce the resistance by when adding more to the system. For example if you have 2 electrodes and measured 20ohm then the total resistance would be (20/2)(*1.16).I feel like by having them all together and doing the 3 point potential fall test should do it but I would need to get really far away. Please see this piece of literature on multiple ground system and distances: http://www.rainbirdservices.com/Training/Documents/Understanding Earth Ground Resistance by AEMC.pdf (page 17 of 39)

 

[W@ttson]

Wherever, sensitive equipment is located they need 5 Ohm or less. (As mentioned previously in this forum, this requirement is debatable but be that as it may, it is what is written in the spec.)

What's the reason the specs want less than 5 ohms?

Roger

 

[roger]

Wherever, sensitive equipment is located they need 5 Ohm or less. (As mentioned previously in this forum, this requirement is debatable but be that as it may, it is what is written in the spec.)

Whoever wrote the spec needs to understand that if they achieve it today it will probably be different (most likely higher) the next day.

Roger

 

[GoldDigger]

I was thinking about that but thought that since all of the literature I could find on the 3 point fall of potential test says that the electrode needs to be disconnected and the test performed on a single electrode that it wouldn't yield a proper result for all of them together.

The problem, which the literature does not explain well, is that the "distance" to use in the three point FOP method is not well defined for a distributed system. You would have the same problem with a single electrode if, for example, it was a ring type or a water pipe.

As you note, get far enough away and it will theoretically work. But the presence of metal pipes, foreign ground systems and many other things across that distance will also make the results unreliable.
FOP explicitly assumes that the only conductive substance between the test points is uniform resistivity earth.

 

[W@ttson]

I am assuming if some of those items that you mentioned below are present, that the readings would be lower than actual??

The problem, which the literature does not explain well, is that the "distance" to use in the three point FOP method is not well defined for a distributed system. You would have the same problem with a single electrode if, for example, it was a ring type or a water pipe.

As you note, get far enough away and it will theoretically work. But the presence of metal pipes, foreign ground systems and many other things across that distance will also make the results unreliable.
FOP explicitly assumes that the only conductive substance between the test points is uniform resistivity earth.

 

[GoldDigger]

I am assuming if some of those items that you mentioned below are present, that the readings would be lower than actual??

If they are close to the GES under test then the reading you calculate could be lower than the actual GES resistance, yes.
If they are somewhere in between, I think they could affect the calculated measurement either way depending on exactly where they were and which direction they ran in. It's complicated.

 

[W@ttson]

I think the below sums everything up.

If they are close to the GES under test then the reading you calculate could be lower than the actual GES resistance, yes.
If they are somewhere in between, I think they could affect the calculated measurement either way depending on exactly where they were and which direction they ran in. It's complicated.

 

[K8MHZ]

Whoever wrote the spec needs to understand that if they achieve it today it will probably be different (most likely higher) the next day.

Roger

The above is probably more important than anything else.

When we did telco work, the engineers would have laughed out loud if anyone ever suggested that 3 driven rods would assure <5 ohms consistently.

A solid <5 ohm electrode system is usually a Ufer, a ground ring or a chemical ground.

I know that there has been articles indicating that the clamps will give just as accurate reading as an FOP test, but I am not convinced.

 

[big john]

You test a GES the same way you do a single electrode: The FOP tester has no idea how big the piece of copper is you hooked it to, the proof of a valid test is in the shape of the resistance plot.

The problem you will run into is that the rule of thumb is your test distance is 10× the size of the electrode. When you start connecting multiple electrodes together then the distance between them becomes "the size of your electrode." This can mean very long test distances.

E.g.: 3 rods spaced across 30 feet. Your test distance might be 300 feet. If the rods are on opposite sides of a 100' building, then in theory you could be doing a 1000' test.

The reality I find is that you often hit remote-earth before you have to go 10× the distance.

 

[cuba_pete]

The above is probably more important than anything else.

When we did telco work, the engineers would have laughed out loud if anyone ever suggested that 3 driven rods would assure <5 ohms consistently.

A solid <5 ohm electrode system is usually a Ufer, a ground ring or a chemical ground.

I know that there has been articles indicating that the clamps will give just as accurate reading as an FOP test, but I am not convinced.

Not necessarily. I have a ground field that consistently, over the last 30 years, has delivered less than 1 ohm. This is an AT&T/WECo engineered system.

i have older facilities dating to the 1950's at or very near 1 ohm.