Ground resistance testing

[ammklq143]

In the past I've always just added the supplemental grounding electrode rather than testing but recently I purchased a tester because a service I installed was surrounded by concrete. Since then I've done several and installed one electrode because the ground resistance test was well under the 25 ohms required. One inspector took my word for it that the test passed and didn't require anything further. Another inspector said the panel had to have a green label placed on it with the ground resistance, date, and contractor name. I haven't found this requirement in the code book but if it is I'm wondering what the requirements are for the label. If it's in the book can someone tell me where?
Thank you.

 

[zog]

Typically the inspector will require to witness the test and puts the sticker on , what type of test are you performing? (Rare to get <25 with one rod)

 

[petersonra]

In the past I've always just added the supplemental grounding electrode rather than testing but recently I purchased a tester because a service I installed was surrounded by concrete. Since then I've done several and installed one electrode because the ground resistance test was well under the 25 ohms required. One inspector took my word for it that the test passed and didn't require anything further. Another inspector said the panel had to have a green label placed on it with the ground resistance, date, and contractor name. I haven't found this requirement in the code book but if it is I'm wondering what the requirements are for the label. If it's in the book can someone tell me where?
Thank you.

there is no code requirement for this. The AHJ can make up any kind of rules it wants to as far as whether they want to witness performance tests or are willing to accept some written or oral statement that it passed. Keep in mind that the inspector has no authority to decide this on his own. it has to be a uniform thing that the AHJ enforces for everyone, not just something made up by an inspector. having said that, if a green sticker satisfies him why not just do it. And I can just about guarantee that it will be more cost effective to just pound in a second rod even in the case you have rather than screwing around with the test and getting it witnessed. Does the AHj have a requirement that this kind of performance testing be carried out by someone who is certified to do so?

 

[K8MHZ]

Typically the inspector will require to witness the test and puts the sticker on , what type of test are you performing? (Rare to get <25 with one rod)

I have my doubts that an accurate test was done, and the measurement was not for an isolated electrode to earth. The OP says he has installed several single rods with less than 25 ohms, which makes me suspicious of the test method.

 

[ammklq143]

Typically the inspector will require to witness the test and puts the sticker on , what type of test are you performing? (Rare to get <25 with one rod)

I'm using a clamp on tester. I recorded the calibration and test on the ground rod for one EI and told him I could send him the video or screenshot of the results. He passed it without needing proof. The other EI said he didn't need to see the test or a recording but wanted me to tag the panel with the ohms, date, and contractor name on a green label.

The most recent test I did I had a between 13 and 14 ohms with one ground rod. I also had a ground conductor tied to a rebar in the driveway just to see what that would test. With that one connected instead of the ground rod I had between 6 and 7 ohms.

The soil is mostly clay in this area, not very sandy so that's probably why several electricians in the area use testers instead of installing the supplemental rod.

 

[ammklq143]

there is no code requirement for this. The AHJ can make up any kind of rules it wants to as far as whether they want to witness performance tests or are willing to accept some written or oral statement that it passed. Keep in mind that the inspector has no authority to decide this on his own. it has to be a uniform thing that the AHJ enforces for everyone, not just something made up by an inspector. having said that, if a green sticker satisfies him why not just do it. And I can just about guarantee that it will be more cost effective to just pound in a second rod even in the case you have rather than screwing around with the test and getting it witnessed. Does the AHj have a requirement that this kind of performance testing be carried out by someone who is certified to do so?

So far the inspectors haven't required the test to be witnessed. The one inspector wanted the label on the panel with the results and the contractors name. I don't think a contractor would want to falsify a test and put their name on it rather than drive a second rod so that's probably why he doesn't require it to be witnessed. The EI is fine with the electrician testing the ground rod and doesn't require someone to be certified. I didn't have a tester before but when I when I would have had to saw concrete, drill a hole through it for the rod, and then filling the saw cut and hole, etc., I decided to buy one and try it.

 

[zog]

I'm using a clamp on tester. I recorded the calibration and test on the ground rod for one EI and told him I could send him the video or screenshot of the results. He passed it without needing proof. The other EI said he didn't need to see the test or a recording but wanted me to tag the panel with the ohms, date, and contractor name on a green label.

The most recent test I did I had a between 13 and 14 ohms with one ground rod. I also had a ground conductor tied to a rebar in the driveway just to see what that would test. With that one connected instead of the ground rod I had between 6 and 7 ohms.

The soil is mostly clay in this area, not very sandy so that's probably why several electricians in the area use testers instead of installing the supplemental rod.

A clamp on test cannot be done on a single ground rod since it relies on a return path. Also there is no way to "prove" your reading is accurate. So every test you have done is invalid and I am surprised any inspector accepted it as a valid test. You need to do a 3 point fall of potential test.

 

[roger]

A clamp on test cannot be done on a single ground rod since it relies on a return path. Also there is no way to "prove" your reading is accurate. So every test you have done is invalid and I am surprised any inspector accepted it as a valid test. You need to do a 3 point fall of potential test.

:thumbsup:

Roger

 

[kwired]

A clamp on test cannot be done on a single ground rod since it relies on a return path. Also there is no way to "prove" your reading is accurate. So every test you have done is invalid and I am surprised any inspector accepted it as a valid test. You need to do a 3 point fall of potential test.

A clamp on tester assumes you will be connected to the grounded service conductor, the return path is via other electrodes in the grid, through distribution grounded conductor and grounded service conductor.

What you can't test with this method is a single rod with no connection to the grid - there is no return path for the test current.

It assumes the grid as a whole will be near zero resistance, so you couldn't accurately test something that is isolated from grid even though it may have an electrode elsewhere to allow for a return path.

Now if you have reinforcement in concrete also connected - the reading will be for more then just the single rod.

this should be giving somewhat accurate reading based on current it is seeing in the test circuit - the key is to understand what is all a component of that circuit and how it can change the reading.

If you had onsite power production and a rod near the source and a rod at some other location with a common conductor between them - then the test is primarily telling you the combined resistance between both rods. If your source is the national grid with a large grounding network, you will have low impedance from the grid and your reading is mostly just the resistance between electrode(s) you are trying to test and the grid. May not be quite as accurate as three point method but is likely pretty close.

 

[zog]

That is a lot of "ifs" "assumes" and "somewhats". lol

Regardless no way to prove your reading like in a 3 pt FOP and clamp on method is not accepted by any testing standard as a valid test.

 

[kwired]

That is a lot of "ifs" "assumes" and "somewhats". lol

Regardless no way to prove your reading like in a 3 pt FOP and clamp on method is not accepted by any testing standard as a valid test.

I agree the 3 point test is going to be most accurate, and there may be some applications where knowing an accurate resistance is important. For most applications however we don't even care what the numbers are - except when it comes to NEC's 25 ohm rule on the first rod. Second rod can still leave you over 25 ohms but NEC says you don't need to improve it any more then it already is - so most of us don't even spend the time or money on testing or equipment to test with and just drive two rods and move on.

 

[GoldDigger]

A clamp on test cannot be done on a single ground rod since it relies on a return path. Also there is no way to "prove" your reading is accurate. So every test you have done is invalid and I am surprised any inspector accepted it as a valid test. You need to do a 3 point fall of potential test.

On the other hand, the FOP test can be invalidated by the presence of a large network of connected grounds or even underground piping within the area you choose for the electrode separation. Or you may not be able to get far enough away from the building to place the two electrodes outside the zoone of influence of the electrode under test.
The theoretical basis of FOP is very strong, but the practical application is not at all as simple except for an isolated electrode surrounded by bare ground.

 

[Ingenieur]

we do a 3 point test, but not 'fall of potential' or walking the center probe out
the testers (megger, fluke, biddle) have a chart the gives you the dist required based on bed size, basically 5 x diagonal
for a single rod 100 ft and 62
for a bed 100' across 500 an 310 (62% x far probe)
getting out far enough makes a big difference

just tested the bed for the highwall miner I posted on
90 rods, connected with 4/0 bare Cu
100' diag
measured at 60/37 >11 Ohm
100/62 6.7
200/125 4.4
we let them stop there (we know it would drop further and the leads z~0.7 Ohm)
at 500' likely less than 2 Ohm