Ground rod tester

E

electro7

Senior Member
Location
Northern CA, US
Occupation
Electrician, Solar and Electrical Contractor
I haven't had to test the 25 ohm resistance on ground rods yet, but I was just asked by an inspector. I was wondering if someone could give me a good reference for which tool to buy for this? Also, what is the process of testing them?

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Testing can be a slippery slope. There are many factors that can effect the outcome of the test, such as ground water or rainy days or been dry for a long time, Soil make up etc. So even if you've tested good on a number of occasions prior to the inspector coming to inspect, the day of the inspection and all of a sudden you fall below the 25ohms and fail the inspection. That is why the simplest is to just drive two and be done.

Mike has a good video of how to test and what it takes to get there:
 
Okay. Would I daisy chain the GEC between both ground rods, or does it loop back to the terminal? They have to be at least 6ft apart, correct?

It's for generators and not sure why the inspector asked. He is new and said he was told to make sure and check the grounds so he asked for a ground rod test

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Does not have to loop back, you can hit one rod, then end at the second, or you can run two separate wires, one to each rod. The former is much easier. Unless there is a local amendment, and there is no other grounding electrodes present. Two and done!
 
The tool you would need to test it is called a fall-of-potential tester and required three points to measure between. It's much more economical to drive two rods.
 
electro7 said:
Okay. Would I daisy chain the GEC between both ground rods,
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Since you have more than one rod there is no reason to test. Sounds like the inspector is not an electrical inspector.
 
Great, thanks, guys.

No, he isn't an electrical inspector, a new guy trying to work his way up:)

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electro7 said:
Okay. Would I daisy chain the GEC between both ground rods, or does it loop back to the terminal? They have to be at least 6ft apart, correct?

It's for generators and not sure why the inspector asked. He is new and said he was told to make sure and check the grounds so he asked for a ground rod test

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Not that common for a generator installation to require a grounding electrode. Can you provide more details of the installation?
 
don_resqcapt19 said:
Not that common for a generator installation to require a grounding electrode. Can you provide more details of the installation?
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Yes, I went into this in another thread - but the engineer has the ground rods and GECs on the plans. He has the neutral being switched at the ATS. It's two larger systems, one a 2000A and the other 1600A, 3 phase120/208V.

He has ground rods at the generators with #4 GECs, but I put in #2 copper because I thought it should be according to 250.66.

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electro7 said:
Yes, I went into this in another thread - but the engineer has the ground rods and GECs on the plans. He has the neutral being switched at the ATS. It's two larger systems, one a 2000A and the other 1600A, 3 phase120/208V.

He has ground rods at the generators with #4 GECs, but I put in #2 copper because I thought it should be according to 250.66.

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#4 is more than sufficient to a ground rod regardless of the amperage. You could run #6, and be compliant.
 
Like most people have mentioned, testing ground rod resistance is typically overkill for most residential or light commercial applications unless you are dealing with industrial facilities or substations. There are companies that specialize in ground system testing and can perform fall of potential tests. For a one time application, it is probably more cost effective to hire them rather than invest in the equipment. That said, I usually recommend just installing two ground rods and calling it a day. If you are still interested in testing, the AEMC Model 6416 clamp-on ground tester is something I have used in the past. It is a convenient option if you are looking for a simpler method without driving test stakes.
 
baserunner4 said:
the AEMC Model 6416 clamp-on ground tester is something I have used in the past. It is a convenient option if you are looking for a simpler method without driving test stakes.
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Read the manual. These clamp-on testers are for use with multi-point grounding it is not for single grounds.
 
jim dungar said:
Read the manual. These clamp-on testers are for use with multi-point grounding it is not for single grounds.
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I agree to a certain extent. Clamp-on ground resistance testers can be used on single-point ground rods but only under specific conditions. These type of testers require a return path to measure resistance accurately, which is typically available in multi-grounded systems as you mentioned. In the case of a single ground rod, the tester will not work properly unless there is a series-parallel resistance path downstream such as a connection to building steel or another low-resistance ground. Without one, the meter may give unreliable results due to the lack of a complete circuit. But it could work if a temporary path can be created by connecting a jumper from the ground rod to a nearby known low-resistance point, like a pole ground or building steel. This would allow the clamp-on tester to simulate the needed return path and produce a valid reading. If you have a simple ground rod, it's probably easier to go with a fall of potential test than to try to explain to an inspector how you got your results.
 
baserunner4 said:
But it could work if a temporary path can be created ...
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So you might be able to use this meter to justify a single ground rod as long as you install a second ground electrode.

These meters have a purpose, but I would not buy one for single ground rod testing to justify <25 ohms.
 
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