Using a megger to get around a 2nd ground rod

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ivsenroute

Senior Member
Location
Florida
Does anyone use a megger (for residential) to prove the 25 ohm rule just to get out of driving in a 2nd ground rod. (plastic water feed, etc.)?

If so, what method do you use for ground reference?
 

brian john

Senior Member
Location
Leesburg, VA
There is more to this than just using a megger. The test set is commonly referred to a a earth megger, earth resistance tester and a 3-point test set. When utilizing the test set test electrodes are driven, typically 33 probes are driven at a minimum.

With residential you can use a clamp on style but unless you have an AHJ that allows self testing, the cost is much less with just driving a 2nd rod.
 

SEO

Senior Member
Location
Michigan
Brian have you ever used a Extech earth/ground resistance tester or one similar? They use three probes so i'm guessing that they would not give as accruate of a test that you are suggesting?
 

ivsenroute

Senior Member
Location
Florida
I figured that the expense and time of doing this testing is much more than just adding the 2nd rod.

I am just curious is this is handled differently across the country vs my area.
 

SEO

Senior Member
Location
Michigan
ivsenroute said:
I figured that the expense and time of doing this testing is much more than just adding the 2nd rod.

I am just curious is this is handled differently across the country vs my area.
I think that the second rod is the most common way to take care of meeting the Nec requirements and by far the most cost effective. Sometimes you run into a case where you have to meet a requirement not required in the NEC and have to test the systems grounding reliability.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
SEO said:
Brian have you ever used a Extech earth/ground resistance tester or one similar? They use three probes so i'm guessing that they would not give as accruate of a test that you are suggesting?

That is what Brian is referring to, it is called a "three point fall of potential" test. There are also 4 point test sets.

Roger
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
SEO said:
Brian have you ever used a Extech earth/ground resistance tester or one similar? They use three probes so i'm guessing that they would not give as accruate of a test that you are suggesting?
I think I have read that the 3 and 4 point testes are more accruate than the clamp on types.
 
here we use a biddle earth megger to do all of our building grid grounding, our ahj, allows us to use a water pipe as one side of the ground, then we test the grounding system grid, on grids that donot have this we use the three point system, and test the grid that way.
 

SEO

Senior Member
Location
Michigan
roger said:
That is what Brian is referring to, it is called a "three point fall of potential" test. There are also 4 point test sets.

Roger
I have heard about the three point fall of potential test and also the 4 point. I was curious about the 33 probes being driven at a minimum?
 

zog

Senior Member
Location
Charlotte, NC
SEO said:
I have heard about the three point fall of potential test and also the 4 point. I was curious about the 33 probes being driven at a minimum?

He meant 3 minimum, 52%, 62%, and 72% of the current probe, which is at least 100 Ft from the rod under test.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
SEO said:
I have heard about the three point fall of potential test and also the 4 point. I was curious about the 33 probes being driven at a minimum?
Whoa, I missed that, that would definately be an expensive test. :D

I guess a "33 point fall of potential test" would give a pretty accurate result though. ;)

Roger
 

zog

Senior Member
Location
Charlotte, NC
roger said:
Whoa, I missed that, that would definately be an expensive test. :D

I guess a "33 point fall of potential test" would give a pretty accurate result though. ;)

Roger

I was going to give him a hard time about that but figured it was a typo, and knowing my typing, I would pay for it over and over.
 

electricalperson

Senior Member
Location
massachusetts
i found a cheap earth resistance tester made in china on ebay for 100 dollars. seems to work and its great to learn on but i wouldnt use it for anything important. i used it to test a ground rod i driven and it seemed to work ok i guess. i seen the megger in a testing book and i think by megger they mean the earth resistance tester made by megger

i think the only time i would use an earth resistance tester is if someone asked me to do it other than that i wouldnt use one
 

zog

Senior Member
Location
Charlotte, NC
electricalperson said:
i found a cheap earth resistance tester made in china on ebay for 100 dollars. seems to work and its great to learn on but i wouldnt use it for anything important. i used it to test a ground rod i driven and it seemed to work ok i guess.

the way you know if it worked it by your plateau region on your plot, if no plateau, no go.

electricalperson said:
i seen the megger in a testing book and i think by megger they mean the earth resistance tester made by megger

That is exactly what they mean.
 
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