Megaohm testing

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mshields

Senior Member
Location
Boston, MA
NEC requires, 25ohms ground resistance at your service right? How is that measured. I mean I know it's with a megaohm tester, but where would one actually put the probes.

Also, is there any value in providing more grounding to lower that value?

And lastly, what's prompted the question is a pad mounted transformer we're putting in which is going to be about 3 feet above ledge. Our standard grounding detail shows 4 10-0' x 5/8th inch copper clad steel ground rods, one on each corner, interconnected with 4/0, Cadweld, the whole 9 yards. The EC wants to know how to install this. I know they can be at angles within reason but we'd practically have to have them lying down. I'm thinking that he can put them in as best he can and then megger it to make sure we meet the 25ohms or better.

What do you think?

Thanks,

Mike
 

zog

Senior Member
Location
Charlotte, NC
NEC requires, 25ohms ground resistance at your service right? How is that measured. I mean I know it's with a megaohm tester, but where would one actually put the probes.

No, you do not use a megohmeter to measure ground resistance. There are several methods but the most common (And most accepted) is a 3 point fall of potential test. It invloves driving a current and potential probe and different distances from the ground rod (Disconnected) under test and taking many different measurements at different distances (% of current probe distance) to prove your results which are typically graphed.

I recommend hiring a testing company to do this, or buy yhe right equipment and do some reading, starting with "A stitch in time" from the Megger website.
 

roger

Moderator
Staff member
Location
Fl
Occupation
Retired Electrician
NEC requires, 25ohms ground resistance at your service right? Mike
No, the NEC only requires 25 ohms if your GE is a single rod, if you have more electrodes there is no code requirement for a minimum. If it's a spec for performance reasons I agree with Zog's recommendation to hire it out.

Roger
 

brian john

Senior Member
Location
Leesburg, VA
The confusion arises from the fact that Megger (the manufacture) use to call their Earth Ground Resistance Tester an Earth Megger or Earth Resistance Megger, one of the two.

And you surely do not want megohm readings from a earth ground resistance test.
 
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