Ground Rods

[macmikeman]

Why is it 25 ohms? Why does code use that value? Seems odd to me, that regardless of the voltage, 25 ohms is the standard.

That value was rumored to have been of importance to the boys stringing telegraph wire back before Alex Bell's day.

 

[brian john]

90% of the installers would not buy the equipment needed to actually do the test properly and if they purchased a 3 point measuring device would probably test that incorrectly anyway.

90% you are being kind today more like 99%, heck I bet 90% do not own meggers.

We own 5 earth ground resistance 3-4 popint testers plus several clamp on testers and testing seems to come in spurts, cell sites, towers, data centers hospitals, Communciation facilities generally as Roger noted to comply with specificiations.

Had a hospital, where the GFP tripped on a fault rather than testing the GFP first, they wanted a complete ground test..Engineers requirement, I try to discuss what I feel are the primary issues I would look at and he slams me HARD as being just an electrician..So I do the test and charge for them. HEY WHAT DO I KNOW.

 

[jim dungar]

In WI we are required to drive two g. rods. Has to do with soil consistency. So if we did an ohms test and failed 25 ohms, We'd end up with three rods.

The state of Wisconsin does not have any ammendments to Article 250.56.

The closest it comes is Comm 16.26(2) which ammends 250.53(D)(2) and says: "Where the supplemental electrode required by NEC 250.53(D)(2) consists of made electrodes specified in NEC 250.52(A)(5), not less than 2 supplementary made electrodes shall be installed."

Effectively WI says if you use a ground rod as a supplemental electrode, you need to install at least 2 of them regardless of the soil resistivity.

 

[radiopet]

Phishwi,

Have you seen the video Mike Holt did on ground rods at his home? I believe it took him nearly 100' of ground rod to get below 25 OHMS....I can't remember the exact amount but it was a COOL video.

Just went to prove in HIS specific location getting the OHMS down to the NEC's Magical 25 OHMS was in reality harder then you think.....remember however Mike is in Florida....

 

[brian john]

In Northern Virginia 2 ground rods will normally get you below 25 ohms, but I have seen some locations around here were numerous rods were required to meet the ohmic reading (SPEC JOB).

 

[jrclen]

In WI we are required to drive two g. rods. Has to do with soil consistency. So if we did an ohms test and failed 25 ohms, We'd end up with three rods.

I'm in the dry sand area of Wisconsin. I can push a rod down at least half way before needing to hit it. In many years of installs, I have only seen one test done. That was done by a local small power company who just bought the new toy. My 2 rods 10 or 12 feet apart failed of course. But the lineman admitted there was nothing he could do about it, so he connected to the meter anyway. This was before our area had inspections and only the power company had to pass your installation for a service. I have never heard of anyone in Wisconsin ever needing to install a third rod until now. Is that common by you phishwi? We do bond to the well pipe or casing. But the last few services I did, had PVC well casing.

 

[TOMWELDS2]

The test, as i remember, consisted of: the ground rod being tested and 2 reference rods..they are labeled as R1 R2 R3..The math formula i dont recall...something like R1+R2/R1-R3 i know thats not it but im sure someone here has it...