earth resistance

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domnic

Senior Member
Occupation
Electrical Contractor
Is their any place in the usa I can drive a 8' ground rod and get 4 ohms or less?
 

al hildenbrand

Senior Member
Location
Minnesota
Occupation
Electrical Contractor, Electrical Consultant, Electrical Engineer
I'll bet that if one drove a rod into a wet area of the Great Salt Lake, it'd come close.
 

11bgrunt

Pragmatist
Location
TEXAS
Occupation
Electric Utility Reliability Coordinator
How do you perform your resistance testing?
Recently, we connected 120 volts to an old galvanized anchor rod near the building to see how much current would flow. 22 amps. We said 120 over 22 equals 5.45 ohms. Thought that can't be right, the resistance is too low. Five foot away, but no closer to the perceived secondary source, an 8' copper rod was driven and 120 volts connected. This time 42 amps flowed through the copper ground rod back to source. 120 over 42 equals 2.85 ohms. Again I think way too low. The pad mount transformer that is the source is 150' away. I know engineers that would be thrilled if we could turn in numbers like that consistently. Part of this testing was to show the value of the ground rod as the only return to source in the event of an open secondary neutral from the POCO or in the service entrance.
Maybe I am more confused than usual. We looked for every spot that return current could get back on another rod and turn into a wire path back to source but never found it. The ground at this location is nothing to brag about but there is a little pond on the property so that suggest some clay. Maybe the ground is that good or my test that bad?
 
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junkhound

Senior Member
Location
Renton, WA
Occupation
EE, power electronics specialty
Maybe if you drive right above a few hundred foot long copper water supply pipe --- and against or thru the water pipe!

Old minuteman missile silos in Grand Forks, ND often measured under 1 ohm resistance to remote ground. However, those were 12 ft dia steel 80+ ft deep, often 75 ft below water table, and so much ammonia salts (from fertilizer in adjacent fields) that leaks formed stalgmites and stalactites, and have seen the ground water etch nickel plating off aluminium forgings in the silos.

My own house measures about 6 ohms - 350 ft of 24 in wide footings below water table with 2 parallel spot welded rebar in the footings.

I did notice that there is an old thread here on XIT rods, a supposed big thing in the early 1970s, never did use one though.

Wahiawah HI naval base built an big pulse antenna over part of the base in the 1970s to test site EMP hardness. Volcanic soil on Ohau very acidic (and conductive), so bad that when the Honolulu airport was expanded in the 60's/70's buried aluminum conduit corroded thru in less than 1 year. Big 100 ft diameter circle of ground rods linked with buried copper wire was needed to get resistance under 100 milliohms, dont recall how many rods.
 

RichB

Senior Member
Location
Tacoma, Wa
Occupation
Electrician/Electrical Inspector
maybe somewhere in Yellowstone--what with all the minerals in the ground,,,,,
 

big john

Senior Member
Location
Portland, ME
...We said 120 over 22 equals 5.45 ohms. Thought that can't be right, the resistance is too low. Five foot away, but no closer to the perceived secondary source, an 8' copper rod was driven and 120 volts connected. This time 42 amps flowed through the copper ground rod back to source. 120 over 42 equals 2.85 ohms....We looked for every spot that return current could get back on another rod and turn into a wire path back to source....
I think you're on the right track.

When I have to do fall-of-potential grounding tests in the city, it's really common to get values of <5 ohms because of short circuits: There's so much buried pipe, rebar, conduit, fencing, etc, that it can be really difficult, if not impossible, to rule out all the influences on the electrode system.

I would be very surprised if a single electrode system was actually <3 ohms to remote earth all by itself: You may have very different geology, but I've never seen any tests begin to approach those numbers without very large engineered grounding mats.
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
I think ground resistance is entirely a byproduct of CMP failing to understand the very code they create.
 
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