Ground rod experiment

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gar

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Location
Ann Arbor, Michigan
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EE
190729-1652 EDT

I have a grounding electrode for my home that can be defined as 150 feet of 1.25" copper tubing. This is my water supply from the street. My home has no ground rod or other grounding electrode. The water pipe is it.

The pole transformer has a ground rod adjacent to the pole. This is 70 to 80 feet from my water entry point, which is also where my main panel is.

Our primary supply is an ungrounded delta, grounded only at the substation. I have one neighbor sharing this transformer. There is no neutral sharing from one neighbor to another, except for those that share a transformer.

So my one neighbor and I share grounding thru the earth and power wiring. We don't share the same water main.

To a large extent I can define my water line as remote earth.

My power transformer ground rod has no effect on my experiment because it is away from my experiment in an opposite location 150 feet or so from my experiment location.

Near the road where my water line enters the property I drove into the earth 9 feet worth of 1/2" copper coated steel ground rod about 25 feet from the water line.

With 120 V applied to the experiment rod current flow was 3.5 A. This is a rod to my defined remote earth earth impedance of about 34 ohms. The water line is more than 6 feet below the earth surface. Possibly 8 feet.

Thru the magnetic field I can detect where the water line is located.

The surface potential gradient was approximately, using eyeball measurement for distance, 99 V to 3 foot radius, 107 V to 6 foot radius.

From the main panel neutral or EGC to a point above the water line the voltage difference was 2 V. My leads were not long enough to measurer from the test ground rod to the point over the water line. This may not make sense, but from the plug into the extension cord that supplied 120 V to the test rod I also had a long EGC wire.

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comparison, concrete encased electrode below water table on cemented glacial hardpan, own house. Ground water high in magnesium salts.

Approx 350 sq ft of footing with 2x 1/2" rebar spot welded at ties (BI said not to weld but did anyway)

Poco ground rods at many locations throughout neighborhood.

apply 120 V 60 Hz to ground unconnected to SE enclosure, trips 50 A QO breaker PDQ (did not time with scope)

Impedance measured using 15 ohms in series as current limiter = 0.6 ohms. From this, assume there must be other below water table grounds in the neighborhood hard wired to neutral.

Sons house, small ADU, 120 sq ft CEE ground, only pulls about 10 A. = 12 ohms.
 
190530-0911 EDT

brichter:

You are listed as an electrician.

Do you ever have to troubleshoot a problem where there are multiple hots, or neutrals feeding the same load? Or where you need to find where wiring travels behind walls, in conduit, or under cement? If so then you could use this information.

Or track current flow on water pipes, or steel framework?

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190730-0924 EDT

junkhound:

Is Renton down near sea level in your area? My sister-in-law lived up on a hill in Renton. Now she is in an assisted living location.

Another rod I had driven years ago in a different area of my yard, and further from my water line by about 50 feet, was between 10 and 20 ohms, possibly a wetter time. Not having any current flow for years this rod is now a very high resistance to earth. I need to measure it again.

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gar said:
190730-0924 EDT

junkhound:

Is Renton down near sea level in your area? My sister-in-law lived up on a hill in Renton. Now she is in an assisted living location.

Another rod I had driven years ago in a different area of my yard, and further from my water line by about 50 feet, was between 10 and 20 ohms, possibly a wetter time. Not having any current flow for years this rod is now a very high resistance to earth. I need to measure it again.

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Click to expand...

I'm 400 ft up a hill. There is a gully 1000 ft from me with year round springs, so soil always wet. Near south end of the Puget lobe glacier area, lots of high electrical conductive minerals in the hardpan, all probably scoured off some Canadian mountain side millennia ago.

Interesting sidelight on minerals - with the advent of high strength rare earth magnets, found it is possible to pick up a NeFeBr small magnet with a common rock, so much iron or nickel in them here.
 
190731-2024 EDT

mikeames:

That is a good video, but I would suggest that the common voltage reference point should be the EGC and/or neutral bus bar in the main panel. This location is what all the points being tested reference to. For most places in a home or yard there should be no current in the EGC, thus no voltage drop.

Separately one wants to evaluate the earth potential at many different critical points relative to the reference point in the first paragraph. By referencing to the EGC bus bar you should see very low voltage from the bus bar to any point that is connected to the bus bar. If there is current flow on the EGC being measured, then there is a voltage drop.

In my main panel the voltage difference between the EGC bus and the neutral bus is about less than 0.001 V, read about 0.7 mV. This is with a high impedance meter, and no filtering at the input.

In my yard from the main EGC bus to a point in the yard I am generally less than 0.1 V until I get close to the pole transformer ground rod. Here the reading grows to about 0.35 V. This is the voltage drop from whatever unbalanced current is flowing on the neutral from the transformer to the main panel.

Some of the readings I took may be greater than the 60 Hz component from high frequency harmonics or noise. Without filtering I have to accept any noise within the bandwidth of the AC meter.

Today I did not take any voltage readings between two probes in the earth at a 12 foot distance. In the past I have seen in the range of 0.1 V. Really need filtering, and possibly a correlation detector to extract what is locally generated.

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