Can a ground fault through dirt trip the OCPD?

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hurk27

Senior Member
The 6 amps there abouts @ 50 volts would give you about 8.3 ohms of the whole circuit Including the wiring, at 120 volts this would provide about 14.5 amps, but one thing I am confused with is the variable 0-120 volt section you have the banana plug in to states @ 5 amps? I have worked with a few lab power supply's and the OCPD has current coils with different ratings for each section IE. 15 amps for the non-variable 120/208, and 5 amps for the variable 0-120/0-208, 8 amps for the non variable 0-120 DC and 2 amps for the variable 0-120 DC, this is so only one reset is required.

I'm not so sure you actually ever reached 120 volts before the OCPD tripped?
 

radiopet

Senior Member
Location
Spotsylvania, VA
Look. Wherever Mike did his test is not the same as where I did my test. My soil conditions are different than his. My test procedure was different than his. I made no claims about Mike being wrong.

The water main in the street has been here since at least 1960 and I am betting it is about an 8" steel pipe that runs fro MILES, buried in the conductive clay soil. Also, the 1 1/4 water pipe off that main to this building is about 120 feet long and buried. Plus the building steel/slab may be contributing also.

So.... 3 feet of 1/2 rigid pipe had 6 amps flowing through it, into the earth, then to all the components that I mention above.

A three point resistance test is not what I performed. My experiment simply determined how much current would flow through MY SOIL CONDITIONS back to Xo of my xfmr.

If you do not believe these results, then I don't know what to tell you. Do I need to do a video like Mike Holt did?

I don't care what my pipe to earth resistance is. All I can tell you is 6 amps was flowing at 50 volts. And it tripped my power supply OCPD at 120 volts.

If Mike came here and did MY test, he would get the same results as I did.

And all my test did was prove to ME that significant current can flow in MY soil. Maybe yours doesn't. Mine does.

come see for yourself.

Dude...relax...I was simply telling you about another person who also did some tests.
 

hillbilly

Senior Member
Great thread.:)
Like you Gary, I'm all about learning.

In the past I set up a similiar test at my shop.

My shop sits by itself, has it's own (overhead) service and the closest residence is my home which is about 200 feet away.
The next house is about 1/4 mile away.
The pole mounted transformer feeding my shop is about 75 feet away.

The county water line is plastic.

The ground rod for my shop sits in a area that's near the discharge of a artesian well.
It's 8 feet long and driven straight down.
The water is heavy in mineral, especially red coloring, which I assume is iron.
The ground is always wet.

I ran a #8 conductor from a 20A breaker in my shop and connected it to the ground rod.
I disconnected the GEC from the rod.

I turned the breaker on, and while standing on a piece of dry plywood, I read the current on the wire.
6 Amps @ 120V....20 Ohms resistance.

I was suprised.
I was certain that the resistance would be lower.

I did this test because this site was having a discussion about ground rod resistance, and I was skeptical about what I was hearing.
I learned something that day.
Don't believe everything that you think you know.:)

Just another perspective
steve
 

crossman gary

Senior Member
I have worked with a few lab power supply's and the OCPD has current coils with different ratings for each section IE. 15 amps for the non-variable 120/208, and 5 amps for the variable 0-120/0-208, 8 amps for the non variable 0-120 DC and 2 amps for the variable 0-120 DC, this is so only one reset is required.

I'm not so sure you actually ever reached 120 volts before the OCPD tripped?
Those are indeed the voltages on my LabVolt power supply, so most likely, we are talking about the same thing. You may be correct on the amperage ratings. We have four of these units. Three of them were purchased back in the 1970s. The one I was using was purchased in 1999. It is somewhat different than the others. The old ones actually have an on-off switch that looks like a breaker and it says 15 amps on the handle, so I always assumed everythng was 15 amps. The newer unit has the "klixon" type reset on it. I never considered that the various voltages could have different OCPD ratings. You may be right on this one. I'll check that on Monday.

I may even redo the experiment and take a little more time with it... I'll record voltages and amps in more places, maybe put the ground rod in various places.
 

crossman gary

Senior Member
In the past I set up a similiar test at my shop.
Hi Steve:

Thanks for the info on your experiment. It gives us some data to compare.

I am guessing that the POCO has a ground at each pole, most likely a little ball of rolled up copper wire on the bottom of the pole. And the poles are about 300 feet apart?

So you had a ground rod in a wet area, with a path to a ball of wire under a pole 75 feet away, which I assume was in dryer soil. I am actually surprised that your resistance was as low as it was. I'm betting that the red soil contains iron oxide, which I think has a pretty significant resistance itself.

Comparing to my experiment, I have underground metal water pipes on 2 sides of the area where I drove the rod as shown in the diagram of the building I posted. I think that could be a major difference between your experiment and mine. The electrode to which your current was traveling was quite different than mine. The path from your rod to your xfmr Xo was much different than the path back to my Xo.

Does your building have a foundation with rebar? a concrete encased electrode? Is it metal framing?
 

crossman gary

Senior Member
Mythbusters....Mike Holt edition. :smile:
Well....:smile:

Actually, I think Mike was the myth buster because in the old days, everyone thought a grounding electrode and the earth was a highly conductive arrangement.

Then people started testing things and folks accepted that electrodes/earth are terrible conductors. And the tide turned the other way, maybe too far.

It is safe to say that the resistance of the electrode/earth as a conductor is a highly variable thing. It depends on the soil type, the moisture, the temperature, the electrode type, the actual method being used in the test, and what the question is that is being tested. The actual resistance of a particular location could be anything from very low to very high ohms.
 

crossman gary

Senior Member
lol.....man I am always relaxed....opps wife came in the room......back to being tense...lol
:grin:

I'll do some more experimentation next week, and I'll take more time and give more concise results.

Does anyone have suggestions on what else I should do? For example, I will get some more pipes and drive them in various locations around the property and see how they check out.
 

crossman gary

Senior Member
I don't know exactly what is in the soil. I did a google search on soil resistivity southeast texas, and houston, various things like that. Didn't find much. I'll search on minerals clay soil and see what I find.
 

crossman gary

Senior Member
I'm still curions about the whole thing. I am at home today. We had a good rain last night. I took two 3 foot pieces of aluminum pipe (aluminum because that is what was handy) and drove them 2 feet deep, about 3 feet apart. I measured the resistance on the Rx1 scale and it was 40 ohms. I thought it would have been lower considering we had a good rain last night.

But again, I only drove them in 2 feet and they went down pretty easy. Plus, I am wondering about the aluminum oxide on the surface of the pipes. Maybe it is better to use galvanized steel for the experiment.

Again, the whole thing has me wondering. More testing is in order.
 
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crossman gary

Senior Member
(I believe your experiment setup is too close to the buildings existing electrode system, which is "augmenting" the current path.
I wanted to revisit this. The original question was "could it ever be possible for a fault with dirt as part of the path to trip an OCPD?"

By definition, the building's electrode system is going to be included as part of the path. The building electrode is not "augmenting the path." it is absolutely part of the path.

Here is that path, starting at X1 of the 208Y/120 xfmr:

X1 to panel

panel to busduct via EMT feeder

busduct to busplug discoonect to subpanel in classroom

20 amp branch circuit to outlet

outlet to labvolt power supply

power supply through ammeter

ammeter to #8 wire laid on floor, out the door, to the driven pipe

driven pipe to building electrode system via dirt path

building electrode system to GEC

GEC to Xo of xfmr

So again, by design, the building electrode system is being used as part of the current path. The test is asking the question of "how much current can flow from a pipe driven in the ground to the building electrode system?"

I am positive that if I were to go out in the middle of the field next door and repeat the test, the current would be significantly lower. But that is not what the question asked.... which roughly was:

can Gary devise an experiment where significant current will flow through the earth from a 120 volt source?
 

crossman gary

Senior Member
Thank you sir.

Here is an addition to today's test where I drove 2 aluminum pipes into the ground at my house.

I had driven those pipes about 8 feet away from my house. I had forgotten that my house slab was built up about 18 inches above existing grade. Then the builder used bank sand to grade a gentle slope back out to the existing soil. Most of the first experiment was testing the sand.

So I did this over. I found a piece of 1/2 EMT, cut it in half, went out in the middle of the yard at the original grade, and drove these down about 3 feet into the earth, and about 3 feet apart.

I mentioned earlier that we have been in the midst of a two+ week drought and the soil had been really dry to the point that there were 1" cracks in the ground all over the place. But over the past two days, it rained pretty good, so there was moisture in the ground.

When I drove the 1/2 EMT pipes, they went in easily for the first 2 feet, then it was like they hit a hard bottom and I had to use the sledge hammer to put them down further.

This leads me to believe that only the top two feet are moist right now. Lower than that, the soil is still dry. The rain has not had a chance to soak in all the way.

Anyway, with the two 1/2 pipes, the resistance was 15 ohms between them. That is pretty good conductivity of the soil considering what others have siad about their soil.

Now, I will be the first to admit that when this clay soil dries completely out, first it shrinks leaving cracks in the ground, then it hardens like concrete and I am sure the resistance would be very high.

But wet, look out!
 

hurk27

Senior Member
Bravo to you crossman. I really enjoyed the pics and explanations so thank you. I have no doubt at all that conditions could be such to get the earth to carry significant current. Great job.

As long as we can keep in mind that the connection to Earth has too many variables to ever depend upon to clear a fault below 600 volts. we must never treat an electrode as such a path.

I have yet to be shown why a ground rod should ever be required in the NEC?

I had a house that a truck backed into a pole and knocked the 7200v primary down onto the neutral to the house that was also severed from the pole, the house didn't have a water pipe electrode because it was a well, but did have two rod electrodes, actually the neutral fed two houses that had the same grounding, just two rod electrodes, the 7200 volts went into these two houses, and arced around in the panel, and because of the common cable and tel-co connections went through every electronic device in these houses and in one case blew apart one TV set, several phone sets actually caught fire but luckily went out on there own as the home owners was too busy running out for their lives, I did the assessment on these houses and found in several places the NM cables had all the insulation blown off the conductors.
It was almost like these houses had been hit by a direct lightning strike, but they were not. the one thing I realized is even with a high voltage source, the rods did very little in protecting these two houses with 4 ground rod in all.
Yes the rods were in a dry sandy soil area, but the NEC does not state the type of soil we must use, it just says if 25 ohms is not reached install another rod and go home. I for one do not like a requirement that does not preform like intended.

The above story is just one of the intended purposes the NEC requires a driven rod, and it failed, lightning is the other, as far as I'm concerned, it does not preform repeatably for either of these, and should be removed from the NEC as it only provides a false sense of security for those who still believe this myth.
 
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