Can a ground fault through dirt trip the OCPD?

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crossman gary

Senior Member
Okay, I did the following experiment which was initiated from the "grounding electrode conductor question" thread. The point was to determine if a fault path through earth could trip an OCPD.

I took a 10 foot piece of 1/2" rigid and cut in in half. I went to a spot outside the door to a grassy area. This is black clay soil in southest texas. The soil was moderately damp. Photo below:



I drove one of the 1/2" rigid pipes about 3 feet into the ground. The pipe is 5 feet long, notice 2 feet still sticking up. It went into the ground fairly easily, but I did use a sledge hammer to get'er in, but nothing difficult. Then I took a simpson meter on the Rx1 scale, zeroed it, and hooked it from the pipe to a brass water spigot on the side of the building.

Reading: 2 ohms approximately

The water piping in the building is steel. It goes up inside the brick wall to the rest of the plumbing - connected to the water main coming in on the other side of the building. I am guessing the city main is steel, and it runs in front of the building along the street.

Electrical service to building is 480Y/277. Inside there is a 208Y/120 xfmr. This xfmr has Xo bonded to the water pipe coming in the building. The point is, there is a whole lot of buried steel pipe acting as the grounding electrode.

So again, 2 ohms from the driven pipe THROUGH THE EARTH to the closest plumbing pipe in the area. Photo below:



Close-up of meter:



So I took a #8 copper wire to the pipe:



Connected it in the building to our experiment machine:



Here is the connection to the voltage source. The power source is fed through a subpanel from the 208Y/120 xfmr. It has a variable supply, I can go from 0 to 120 volts to neutral/ground. There is an OCPD in the power supply, it is supposed to be 15 amps but I suspect it trips a little sooner than that. I connected one phase of the supply through an ampmeter to the pipe via the #8.



As I slowly turned the voltage up to 50 volts, the current slowly went up to 6 amps. (these meters have not been calibrated in awhile, so I guess we may have a +/- 10% error here.



After playing around with these voltage/current levels a bit, I went ahead and cranked the voltage source up to 120 volts. The OCPD kicked out. Math says that if 50 volts gives 6 amps, 120 volts would be about 14 amps. Like I said, the meters are not calibrated, and the OCPD is not a normal breaker, it is more of a "reset" type button that kicks out when there is too much current.

Yeah, this is a fairly non-rigorous experiment, but a good amount of current certainly was flowing through the earth path.

Never say that 120 volts cannot trip an OCPD device through the earth. It can under proper circumstances.

Also, I did not "cheat" on any of this. No hidden wires, nothing like that. This is legit.
 

Dennis Alwon

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Location
Chapel Hill, NC
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Electrical Contractor
You would have to have 1 ohm or less to trip a ocpd going to a rod and then it would take awhile. I find it hard to accept a 2 ohm reading on a ground rod. You need to do a 3 point test on the rod
 

radiopet

Senior Member
Location
Spotsylvania, VA
Are you saying OHMS LAW is incorrect?

In a perfect world I have a ground rod with 25 ohms. I have 120V source and in doing OHMS LAW you would have 120V/25ohms= 4.8A....do you think 4.8 ohms will trip a standard 15 A OCPD.....Do you really think this?

This example if not really a controlled test. I doubt you are going to have this situation in most cases of something as low as 2 ohms and we have to deal in the norms or reality and since you did not really use a ground resistance imp. meter or 3 point fall....very hard to lay claims to this test my friend.
 
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crossman gary

Senior Member
Concerning the 2 ohms.

That 2 ohms was from the driven pipe through the earth to the water main and back on the waterpipe at the faucet.

The actual path for current flow was not through that path. It was from the xfmr to the variable supply down the #8 then to the pipe then through the earth to the main then back on the GEC to the xfmr Xo. Considerably a longer path and many more components, plus I suspect the variable supply (contains a rheostat) has a considerable voltage drop.

Sounds like some of you don't believe me? Would you care to come to Houston and see this for yourself. If it doesn't work as i have stated, I'll pay for your plane trip.

I can't vouch for other types of soils. All I am saying is that southeast texas gumbo clay is a friggin good conductor. I think this area, millions of years ago, was under the ocean, and therefore we have lots of minerals, salt, ions, good conductive stuff when dampened with H2O.


Here is a rough diagram of the circuit.
 

crossman gary

Senior Member
You would have to have 1 ohm or less to trip a ocpd going to a rod and then it would take awhile. I find it hard to accept a 2 ohm reading on a ground rod. You need to do a 3 point test on the rod
Dennis, all I am saying is that the experiment showed 6 amps at 50 volts. It tripped my power supply OPCD when I turned it up to 120 volts.

And the 1/2 rigid pipe was driven in only 3 feet.

Believe it or not.
 

crossman gary

Senior Member
Are you saying OHMS LAW is incorrect?

In a perfect world I have a ground rod with 25 ohms. I have 120V source and in doing OHMS LAW you would have 120V/25ohms= 4.8A....do you think 4.8 ohms will trip a standard 15 A OCPD.....Do you really think this?
A perfect world has ground rod resistance to earth at WAY less than 25 ohms.

So are you saying that ground rods are automatically 25 ohms???
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Electrical Contractor
Well if you get 2 ohms with a 3' rod in the ground then that is the best soil I have even come across. BTW my statement about 1 ohm was inaccurate.
 

Dennis Alwon

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Location
Chapel Hill, NC
Occupation
Electrical Contractor
That is why I used my 0 to 120 volt supply. I start out at very low volts, and go from there to see what she is capable of handling.
You did good I was referring to Bucks post. I do believe you but it is hard for me to phathom that good a soil. Around here with 2 - 8' rods you would be lucky to get under 70 ohms.
 

crossman gary

Senior Member
This example if not really a controlled test. I doubt you are going to have this situation in most cases of something as low as 2 ohms and we have to deal in the norms or reality and since you did not really use a ground resistance imp. meter or 3 point fall....very hard to lay claims to this test my friend.
Errr..... didn't I say that this was not a scientifically rigorous test???

The only thing I am claiming is that 50 volts caused 6 amps to flow. A portion of that current path was black clay soil. Also, the OCPD in my supply tripped when I turned the voltage up to 120.

Those are my claims because I saw it happen. If you care to see it for yourself, then PM me and come on down! If it don't work, I'll pay your air fare.
 

iwire

Moderator
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Location
Massachusetts
A perfect world has ground rod resistance to earth at WAY less than 25 ohms.

So are you saying that ground rods are automatically 25 ohms???

I doubt many rods even reach 25 ohms.

Unless you did a three point fall of potential test it is my opinion you have no idea of that rods resistance.


Your experiment figures (50 volts = 6 amps) suggest a ground rod resistance closer to 8.33 amps which is still mazing low.

Few areas can even approach that with a full depth rod.
 

radiopet

Senior Member
Location
Spotsylvania, VA
A perfect world has ground rod resistance to earth at WAY less than 25 ohms.

So are you saying that ground rods are automatically 25 ohms???

No I am saying using accepted " stopping point" of a single ground rod, plate or pipe in the eyes of the NEC's perfect world. As stated in 250.56

250.56 Resistance of Rod, Pipe, and Plate Electrodes. A
single electrode consisting of a rod, pipe, or plate that does
not have a resistance to ground of 25 ohms or less shall be
augmented by one additional electrode of any of the types
specified by 250.52(A)(4) through (A)(8). Where multiple
rod, pipe, or plate electrodes are installed to meet the requirements
of this section, they shall not be less than 1.8 m
(6 ft) apart.

So in the eyes of many if not all, reaching the magic number of 25 ohms for a "single" ground rod would be the equivalent of a perfect world unless they choose to beat the issue with a steel horse and try to lower it even more. Did you ever watch mike holts video on ground rods resistance.......and the nearly over 100+ feet he had to drive to get below 25 ohms?
 

crossman gary

Senior Member
Did you ever watch mike holts video on ground rods resistance.......and the nearly over 100+ feet he had to drive to get below 25 ohms?
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.
 

wwhitney

Senior Member
Location
Berkeley, CA
IUnless you did a three point fall of potential test it is my opinion you have no idea of that rods resistance.
The simple test with an ohmmeter that the OP described does tell you something about the rod's resistance. It basically measures that rod in series with the other ground electrode, the water system. So it tells you that the rod's resistance is less than the measured resistance, 2 ohms.

Of course, that doesn't jibe with the results of 50 volts @ 6 amps, or 8.33 ohms. So either the extra circuit elements in that test contributed 6.33 ohms, or the resistance of the ground electrodes increases with increasing current.

Cheers, Wayne
 

charlie b

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Location
Seattle, WA
Occupation
Electrical Engineer
I don?t understand why anyone is giving Gary a hard time on this thread. His test used a perfectly acceptable method to answer the question that it was intended to answer. This test is not about a normal installation. It is about the following question: Is it possible, or is it not possible, for a 120 volt source to trip a 15 amp circuit breaker, if planet Earth is part of the current path? The answer has been proven to be, ?yes, it is possible.? That is all that is being presented here.

This test has nothing to do with the NEC?s 25 ohm rule. There is no expectation that this test could be repeated in any other area of the country, using any other type of connection between power source and dirt, or using any other method of determining the resistance to ground of the electrodes, and having the test yield the same result.

Gary, I thank you for making the effort to perform the experiment and for reporting the results. :smile: You showed what I already knew to be true, and what anyone should expect to be true, if they but consider the question in its intended light.

Now, does anyone want to start a proposal for the 2014 NEC to allow dirt to become part of a recognized fault path for 120 volt systems? No? I suppose that is OK by me. I won?t offer such a proposal either. :wink:
 

iwire

Moderator
Staff member
Location
Massachusetts
It is about the following question: Is it possible, or is it not possible, for a 120 volt source to trip a 15 amp circuit breaker, if planet Earth is part of the current path?

I have no problem believing an experiment could make that happen, it apparently did happen. You can set up an experiment to come up with almost any result without putting some parameters to it. :smile:

What I am having a hard time with is his incredibly good soil conditions with just 3' of electrode. I am not saying Gary faked anything but something is odd here. :smile:
 
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