Can a ground fault through dirt trip the OCPD?

[hurk27]

Actually I should have said that the wording of the rod requirement should be changed to something like:

If a driven electrode can be proved to continuously hold a low impedance path to Earth, it can be use as one of the 250.52(A)(4) Thu(A)(6) electrodes

But can always be used as a supplemental electrode.

 

[LarryFine]

Don't believe everything that you think you know.

Are you kidding? I don't even believe everything I tell myself! :grin:

 

[LarryFine]

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

I'm gonna tell her you said that!

 

[LarryFine]

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.

How about a single 'real' ground rod, and then with two, as the NEC calls for.

 

[LarryFine]

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 primary fuse should have blown.

 

[Dennis Alwon]

Larry, are you having fun meeting your post quota for the day. :grin:

 

[hurk27]

The primary fuse should have blown.

Not in this area, they seem to have a burn free type of mind set, NIPSCO is not a Co-op and they have to meet the state set outage minimum, so they fuse everything high, T-15's for most transformers Etc... try to get a T-15 to blow actually I was told this line laid on the ground on top of this neutral throwing sparks and arcing for about 2 to 3 minutes before the other end burned off the next pole

I could have imagine what the home owners went thought, buzzing and snapping and burning only to run out of the house to be met with the same thing, they had to think it was the end of the world

 

[crossman gary]

Everyone:

A few minutes ago, I was out in the yard pulling up the 1/2 emt pipes from my last test. While doing so, it dawned on me:

This is not a ground rod test.

It is a test of how much current can flow through the soil at my building under a 120 volt fault with earth as part of the path.

Somehow I let the discussion diverge to ground rod resistance. I purposely did not use a ground rod because I was trying to mimick some goofy possibility of a hot wire going to ground (meaning the earth). If I had used an 8 foot ground rod, most certainly one of you would have hit me with "Yeah, but when have you ever seen a 120 volt circuit get mashed against a ground rod? This isn't a real test of anything."

Here is an example of what I was trying to model: Hypothetically, let's say an electrician (?) was installing a receptacle located at some distance from the building. Say he drove a piece of unistrut into the ground about 3 feet, with 1 foot sticking up, ran a PVC conduit underground to that spot, mounted a metal bell box on the unistrut, and pulled only a hot and a neutral in the pipe. Then sometime later the hot wire got mashed up against the bell box.

Again, my point was not to make any statement about ground rod resistance. My point was to determine what current might flow under such a condition as above, considering the black clay soil here in Houston.

Obvioulsy the entire grounding electrode system at the building will play a role in the amount of current that flows in the above scenario.

I am not advocating that the earth is enough for the safety of an installation. Quite the contrary. It is all too obvious that the proper grounding as established in the NEC must be there for safety.

 

[crossman gary]

How about a single 'real' ground rod, and then with two, as the NEC calls for.

Larry, I really don't want to spend the money for the ground rods. Nor do I want to drive them!

As mentioned in the previous post, I am not trying to test "ground rod resistance."

If I did use the ground rods, I would expect that even more current would flow since an 8 foot deep rod would almost assuredly provide a better path than 3 feet of 1/2 rigid.

If you want, I could drive the 1/2 rigid deeper, and also do multiples of them. I have 1/2 rigid laying around, but I don't have ground rods unless I go buy them.

 

[crossman gary]

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

If the soil conditions in most parts of the country are as bad as y'all say, with 2 rods not even meeting the NEC 25 ohm requirement, then it seems like an almost pointless exercise.

Of course, most new installations nowdays should not even need a ground rod by code.

 

[480sparky]

If the soil conditions in most parts of the country are as bad as y'all say, with 2 rods not even meeting the NEC 25 ohm requirement, then it seems like an almost pointless exercise.

Of course, most new installations nowdays should not even need a ground rod by code.

Two rods are not required to meet the 25-ohm mark. Only if there's one... 250.56.

 

[hurk27]

Everyone:

A few minutes ago, I was out in the yard pulling up the 1/2 emt pipes from my last test. While doing so, it dawned on me:

This is not a ground rod test.

It is a test of how much current can flow through the soil at my building under a 120 volt fault with earth as part of the path.

Somehow I let the discussion diverge to ground rod resistance. I purposely did not use a ground rod because I was trying to mimick some goofy possibility of a hot wire going to ground (meaning the earth). If I had used an 8 foot ground rod, most certainly one of you would have hit me with "Yeah, but when have you ever seen a 120 volt circuit get mashed against a ground rod? This isn't a real test of anything."

The reason the thread diverged to ground rod resistance is because it is this resistance that ohms laws will tell us how much current will flow in a given circuit, of a certain voltage, and when this is known you would have had your answer.

In your original post you asked "Can a ground fault through dirt trip the OCPD? "

While you got your answer of "yes if the resistance of the contact point is low enough" yes it can open an OCPD, but you also got the answer that because of the many types of soils and other circumstances that with these variables it can not be safely depended upon to always serve this function, so from a safety stand point the answer would be "no".

Now while at it, it does allow an easy method of finding the resistance of a ground rod, while not an accurate method, it serves to find how much current will pass through this point on its way back to the transformer, thus giving you an approximate of the resistance value of it's connection to Earth.

remember after you are 6' from the rod almost all the resistance of the connection point has diminished, and for the rest of the pathway back to the transformer will be at the given Earth over all resistance of 0 ohms, so it is only with in this close proximity of the rod that other metal pathways will even interfere with your over all measurement using the voltage injection method.

that close proximity of the rod is called "the sphere of influence" and there usually are two of them in each Earth circuit, one under the transformer pole, and one under the rod. If your rod is close (within the sphere) to other metallic paths also bonded to the X0 of the transformer then there can be more which can lower the resistance reading. but in general it will be toward the transformer pole.:wink:

 

[jeremysterling]

I don't know exactly what is in the soil...

...I just think there is something unaccounted for here.

Could be that Houston, my hometown and second only to Chernobyl in being made uninhabitable by man, has so many heavy metals (like Mercury) and synthesized chemicals in its "soil".:smile:

 

[Twoskinsoneman]

I have no clue how a power generating plant wires up their generators to power a grid but I wonder how much current returns to it through the earth...probably a significant amount.

Anyone have any pertinent info that might shed light?

 

[crossman gary]

Two rods are not required to meet the 25-ohm mark. Only if there's one... 250.56.

I'm familiar with that one. Perhaps I should have been more detailed in my statement:

"with 2 rods not even meeting the NEC single rod 25 ohm requirement, then it seems like an almost pointless exercise."

 

[crossman gary]

The reason the thread diverged to ground rod resistance is because it is this resistance that ohms laws will tell us how much current will flow in a given circuit, of a certain voltage, and when this is known you would have had your answer.

But in my hypothetical example of the unistrut with the bell box, there isn't even necessarily a ground rod in the equation.

And in the actual experiment, a ground rod was not involved either. The building doesn't have a ground rod. It uses other items allowed by Code for the electrode.

My experiment wasn't intended to say anything about ground rods.

Now, there isn't any harm in the "ground rod resistance" discussion, just that it doesn't apply to the actual experiment because I was not creating/using a ground rod. Now, vice versa, my experiment may certainly be extrapolated to say something about ground rods. And this is where some of the discussion went.

That's okay.

 

[crossman gary]

In your original post you asked "Can a ground fault through dirt trip the OCPD? "

I appreciate the comments, Hurk. And something else dawned on me from reading your post.

I can see that some would have taken this thread to be a blanket statement from me that an OCPD could be tripped through earth anywhere on the planet. That was not my intention. I understand that everyone nowadays is taught that the earth /electrode sucks as a conductor at lower voltages. I think this is why some people got upset. My result went against the norm and the beliefs. What my test showed is that it isn't quite so cut and dried.

With the limitations of the written word on the net, as opposed to direct talk, my question was meant to be "Is there even a single case possible where a fault through dirt could trip a breaker at 120 volts?" I never ever believed or even implied that dirt/electrodes the world over could carry enough current to trip the OCPD. Before I even did the test, my belief was that most areas had soil with very high resistance. I also knew that my black clay soil was a pretty good conductor and I wanted to test it.

The whole thing started from that "grounding electrode question" thread when Iwire said that a ground fault through earth, under 600 volts, could never trip an OCPD.

So my experiment wasn't a blanket statement of proving that this would work anywhere. Essentially I was proving to myself that there is at least one place in the world where current through dirt tripped my OCPD.

 

[crossman gary]

Could be that Houston, my hometown

Coming to Houston anytime soon? If so, let me know, we'll do the experiment again. Unless you are afraid of getting cancer from touching the dirt!:wink:

 

[LarryFine]

Larry, are you having fun meeting your post quota for the day. :grin:

Hey, each of my posts is a legitimate response.

 

[LarryFine]

Not like this one.