Grounding electrode conductor question

[tom baker]

I thought grounds were used to provide a path of low resistance path to ground for fault currents.

Never at 600 v and below
Ohms Law
120V
25 ohms ground rod resistance
What is the resulting current?
Will that trip a breaker or make a lot of money for the POCO?

 

[macmikeman]

250.4(A)(1) says ?Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.? This is grounds purpose; its conductor must contribute integrally.

The part in red is the part I never could quite understand what is meant by it. I have seen it in Soares book and other places but does it really happen or is it all theory?

 

[tryinghard]

The part in red is the part I never could quite understand what is meant by it. I have seen it in Soares book and other places but does it really happen or is it all theory?

I really don?t know :-? How is the ground going to stabilize the voltage during normal operation?

Maybe it is such that the resistance is the same though earth allowing the same potential throughout, this doesn?t make sense either what if its an ungrounded system? Its an enigma and inquiring minds would like to know :grin:

Smart people out there must have this answer?

 

[roger]

Smart people out there must have this answer?

I hope it's not the same people that are holding on to the mysterious 25 ohm requirement. :grin:

Roger

 

[Pierre C Belarge]

A couple of points.
I also disagree with the question's validity, but... a multiple choice test demands one to answer the question that is the most correct or closest to being correct.
(the above test question, in my opinion, HAS to be a misprint)

Gary
The resistance to earth of the rod is one thing, rarely even coming close to 25 Ohms. But that is only part of the path back to a transformer. The resistance of the earth itself is extreme high, and will most likely only permitt miliamps of current to flow. That is the reason the NEC says that earth is not permitted, not the electrical connection of the rod to the earth.


If one has a 120v or 277v circuit opening the overcurrent device during a fault, there is most likely another path (parallel path) that is "assisting" in the process.

 

[crossman gary]

Pierre,

I'm really just being contrary. Iwire said that at anything under 600v, the earth path will not carry enough current to trip the OCP. I am just saying "never say never."

I agree with you that under typical scenarios and normal size OCPD, having the earth as part of a fault current path is not going to trip the breaker. But going out on a limb and saying it is absolutely impossible may be pushing it...

Y'all are going to force me to do an experiment, aren't you.

 

[wwhitney]

I really don?t know :-? How is the ground going to stabilize the voltage during normal operation?

I think "stabilize the voltage" should be read as "stabilize the voltage relative to ground". Obviously grounding one of the conductors will do that. If none of the conductors is grounded, then the system can float to some higher voltage above ground. Should one of the conductors be near a grounded object, then conceivably the voltage to ground could float high enough to exceed the insulation rating of the conductor.

Cheers, Wayne

 

[charlie b]

The resistance of the earth itself is extreme high. . . .

I disagree. To start with, we can take note of the fact that utilities use planet Earth as a neutral return path. They couldn't do that, if the resistance of dirt was extremely high.

We all know that when you put two resistances in parallel, the net resistance gets smaller than either of the two. The more resistance elements you put in parallel with each other, the smaller the net resistance becomes. Now think of all the "parallel paths" that a planet the size of ours provides. Stick two 8-foot long electrodes in dirt a long distance apart. Current can travel between the two points via a straight line that is one inch below the surface, or via a straight line that is two inches below the surface, or via a slightly curved line that is 10 feet below the surface, or via a more curved line that is 20 feet below the surface, or via a very curved line that is 5 miles below the surface. There are a nearly infinite number of parallel dirt paths between the two points. I infer that the resistance between the two is extremely small.

 

[Pierre C Belarge]

The earth as a WHOLE is a considered a good conductor. Utility companies use the earth, as the "push" of the higher voltages will permit the flow of current; I am sure you are well aware of that.

In regards to voltages below 600v, there is very little, if any chance at all the voltage will push any significant amount of current through the earth.

The point of my earlier post is that the resistance of the earth contact of ground rods is not the only consideration, the resistance of earth is a contributing factor as well.

 

[charlie b]

In regards to voltages below 600v, there is very little, if any chance at all the voltage will push any significant amount of current through the earth.

That is only because, in order to push current through dirt, it has to first push current through the ?grounding electrode-to-dirt? interface. That is where the 25 ohms we have been discussing come into play. If I could manage to pound two rods into the ground, 100 feet apart, and somehow magically achieve a ground resistance of 1 ohm each, a 120 volt source connected between the two would yield a current around 60 amps. That would probably trip a 20 amp breaker, eventually. Of course, we are not permitted to take credit for the earth as a part of the current path for tripping breakers. That does not mean that it is impossible for a 120 volt source pushing current through dirt to trip a breaker.

The point of my earlier post is that the resistance of the earth contact of ground rods is not the only consideration, the resistance of earth is a contributing factor as well.

That is the part with which I am disagreeing. The resistance of the earth is so much smaller than the resistance of the interface between rod and earth as to be negligible.

 

[macmikeman]

I think "stabilize the voltage" should be read as "stabilize the voltage relative to ground". Obviously grounding one of the conductors will do that. If none of the conductors is grounded, then the system can float to some higher voltage above ground. Should one of the conductors be near a grounded object, then conceivably the voltage to ground could float high enough to exceed the insulation rating of the conductor.

Cheers, Wayne

Wayne, I apreciate your reply to my query, however I got curious in the past and have taken the time to attempt to create just such a scenerio using small (3kva and less) transformers at various voltages below 600 volts to see if while having no conductor grounded, I could record or read any wild swings on one or across both the two secondary conductors with respect to ground. I could not. I did not however attempt to try a "fault condition" at any time, so the possibility of this condition occuring during a fault (high voltage??) may still be what that statement is in there for.

 

[steve66]

I think B is the correct answer.

A, C, and D are obviously false. That leaves B by elimination.

The only objection I've seen to answer B is the term "high resistance".

The terms high or low are always relative. A 25 ohm impedence to ground is "high" compared to a normal metal-metal electrical connection. And most of us agree the impedence would be high enough that it probably wouldn't draw enough current to trip a circuit breaker.

So I think the question and answer are correct as written.

But I do agree it isn't the best question. Many people would be thinking "low resistance" because we want to create the lowest resistance to ground that we can. But its important to know that no matter how many ground rods you dirve, the impedence will still be high compared to any normal connections.

 

[wwhitney]

However I got curious in the past and have taken the time to attempt to create just such a scenerio using small (3kva and less) transformers at various voltages below 600 volts to see if while having no conductor grounded, I could record or read any wild swings on one or across both the two secondary conductors with respect to ground. I could not.

Well, this has gotten outside of my area of knowledge. But doesn't the act of connecting the the voltmeter between one supply conductor and ground provide a high impedance connection that would dissipate any voltage difference? I mean, there's obviously no EMF source driving the supply conductor to a particular voltage relative to ground, so any voltage difference that arises is transitory and wouldn't be measurable. It would be like trying to measure static electricity with a voltmeter, I think.

Cheers, Wayne

 

[macmikeman]

Well, this has gotten outside of my area of knowledge. But doesn't the act of connecting the the voltmeter between one supply conductor and ground provide a high impedance connection that would dissipate any voltage difference? I mean, there's obviously no EMF source driving the supply conductor to a particular voltage relative to ground, so any voltage difference that arises is transitory and wouldn't be measurable. It would be like trying to measure static electricity with a voltmeter, I think.

Cheers, Wayne

Good call Wayne. I thought of that also, so I just pulled up a lawn chair, put on my Coast to Coast AM tin foil hat:grin:, let the test run with no measuring for a spell, and waited for the catastrophic insulation failure once the "voltage with respect to the earth" swung up a billion or two volts. While sitting there in deep contemplation, I realized that the same exact thing would happen to doorbell transformers if any of that theory about the insulation failures resulting from simply not grounding the system held any water. So I figure it doesn't hold any water, and there must be another reason they put that line about "stabilizing" the voltage to earth in the code book. I just cannot find out what it is from any source so far. Wayne, you have been a good sport, and once again I thank you for trying to explain it to me. I just think there must be more to it or else it is plain baloney, and needs to be removed from the code to lessen any confusion.

 

[tryinghard]

I think "stabilize the voltage" should be read as "stabilize the voltage relative to ground". Obviously grounding one of the conductors will do that.

How so you mean like a corner grounded system, "B" phase will have zero reference to ground but A & C will have potential?

If none of the conductors is grounded, then the system can float to some higher voltage above ground. Should one of the conductors be near a grounded object, then conceivably the voltage to ground could float high enough to exceed the insulation rating of the conductor.

How can the voltage float higher if it?s a true ungrounded system there is no reference to ground? Ungrounded systems are used often for critical equipment/machinery?

I don?t get maybe I?m missing something here?

 

[LarryFine]

How so you mean like a corner grounded system, "B" phase will have zero reference to ground but A & C will have potential?

With a solidly-groubded conductor, the voltage to ground on any other conductor, while obviously unavoidable, is predictable. A floating system should equalize via capacitance, but it's a high impedance.

How can the voltage float higher if it’s a true ungrounded system there is no reference to ground? Ungrounded systems are used often for critical equipment/machinery?

True, but such systems require fault-detection equipment. While the 'first' fault won't cause interruptions, a second one would, creating the shutdown not bonding is to avoid.

Another reason to bond: Imagine what would happen if a primary conductor accidentally contacted a secondary conductor of a floating system. Instant insulation tester.

 

[tryinghard]

With a solidly-groubded conductor, the voltage to ground on any other conductor, while obviously unavoidable, is predictable. A floating system should equalize via capacitance, but it's a high impedance.

So in my limited understanding of this phenomenon I ended up at referance.com and this is how they describe capacitance:

Property of a pair of electric conductors separated by a nonconducting material (such as air) that permits storage of electric energy by the separation of electric charge, and that is measured by the amount of separated charge that can be stored per unit of electric potential between the conductors. If electric charge is transferred between two initially uncharged conductors, they become equally (but oppositely) charged. A potential difference is set up between them and some of the electricity can be stored. Capacitance math.C is the ratio of the amount of charge math.q on one of the conductors to the potential difference math.V between the conductors, or math.C = math.q/math.V. The unit of capacitance is coulombs per volt (C/V), or farads (F).

Somehow grounding will stabilize; but this is for a “grounded system”.

An “ungrounded” system is intentional and there is no reference to ground. I would not think of an intentional ungrounded system as floating, would you? (Actually 250.4(B)(1) is devoid of the word stabilize.)

I’m on the way but I aint there yet :-?

 

[wwhitney]

While sitting there in deep contemplation, I realized that the same exact thing would happen to doorbell transformers if any of that theory about the insulation failures resulting from simply not grounding the system held any water.

OK, so let's say insulation failure is out. What is the point of stabilizing the voltage relative to ground? Or is the only point of earthing to deal with lightning or accidental overvoltage?

Cheers, Wayne

 

[crossman gary]

I did the experiment to see if the earth path can trip the OCPD. I placed it in a new thread:

http://forums.mikeholt.com/showthread.php?p=1078063#post1078063

Under the soil conditions around me, the earth carries substantial current.

 

[macmikeman]

OK, so let's say insulation failure is out. What is the point of stabilizing the voltage relative to ground? Or is the only point of earthing to deal with lightning or accidental overvoltage?

Cheers, Wayne

All tomfoolery by me aside, that is exactly my point in all this. Will there really be any insulation failure incidents pertaining to the subject non grounded system running without reference to ground, and if not, then why is it in the code book at all. Either there is a real demonstratable reason, or the wording I highlighted into red is a mumbo jumbo put in that possibly continues a theoritical myth. I am no expert, and this has bugged my curiosity for some years now. I wish I knew the real answer. Maybe Larry has the answer, but that is past my knowledge base level. Cheers back, Mike