Purpose of ground rod?

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Guest

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Re: Purpose of ground rod?

I didn't say it WILL clear a fault. I said it can clear a fault. Maybe I should have said it MAY clear a fault. It will only come into play if there is an open neutral to the PoCo. My point is that using numbers like 25 and 20 can be deceiving, when numbers like 5 and 15 might work. 5 and 15 (5 Ohms & 15-amp OCPD) works on paper :)

As for circuit breakers that trip in 45-seconds to never: What kind of OCPD is that? That's like selling parachutes that "might open, or might not". Maybe it's time to go back to fuses. Ed?

Even a 1-Ohm grounding electrode won't meet NEC. That's understood. But it will work on paper. That's why people are confused by the purpose of the GEC. Best case, it works-- on paper.

Please don't tell me it can't work. Please do tell me not to rely on it working :)

Your mileage may vary. Happy Trails!
 

hurk27

Senior Member
Re: Purpose of ground rod?

The idea is not to depend on the earth as a fault clearing path. the main reasone is the resistance of earth is a varible and no one can say what the resistance would be from one point in time to the next. It would be kind of like the weather. A big gamble and with peoples life a risk. It would not be dependable enough to risk it.

Let's say when you install this GE you got a reading of 2 ohm's now the weather changed and your in a bad dry spell, are you going to go back and try to redo the GE? This is why the NEC don't wan't us to depend on the earth as the sole means for a fault current path.
 

iwire

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Re: Purpose of ground rod?

Originally posted by awwt:
As for circuit breakers that trip in 45-seconds to never: What kind of OCPD is that? That's like selling parachutes that "might open, or might not". Maybe it's time to go back to fuses. Ed?
Wayne as strange as this sounds right off the bat all OCPDs have a time / current trip curve.

Even a fuse can hold above it's rating for a predetermined time.

This is not a bad thing, would you want your OCP to trip every time your air conditioner kicked on? :p

Go to the Bussman web site and you can get the time current curves for the different types of fuses or go to Square D and check out the time current curves for breakers.

It is all pretty interesting.

Here is a link to a thread with more info on this.
Breaker not Tripping
 
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bthielen

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Re: Purpose of ground rod?

I'm not so sure that I agree with the statement "current is trying to get back to its source." If this were true, why would it leave its source in the first place? The truth is, current (electron flow theory) is the result of a difference of potential between two points. When the difference is sufficient enough to overcome the resistance to the current flow, then the electrons will attempt to achieve a balance thereby eliminating the potential difference.

Think of it this way. Place two 50 gal. drums with open tops side-by-side with a pipe on the bottom connecting the two. Place a valve on the pipe to control the resistance. Fill one of the drums with water. We now have a difference of potential between the two drums. Opening the valve will reduce the resistance. The potential difference is able to overcome the remaining resistance and the water begins to flow. Without providing a means to maintain the potential (maintaining the water level on the supply side), the water will eventually balance between the two drums eliminating the potential difference.

Electricity works in a similar way. So long as we can maintain the potential difference between two points at a constant, the current will continue at a constant. Change the potential or change the resistance, and the current will change. Opposite forces attract, like forces repel. The electrons are not trying to RETURN to the source, they are trying to LEAVE the source. In an AC system the potential is constantly reversing polarity, which causes the electron flow to do likewise.

Bob
 

karl riley

Senior Member
Re: Purpose of ground rod?

The rule of thumb for breaker clearing is that you want 4 to 5 times the amps of the breaker to flow in order to clear fast enough to prevent a fire and other damage. So for a 15A breaker you need at least 60A for a reasonable trip time. For a 20A breaker you want 80+A. This means a 1.5 - 2 ohm impedance.

This would have to include the ground rod at the transformer also, so the Ufer at the building plus the ground rod at the transformer would have to add up to no more than 2 ohms. Get the idea?

Why would anyone want the earth to do the job of the service drop neutral? Maybe it's the Mother-Earth-as-a-huge-warm-sponge syndrome. Psychologists could add this to their list.

Karl
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
Re: Purpose of ground rod?

Originally posted by bthielen: I'm not so sure that I agree with the statement "current is trying to get back to its source." If this were true, why would it leave its source in the first place?
The more deeply you attempt to delve into the precise nature of Nature, the less firmly you must hold to your favorite clich?s. We would be well served if we could take the clich?, ?electricity takes the shortest path to ground,? and forever obliterate it from our language. It conveys very little truth, and opens the door to many misunderstandings. However, I would hesitate to cast doubts upon the clich?, ?electricity is seeking its own source.? It is, in fact, untrue! But we can safely ignore that fact. For the purposes of our profession, it holds enough truth, and presents very few opportunities for misunderstanding. What follows is a deeper delving into the truth of electron flow than is needed for a member of the electrical profession. Feel free to stop reading, if you fear becoming confused.

In the presence of an electric field (as may be imposed by a battery), an electron will feel a force. [Aside: So too with a proton that sits in the nucleus of a copper atom along the length of a copper conductor. But the protons are not as free to move about as are the electrons, so we?ll speak of them no more.] In the presence of a magnetic field that is ?moving? (i.e., varying in strength or direction, as may be imposed by the stator of a generator), an electron will feel a force. If either force is strong enough to overcome the attraction that the electron has to its host atom, the electron will move. That creates two unstable entities: One is the electron, which would be happier (i.e., stable) if it were to be attached to an atom. The other is the atom from which the electron moved away. The atom would be happier if it were to gain back an electron. Nature doesn?t like such instabilities, and quickly acts to eliminate them. It does so by having the electron attach itself to the next atom in line, and by having some other electron (from further back along the wire) attach itself to the atom that lost the first electron. Since the electric field (or the moving magnetic field) is still present, electrons will continue to be knocked free of their host atom, and will continue to jump from atom to atom along the line.

So is any given electron trying to get back to its source? Well, the answer is ?No,? if by the question we mean, ?Is the electron trying to get back to the atom to which it was attached before we threw the ON switch?? No, once any electron has found another atom that had been missing an electron, whether that atom is part of the copper wire, or part of the battery, or part of a human, or part of planet Earth, then it will be perfectly happy to remain there forever. But it might not remain there, if the electric field (or moving magnetic field) is still present.

So when we say that ?current is trying to get back to its source,? the truth is that the source is capable of imposing the electric field (or moving magnetic field) along the entire closed path of the circuit for the one any only one reason that the path is, in fact closed. If you break the path (e.g., by opening the switch), then the source cannot impose its field along the entire path. Therefore, electrons do not feel the force, and remain happily attached to their host atoms. But so long as the path is complete, so long as the field is present along the entire path, so long as electrons are kicked out of their happy homes (i.e., their host atoms), then the most readily available place an electron can find to re-attach itself to a different host atom is along the path. In the due course of time, moving along the path will bring the electron back to the source.
 
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bthielen

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Re: Purpose of ground rod?

It would appear that the definitions of ?source? and ?ground? may even be at question. The terms ground, source, and common are not necessarily one and the same. My understanding is that ?ground? is related to earth, source is a generator of potential, and common is an ungrounded path to close a circuit to provide a potential. Obviously, the potential of our source can be positive or negative with respect to ground or common. Using this definition, electricity does not require a path to ground in order to find enough potential force, either push or pull, both of which are at play, to initiate current flow. Static electricity is a very good example of this. The discharge can happen without a path to ?ground?. There only needs to be a difference of charge or potential between the ?source? where the electrons are excessive to the object of their desire. A battery is another excellent example. The positive terminal is not ground. It only provides the area that presents an imbalance and the resulting forces between the electrons and the more positively charged area attracts the electrons who attempt to seek the balance. Close the circuit between the negatively charged terminal and the positively charged terminal and you will have current flow without ground.

My comments were meant to dispute that the current is trying to do anything. I have seen too many close calls by individuals that believe they cannot experience electric shock if they are not grounded. The electrons, which are somewhat free to roam, are trying to find a place where they can remain in a balanced energy field. Incidentally, the idea of proton flow would disrupt the conductor enough that it would cease to exist. The removal of even one proton out of an atom, let?s say a copper atom, would result in the copper no longer existing.

Bob
 

gwz2

Senior Member
Location
Indiana
Re: Purpose of ground rod?

Title of this thread is " Purpose of Ground Rod "

would it not be to have an equalpotential plane ( at ground potential ) around the rod area to the the other end of the GEC ? Or at least nearly-so.
 

iwire

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Location
Massachusetts
Re: Purpose of ground rod?

Charlie B. That was great, I think I even understood it. :)


Originally posted by bthielen:
I have seen too many close calls by individuals that believe they cannot experience electric shock if they are not grounded.
Bob can you explain this more? :confused:
 

friebel

Senior Member
Location
Pennsville, N.J.
Re: Purpose of ground rod?

Hello to all, My only comment on this issue of the groundrod is this. I just wish that we were all able to attend one of Mike's Grounding & Bonding Seminars. I attended one on Nov.6th,2003 in Newark, Delaware. Mike, will surely convince you that all current is trying to get back to the source, and that path is on the grounded conductor that goes back to the source. Now the main purpose of the main ground rod is for lightning protection, and for any high voltage surges. Stop, and think about it. A fault current on a 120-volt system trying to get back to the source from a 25 ohm ground rod. Using Ohms law, the current will only be 4.8 amps. Now, as we must know, this will never trip any 15 amp breaker.

[ December 03, 2003, 04:17 AM: Message edited by: friebel ]
 

pierre

Senior Member
Re: Purpose of ground rod?

Charlie from Washington
That was very well stated! :D As far as what an electron is actually doing I can say that is the best explaination I have read yet.

In wiring of premises, and COMPLETING CIRCUITS to allow current to flow in our scheme of wiring, we can safely say that the electrons are flowing from the source, to the source, by virtue of the potential of difference between the two. Would you say that is a correct statement?

It is almost (I mean almost) like playing musical chairs. Electrons go round and round until the circuit is open (the music stops) or enough resistance is imposed on the circuit, then the electron will stop where it is.

Pierre
 

Ed MacLaren

Senior Member
Re: Purpose of ground rod?

This sketch might complement Charlie B's excellent explanation of electron behavior.

Electron1.gif


Ed
 

charlie b

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Lockport, IL
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Retired Electrical Engineer
Re: Purpose of ground rod?

Originally posted by pierre: . . . we can safely say that the electrons are flowing from the source, to the source, by virtue of the potential of difference between the two. Would you say that is a correct statement?
Yes I would. But please allow me to add a few words, in an effort to add a bit of clarity: ?The electrons are flowing from one end of the source, along a complete path (i.e., that includes wires and the load), back to the other end of the source, by virtue of the difference in potential that is generated internal to the source.?
 
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bthielen

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Re: Purpose of ground rod?

Ed,
I would like to attempt to shoot a hole in your theory, somewhat. No pun intended...maybe. In order for the free electron to move from one atom to the next it must be forced to do so (they don't do this just because they feel like it). This means that its current location is repelling it OR its resulting location was attracting it, more than the bond which held it with its original atom. It is not the holes that are moving, it is the electrons. To my understanding, conventional theory has been proved incorrect for years and now the accepted theory is electron flow.

Charlie b,
I believe your latest comments are accurate provided there is a closed loop system. What about an open loop system? Let me try to expand.

If I measure the voltage potential between me and earth, I will likely find a very low potential even if I am insulated from earth. Now, if I make contact with a positively charged surface with respect to earth, and measure the voltage potential between me and earth, there will be potential difference between me and earth. Electrons have been forced to move to a state of equilibrium with the charged surface and therefore, I am now also charged. Current has flown through my body. If I make contact with a surface that is less positively charged than myself, but not necessarily earth, there is a potential for electric shock as the electrons are forced to move to balance the forces acting against them. Depending on the potentials in question, the electric shock could be quite significant. Lightning comes to mind. It doesn't always arc to earth. It arcs from one area of ionic charge to an opposing area of charge and it doesn't care where that is so long as the electrons can find a balanced state.

Bob
 

Ed MacLaren

Senior Member
Re: Purpose of ground rod?

In order for the free electron to move from one atom to the next it must be forced to do so (they don't do this just because they feel like it).
Of course. I assumed that would be common knowledge.

It is not the holes that are moving, it is the electrons
My reference to holes was for those who might be reading the many electronics texts that refer to hole flow.

Ed
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
Re: Purpose of ground rod?

Originally posted by bthielen: In order for the free electron to move from one atom to the next it must be forced to do so (they don't do this just because they feel like it). This means that its current location is repelling it OR its resulting location was attracting it, more than the bond which held it with its original atom.
Actually, neither is true. The force comes from outside both atoms (i.e., the atom the presently holds the electron, and the atom the electron moves to). The force is called into existence by the power source (e.g., the battery, generator, or transformer). The method by which the force is imposed is either by virtue of an electric field or by virtue of a moving magnetic field. Yes, the fact that electrons move and not positive charges is well known, and yes you can blame Benjamin Franklin for an incorrect guess. That is neither relevant nor important. An electron moving left and a hole moving right have the same impact, and are analyzed in the same manner.

What about an open loop system?
The rule about ?current needs a complete path? is true of a system in equilibrium. During transient periods (i.e., between one equilibrium state and another equilibrium state), current can flow (e.g., electrons can move) from one point to another, without the need for a return path. Such transients generally last a very small fraction of a second. Lightning is an example of such a transient. The static electricity experiments you describe are other examples.
 
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