Purpose of ground rod?

[mrhanky]

I recently attended a seminar in a major city that was instructed by that citys chief electrical inspector. It was a 2002 NEC update course. In it he explained how fault current trips a breaker. After reading the Soares book on grounding and several Mike Holt books on the subject I didnt see how his theory would work. 1st he stated that ALL current is trying to find a path to earth. I thought current was trying to find a path back to its source.
2nd he stated that when you have a short circuit or a ground fault the fault current travels back to the main panel, down the ground rod and then quickly said "and trips the breaker". I thought the fault current goes back to the main panel where it travels back to the transformer on the neutral, then it returns on the ungrounded conductor to the main panel thus tripping the breaker in which ever circuit had the fault. Thanks

 

[russ]

Re: Purpose of ground rod?

You have it right, the inspector at the seminar was in error.

Russ

 

[charlie]

Re: Purpose of ground rod?

I agree with Russ, the ground rod doesn't really even help with fault current. This guy needs to take some electrical training, I'll bet he is sharp on the Code though.

 

[pierre]

Re: Purpose of ground rod?

It never ceases to amaze me how the train of thought that ground fault current wants to go to earth is still even a thought. But the reasons are old habits never die easy, and there are several books on the market, by major publishers, that still teach this. With that in mind I can see this train of thought will still be passed on.
Simple Ohms law will reveal the amount of current that the earth and the connection to earth, how much current can flow.
Lets say you have 120 volts and the resistance of the earth is 20 Ohms: 120/20 = 6 amps of current flow. If that is connected to a 15 ampere breaker, that will not trip that breaker.
BTW - We have measured many grounding connections around here and the average is around 70 Ohms. 120/70 = 1.7 amperes of current flow
Theory: All electrons that leave the source (battery, transformer, generator, etc..) want to get back to the source. The electrons will travel every available path, regardless of the resistance. So some will travel the earth path, but most will follow the path of 'least resistance', which hopefully will be the intended path.
Mike Holt's Theory book is very good for electricians, and has good illustrations that we can relate to on an everyday basis.

To the ones interested in learning the NEC, if you are studying the code, that is great, but you should also be studying theory, this will greatly enhance your understanding of the code as well.

Pierre

[ November 30, 2003, 08:45 AM: Message edited by: pierre ]

 

[hurk27]

Re: Purpose of ground rod?

I was once told by an inspector the reason he wouldn't release a service to be heated up by a power company was there was no ground rod installed yet and without it the breakers could not trip. this was in the middle of winter when there was no chance of a lightning storm. and the ground was so frozen I needed a power driver to install the rod.

 

[tom baker]

Re: Purpose of ground rod?

MRhanky
Your are 100% correct. Current is trying to get back to its source. In a fault we want the fault current to take the intentionally designed low impedance path back to the source, the resistance of the earth is so high that it will not allow sufficent current to flow and trip the overcurrent device. What is the current with a 25 ohm ground resistance and 120V?

 

[gwz2]

Re: Purpose of ground rod?

hurk27,

Are you indicating that an Inspector should approve a SERVICE that does NOT have a Grounding Electrode System as per 250.50 ?

 

[ryan_618]

Re: Purpose of ground rod?

Originally posted by gwz2:
hurk27,

Are you indicating that an Inspector should approve a SERVICE that does NOT have a Grounding Electrode System as per 250.50 ?

I don't think wayne is saying that at all. If the rod isn't availabe (already driven into the earth), that it is not required (unless as a supplement to the water pipe).

 

[earlydean]

Re: Purpose of ground rod?

What I have run into is: whenever I allow a contractor a little slack because of weather, or time constraints, or whatever. He seldom comes through. I have allowed the power company to re-energize a service that was disconnected so the siding could be replaced, but the SEU was not re-fastened to the house, because the siding guy assured me it would be done next week when the electrician returned. Guess what, it never got done. The inspector's job is to verify that the job had been done to code, not will be done to code.

 

[hurk27]

Re: Purpose of ground rod?

No Glen we had a tempoary #4 to the water line stubed into the crawl space. it was the reasone behind why the inspector wouldn't release the service. He thought that without the rod the breakers would not trip.
I tried to explain that the water line which ran to the street but not tapped yet would serve just as well but he insisted on having the rod installed. as made the statment that the breakers would not trip with out it.

 

[Guest]

Re: Purpose of ground rod?

Originally posted by pierre:
It never ceases to amaze me how the train of thought that ground fault current wants to go to earth is still even a thought. <big snip>

Herbert Ufer designed his Ufer Ground to be less than 5 Ohms. That quality of ground can clear a fault. For example: 120 volts divided by 5 Ohm's equals 24 amps. This was in dry, poor soil. With more moisture the impedance would be even lower.

A New Look at the Ufer Ground System

 

[don_resqcapt19]

Re: Purpose of ground rod?

awwt,
You're missing the point. The current only wants to return to its source. While the grounding electrode does provide an indirect path back to the source, that is not the code indented fault clearing path. The path back to the source via the grounding electrode system should be a much higher impedance path than that via the equipment grounding conductor and the main bonding jumper. In fact the code required fault clearing path in a properly installed system will be hundreds of times better than the path via the grounding electrode system.
A 5 ohm grounding electrode system (if that is the only fault clearing path) might never clear a fault even on a 20 amp breaker and would place a lethal voltage on everything connected to the electrical system. The trip time for one name brand 20 amp breaker is between 45 seconds and never. The hazard exists until the fault is cleared.
Don

 

[roger]

Re: Purpose of ground rod?

Awwt, we can do the same thing with any load bank.

Ufer has perfected this method of the made electrode as we know or refer to it, but this doesn't change the fact that current "must" return to source.

If you were to provide a 50 amp OCPD and measure voltage gradient in the most conductive path back to said source from the ufer, you would see it didn't stop at this point.

Roger

 

[don_resqcapt19]

Re: Purpose of ground rod?

In my opinion, this is just another example of the confusion that is caused by using the term "equipment grounding conductor". The use of the words "equipment grounding conductor" lead many to believe that the grounding electrode system and the connection to earth plays a major part of the fault clearing process on premises wiring systems. I think that some of this confusion would go away if the code would change "equipment grounding conductor" to "equipment bonding conductor"
Don

 

[gwz2]

Re: Purpose of ground rod?

hurk27,

Thanks for the further comment on the installation.

If the service is being used as a "early service" for construction of the building, I would undoubtedly permit it.

But, if this is the final inspection at that site - - - 250.50 prevails.

As one of several examples ( several years ago when I became an inspector ):

Being a part-time inspector, and the only electrical inspector, made inspections in the evening. I had a "rough".
Two electricans were wotking there that evening and obviously 'almost done'. I mentioned they needed a extra recp under the long 'picture window, and a wall switch for that room. Left a "rough" approved tag.

When the final inspection was conducted, neither item was corrected. I am guessing that as soon as I posted the Green Tag, They went home.

Their Boss told me when rejecting the final , " I got a green tag showing the Rough was approved."

Was "stung" only one more time by that contractor.

gwz2

 

[dereckbc]

Re: Purpose of ground rod?

Herbert Ufer designed his Ufer Ground to be less than 5 Ohms. That quality of ground can clear a fault. For example: 120 volts divided by 5 Ohm's equals 24 amps. This was in dry, poor soil. With more moisture the impedance would be even lower.

AWWT, Mr. Ufer designed his ground system for bomb shelters in the Arizona desert to protect them from accidental denotation from lightning and static discharge, period... A fault clearing path was not his intention.

[ November 30, 2003, 10:24 PM: Message edited by: dereckbc ]

 

[hurk27]

Re: Purpose of ground rod?

No problem Glen. I know how hard it is to trust people to there word. and I see it all the time when I send someone out to do a job only to find out later that he tried to cheat or just didn't do it. The problem is that it ruins it for everyone else. I myself try to keep a non-stereo type mind and give everyone a chance but it is hard.

 

[karl riley]

Re: Purpose of ground rod?

It's good to see that others share my amazement and frustration with electrical "authorities" who believe the myth that the purpose of grounding electrodes is to clear a fault. And yes, I also have seen this in Time/Life series electrical books.

Uffer, by the way, devised his ring to discharge static and lightning in ammo bunkers, not to clear faults.

Let me add a point about electrical circuits. All of us are used to saying that the neutral current is "trying to" get back to the source. A physicist would say that is not an accurate way to say it, since electricity flows from higher potential to lower. It doesn't actually try. It flows. This means that the transformer XO point is at a lower potential than the neutral bus at the main, and also lower than the local earth. So it acts like a voltage sink, and the electrons flow to it.

But how do we say this in a simple way? We might say the neutral current flows to the low voltage point of the Tformer, thus completing the circuit. Most of it flows on the lowest-impedance path, which is not the earth.

Karl

 

[karl riley]

Re: Purpose of ground rod?

Oops, Dereck, I missed your post. Seems we both had the same thought about Uffer.

Karl

 

[brian john]

Re: Purpose of ground rod?

You mean the earth isn't waiting for all her little electrons to return home?

I have thought about what a mess it might be if the earth was an excellent conductor. Think about all the parallel paths.

[ December 01, 2003, 09:31 PM: Message edited by: brian john ]