25 ohms

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Mule said:
I remember reading that in a fault condition, 25% of the fault current typicaly travels down through the electrode, while the remainder of the current, 75%, goes up the system neutral back to the transformer and its system grounding methods,
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Well if you put some thought into that idealology, you would quickly dismiss that notion. All you have to do is compare the imepedance of the EGC to a 25-ohm earth connection. An EGC path impedence will be less than 1-ohm compared to very good earth ground of 25-ohms. So do some math and tell us what you come up with using 120 volts.

120/1 = 120 amps
120/25 = 4.8 amps.

Humm, 4.8 sounds like a normal load to me on a 20-amp circuit, and is only 4% of 120-amps. So which conductor would you prefer, the EGC or Earth?
 
dereckbc said:
Well if you put some thought into that idealology, you would quickly dismiss that notion. All you have to do is compare the imepedance of the EGC to a 25-ohm earth connection. An EGC path impedence will be less than 1-ohm compared to very good earth ground of 25-ohms. So do some math and tell us what you come up with using 120 volts.

120/1 = 120 amps
120/25 = 4.8 amps.

Humm, 4.8 sounds like a normal load to me on a 20-amp circuit, and is only 4% of 120-amps. So which conductor would you prefer, the EGC or Earth?
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Humm, your so smart ...you tell us....what the percentages would be...I dont really care to...I was just coveying something I had read. :)

I think in that split instance before a pole fuse blows, there is a enormous amount of current delivered from the transformer to the fault, depending on impedence, KVA rating, length and resistance of the service entrance coductors, bla bla.... and I for one would not be wanting to be touching any thing in close proximity without the integrity of a good ground rod system being present.AND if that fault happened to be a lighting strike, it would probably toast your calculator as well....:D Humm.....
 
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Mule said:
I think in that split instance before a pole fuse blows, there is a enormous amount of current delivered from the transformer to the fault, depending on impedance, KVA rating, length and resistance of the service entrance coductors, bla bla....
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... and the resistance of the ground fault path.

Assume a 120 volt fault into 25 ohms, you get 4.8 amps maximum current no mater how much fault current is available from the transformer.
 
Mule said:
I for one would not be wanting to be touching any thing in close proximity without the integrity of a good ground rod system being present.
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Actually, I could care less whether a pole has an earthing connection if I were to touch it during a fault. I would be very concerned about whether the pole were connected to a good EGC, however.
 
iwire said:
... and the resistance of the ground fault path.

Assume a 120 volt fault into 25 ohms, you get 4.8 amps maximum current no mater how much fault current is available from the transformer.
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So for that I only need a #22 guage bare copper to connect to the rod? right?

I fully understand the calculation, but if you have "hungry for earth" fault current, My point is we do need a GOOD ground rod system, because IF the system neutral happen to be a bad connection and we dont blow that fuse, then it remains a fault, just waiting for someone to come along and touch it.

But at 4.8 amps well never blow the fuse?....so we should just quit putting in ground rods?
 
George Stolz said:
Actually, I could care less whether a pole has an earthing connection if I were to touch it during a fault. I would be very concerned about whether the pole were connected to a good EGC, however.
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are they not bonded together? geeze I have three moderators at me now :confused:

Ok your right, the fault current is going to focus back to XO not the butt rap, unless the neutral integrity is questionable.

Lets just get the fuse blown and we'll argue about it later....!!
 
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yankj said:
Ding, ding, ding! You win!
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I HIGLY doubt that....:D ...You know I had the lack of a ground rod, put me in a very dangerous situation some time back. I could have been killed, and I just think there is a purpose for a healthy grounding system...ALL OF IT
 
Here is something to think about. Following is a proposal I submitted to change 250.58 of the NEC. While it may not appear to have anything to do with this discussion, please notice the very last statement made by the CMP for which I have made BOLD:

5-176 Log #386 NEC-P05 Final Action: Reject
(250.58)
____________________________________________________________
Submitter: Bryan P. Holland, Holland Electric

Recommendation: Add new text as follows:

Where separate services, feeders, or branch circuits supply a building and are
required to be connected to a grounding electrode(s), the same grounding
electrode(s) shall be used and the provisions of (1) are met:

(1) An approved audible or visible alarm shall be installed at each service to
indicate the grounded conductor brought to the service has opened.

Exception No. 1: In industrial installations, with written safety procedures,
where conditions of maintenance and supervision ensure that only qualified
persons service the equipment.

Exception No. 2: Where electric service and electrical maintenance are
provided by the building management and where these are under continuous
building management supervision.

Substantiation: Upon the opening of the grounded (neutral) conductor at one
service, potentially dangerous current will flow between the common
electrodes to both services. Any person who comes in contact with exposed
metal parts at the service equipment or grounding electrode system could be
exposed to lethal current.

Panel Meeting Action: Reject

Panel Statement: The submitter has not provided any technical substantiation
to require monitoring grounding electrode and grounding electrode conductor
integrity by audible and visible alarms. The Code is not structured to protect against abnormal conditions such as open neutrals that may develop.

Number Eligible to Vote: 15
Ballot Results: Affirmative: 15
____________________________________________________________
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So let's go all the way back to the very first post that started this discussion before open neutrals and Iraq got involved.

Here's what I know:

1. The Chair of CMP 5 - Ronald Toomer cannot explain why 25-ohms is significant.
2. The Chair of the TCC - James Carpenter cannot explain why 25-ohms is significant.
3. Nor has Mark Earley of the NFPA.
4. Nor has Mark Ode of UL.
5. Nor has John Minick of NEMA.
6. Nor has Maichael Johnston of NECA.
7. Nor has Mitchell Guthrie - TC Chair of the NFPA 780.
8. Nor has Donald Zipse of IEEE.

AND, I have failed to mention at least a dozen more EXPERTS. These are individuals I have had one on one conversations with on this topic and this topic alone.


(PS - Let's not get into a conversation about my proposal. I am not good with rejection and I would have prefered it was never brought up. Please keep to the topic at hand.)
 
Oh know...a fourth moderator....I'm running...got to go to work :D


Look guys, maybe Im nieve here and need to read up on the matter more....I was just trying to make a point that ground rods ARE important....In a lighting strike on POCOs line...we need all of that grounding system to disapate all that energy away as safely as possible, dont we?
 
Bryan I commend you on all your hard work on your proposal. I've had proposals rejected for a lot better reasons than yours and it is still frustrating. Just wanted to let you know that I appreciate what you do.
 
Mule said:
I was just trying to make a point that ground rods ARE important....In a lighting strike on POCOs line...we need all of that grounding system to disapate all that energy away as safely as possible, dont we?
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In my opinion it is important that the utilities distribution system be connected to earth.

But are one or two electrodes that important?

In my opinion no.
 
Mule said:
I was just trying to make a point that ground rods ARE important....In a lighting strike on POCOs line...we need all of that grounding system to disapate all that energy away as safely as possible, dont we?
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Now you are on the right track. :D In low voltage systems like residential the main purpose of the earth connection is to provide a fault path for lightning, and accidental contact with high-voltage primary conductors on the same distribution system.

It is high voltage systems that earth comes into play. This is why utilities use earth as a conductor hence the multi-ground neutral system used by POCO's. But in NEC applications earth is forbidden to be used as a conductor for fault paths or normal load current.
 
SEO said:
Bryan I commend you on all your hard work on your proposal. I've had proposals rejected for a lot better reasons than yours and it is still frustrating. Just wanted to let you know that I appreciate what you do.
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I ment to say that Bryans proposal was rejected for a reason that was not as satisfactory of a reason as some of the reasons given to me for my proposal rejections. Not that my proposals were better than his. After I re-read my post I see that it could have been taken out of context.
 
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