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#128983 - 06/19/04 07:21 PM Does ground rod work??
ayrton Offline
Member

Registered: 12/21/01
Posts: 205
Loc: Pa
How much does the ground rod really do?
In thoery the current if there is a fault, is to travel through the rod, through the ground and back to the source.
If the the Service equipment is bonded correctly, shouldnt the current travel through the service ground back to the source??
Opinions wanted

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#128984 - 06/19/04 07:32 PM Re: Does ground rod work??
ayrton Offline
Member

Registered: 12/21/01
Posts: 205
Loc: Pa
I had a service call today, where the pole by the street was hit and completly taken out. A triplex went to my customers building, feeding a single phase service.
A second Quadplex went to the building for a three phase Delta service. When the pole was hit, it tore the triplex off the building, which completly chafed the service conductors. The quadplex pulled on the 500's in the service conduit but did not damage the conductors. Now each service has its own ct cabinet. Neither was bonded. Second there are no ground rods. Only a building steel ground bonded to one of the single phase service panels. The ground wire in the delta service was burnt from the first service disconnect through the cabinet and up the service pipe.
Now it appears to me that when the single phase service conductors chafed and where touching the conduit this caused a short. Which in turn sent the current through the delta service grounding electrode.
Now if the cabinets where bonded, the current should of went through the conduit and through the cabinet and back through the service ground.
How much would a ground rod have really done??

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#128985 - 06/19/04 10:28 PM Re: Does ground rod work??
nesparky Offline
Member

Registered: 06/21/01
Posts: 650
Loc: omaha,ne
Depending on soil conditions and distance probably not much. The ground rod is more for earth reference and lighting disappation than for clearing a fault. The earth is a poor conductor.
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ed

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#128986 - 06/20/04 04:52 AM Re: Does ground rod work??
ayrton Offline
Member

Registered: 12/21/01
Posts: 205
Loc: Pa
Why in the NEC, does it state 250.66 that size of electroce conductor should be 1/0 cu
for service conductors over 350-600kcmil. Yet 250.66(A) states the conductor that is sole connection to the grounding electrode shall not be required to be larger than 6awg cu.
Now if you say, run the 1/0 into the building from the service head and than in the service enclosure, where bonded cgange to 6awg, that contradicts 250.64(C) where it states grounding electrode should be continuous without splice unlesss compression type connectors are used.

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#128987 - 06/20/04 05:09 AM Re: Does ground rod work??
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
The grounding electrode conductor running to the water service pipe may very well have to withstand every bit of the fault current the utility transformer can deliver.

This is possible because that metal water pipe may be electrically continuous with the other structures around it. Those other structures will also be bonded to the metal pipe providing a low resistance fault path.

On the other hand a ground rod or a concrete encased electrode can only be expected to carry the current it can pass into the soil which is a poor conductor.

Lets say you have a ground rod with 25 ohms of resistance, a 120 volt fault to this ground rod will result in only 4.8 amps of current flow. (120 V / 25 O = 4.8 A)

Even if you happen to get the rods resistance down to 3 ohms (unlikely) a 120 volt fault will result in 40 amps of current on the GEC.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#128988 - 06/20/04 05:15 AM Re: Does ground rod work??
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
Now if you say, run the 1/0 into the building from the service head and than in the service enclosure, where bonded cgange to 6awg, that contradicts 250.64(C) where it states grounding electrode should be continuous without splice unlesss compression type connectors are used.


No it does not contradict itself.

The conductor from the service head to the panel is a 'Service Conductor' it is not the 'Grounding Electrode Conductor'

The Grounding electrode conductor starts at the bonding point in the service disconnect enclosure and ends at the grounding electrode.

If you chose to daisy chain more than one electrode together the GEC is the conductor from the bonding point to the first electrode. The conductor between electrodes is a bonding jumper forming the grounding electrode system it can have splices.

Bob
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#128989 - 06/20/04 05:45 AM Re: Does ground rod work??
ayrton Offline
Member

Registered: 12/21/01
Posts: 205
Loc: Pa
Where does it state that?

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#128990 - 06/20/04 06:17 AM Re: Does ground rod work??
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
I can not say that "it states that" in any one place.

You got to jump around.

 Quote:
Grounding Electrode Conductor. The conductor used to connect the grounding electrode(s) to the equipment grounding conductor, to the grounded conductor, or to both, at the service, at each building or structure where supplied from a common service, or at the source of a separately derived system.


 Quote:
Service Conductors. The conductors from the service point to the service disconnecting means


Now take a look at part of 250.50

 Quote:
If available on the premises at each building or structure served, each item in 250.52(A)(1) through (A)(6) shall be bonded together to form the grounding electrode system.[/b]


Bonding jumpers are used to bond the electrodes together into a grounding electrode system.

Once you have made this grounding electrode system the conductor from any point in this system back to the service disconnect is the grounding electrode conductor.

Bob



[This message has been edited by iwire (edited 06-20-2004).]
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#128991 - 06/20/04 08:58 AM Re: Does ground rod work??
ayrton Offline
Member

Registered: 12/21/01
Posts: 205
Loc: Pa
I understand and have practiced evrything you said. I am trying to get specific articles. I am weel awaree of the bonding process in the service. I overkill, as to sleep at night lets say.
The code states the GEC should not be broken but continuous with exception of crimps.
This I interpret as from the ct cabinet if used, bonded to neutral continuous to disconnect or MDP,on to ground rod,to building steel and water pipe.
We both know nobody does this, but I interpret this article as stating to do so.
I originaly posted because I do not beleive that in circumstances I presented with the burnt ground, that a rod would not of prevented this, but proper bonding of the service entrance neutral or ground, to the cabinet and disconnect would have.
Agree or disagree?

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#128992 - 06/20/04 09:25 AM Re: Does ground rod work??
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Hopefully some others will jump in and offer some other suggestions, the info you want is spread out throughout Chapter III and Chapter V of Article 250.

If you have access to the NFPA handbook you can see a lot of different ways of installing a grounding electrode system.

I could run a GEC from the Service Disconnect enclosure to building steel, then on the other side of the building I could run a bonding jumper from building steel to the water service pipe.

This is all shown in the handbook.

Here is a relevant section.

 Quote:
250.53 Grounding Electrode System Installation.

(C) Bonding Jumper. The bonding jumper(s) used to connect the grounding electrodes together to form the grounding electrode system shall be installed in accordance with 250.64(A), (B), and (E), shall be sized in accordance with 250.66, and shall be connected in the manner specified in 250.70.


Now we have to decide, are we going to talk about how you and I may do it beyond code or are we going to stick with what is required.

I will usually jump the water meter without a splice but we are not required to.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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