250.81

[bennie]

250.81 1996 Edition...The unspliced grounding electrode conductor shall be be permitted to run to any convenient grounding electrode available in the grounding electrode system. It shall be sized for the largest grounding electrode required among all the available electrodes.

250.81 changed to 250.50 1999 Edition. An unspliced grounding electrode conductor shall be permitted to be run to any convenient grounding electrode system or to one or more grounding electrodes(s) individually. It shall be sized for the largest ground electrode conductor required among all the electrodes connected to it.

The submitter for the change in 1998 ROP's wrote "While this change is primarily editorial, it will clarify that individual grounding electrode conductors are permitted to run to one or more electrodes. A single grounding electrode conductor should not be required to run to all the electrodes.

This change was done by one person. Electrical technology and the science of current flow is proof that this change is wrong. The original wording should have remained. There was never any intent to feed each electrode individually.

 

[Ed MacLaren]

Re: 250.81

I don't see what problem is caused by running two separate grounding electrode conductors as permitted by the change you referred to.

Could you explain it, using the sketch of a typical service below?

In the sketch, both conductors going to the grounding electrode system are connected at the same point on the grounded (neutral) conductor.

Ed

 

[don_resqcapt19]

Re: 250.81

Bennie,
The lack of a panel statement or comment on proposal 5-227 for the wording change indicates that the panel agrees with the submitter that the previous codes permitted multiple grounding electrode conductors. When the panel members do not agree with the substantiation for a change, there is always a panel or panel member comment to that effect. There is no such comment in this case.
Don

 

[bennie]

Re: 250.81

Don: No one on the panel is qualified to question the submitter.

 

[don_resqcapt19]

Re: 250.81

Bennie,
Did you say that because the submitter was the chairman of CMP 5?
don

 

[bennie]

Re: 250.81

It is clear to me that the original description does not indicate more than one grounding electrode. The editoral change does approve more than one.

Please explain how the original description suggests more than one conductor to the electrodes.

 

[bennie]

Re: 250.81

Ed: The normal neutral load current will take every path available to return to the source.

Place an impedance value on each earth point and on the neutral conductor. The current will divide according to this impedance value.

Current flow from the neutral to ground is common mode current. Common mode current produces EMF.

Differential mode current is a circuit with balanced current. A balanced current circuit will have equal and opposite magnetic fields. The EMF is neutralized.

When common mode current is present in the ground system, the same amount is present in the active line side.

This is objectionable current that can be reduced by proper design. All common mode current can not be eliminated.

[ May 02, 2003, 10:17 PM: Message edited by: bennie ]

 

[don_resqcapt19]

Re: 250.81

Bennie,
It doesn't say that you can have more than one GEC in the 96 code, but it also doesn't say that you can't have more than one. The submitter wanted to make it clear that more than one GEC is permitted so he sent a proposal saying that. Even the code handbooks edited by Joe McPartland for 1996 and earlier codes show the use of multiple GECs.
Don

 

[bennie]

Re: 250.81

1984 Edition of McPartland handbook. Page 383,& 384

When two or more grounding electrodes are to be combined into a "grounding electrode system" The size of the bonding jumper between electrodes shall not be smaller than indicated on table 250-94.

The unspliced grounding electrode conductor may be connected to which ever interbonded electrode that is most convenient.

A bond conductor is a short circuit, a ground electrode conductor is an inductor. This is a recognized fact in the science of electricity.
There is an IEEE standard to this effect. I will try to locate it.

 

[bennie]

Re: 250.81

The schematic shows the ground electrode conductors bonded(shorted) not the ground electrodes.

 

[don_resqcapt19]

Re: 250.81

Bennie,
Look at the figure on page 387 of that book. This figure shows one GEC to the water pipe and a second GEC to the ground rod. Neither the text in the McGraw-Hill handbook or the NEC itself states that there can only be one grounding electrode conductor.
Don

 

[don_resqcapt19]

Re: 250.81

Bennie,

A bond conductor is a short circuit, a ground electrode conductor is an inductor.

I still don't understand that concept. How does adding a splice to a wire make it an inductor?
Don

 

[wocolt]

Re: 250.81

Current flow from the neutral to ground is common mode current. Common mode current produces EMF.

All good points, is it not true that system itself is its worst enemy. How long have we been grounding systems.
Everytime we go to the waterline, and drive ground rods we create a parallel path for the neutral return current and by that alone we effectively have created a condition where we are actually standing everyday on the neutral.
Is ther a solution ? an ungrounded system ? where do we start?

WOC

 

[Ed MacLaren]

Re: 250.81

Place an impedance value on each earth point and on the neutral conductor. The current will divide according to this impedance value.

I have chosen some arbitrary values for the grounding path components. They are DC resistance values. 60 Hz impedance values would be a little higher.

This is objectionable current that can be reduced by proper design.

Questions -
1. What design changes would reduce the current returning to the source through the grounding system? (common mode current)


2. How would a single conductor between the service neutral buss and the grounding electrode system reduce the portion of unbalanced load current returning to the source through the grounding system? (common mode current)

3. Is this amount (12 milliamps out of 100 amps) of current on the grounding system objectionable?

Ed

[ May 03, 2003, 02:39 PM: Message edited by: Ed MacLaren ]

 

[wocolt]

Re: 250.81

As usual Ed great graphics and illustration, only problem is the Delta infront of everything.
I see alot of the problem as being generated by the utility company insisting that they ground the primary neutral to the secondary neutral. Even if you have a balanced load on the secondary there still primary neutral current.
I think Bennies' whole point is the grounded system, in that it makes its own problems just by virtue of being grounded.

WOC

 

[bennie]

Re: 250.81

wocolt: You are correct. My point is the currents and voltages developed on the ground system, are more complex than just sticking a wire in the dirt.

The two conductors, and ground points, will produce voltage gradients in the surrounding terrain, during a fault such as a HV line contact.

Current from a lightning strike will divide and create a voltage gradient.

A fault on the primary distribution system will produce current flow in both paths.

In many cases the water pipe current will be high and should be kept outside the building. The water pipes are the largest source of EMF. This is why the 5 foot connection point.

All conductors with common mode current flow should be routed as carefully and designed to channel the EMF's generated, to outside locations.

ANSI/IEEE C57.105-1978, Paragraph 2.1.2. Types of sources. The effectively grounded distribution system includes a solidly grounded neutral conductor derived from the source. Usually the neutral is multigrounded along the feeder.
Such systems are referred to as "grounded Y" systems.

This system will produce common mode current on all ground points. All distribution systems are grounded wye, except in unique situations.

 

[bennie]

Re: 250.81

Don: You are correct the handbook does show a separate ground conductor to each of the electrodes.

This was before the 5 foot water pipe rule. The 5 feet is for bonding the ground electrodes by interconnecting them, making them one shorted electrode, and one current path.

I think the same person who said "the 5 feet is due to the possibility of changing the interior piping to plastic", is the same one who says "the reason for 3 wires to a range is because of the war". WAG without research.

 

[iwire]

Re: 250.81

Originally posted by bennie:
The 5 feet is for bonding the ground electrodes by interconnecting them, making them one shorted electrode, and one current path.

My take on this was to prevent one grounding connection near the meter and another at some hidden point in the dwelling.

This could present a hazard to anyone that cuts into the piping for repair on changes.

With the 5' rule all grounding connections to the piping should be in sight of each other.

But this is purely an assumption.

 

[bennie]

Re: 250.81

Bob: We need the 1992 panel comments. This change was likely done by the Technical correlation committee, with no explanation.

I am basing my findings on the papers of Kirchoff.

 

[don_resqcapt19]

Re: 250.81

Proposal 5-141 for the '93 code was to add the following to 250-81 after the existing first sentence:
"Interior metal water piping is not to be used as a conductor to connect the required grounding electrodes and their conductors together.
Ex. In industrial buildings where the entire length of the water pipe that is being used for the conductor is visible."
Substantiation: "Electricians do not have total control of the water piping system. Many unqualified persons may modify the water piping system. By allowing the water piping to serve as the only conductor, it may be possible to isolated the required grounding electrode conductors which is a hazardous condition. Sometimes this happens in concealed areas not visible on reinspections."
Panel Action: Accept in Principle.
Change the new sentence as follows: "Interior metal water piping shall not be used as a conductor to interconnect the electrodes and the grounding electrode conductor. Number existing exception as Exception #1, and add new Exception #2 as follows: Exception #2. In industrial and commercial buildings where conditions of maintenance and supervision assure that only qualified persons will service the installation and the entire length of the interior metal water pipe that is being used for the conductor is visible.
Panel Statement: "Panel believes the recommended change to 250-81 was too restrictive not to include industrial and commercial buildings, but the panel feels that supervision and maintenance is necessary for this provision.
Vote on Panel Action: Unanimously Affirmative.


Comment 5-70Recommendation. The proposal should be accepted in principle, reworded as follows: Interior metal water piping greater than 5' from the point of entrance into the building, measured along the pipe, shall not be used as the deliberate grounding path between a grounding electrode and the earth, or between any two grounding electrodes.
Exception #2: In industrial and commercial buildings where conditions of maintenance and supervision assure that only qualified persons will service the installation, and in residential buildings where the entire length of the metal water piping that is used as a conductor is exposed, interior metal water piping shall be permitted to serve as a grounding path.
Substantiation: The panel wording supposes that the internal water piping is not interpreted as an extension of the contact area of the electrode. Many would so understand it. This comment avoids that interpretive problem while allowing a clear area for connections, very tightly defined.
In addition, the exception is somewhat liberalized to cover all pipes in industrial/commericals with qualified supervision, and residentials with the pipe exposed, not visible. Exposed means that a suspended ceiling could cover the pipe, which does not seem unreasonable. If the panel chooses not to accept the residential allowance, the word "exposed" should still replace "visible" in the original.
Panel Action: Accept in Principle in Part.
In the panel action of proposal 5-141 revise to read as follows: "Interior metal water piping located more than 5' from the point of entrance to the building shall not be used ... " Also revise EX. #2 in the Panel Action as follows: Change the last word "visible" to "exposed".
Panel Statement: CMP-5 rejects the part in the commenter's proposed Ex. #2 that includes residential buildings. See Panel Action and Statement on Comment 5-73. The panel believes that the panel action otherwise meets the submitter's intent.
Vote on Panel Action: Unanimously Affirmative