250.81

[iwire]

Re: 250.81

I bought a house water filter and was surprised and impressed that it came with 2 good quality ground clamps, a piece of green (should it be green? ) 6 AWG prestripped and instructions to install these before cutting the pipe.

I can only wonder what made them do this, what kind of accident?

 

[don_resqcapt19]

Re: 250.81

Ed,
In areas where there is a common undergound metal water piping system, the resistance of the water pipe path back to XO will often be only 3 or 4 times that of the grounded conductor resulting in 25% to 33% of the unbalanced current flowing on the water pipe.
Don

 

[Ed MacLaren]

Re: 250.81

Don,

where there is a common undergound metal water piping system, the resistance of the water pipe path back to XO will often be only 3 or 4 times that of the grounded conductor

I agree. In the example, (sketch above) this is the only service supplied from the transformer.

The main point was the question - "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?"

Ed

 

[dereckbc]

Re: 250.81

Ed great drawing. Two comments.

1. R2, R3, and R8. The resistances are so low and insignificant I think it could have been left out to simplify the circuit.

2. R6 of 100 ohms for earth I think is an error. Shouldn't it be common point just like the junction of R4 & R5? You already established the earth resistance with R4, R5, and R7 This would raise your ground current to .039 amps. Still no problem

Bennie, by establishing two separate electrodes only lowers the impedance. So what? If you went in a serial fashion rather than parallel you accomplish the same thing except a slightly higher resistance from the series connections. Since it is not paired with current carrying conductors there is going to be EMF either way you do it.

[ May 03, 2003, 04:03 PM: Message edited by: dereckbc ]

 

[bennie]

Re: 250.81

Dereck: By having only one ground electrode conductor, there is only one source of EMF to resolve. The one conductor is likely run outside the building. Two or more conductors will spread the EMF to a larger area.

Don: Thanks for the documents. There is more to this than the items in the papers. I will stick my neck out...I bet the submitter was either a member of, or the code panel itself.

Ed: The common mode current, in the ground side, will have an effect on the service entrance conductors.

 

[don_resqcapt19]

Re: 250.81

Bennie,
5-141 was by Mark Shapiro and James Worden. The comment was by Fred Hartwell. None were on CMP 5 for that code cycle. Hartwell was on CMP 9 at that time.
Don

 

[bennie]

Re: 250.81

Thanks Don, this is what I thought. There is more to this issue, than I am prepared to address at this time.

Ed: The ground wires are all bonded(shorted)together at the neutral/ground bus. The ground electrode conductors are separately feeding two independant electrodes.
Bond the two electrodes at the earth level, and remove the second conductor. This prevents a multi-ground point and resultant ground loop.

 

[Ed MacLaren]

Re: 250.81

R6 of 100 ohms for earth I think is an error. Shouldn't it be common point?

There seems to be many opinions about whether the earth path between two electrodes actually has resistance or not.
An example is this quote in some material from Chance, a division of Hubbell,

“The earth’s resistance is by far the major component of resistance in a grounding system. So,
from an electrical standpoint, galvanized and
copperclad rods are nearly identical.”

but they could be referring to the rod-to-earth contact resistance.

Because the specific path that a current would follow between two given electrodes cannot be isolated from the body of the earth, it’s resistance can’t be measured.
I am also of the opinion that it would be quite low, because of the almost infinite number of “parallel paths” between the electrodes.

I showed it as 100 ohms in the example (sketch) because I thought there would be many arguments against not indicating it at all.

And, like Dereck said, even when the 100 ohms was omitted, the current in the grounding system was less than .5 amps.

Ed

 

[Ed MacLaren]

Re: 250.81

Bennie,
I know we've flogged this horse before, but here goes again for the new guys.

I don't see that as a multi-point ground.

A multi-point ground would only exist if grounding electrodes were connected to the grounded (neutral) conductor at more than one point.

In other words, the term "single-point grounding"
refers to the number of connections made to the grounded (neutral) conductor, not to the number of connections made to the earth.

Ed

 

[bennie]

Re: 250.81

You're right Ed: I'll kick it again...The ground electrode conductor, equipment ground, and ground/neutral are bonded (shorted) at the service bus, not grounded.

The only grounding is the earth itself, and I count two locations.

 

[bennie]

Re: 250.81

The earth is a common impedance. Only a low impedance ground plane can bond two or more electrodes.

The earth connection can be 10 ohms, a metal ground plane can be 100 milli-ohms across a 10 meter span.

Two contact points, across a common impedance, will have a different potential and result in current flow.

[ May 04, 2003, 02:41 PM: Message edited by: bennie ]

 

[bennie]

Re: 250.81

To further substantiate the schematic as being multi-point grounding.

Feature a lightning strike on the lines. The charge will divide to earth, in proportion to the individual impedance of the two electrodes.

The voltage gradient will be lethal, and has killed many cows on farms. One earth connection would produce an elevated voltage but not a difference in potential.

This drawing is showing the neutral being grounded at two locations.

 

[Ed MacLaren]

Re: 250.81

Here's a sketch to illustrate what I meant in my last post.

I don't believe a "ground loop" exists unless it includes a "normal-current-carrying" circuit conductor.

Ed

[ May 04, 2003, 11:30 AM: Message edited by: Ed MacLaren ]

 

[bennie]

Re: 250.81

Ed: The drawings are electrically the same.

 

[bennie]

Re: 250.81

Run the conductor from the water pipe to the rod. Remove the conductor from the panel to the rod. Now analyze the circuit.

 

[wocolt]

Re: 250.81

From the latest drawings are we not going back to the definition of a service panel as opposed to that of a subpanel ?
If in the same building the service is already grounded at the disconnect and the distribution panel becomes a "Virtual" Sub-panel only without the ground rod.
If in a separate building now add the ground rod.
The whole debate is about objectionable current unless we change the entire system that ground current will always be there, the Common Mode current, the EMF generator type, and that would have to start with the utility. I really believe NOW that the extra ground rod, '99 code, was highly influenced by the collective utility board
on the code making panel. It was a quick and cheap way to increase the earth as a conductor for return currents to their source, without having to increase the size of their existing neutals.
I mean why do we drive ground rods, the common answer is for lightning. How often do they work as far as shunting a lightning strike to ground as is the premise for the ground rod. In fact when in Fredricksburg National park, the government went so far as to ground trees( they were as old as the battlefield) The cable from the top of the tree was the braided type, all copper to a huge ground rod,in an effort to preserve them and some still got struck.

Just my two cents

Wm.Colt

 

[bennie]

Re: 250.81

The concept of single point grounding pertains only to the earth connection point.

The equipment ground conductors return over a separate set of conductors to a central earth point.(a star connection to a Mecca).

The ground conductors are bonded(shorted) at the service bus, for fault clearing. The earth connection is for zero reference.

 

[bennie]

Re: 250.81

The bonding (shorting) effect, of the equipment ground conductors, have no bearing on the earth connection.

To establish a reference point, there can only be one earth connection point, not two or more.

[ May 04, 2003, 01:40 PM: Message edited by: bennie ]

 

[dereckbc]

Re: 250.81

Ed, your last drawings are correct examples of single-point and multi-point grounding. With the single-point system you drew, it is possible for common mode current to flow between the two ground electrodes, but as long as the two electrode terminations are made adjacent to each other, any voltage developed would not have any effect, as the neutral and EGC's would float above this point and not interfer with any downstream equipment operations.

In your multi-point drawing, the the common mod current could flow between the two electrodes and develop some voltage that might be seen downstream from the N-G bonding point, and possible cause problems in sensitive equipment. However it is completly code compliant and from a safety perspective it is effective. It is only from a performance/design perspctive that issues could be raised, which is not in the scope of NEC

 

[bennie]

Re: 250.81

The two earth connections, referred to as one connection, can only be considered one point, when connected to a ground plane.

The earth is not a low impedance ground plane.

[ May 04, 2003, 02:28 PM: Message edited by: bennie ]