Grounding

[dnem]

I still fail to really understand why the possibilty of a path that is in parallel with the grounded conductor on the load side of the service disconnect is such a big problem, given that the code requires the creation of parallel grounded conductor paths on the line side of the service disconnect.
Don

The big problem with parallel paths is the imbalance of current. . If it's the earth itself or a steel conduit, the chances of having a problem are greatly reduced. . But if we're talking about conductors in parallel with each other, then we're talking 310.4

If the 250.32(B)(2) feeder neutral between buildings loses full contact on either end, not even a complete loss of contact but just a loose lug on one end, then the shield of the LV might pick up significant neutral/imbalanced current.

I've never heard of an inspector taking LV into consideration when considering 250.32(B)(2) and I'm not going to be the first one to do so. . I don't disagree with the concept of considering LV shields and would support it if asked, but I'm not going to jump out in front of the pack.

David

 

[don_resqcapt19]

David,

If the 250.32(B)(2) feeder neutral between buildings loses full contact on either end, not even a complete loss of contact but just a loose lug on one end, then the shield of the LV might pick up significant neutral/imbalanced current.

My problem is that the code requires the creation of these parallel paths on the line side of the service disconnect. Two common ones are the cable TV shield which is bonded at each building and makes a parallel path for grounded conductor current and the same happens with underground phone cables where there is a metallic gopher shield. If this is not a problem on the line side, where in my opinion an open gronded conductor is more likely, why is it a problem on the load side?
Don

 

[dnem]

My problem is that the code requires the creation of these parallel paths on the line side of the service disconnect.

I agree

Two common ones are the cable TV shield which is bonded at each building and makes a parallel path for grounded conductor current and the same happens with underground phone cables where there is a metallic gopher shield. If this is not a problem on the line side, where in my opinion an open gronded conductor is more likely, why is it a problem on the load side?

"If this is not a problem on the line side, ?.."
I don't think that the code is saying that it isn't a potential problem on the line side. . Multiple grounding points and parallel paths are always potential problems. . Of course 250.142(A) highlights supply or line, while 250.142(B) highlights the load and while dealing with one building or structure, that breaking point is useful.

What I believe the code panel is saying in places like 250.24(A)(2) is that the potential for problems is low enough in outside locations so as to be offset by the advantages of ensuring that the potential to earth remains at zero throughout the property.

In my opinion that?s where the thought in 250.32(B)(2) originally came from. . It isn?t so much a line or load breaking point, it?s an inside or outside breaking point. . The balance between conflicting considerations changes when you get outside.

David

 

[don_resqcapt19]

David,

... offset by the advantages of ensuring that the potential to earth remains at zero throughout the property.

That is not possible how ever you make the installation.

In my opinion that’s where the thought in 250.32(B)(2) originally came from. . It isn’t so much a line or load breaking point, it’s an inside or outside breaking point

How do you apply 250.32 if you don't have an inside or outside breaking point? The section only applies to seperate structures.
Don

 

[M. D.]

I still fail to really understand why the possibilty of a path that is in parallel with the grounded conductor on the load side of the service disconnect is such a big problem,.....Don

This is what I have been taught the reasons are.

BY MIKE HOLT

"A load side neutral-ground bond is a common

cause of power quality problems. Such a bond
creates ground loops, which allow undesired
current to circulate in the system. Power quality
problems often lead to the discovery and
removal of such a bond. But, don't wait for
power quality problems to reveal the bond.
Another concern makes corrective action
imperative. Load side neutral-ground bonds
allow objectionable current to flow on
conductive metal parts of electrical equipmentthereby
violating 250.6(A). This objectionable
current can cause lethal electric shock (Figure
250-55). And, it sets the stage for inadvertent
flashovers, overheating of equipment, and other
problems stemming from electricity in the wrong
place.
So, don't make (or allow) a neutral-ground
connection on the load side of the service
disconnect [250.24(A)(5)]. There are two
exceptions to this [250.142]. You can make this​

connection for:
Separately derived systems if you follow the
requirements of 250.30(A)(1).
Separate buildings, if you follow the
requirements of 250.32(B)(2)"​

 

[tryinghard]

If the shield of the Cable (for Cable TV) is bonded on both ends, there will be neutral current flowing on it. This is a valid metallic path.

Is it true that current will follow ALL conductive paths not just the path of least resistance?

 

[stickboy1375]

Is it true that current will follow ALL conductive paths not just the path of least resistance?

yep... .................

 

[tryinghard]

If the 250.32(B)(2) feeder neutral between buildings loses full contact on either end, not even a complete loss of contact but just a loose lug on one end, then the shield of the LV might pick up significant neutral/imbalanced current.

Actually ANY items that are conductive, and metallic, bonded to the ground system at both ends WILL have current if the neutral has returning current! The neutral does not have to be loose it can be tight!

I've never heard of an inspector taking LV into consideration when considering 250.32(B)(2) and I'm not going to be the first one to do so. . I don't disagree with the concept of considering LV shields and would support it if asked, but I'm not going to jump out in front of the pack.

Do not misunderstand this hazardous installation to be jumping in front of any pack, you would simply be doing you minimum job because this type of situation clearly and intentionally installs current where it does not belong.
250-32(B)(2),(2): ?there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved?

 

[don_resqcapt19]

M.D.,
I fully understand the reasons...I just don't buy the idea that the electrons behave differently on the line and load side of the service. This same serious hazard that the code is trying to eliminate on the load side of the service disconnect is required by the very same code on the line side of the service. If it was really an issue, we would require an EGC to be run with the service conductors and only bond at the utility transformer.
Don

 

[peter d]

M.D.,
This same serious hazard that the code is trying to eliminate on the load side of the service disconnect is required by the very same code on the line side of the service. If it was really an issue, we would require an EGC to be run with the service conductors and only bond at the utility transformer.
Don

Do you suppose it's because parallel paths are considered to be "less dangerous" outdoors than indoors? (neglecting stray voltage issues and farm animal issues for a moment)

 

[don_resqcapt19]

Peter,

Do you suppose it's because parallel paths are considered to be "less dangerous" outdoors than indoors?

The parallel path that the code is getting rid of this cycle is outside...second buildings.
Don

 

[peter d]

Peter,

The parallel path that the code is getting rid of this cycle is outside...second buildings.
Don

I was not aware of that as I haven't been up on the '08 changes. It doesn't really make sense either way, as parallel paths are still an issue with many NEC services with a common metal water supply system and all utility installations.

 

[M. D.]

M.D.,
I fully understand the reasons...I just don't buy the idea that the electrons behave differently on the line and load side of the service. This same serious hazard that the code is trying to eliminate on the load side of the service disconnect is required by the very same code on the line side of the service. If it was really an issue, we would require an EGC to be run with the service conductors and only bond at the utility transformer.
Don

The potential for harm as a result of parallel paths is much greater on the load side

" the practical safeguarding ..."

Is there any U.E. on the line side??

I'm not sure why an EGC was never required with the service entrance conductors , good question though probably has to do with who owns what and where

 

[M. D.]

"If it was really an issue"

I think it really is an issue

 

[don_resqcapt19]

M.D.

The potential for harm as a result of parallel paths is much greater on the load side

I don't think so. The hazard is equal or greater on the line side.

I think it really is an issue

And I don't. When they change the code rule for ranges and dryers to require the use of an EGC, they tried for 3 or 4 code cycles. One of reasons that it took so long to get the change made was the the submitters could not show real world problems to the code making panel. They could only show theoretical problems. The same with this change, there was no citation of real world problems, and as a former member, Bennie, often said, "why would you want a smaller fault clearing conductor?". The EGC will be smaller than the grounded conductor and under the existing rule you are permitted to use the grounded conductor as the fault clearing path for a second building, and under the 2008 rule the only fault clearing path is the EGC.
Don

 

[M. D.]

I'm sure I have only a fraction of the knowledge that you have on this issue ,..
but this makes perfect sense to me. What you are saying does not, not your fault ,there is much to learn in the world and I have only just begun.

http://forums.mikeholt.com/archive/index.php/t-72499.html

 

[dnem]

Don,

I want to jump in and comment on some things that different people have said but first I wanted to say that your comments strike me as leaning toward questioning the need for a grounding conductor that's separate from the grounded conductor in any location.

The example was given of the dryer housing grounding being handled by the grounded neutral for so many years. . I don't think people were being shocked by dryers/ranges bonded to the neutral but the change was made anyway.

The answer is that there really isn't a need for a separate grounding conductor. . The grounded conductor can easily do "double duty" and carry unbalanced current and also carry fault current when needed.

I don't see this as a question of need, I see it as a question of better or worse. . The grounded always has the ability to carry fault current. . But is that the best option ? . If you use multiple point grounding with electrodes at multiple locations and no neutral terminations are ever loose for the life of the installation, then I think "double duty" for the grounded isn't a problem. . But do you have perfect terminations everytime ?

I've thought about 3rd world installations before and my guess is that there are places where running a separate grounding wire isn't common practice. . I'll bet that 90 to 95% of the time those installations are totally fine. . I just don't think even 95% is acceptable in 21st century USA.

David

 

[tryinghard]

I'm sure I have only a fraction of the knowledge that you have on this issue ,..
but this makes perfect sense to me. What you are saying does not, not your fault ,there is much to learn in the world and I have only just begun.

http://forums.mikeholt.com/archive/index.php/t-72499.html

Great site, Thanks! Also notice the site at the bottom of Mike Holts article: www.mikeholt.com/Powerquality/Powerquality.htm

 

[dnem]

David,

... offset by the advantages of ensuring that the potential to earth remains at zero throughout the property.

That is not possible how ever you make the installation.

Of course an absolute isn?t possible but approaching the target of totally zero is a worthwhile attempt. . That?s the point of equipotential bonding in 547.10, 680.26, + 682.33. . You never achieve total absolute equal potential but getting close is worth the effort. . If you don?t achieve zero throughout the property, that doesn?t mean you failed hitting your objective.

In my opinion that?s where the thought in 250.32(B)(2) originally came from. . It isn?t so much a line or load breaking point, it?s an inside or outside breaking point

How do you apply 250.32 if you don't have an inside or outside breaking point? The section only applies to seperate structures.
Don

Reread the sentence again. . I think you missed my point.
?It isn?t so much a line or load breaking point, it?s an inside or outside breaking point?
I?m saying that it is an inside / outside thing.

David

 

[dnem]

If the 250.32(B)(2) feeder neutral between buildings loses full contact on either end, not even a complete loss of contact but just a loose lug on one end, then the shield of the LV might pick up significant neutral/imbalanced current.

Actually ANY items that are conductive, and metallic, bonded to the ground system at both ends WILL have current if the neutral has returning current! The neutral does not have to be loose it can be tight!

Your terms ANY and WILL are correct but notice that I used the word ?significant?. . Accounting for every conceivable small stray current isn?t necessary. . It?s only if there?s a problem with a connection in the 250.32(B)(2) neutral that you get significant current flow on other paths.

This goes back to the inside / outside discussion. . When outside, multiple point connections to earth are standard procedure and with multiple points inevitably come multiple paths. . But most of those paths will have too high of a resistance to be more than a small stray current that is no shock hazard. . It can screwup electronic equipment and/or data streams but it isn?t a shock hazard.

There?s pros and cons each way and no 100% that exists without any conflicting results. . best way that works everywhere. . In the end analyses, when outside, multiple point bonding has considerable advantages that outweigh the disadvantages. . When inside, single point bonding has considerable advantages that outweigh the disadvantages.

David