250.6 Objectionable Current

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jxofaltrds

Inspector Mike®
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Mike P. Columbus Ohio
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ESI, PI, RBO
I have been told that my thoughts on 250.6 are unique. So I would like to hear how others read (A) & (B) below.

When does 250.6 apply? Sounds like some say never.

(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, surge-protective devices, and conductive normally non? current-carrying metal parts of equipment shall be installed and arranged in a manner that will prevent objectionable current.

(B) Alterations to Stop Objectionable Current. If the use of multiple grounding connections results in objectionable current, one or more of the following alterations shall be permitted to be made, provided that the requirements of 250.4(A)(5) or (B) (4) are met:
(1) Discontinue one or more but not all of such grounding connections.
(2) Change the locations of the grounding connections.
(3) Interrupt the continuity of the conductor or conductive path causing the objectionable current.
(4) Take other suitable remedial and approved action.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
As I read it, if you have a system with _multiple_ connections between the grounded circuit conductor and 'grounded metal' (including bonded metal, piping, etc) then you are required to prevent neutral current from flowing on that 'grounded metal', and are permitted to make various changes to the system as long as you maintain a suitable path for fault current.

The thing that you said which made me say 'hmm interesting', is applying this section of code to residential systems where there is only a _single_ ground to neutral bond in each residence, but where the transformer secondary has _multiple_ ground to neutral bonds.

The typical interpretation is that the NEC stops at the point of connection to the utility, and that the NESC applies to the service drop and transformer.

Yet the NEC applies inside the structure, and arguably this includes 'objectionable current' flowing from 'NEC land' to 'NESC land'. One could argue as follows:
The NESC supplied service conductors do not have a separate ground and neutral, and the NEC cannot force a change to the service conductors.
Multiple bonding to common metallic water piping constitutes multiple grounding connections as described in 250.6/
An effective ground fault current path is required in each residence, including on interior metal water piping, so interior piping _must_ be bonded.
Therefore some sort of dielectric must be inserted in the common metallic water piping, to eliminate the multiple grounding connections.

-Jon
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
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ESI, PI, RBO
I'm not talking about a N/G bond.

(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems, circuit conductors, surge arresters, surge-protective devices, and conductive normally non? current-carrying metal parts of equipment shall be installed and arranged in a manner that will prevent objectionable current.

Now if another Article causes objectionable current (B) Alterations to Stop Objectionable Current kicks in.

IMHO 250.6 applies to all installations. Why is it so early in the Article????? :slaphead:

What about 250.53?

(D) Metal Underground Water Pipe. If used as a grounding electrode, metal underground water pipe shall meet the requirements of 250.53(D)(1) and (D)(2).

Sounds like there may be cases where it may not be used. ?????
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
I'm not talking about a N/G bond.

What significant sources of 'objectionable current' are there other then multiple N/G bonds?

What about 250.53?

(D) Metal Underground Water Pipe. If used as a grounding electrode, metal underground water pipe shall meet the requirements of 250.53(D)(1) and (D)(2).

Sounds like there may be cases where it may not be used. ?????

Agreed. There are situations where a metal underground water pipe does not qualify as a grounding electrode, and may not be used as such. Quite likely such a pipe would still need to be _bonded_.

The suggestions in the other thread were mostly focused on rendering the underground water pipe _not_ a required grounding electrode and _not_ a continuous metallic path to other residences.

-Jon
 

mbrooke

Batteries Included
Location
United States
Occupation
Technician
I think the article is intentionally left up to interpretation, especially what constitutes as "objectionable" current because the scope of the topic is so broad, having a different meaning to different people.











Make all loads phase to phase and you could get rid of a lot of that objectionable current. Either that or rewire the entire power grid. What else are you going to do?

Ive always wondered why we never did when it comes to residential and light commercial. We have 3 wire residential service drops, and the rest of the world already uses 240 volt appliances :? Seems like it would solve a lot of issues, especially open neutrals.
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
The suggestions in the other thread were mostly focused on rendering the underground water pipe _not_ a required grounding electrode and _not_ a continuous metallic path to other residences.

-Jon

Exactly. Elimination the metal water line as an electrode. If it is the only electrode then other means are needed to comply with 250.50.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Exactly. Elimination the metal water line as an electrode. If it is the only electrode then other means are needed to comply with 250.50.

Right, but this discussion thread is about the _justification_ for elimination of the metal water line as an electrode, on the basis of 250.6.

IMHO the source of the 'objectionable current' described in 250.6 is the existence of multiple N/G bonds.

The normal resolution of the objectionable current is the removal of the multiple N/G bonds. Many other parts of article 250 prohibit multiple N/G connections, and article 250 has evolved to remove more and more potential N/G connections. See, for example, the removal from code of the use of the neutral to bond the frames of large appliances, or the use of neutral for fault current in feeders to detached structures.

The only place where multiple N/G bonds is the norm is in residential services where multiple homes share a common service transformer. Your application of 250.6 to this situation IMHO has merit but is not the norm.

-Jon
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
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ESI, PI, RBO
Right, but this discussion thread is about the _justification_ for elimination of the metal water line as an electrode, on the basis of 250.6.

IMHO the source of the 'objectionable current' described in 250.6 is the existence of multiple N/G bonds.

The normal resolution of the objectionable current is the removal of the multiple N/G bonds. Many other parts of article 250 prohibit multiple N/G connections, and article 250 has evolved to remove more and more potential N/G connections. See, for example, the removal from code of the use of the neutral to bond the frames of large appliances, or the use of neutral for fault current in feeders to detached structures.

The only place where multiple N/G bonds is the norm is in residential services where multiple homes share a common service transformer. Your application of 250.6 to this situation IMHO has merit but is not the norm.

-Jon

250.6 is about grounding.

(A) Arrangement to Prevent Objectionable Current. The grounding of electrical systems.......


We ground first -then we bond.

Not to insult but Part 1 gives us "General" rules. I look at them as guidelines.

250flow.JPG
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
I am not sure if the verbiage was like this or when it changed.

250.6. Objectionable Current Flowing Through The Grounding Path
(A) Arrangement to Prevent Objectionable Current. To prevent a fire or electric shock, the grounding of electrical systems, circuit conductors, electrical equipment, and conductive metal parts must be done in a manner that objectionable current will not flow over the effective fault current path.

Author?s Comment: Objectionable neutral current on the grounding path is often created by improper wiring of the electrical system:
An improper neutral-to-ground connection,
Errors in the wiring installation, or
Improperly using the grounding path to carry neutral current.

http://www.mikeholt.com/mojonewsarchive/GB-HTML/HTML/NECArticle250Sections250.6-250.12~20020125.htm

That is what we are discussing.
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
250.6 is about grounding.

[...]
We ground first -then we bond.

Not to insult but Part 1 gives us "General" rules. I look at them as guidelines.

Let's not get sidetracked by a semantics argument.

I agree: grounding is what we do when we connect a circuit conductor to earth electrodes.

_Bonding_ is what we do when we connect non current carrying pieces of metal together with conductors to limit voltages between those pieces of metal.

250.6 talks about grounding, not bonding.

In my defense, take a look at 250.28. What is the term used for the item used to _ground_ the electrical system?

To repeat my point without using the term 'neutral/ground bond':

Normally the source of 'objectionable current' is the _grounding_ of the electrical system at multiple different locations on the grounded conductor. This is done by multiple connections between the grounded conductor and 'non current carrying' bonded metal. These multiple connections might be from the use of the grounded conductor as the equipment grounding conductor, wiring errors, or multiple (un?)intentional connections to the grounded conductor. As the NEC has evolved, the use of the grounded conductor for fault current has been eliminated, removing many allowed forms of multiple grounding of the grounded conductor.

What, other than multiple grounding of the grounded conductor results in 'objectionable current'?

-Jon
 

Dennis Alwon

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Staff member
Location
Chapel Hill, NC
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Retired Electrical Contractor
Every commercial building I have seen has objectionable current (if you want to call it that) in that it has a parallel path between the meter and the service disconnect. The metal raceway and the neutral make a parallel path. Obviously the NEC does not see that as objectionable
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
Every commercial building I have seen has objectionable current (if you want to call it that) in that it has a parallel path between the meter and the service disconnect. The metal raceway and the neutral make a parallel path. Obviously the NEC does not see that as objectionable

Yes!
http://iaeimagazine.org/magazine/2006/05/16/current-in-the-grounded-conductor/

But it also requires us to remedy:

"What About Objectionable Current?

Article 250 includes a few key requirements that apply to grounded conductor connections that when properly applied can minimize the amount of objectionable current present in conductive paths, including the equipment grounding conductors, that are not usually intended for normal operating current. The Code addresses this objectionable current by requiring the grounding of systems, circuit conductors, surge arresters, surge protective devices, and non-current-carrying materials and equipment be installed and arranged in a manner that prevents objectionable current over the equipment grounding conductors or other grounding paths [see Section 250.6].

Section 250.6(B) provides a few alternative remedies for stopping objectionable current, but clearly requires the effective ground-fault current path required by 250.4(A)(5) and the ground-fault current path required by 250.4(B)(4) to remain intact and effective. It is important that the remedies or solutions attempted to reduce objectionable current meet the basic requirements of 250.6(B), which includes maintaining the equipment grounding conductor path. This ensures effective grounding of equipment and also ensures the integrity of the effective ground-fault current path to facilitate overcurrent device operation should a ground-fault condition develop."

Removing a GEC does not effect any see blue above^^^^^^
 

winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
The consensus of the previous discussions that I've read on this topic suggest doing something to eliminate the common metallic piping system as a viable grounding electrode, so that it need not be connected. This elimination would involve digging up pipe and replacing it with a significant length of plastic.

If we accept that 250.6 applies, then the common metallic piping system could simply be disconnected. This would certainly make it much easier (and less expensive) to remove this very common parallel neutral path.

Since the interior water piping is required to be _bonded_, then unless a dielectric union is introduced into the piping somewhere, you will have inadvertently used the water pipe grounding electrode anyway.

Taking this as a premise for further discussion: what would the safety implications be to adding a dielectric union in the water piping, _bonding_ only the interior water piping, but _not_ making a GEC connection to the incoming metallic water line?

In the event of an open neutral situation, the (presumably short) section of exterior piping inside the home would be maintained at '0V' by external connections to the neutral. The interior piping would float to whatever voltage resulted from the open service neutral. So there would be a shock hazard at this point.

Lighting ground currents could easily arc across this gap; my guess is that earth currents of one sort or another are one of the reasons for connecting _all_ available electrodes together.

Are there any other safety implications to simply disconnecting this electrode? Would the somewhat more accepted approach of rendering the underground water pipe _not_ a suitable electrode and then disconnecting it be any safer?

-Jon
 

George Stolz

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Staff member
Location
Windsor, CO NEC: 2017
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Service Manager
Yes!
http://iaeimagazine.org/magazine/2006/05/16/current-in-the-grounded-conductor/

But it also requires us to remedy:

"What About Objectionable Current?

Article 250 includes a few key requirements that apply to grounded conductor connections that when properly applied can minimize the amount of objectionable current present in conductive paths, including the equipment grounding conductors, that are not usually intended for normal operating current. The Code addresses this objectionable current by requiring the grounding of systems, circuit conductors, surge arresters, surge protective devices, and non-current-carrying materials and equipment be installed and arranged in a manner that prevents objectionable current over the equipment grounding conductors or other grounding paths [see Section 250.6].

Section 250.6(B) provides a few alternative remedies for stopping objectionable current, but clearly requires the effective ground-fault current path required by 250.4(A)(5) and the ground-fault current path required by 250.4(B)(4) to remain intact and effective. It is important that the remedies or solutions attempted to reduce objectionable current meet the basic requirements of 250.6(B), which includes maintaining the equipment grounding conductor path. This ensures effective grounding of equipment and also ensures the integrity of the effective ground-fault current path to facilitate overcurrent device operation should a ground-fault condition develop."

Removing a GEC does not effect any see blue above^^^^^^

It would make it a damned sight easier to understand what you are trying to say if you would leave quoted text alone, perhaps use an excerpt instead of the whole piece, and then write your response in full sentences under the quoted text.

There are a billion ways to configure electrical systems under the NEC. There may be times when a somewhat unusual but compliant system has unexpected consequences, and 250.6 gives us latitude to address them.

Levying it against normal mundane installations as you have pointed to is incorrect in all the ways that have been previously pointed out to you.

I have had people try to claim that a meter socket connected to a service disconnect with a metal nipple violates 250.6. Simply put, that is wrong for the simple fact that if the NEC found that to be objectionable, 250 Part V would look entirely different, banning the metal nipple.

250.6 is general because it is a general requirement in Part I. It does not replace rules in later Parts. There is no wording to back that up, such as the recipe used in Article 680 to accomplish just that.

250.6 is just as simple as it looks, and there's a reason it has dust on it; there's few occasions to put it to use.
 

jxofaltrds

Inspector Mike®
Location
Mike P. Columbus Ohio
Occupation
ESI, PI, RBO
there's few occasions to put it to use.

That is what I am talking about. The few occasions. How you got the idea that I think that is common place is beyond me.

If you have objectionable current entering the structure via the public water supply removing the water line electrode is allowed per 250.6.

If it is leaving the structure via the water lines then you probably have a neutral problem.

Either way current on the water lines should be removed.
 
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