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3 minutes ago, Marc said:

No.  Neutral is always a grounded conductor.

Why?

And then let me push back a little bit.  From the NEC, the definition of neutral conductor and neutral point are:

Neutral Conductor: The conductor connected to the neutral point of a system that is intended to carry current under normal conditions.

Neutral Point: The common point on a wye-connection in a polyphase system or midpoint on a single-phase, 3-wire system, or midpoint of a single-phase portion of a 3-phase delta system, or a midpoint of a 3-wire, direct current system.

FPN (fine print note):  At the neutral point of the system, the vectorial sum of the nominal voltages from all other phases within the system that utilize the neutral, with respect to the neutral point, is zero potential. 

These definitions don't seem to require any type of grounding.  At the neutral point of a standard 120/240 system, where our "neutral" conductor attaches at the transformer, isn't the vectorial sum of the voltages from the other phases equal to zero, regardless of whether or not this conductor is grounded?

 

neutral point.png

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Grounding has nothing to do with the lack of a voltage vector on the neutral of a balanced configuration.  Not all configurations are balanced. A three wire delta configuration with the neutral taken at the center-point of one of the legs (which creates a high leg) is a common example of an unbalanced system.

Neutral grounding requirements are found elsewhere in the NEC.

Edited by Marc
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Vector math becomes very simple when done graphically. Vectors are just magnitude, like any ordinary number, but with a direction.  A car going North at 60 mph is a vector. 60 mph is the magnitude and North is the direction.

https://www.khanacademy.org/math/precalculus/x9e81a4f98389efdf:vectors/x9e81a4f98389efdf:vector-add-sub/v/visually-adding-and-subtracting-vectors

 

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1 hour ago, Marc said:

Grounding has nothing to do with the lack of a voltage vector on the neutral of a balanced configuration. 

Yes, but that's my point.  Or my question.  Or something.  Grounding has nothing to do with whether or not the point X0 in my diagram is the neutral point.  If that's true then the conductor coming from that X0 point is neutral whether or not the system is grounded.  

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8 hours ago, SNations said:

Would what we colloquially call the "neutral conductor" still be neutral if it wasn't grounded?

Steve

Before 2008, the XO point on your drawing would not have been considered a neutral point and the wire connected to it would have been properly called the "grounded conductor." There's nothing particularly "neutral" about that conductor (unless it happens to be part of a multi-wire circuit). However, *everyone* called it the "neutral" so the NEC decided to go with the flow and adopt the name in 2008. 

In a single-phase, 120/240 volt system that conductor must be grounded and it may be called the neutral. Of course, the presence or absence of grounding doesn't change it's "neutralness," which it doesn't really have in the first place. 

A better question might be why the ungrounded conductor is called a neutral when it's not neutral. (And the answer is: because everyone calls it that.) 

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Thanks for the reply.

6 hours ago, Jim Katen said:

Before 2008, the XO point on your drawing would not have been considered a neutral point and the wire connected to it would have been properly called the "grounded conductor." 

I understand that this definition was added in 2008, but nothing physically changed then, right?  So if this was a neutral point in 2008 then it was a neutral point in 2007, wasn't it?  But specifically to the point, doesn't it look like at the neutral point the vectorial sum of the nominal voltages from the two live phases is zero?

6 hours ago, Jim Katen said:

 There's nothing particularly "neutral" about that conductor (unless it happens to be part of a multi-wire circuit).

But isn't that the case?  Isn't the service entrance just a big multi-wire circuit?  That seems to be the crux of the issue.

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Jim, your reply actually helped me quite a bit to understand this better.

But I might not have been clear in my original question, if so I'm sorry about that.  I'm not so much interested in some random "neutral" wire running through your house.  I'm interested in the grounded service entrance conductor running from the transformer in to your service panel.  Would that conductor still be neutral if it wasn't grounded?  

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  • 2 weeks later...

Steve, to add more confusion to your question. The grounded conductor is not always a "neutral". This photo is of a corner grounded delta 3 phase panel. The white marked wires on the "neutral bar" are actually grounded phase conductors for this 3 phase system. It is required to be marked white per code because it is a "grounded conductor". It however, is not a neutral. It supplies the third leg of this system to power 3 phase equipment. Just thought I'd share.

Corner grounded delta photo.docx

Edited by mtwitty
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2 hours ago, mtwitty said:

Steve, to add more confusion to your question. The grounded conductor is not always a "neutral". This photo is of a corner grounded delta 3 phase panel. The white marked wires on the "neutral bar" are actually grounded phase conductors for this 3 phase system. It is required to be marked white per code because it is a "grounded conductor". It however, is not a neutral. It supplies the third leg of this system to power 3 phase equipment. Just thought I'd share.

Corner grounded delta photo.docx 320.13 kB · 3 downloads

Why would anyone choose a corner-grounded 3 phase delta configuration?

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Good question Marc. Probably it was used to save the cost of the extra wire. Only need 3 as opposed to 4 for a "high leg". It also was considered safer than an ungrounded delta, that is often used in industrial installations. This link is to a good article on this subject.

https://iaeimagazine.org/magazine/2002/01/16/installations-and-inspections-of-corner-grounded-systems/

 

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I'm not quite as deep as Marc and others on this subject, but while I understand you are trying for a deeper understanding of "neutral" in theory, the reality is in residential (not  3 phase) the service neutral is ALWAYS grounded both by definition and practice. Even if not grounded in a particular house service, the system is grounded at other houses, pole, etc. For my sanity I let is stop there.

Edited by inspector57
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On 11/20/2019 at 8:21 PM, mtwitty said:

Steve, to add more confusion to your question. The grounded conductor is not always a "neutral". 

 

This was not part of the original question that prompted my post, but now I'm curious about this:

I fully understand the ins-and-outs of equipment grounding with a standard residential 120/240 volt single phase system.  Basically you're connecting a wire from the appliance case back to the neutral bus bar in the main panel.  (We can skip the details.)  And we can do that because the grounded conductor is both grounded and neutral.  But what about a 3-phase system where the grounded conductor isn't necessarily neutral?  You can't just connect a wire back to this grounded conductor if it's not neutral, right?   Or how about in places that use a single phase system that isn't even a grounded system?  I don't think that I understand how equipment grounding works in those cases.

Any help?

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1 hour ago, SNations said:

This was not part of the original question that prompted my post, but now I'm curious about this:

I fully understand the ins-and-outs of equipment grounding with a standard residential 120/240 volt single phase system.  Basically you're connecting a wire from the appliance case back to the neutral bus bar in the main panel.  (We can skip the details.)  And we can do that because the grounded conductor is both grounded and neutral.  But what about a 3-phase system where the grounded conductor isn't necessarily neutral?  You can't just connect a wire back to this grounded conductor if it's not neutral, right?   Or how about in places that use a single phase system that isn't even a grounded system?  I don't think that I understand how equipment grounding works in those cases.

Any help?

Equipment grounding conductors in intentionally ungrounded systems are designed to actuate an alarm (ground detect devices). They do not cause overcurrent devices to open as they do in grounded systems. The purpose is to allow continuation of operation when one phase develops a ground fault. The grounded leg (fault) is at ground potential, so is not necessarily a shock hazard to personnel. The alarm should be addressed and the fault found and repaired ASAP. If a second phase develops a ground fault, this causes a line to line short and shuts down via the OCPD. Ground faults are common in industrial facilities and this ungrounded system is used to allow uninterrupted production until the fault is found. It's not the safest system, but is common. The "corner grounded" system like in the photo I posted intentionally grounds one phase leg and will open the OCPD if either of the other 2 ungrounded phase legs have a ground fault. (line to line short) An ungrounded system basically becomes a corner grounded system if one leg develops a ground fault. As far as single phase systems that are not grounded....that would be a faulted system and not by design. I know this is way over the top for residential and home inspection discussion but it is good to have a basic understanding of IMO. If you look at the photo I posted, at a glance, it looks like a single phase sub panel that we see everyday. Only giveaway is all 2 pole breakers and some blue wires. Although very uncommon, this could be present in an inspection of a commercial property. Hope this makes some sense and answers your question. I apologize for side-tracking this thread. Didn't mean to get so deep in this. 

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8 hours ago, mtwitty said:

Equipment grounding conductors in intentionally ungrounded systems are designed to actuate an alarm (ground detect devices). They do not cause overcurrent devices to open as they do in grounded systems. The purpose is to allow continuation of operation when one phase develops a ground fault. The grounded leg (fault) is at ground potential, so is not necessarily a shock hazard to personnel. The alarm should be addressed and the fault found and repaired ASAP. If a second phase develops a ground fault, this causes a line to line short and shuts down via the OCPD. Ground faults are common in industrial facilities and this ungrounded system is used to allow uninterrupted production until the fault is found. It's not the safest system, but is common. The "corner grounded" system like in the photo I posted intentionally grounds one phase leg and will open the OCPD if either of the other 2 ungrounded phase legs have a ground fault. (line to line short) An ungrounded system basically becomes a corner grounded system if one leg develops a ground fault. As far as single phase systems that are not grounded....that would be a faulted system and not by design. I know this is way over the top for residential and home inspection discussion but it is good to have a basic understanding of IMO. If you look at the photo I posted, at a glance, it looks like a single phase sub panel that we see everyday. Only giveaway is all 2 pole breakers and some blue wires. Although very uncommon, this could be present in an inspection of a commercial property. Hope this makes some sense and answers your question. I apologize for side-tracking this thread. Didn't mean to get so deep in this. 

Thanks for responding.  I learned something.

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  • 2 weeks later...
15 hours ago, Chad Fabry said:

Corner grounded three phase in a single phase panel. Notice they re-identified the ground wire black.

corner grounded three phase.JPG

Yep, the grounded conductors should be identified white or gray. They are not neutrals, but are required to be identified as current carrying grounded conductors.

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Marc, no 120 volt is available in a 240 volt corner grounded delta configuration. 240 volts from each phase to the other, including the grounded phase.

That's what threw me for a loop. I metered the panel and there was 240v between any of the terminals. Luckily, Douglas Hansen answered his phone and told me what I was looking at- once he explained it, I sort of understood it- I had to see the schematic to get it.  

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