Neutral Current on a 3-wire system

[physis]

Are you saying that a transformer is not SDS?

Don, I'm only refering to the definition of SDS in Article 100. A transformer can be an SDS sourse, depeding on where it's fed from.

"A premises wiring system whose power is derived from a sourse ............. other than a service."

I'm also thinking a VFD can be treated the same as a transformer.

I guess the arguement is that a transformer fed from a service is sufficient isolation to create the "seperation" to call it seperate. Maybe it's just me, but I don't read it that way. If the power is "derived from the same sourse".

But then again, maybe it's just me. To be honest, I haven't worked with a lot of SDS's.

 

[don_resqcapt19]

Sam,
Did you spend a lot of time talking with Bennie when he was here?
It is the intent of the code that if the secondary of the transformer does not have a circuit conductor that is common to both the primary and the secondary it is a SDS. In general all transformers other than autotransformers are SDS per the NEC. I do agree that the SDS definition in Article 100 could use some work or a FPN to make it easier to understand. Maybe a FPN along the lines of the handbook commentary for that definition.

Some examples of a separately derived system may include a generator, a battery, converter windings, a transformer, and a solar photovoltaic system, provided they ?have no direct electrical connection? to another source.

Bennie's point was that there is a grounding connection between the primary and the secondary and therefore a transformer is not SDS. The grounding connection is not a circuit conductor.

 

[physis]

Sam,
Did you spend a lot of time talking with Bennie when he was here?

Oh man, as bad as I can be at times, as much as I loved Bennie's enthusiasm and grit, I'm no match. He was certainly something to be reconed with at times. Rest his soul.

It is the intent of the code that if the secondary of the transformer does not have a circuit conductor that is common to both the primary and the secondary it is a SDS.

Then why in the world doesn't the definition say that? It in fact says the opposite. Here's a definition of derived:

de⋅rive   /dɪˈraɪv/ Show Spelled Pronunciation [di-rahyv] Show IPA verb, -rived, -riv⋅ing.
–verb (used with object)

1. to receive or obtain from a source or origin (usually fol. by from).
2. to trace from a source or origin.
3. to reach or obtain by reasoning; deduce; infer.
4. Chemistry. to produce or obtain (a substance) from another.

–verb (used without object)

5. to come from a source or origin; originate (often fol. by from).

Add seperately to that.

I admit that I don't have a lot of experience with SDS's but I do know English pretty well (I mean good :grin.

In general all transformers other than autotransformers are SDS per the NEC.

Well, OK, but where in the NEC is this put forth and why does it differ, at least as far as I can figure, from Article 100? We both know how nearly unchangeable this article is, therefore, you might think it's a really, really important article.

I do agree that the SDS definition in Article 100 could use some work or a FPN to make it easier to understand. Maybe a FPN along the lines of the handbook commentary for that definition.

I think it would be better to remove the word derived rather than explain that that isn't what is meant sometimes in an FPN.

Bennie's point was that there is a grounding connection between the primary and the secondary and therefore a transformer is not SDS. The grounding connection is not a circuit conductor.

I agree with you there, the whole universe is basically grounded, or it at least is supposed to be.

 

[LarryFine]

Here's a simple SDS test: touch each hot wire to the system ground. If there's a spark, it's not an SDS.




Disclaimer: Don't actually do it.

 

[physis]

Here's a simple SDS test: touch each hot wire to the system ground. If there's a spark, it's not an SDS.




Disclaimer: Don't actually do it.

I'm sure that's the code's answer to it's definition. :grin::grin:

 

[winnie]

Sam,

My understanding of the term SDS is based upon my understanding of the requirements for grounding and bonding SDSs, and my understanding of the 'switching the neutral' requirements for generators.

I agree with your read of the article 100 definition of SDS, to the point where I am now forced to argue that the article 100 definition is not consistent with the code requirements relating to SDSs.

My understanding: 'separately derived' can really only be used to describe the mutual state of two systems. Two systems are separately derived if a single connection between them does not result in a closed electrical circuit. I agree with Bennie (as reported by Don); one you make the grounding connection, the systems are rendered no longer separate. IMHO the rules regarding SDSs are there to make sure that if you have two _potentially_ separately derived systems, you make one, but only one connection between the two.

-Jon

 

[physis]

I agree with your read of the article 100 definition of SDS, to the point where I am now forced to argue that the article 100 definition is not consistent with the code requirements relating to SDSs.

It is interesting huh.

 

[physis]

Sam,


My understanding: 'separately derived' can really only be used to describe the mutual state of two systems. Two systems are separately derived if a single connection between them does not result in a closed electrical circuit. I agree with Bennie (as reported by Don); one you make the grounding connection, the systems are rendered no longer separate. IMHO the rules regarding SDSs are there to make sure that if you have two _potentially_ separately derived systems, you make one, but only one connection between the two.

-Jon

Here's what I beleive, a seperately derived system is. And this is based on simple English. :

Two completely differnent power sources that have nothing to do with each other, aside from being grounded and bonded, cause you just plain can't escape that, so it must be.

I dare anybody to show me that an SDS and another source can originate from the same point and pass code. This is what I've been asking all along.

 

[physis]

I dare anybody to show me that an SDS and another source can originate from the same point and pass code.

I have to correct myself, I should have said "be considered an SDS".

 

[don_resqcapt19]

Here's what I beleive, a seperately derived system is. And this is based on simple English. :

We are dealing with the code here and simple English does not apply. The term is defined in Article 100 and only that definition applies.

Two completely differnent power sources that have nothing to do with each other, aside from being grounded and bonded, cause you just plain can't escape that, so it must be.

Because the primary and secondary are not electrically connected to each other they are separate systems. The key is the works "direct electrical connection" in the definition. The primary and secondary are magnetically connected, not electrically connected.

I dare anybody to show me that an SDS and another source can originate from the same point and pass code. This is what I've been asking all along.

Just look at any common transformer installation. If a transformer is not SDS, then how are we permitted to bond a secondary conductor to create a grounded system? That is prohibited by 250.24(A)(5) unless the secondary is SDS then the bonding is required in many cases by the rules in 250.20.

That being said, the definition in Article 100 could use some work. A good project for the 2014 code. I don't think there were any proposals on this subject submitted for the 2011 code.

 

[hardworkingstiff]

I'm a little dense at times, and I'm not sure I totally follow this thread/problem. As I understand it, there is 5-amps on a neutral that is not being used.

Just a thought (hope it's not dumb).

As we know, electricity takes all paths back to the source. Could it be as simple as there is enough of a path back to the "source" from the load side grounding of the transformer to allow the high side grounding to send this current back to the source through the load side grounding?

 

[physis]

We are dealing with the code here and simple English does not apply.

Don, the code is made out of English, perhaps not simple, but none the less.

The term is defined in Article 100 and only that definition applies.

Exactly my point.

Because the primary and secondary are not electrically connected to each other they are separate systems. The key is the works "direct electrical connection" in the definition. The primary and secondary are magnetically connected, not electrically connected.

The second paragraph of the definition for SDS does not make the first paragraph go away. "derived from a sourse ........... other than a service" is still there. I've already posted a definition of derived, you can't escape the fact that code is built from English.

Just look at any common transformer installation. If a transformer is not SDS, then how are we permitted to bond a secondary conductor to create a grounded system? That is prohibited by 250.24(A)(5) unless the secondary is SDS then the bonding is required in many cases by the rules in 250.20.

This has to be done because the secondary is floating until something is done about it, not because of the wording of Article 100, it's because of Article 250.

When you quoted me saying this:

I dare anybody to show me that an SDS and another source can originate from the same point and pass code. This is what I've been asking all along.

Don't forget that I had to add a correction to that.

That being said, the definition in Article 100 could use some work. A good project for the 2014 code. I don't think there were any proposals on this subject submitted for the 2011 code.

I agree with that. And I do beleive that what is intended is that a transformer be considered seperately derived, I just don't beleive that is what's said.

 

[don_resqcapt19]

...
The second paragraph of the definition for SDS does not make the first paragraph go away. "derived from a sourse ........... other than a service" is still there. I've already posted a definition of derived, you can't escape the fact that code is built from English.

There is no direct electrical connection to the service so the secondary of a transformer is derived from a source other than the service. The second sentence does not make the first go away...it only defines its meaning

This has to be done because the secondary is floating until something is done about it, not because of the wording of Article 100, it's because of Article 250.

If a the secondary of a transformer is not a SDS Article 250 prohibits the grounding of any of the secondary conductors. It is only a result of the fact that Article 100 says the secondary of a transformer is a SDS that you can ground the secondary of the transformer.

Don't forget that I had to add a correction to that.

I missed the correction, but again it is separate because there is no direct electrical connection. It really has nothing to do with the actual source of the power, which in the case of the transformer is the service. It only has to do with a power source that does not have a direct electrical connection to any other power source.

 

[physis]

There is no direct electrical connection to the service so the secondary of a transformer is derived from a source other than the service.

The lack of a direct connection isn't what makes it seperatey derived, it's only a quality of an SDS. And I disagree, the current is "derived" from a seperate sourse, I see it as being "delivered" from a seperate sourse. And that's still pushing it.

The second sentence does not make the first go away...it only defines its meaning

It may attemp to clarify it but in my opinion it fails.

If a the secondary of a transformer is not a SDS Article 250 prohibits the grounding of any of the secondary conductors.

You can't justify an article by it's reliance on another if there's a problem with one of the two.

It is only a result of the fact that Article 100 says the secondary of a transformer is a SDS that you can ground the secondary of the transformer.

It's pretty clear that we don't agree on that.

I missed the correction, but again it is separate because there is no direct electrical connection. It really has nothing to do with the actual source of the power, which in the case of the transformer is the service. It only has to do with a power source that does not have a direct electrical connection to any other power source.

Then loose the word derived and I'll agree with you.

The only reason I pick apart your posts so much is because there's always a clear logical thread to follow and it makes having an argument with you rather clean. I hope I'm not getting you mad at me Don, if I am I don't mean to. :smile: