(Earth) grounding transformers

[sandsnow]

Re: (Earth) grounding transformers

Originally posted by charlie b:



Does this answer your question?

Yes, thank you

I agree with Don about using the EGC as a GEC especially when the XFMR is right next to the service equipment. Why should the GEC run all the up to Bldg Steel, when the UFER is sticking out of the slab two feet away? Answer, only to answer the letter of the Code.

 

[benaround]

Re: (Earth) grounding transformers

EGC as GEC, When you draw that out on paper it sure don't look like the ground path back to the source that NEC is talking about to me !

 

[hurk27]

Re: (Earth) grounding transformers

By benaround: EGC as GEC, When you draw that out on paper it sure don't look like the ground path back to the source that NEC is talking about to me !

There has to be a fault path back to the primary source ran with the primary conductors. The secondary conductors fault path is only to this transformer as this is the new source for the secondary conductors so there will be a EGC with them as well. The requirement for bonding to building steel or a ground rod is the same requirement we have for services because each SDS is a new source.

But I do believe that the exception to 250.30(A)(4)(3) Allows the EGC feeding the SDS to also be used for the GEC when the SDS is fed from a (service equipment) rated panel that has a GEC connected to the EGC. as long as the primary EGC for the SDS is not smaller than the required GEC for the secondary which would have to be sized to table 250.66.

If we look at ARTICLE 100 Definitions for Grounding Conductor, Equipment. or Grounding Electrode Conductor. we will see where each of these conductors can be both at the same time.

[ December 28, 2004, 02:14 AM: Message edited by: hurk27 ]

 

[don_resqcapt19]

Re: (Earth) grounding transformers

Ben,

EGC as GEC, When you draw that out on paper it sure don't look like the ground path back to the source that NEC is talking about to me !

The GEC has nothing to do with the fault clearing path...it is just a connection to earth. The fault clearing path on the secondary side of a SDS(transformer) is..fault to EGC..to bonding jumper...to transformer XO. A small amount of fault current would flow on the GEC as the result of parallel paths, but the GEC is not an intentional part of the fault clearing path. The primary EGC would be used to provide the secondary connection to earth. If the SDS is in the same building as the service, the most current that the primary EGC should ever see would be from a fault of the primary ungrounded conductors. It is sized to handle that current per 250.122. I see no electrical reason to require a larger GEC for the secondary of this transformer.
Don

 

[hurk27]

Re: (Earth) grounding transformers

By Don: I see no electrical reason to require a larger GEC for the secondary of this transformer.

What would happen if the secondary did happen to short to a water pipe or building steel or some other metal that is only connected at the main service to the primary EGC as a GEC. This fault path would allow the secondary fault current to be placed on the primary EGC. It's a far reach but it's the only one I can think of as to why this requirement was put in the exception I posted?

 

[benaround]

Re: (Earth) grounding transformers

hurk27 -----,the secondary conductors fault path is only to this transformer as this is the new source for the secondary conductors,so there will be a EGC with them as well.--- This is all good info,as long as you go from XO to building steel,not primary EGC ,for transfomer GEC. Anyway,My problem with using primary EGC is the fault path for the secondary conductors.For example:secondary conductor "somehow" comes into contact with the "sprinkler system pipe" .The way I see it,the fault path using the EGC for GEC method would be--sprinkler pipe to primary bonding conductor,thru the primary equipment ground buss,to EGC,to XO source? Using the Building steel method I see the path as--sprinkler pipe,to steel, to XO?? How much current would travel over EGC in this case. hurk27,Don19, let me add that I have the highest respect for both of you guys,this is not a thing about being "right",It's about Me getting it right. Thanks,Frank

 

[dereckbc]

Re: (Earth) grounding transformers

I have read through this post, and will weigh in my two cents worth. The question has been answered in bits and pieces with some reasonable disagreements. IMO the a transformer xo is bonded for the exact same reasons the service is, most importantly

Lightning protection.
Voltage stabilization.
Accidental contact with higher voltages, primary/secondary failure, or high current ratio transformers.

All of these have been discussed to some degree with varying opinions as to whether they are needed or not. I think what has to be kept in mind is the code has to cover all installation variables.

Start with lightning protection. True in high-rise or otherwise protected environments, lightning is not an issue. But from a code perspective it could be anywhere from outside to being protected inside. No way of knowing from a code writing position. CYA

For system stabilization, the EGC would short out any stray capacitance and suffice, so no real issue here other than it needs a dirt reference with all other parts properly bonded.

My main issue is with accidental contact, primary/secondary failure, or high current ratio transformers. Who says the transformer is a 480/208 device? Why not 4160/208 of something with higher current ratios? If that is the case, the EGC would not have the thermal capacity to clear the fault IMO, therefore a GEC based on secondary conductor size is needed. The other point as Wayne mentioned is it is possible for the secondary feeders to come in contact with building systems that are not intentionally bonded the transformer secondary, so a planned secondary path is warranted.

So to sum up the NEC is trying to cover all possibilities. Otherwise article 250 would be a book in itself to cover all the variables one could find.

[ December 28, 2004, 08:47 PM: Message edited by: dereckbc ]

 

[don_resqcapt19]

Re: (Earth) grounding transformers

hurk,

What would happen if the secondary did happen to short to a water pipe or building steel or some other metal that is only connected at the main service to the primary EGC as a GEC.

250.104(A)(4) covers the water pipe part of your example. 250.104(C) could be changed to require local steel bonding.
Don

 

[don_resqcapt19]

Re: (Earth) grounding transformers

Dereck,

If that is the case, the EGC would not have the thermal capacity to clear the fault IMO,

What type a fault are you talking about?
Don

 

[don_resqcapt19]

Re: (Earth) grounding transformers

Dereck,

But from a code perspective it could be anywhere from outside to being protected inside.

I tried to address that issue in my proposal(5-99)on this subject for the 2005 code. Here is the text that I proposed to add to 250.30(A)(4).
"Exception No. 2: Where a transformer is used as a separately derived system and where the transformer primary feeder circuit originates in the same building or structure, a grounding electrode shall not be required."
Don

edit to add left out word

[ December 29, 2004, 07:14 AM: Message edited by: don_resqcapt19 ]

 

[hurk27]

Re: (Earth) grounding transformers

Don I was commenting on your response to Pierre:

The question at this point that begs answering is: what if there is no means of grounding in the vicinity of the transformer (no steel,cold water, etc...), and a grounding electrode conductor has to be installed 3 floors downstairs to ground rods outside? What does this really accomplish?

Pierre

Your response:

Pierre,
For the type of installation that you have described, I see no electrical reason(only a code reason) why the primary EGC cannot be used as the secondary GEC . I submitted a proposal about this but CMP5 rejected it.
Don

And what I was trying to convey:

But I do believe that the exception to 250.30(A)(4)(3) Allows the EGC feeding the SDS to also be used for the GEC when the SDS is fed from a (service equipment) rated panel that has a GEC connected to the EGC. as long as the primary EGC for the SDS is not smaller than the required GEC for the secondary which would have to be sized to table 250.66.

250.30(A)(4)(3) Exception to (1), (2), and (3): Where a separately derived system originates in listed equipment suitable for use as service equipment, the grounding electrode used for the service or feeder shall be permitted as the grounding electrode for the separately derived system, provided the grounding electrode conductor from the service or feeder to the grounding electrode is of sufficient size for the separately derived system. Where the equipment ground bus internal to the service equipment is not smaller than the required grounding electrode conductor, the grounding electrode connection for the separately derived system shall be permitted to be made to the bus.

And I was wondering if this exception would allow the EGC with the feeder to the SDS to be used also for the GEC back to the service GEC, as long as the feeder EGC is sized for the secondary of the SDS?

And what are all these new buttons for?

 

[don_resqcapt19]

Re: (Earth) grounding transformers

Wayne,

And I was wondering if this exception would allow the EGC with the feeder to the SDS to be used also for the GEC back to the service GEC, as long as the feeder EGC is sized for the secondary of the SDS?

I think that it does, but I see no need for a GEC larger than what is required for the primary EGC.
Don

 

[charlie]

Re: (Earth) grounding transformers

Wayne, the effect of the list start and end is to start and end a list like: </font>

• <font size="2" face="Verdana, Helvetica, sans-serif">The list item button gives you a bulleted item</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">Doing it again gives you another</font>
• <font size="2" face="Verdana, Helvetica, sans-serif">And another</font>

<font size="2" face="Verdana, Helvetica, sans-serif">List end just stops the process.

I don't know what code does except mess up the page width. I have edited out the "code" and replaced it with "quote" to fix the wide page problem.