Transformer XO Bonding

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marcus

New member
Location
Texas
As I interpret the NEC, specifically 250-30, in terms of grounding the secondary of a separately derived system, the grounding electrode conductor must terminate at the system ground (XO/neutral) directly from the building ground grid.
1) Is there any provision, or is it even wise to land the grounding electrode conductor on the equipment ground bar in the transformer, then bond XO to the ground bar?
2) While this creates a different path for phase/neutral fault current than directly to building steel, is the path not mechanically sound? What are the implications of such a set up?
I am interested in learning the reason/theory behind bonding XO directly to building steel. Any assistance would be greatly appreciated.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Transformer XO Bonding

1) I don't see any provision to make the grounding electrode connector to the grounded conductor via a bonding jumper. To do so could be considered a violation of 250.64(C).
2) As far as this part of the question, the grounding electrode conductor and the grounding electrode should not be carrying a lot of fault current. The fault clearing path should be via the equipment grounding conductor to the bonding jumper to the grounded conductor. The only time that the grounding electrode conductor should be carrying fault current in a SDS is the rare case where the fault is between the hot conductor and grounding electrode itself. In your example, this could occur if an ungrounded conductor would fault to the building steel.
Don
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Transformer XO Bonding

There are 2 locations where the GEC can be landed on a SDS

1. The Xo terminal in the transformer.
2. The neutral buss in the first disconnect device.

However if you read 250.30(A)(2). It does say "except as permitted in 250.24(A)(4). There you will find you can terminate the GEC to the ground buss if the conditions are met. The catch in my oppinion is it has to be service supplied equipment. I am sure there will be some debate on this matter. Personnaly I specify the Xo in the transformer or the neutral buss in the disconnect.

[ February 24, 2003, 01:40 PM: Message edited by: dereckbc ]
 

scott

Member
Location
Colorado
Re: Transformer XO Bonding

I believe the exception to apply to the BJ and GEC landing in the same place, not disallowing both to land at the ground bus in the SDS or disconnect.

In other words a two barrel lug on the XO of a transformer. One barrel used for the derived neutral, the other barrel for a properly sized BJ to the ground bus. All GEC's, and equipment grounds could land in the ground bus. This would be code compliant by 250.30(A)(2)(a) referring to 250.24(A)(4).

Enjoy your day!

[ March 01, 2003, 07:09 PM: Message edited by: scott ]
 

bennie

Esteemed Member
Re: Transformer XO Bonding

Gentleman: The definition in Article 100, for a separately derived system, clearly states " A premises wiring system is a separately derived system. A transformer is only the power source.
 

hornetd

Senior Member
Location
Maryland
Occupation
Journeyman Electrician, Retired
Re: Transformer XO Bonding

Originally posted by bennie:
Gentleman: The definition in Article 100, for a separately derived system, clearly states " A premises wiring system is a separately derived system. A transformer is only the power source.
Is this a point you would care to expand on?
--
Tom
 

Ed MacLaren

Senior Member
Re: Transformer XO Bonding

Originally posted by bennie:
Gentleman: The definition in Article 100, for a separately derived system, clearly states " A premises wiring system is a separately derived system. A transformer is only the power source.
What would be the point in any distinction between a "premises wiring system" and its "power source"?

What possible function could one have without the other?

I would think that the definition of any wiring system would include it's power source.

From the Merriam-Webster on-line dictionary-

system - "a regularly interacting or interdependent group of items forming a unified whole".

Ed
 

bennie

Esteemed Member
Re: Transformer XO Bonding

A separately derived system is a premises wiring system with no solid electrical connection to the utility supply.

A generator can be the source, and a component, of a separately derived system.

A transformer can be the source, and a component of a separately derived system, where the only coupling is by magnetic induction.
 

bennie

Esteemed Member
Re: Transformer XO Bonding

The March 1999 edition of EC&M Magazine has a good article about "Should you connect transformer ground to building ground?"

Two point grounding is created by transformers being called separately derived systems.

This is a gross mis-representation of electrical science and technology. There are many hazardous components to this irresponsible interpretation.

[ April 01, 2003, 10:25 PM: Message edited by: bennie ]
 

dereckbc

Moderator
Staff member
Location
Plano, TX
Re: Transformer XO Bonding

Where is the link to ECM Bennie? I would be interested in reading it. I still believe a transformer with a delta primary and wye secondary is SDS and does not constitute two grounds on the neutral.
 

bennie

Esteemed Member
Re: Transformer XO Bonding

Thanks Mike, I heard your typing, so I came back ;)

The subject of the article is exactly what I have been yelling about.

The NEC states the premises wiring, if it's a separately derived system, requires a ground electrode system.

The Handbook, and Soares Book, call a transformer a separately derived system and needs its own ground electrode.

The service ground and transformer ground make two earth connections.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Transformer XO Bonding

Bennie,
The service ground and transformer ground make two earth connections.
But these two earth connections are from different grounded conductors. There is no current flow from the service grounded conductor to the transformer secondary grounded conductor. There may be some service grounded conductor flowing in the primary EGC to earth and back to the utility transformer via the transformer grounding electrode system. This happens in all grounded systems anytime you have an EGC that is connected to any conductive object that is in contact with the earth.
Don
 

bennie

Esteemed Member
Re: Transformer XO Bonding

Don: You are correct. However an earth contact does not constitute an effective ground. It is usually a high impedance.

Building steel, that is effectively grounded, has a low impedance connection.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Transformer XO Bonding

Bennie,
Building steel, that is effectively grounded, has a low impedance connection.
So any time that we run metallic conduit on building steel we create this problem? These paths have always existed and will contnue to exist until we have the service grounded conductor bonded to earth only at the utility transformer and we install both a grounded and a grounding conductor from the utility transformer to the building. There is no other way to prevent this small amount of grounded conductor from flowing in the grounding system.
Don
 

bennie

Esteemed Member
Re: Transformer XO Bonding

Michael: Please excuse me. I am only trying to increase my knowledge to your level.
 

michael nye

Member
Location
California
Re: Transformer XO Bonding

sorry, I did not mean it to be demeaning or offensive. I am the first one that could use a little therory, and the breaking down of why we do what we do not just put that wire there and that wire there without the understanding of what's going on. Again I apologize
 

bennie

Esteemed Member
Re: Transformer XO Bonding

Can anyone explain to me why the X-O of the premises transformer must have it's own ground electrode?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Re: Transformer XO Bonding

Bennie,
That is a very good question. I have submitted a proposal that would permit the primary EGC to also serve as the transformer secondary GEC where both the service disconnect and the transformer are located in the same building.
Don
 
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