Transformer XO Bonding

[marcus]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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.

[mikeackley]

dereck: In case bennie has logged off, here's the link. http://ecmweb.com/ar/electric_connect_transformer_ground/index.htm

[bennie]

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]

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]

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]

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

[michael nye]

Just put the gec on the xo and quit making rocket science out of it

[bennie]

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

[michael nye]

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]

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

[don_resqcapt19]

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

[bennie]

Don: I agree completely. There is no mention that a transformer have it's own ground electrode.

The code reads separately derived system shall be grounded. This is why I maintain that a transformer is not a separately derived system.

[don_resqcapt19]

Bennie,
250.30(A)(4) seems to require a grounding electode in the area of the transformer.
Don

[bennie]

Don: Yes, for a separately derived system. I fail to see the words "transformer" :confused:

[iwire]

Bennie, I know I come late to all of this and I do not mean any disrespect, but you confusing the heck out of me. :confused:

You obviously have put a lot of thought into this, if you got the chance to change things what do you see as wrong and what would you do about it?

[bennie]

Bob: I have researched the past history and origin of the separately derived system. I do not agree with the general understanding and illustrations in the Handbook, and the Soares Book.

I have no confidence in the Handbook since Joe McPartland was the author. Some of his technical descriptions have been incorrectly changed.

I have been on a crusade to correct what I believe to be a corruption of electrical science and technology.

[bennie]

Bob: The origin of a separately derived system was " Alternating-current Systems Without External connections".

In 1956 this title changed to " Separately derived systems to include generators. The other power sources in the definition are not AC sources for premises wiring.

The designation " Separately derived systems" was assigned for technical correctness and not by panel action.

Separately derived systems are premises wiring systems, not the power source. The power source is a component. The grounding requirements are for the premises wiring system not the power source.

When the premises wiring system is properly grounded, the power source will be properly grounded when connected.

There is a violation when grounding a transformer two times at two locations.

[bennie]

Please read 250.30(6) for substantiation concerning a transformer being a source for a separately derived system.

[don_resqcapt19]

Bennie,
The transformer itself is not the complete SDS, but it is the source of power for the SDS and is a part of the SDS. The required connection to ground for the SDS must be at a point from the source of power for the SDS to the first OCPD for the system. This requires a grounding electrode connection.
Don

[bennie]

Don: The first overcurrent device is the premises service. The ground electrode connection is at that point, not at the source, although it can be at the source.

A premises wiring system is all inclusive, it does not say one, or some, of the premises wiring systems.

The code making panel will rule in support of the popular belief that a transformer is a separately derived system, and requires grounding according to 250.30.

This will be a case of the the NEC changing electrical science to meet the explanation, instead of making the explanation meet the definition.

All of the substantiation documentation is in the NEC Archives, of my viewpoint. There is nothing to support the text in the handbook or Soares.

The auto-transformer reference is absurd, fictitous, and fabricated, in someone's imagination.

[ April 03, 2003, 09:34 PM: Message edited by: bennie ]