Grounding Separately Derivied Systems Article 250.30

[wellke]

In an older existing precast concrete building (no effectively grounded structural steel)with all of the interior piping insulated can the primary equipment grounding conductor (EG) if sized correctly per table 250.66 be used as the grounding electrode conductor (GEC) for the secondary XO connection. This proposed GEC would connect to the service grounding bus bar and the ground bus for the service is not smaller than the required grounding electrode conductor.
What do you all think is this a code compliant system?

 

[bennie]

Re: Grounding Separately Derivied Systems Article 250.30

I promised to leave this subject alone, but I'm obsessed.

The procedure you describe is the only correct way to ground the X-O, core, and enclosure, of a transformer.

The only reason for grounding a transformer, is to provide a clearing path for primary ground faults, and winding to winding faults. There is no other purpose.

Sticking one conductor in the dirt by itself is nonsense, unless you have lightning in your building. The equipment ground conductor can be sized as an equipment ground conductor not as a ground electrode conductor. A transformer does not need a ground electrode conductor.

This is my opinion, it is substantiated by engineering standards. The NEC text is correct, only the interpretation is wrong.

 

[pierre]

Re: Grounding Separately Derivied Systems Article 250.30

Bennie

We are sending the 'Grounding Doctor and Couch' right away, it may not be too late.
All kidding aside, would you not also be concerned with voltage stabilization on the secondary side?

Pierre

 

[bennie]

Re: Grounding Separately Derivied Systems Article 250.30

What destabilizes the voltage?

 

[roger]

Re: Grounding Separately Derivied Systems Article 250.30

I try to stay away from Bennies posts for the fact that I usually agree, and don't want to appear to be backing him up for the sake of such.

With that said, I agree with Bennie.

Bennie, what destabilizes voltage is unequal loading of the voltage boat.


Roger

[ July 02, 2003, 09:27 PM: Message edited by: roger ]

 

[pierre]

Re: Grounding Separately Derivied Systems Article 250.30

Roger

your response answered my question, the only problem is my ignorance of transformer theory. Time to go 'back to the books'.

Bennie

My intention was not to question you, but to try and see if I understood, and I didn't. The only dumb question is the one not asked, and as you can see I ask alot of questions (sometimes disguised as a statement).

Pierre

 

[don_resqcapt19]

Re: Grounding Separately Derivied Systems Article 250.30

Actually in this case, I too, agree with Bennie. I see no electrcial or safety reason to have a grounding electrode and grounding electrode conductor for any transformer that is in the same building as primary feeder OCPD. The EGC run with the primary feeder circuit will provide all of the grounding that is required. I submitted a proposal (5-99) on this subjet for the 2005 code.
Don

 

[dereckbc]

Re: Grounding Separately Derivied Systems Article 250.30

Although I agree with Bennie in theory, it would be safe to do it this way. However I do not think it meets current code. Local AHJ may squeal.

But give it a shot and run the GEC with the primary conductors and size the GEC per the secondary conductors and 250.66. The only concern I would have is that the GEC be bonded to both ends of the conduit if used. You could claim the metalic conduit serves as the EGC for the primary, and the GEC is for the secondary. The local AHJ may approve. I would.

Lets us know how it works out.

 

[kqresq]

Re: Grounding Separately Derivied Systems Article 250.30

I also agree that this installation should be safe but, 250.30 clearly requires a Grounding Electrode Conductor sized per 250.66 to be connected to the Grounded Conductor. As such, I could not possibly pass that installation as "Code Compliant".

 

[tom baker]

Re: Grounding Separately Derivied Systems Article 250.30

The grounding electrode for a SDS system is installed for the same purpose as on a serive: to stablize the voltge to ground and to protect against overvoltage (lightning-transients). Primary faults are cleared by the equipment grounding conductor to the primary source.

 

[bennie]

Re: Grounding Separately Derivied Systems Article 250.30

kqresq: In 250.30 where do you see anything about a ground electrode conductor on the X-O of a transformer?

I know this topic has been around the block many times. I feel it is important to clear up the issue and definition of a separately derived system. I know it has been corrupted through time.

 

[pierre]

Re: Grounding Separately Derivied Systems Article 250.30

Bennie

250.30(A)(2)
The last sentence before the exception.
"Except as permitted by 250.24(A)(3) or (A)(4), this connection shall be made at the same point on the separately derived system where the bonding jumper is installed."

Isn't that the x/o?

Pierre

 

[bennie]

Re: Grounding Separately Derivied Systems Article 250.30

pierre: 250.24(A) 2, applies to an outdoor transformer. The NE (neutral earth) connection is required to prevent a fault from appearing on the premises wiring. This specifically is for a transformer, not a separately derived system.

250.30(A)1, Bonding Jumper...Connection of ground electrode shall be at any point from the source(transformer) to the first overcurrent device.(paraphrased). This is the normal connection method, of a service, from the utility.

The reference to grounding a separately derived system applies only to a premises being supplied by an isolated AC source. This can be a generator or transformer. The transformer, or generator, can not be electrically connected to the utility power system to be defined as a separately derived system. Electrically connected means "capable of carrying current".

The origin of Separately Derived System, was "AC systems without external connections".

 

[wellke]

Re: Grounding Separately Derivied Systems Article 250.30

Thanks for all the input. I will keep looking to see what else others say. I know there is no safety problem this is a general practice for these installations but does it meet the 2002 code?

 

[kqresq]

Re: Grounding Separately Derivied Systems Article 250.30

"250.30 (A)(2)(a) A grounding electrode conductor for a single separately derived system shall be sized in accordance with 250.66 for the derived phase conductors and shall be used to connect the grounded conductor of the derived system to the grounding electrode as specified in 250.30(A)(4). Except as permitted by 250.24(A)(3) or (A)(4)., this connection shall be made at the same point on the separately derived system where the bonding jumper is installed."

 

[don_resqcapt19]

Re: Grounding Separately Derivied Systems Article 250.30

wellke,
The installation in your original post is code compliant as long as the power source for your transformer originates in equipment that is listed "suitable for use as service equipment". See the exception to 250.30(A)(4).
Don

 

[wellke]

Re: Grounding Separately Derivied Systems Article 250.30

Thanks I think I have it now. Well keep watching for any additional comments.

 

[brian john]

Re: Grounding Separately Derivied Systems Article 250.30

Pierre:

Bennie states that the "XO", core and enclosure, that means the secondary is grounded not floating.
While on occasion Bennie has pulled my string on this he is right, IMHO. Sticking that rod in the ground especially in the middle of a building would buy you nothing, and per the NEC this rod would have to be connected to any other electrodes. Of course it would be through the feeder GEC, not sure if your AHJ would buy that?

 

[wellke]

Re: Grounding Separately Derivied Systems Article 250.30

Thanks again to all the answer I was overlooking was "Except as permitted by 250.(A)(3) or (A)(4)"
this gets me back to the sevice panel with the GEC.

 

[bennie]

Re: Grounding Separately Derivied Systems Article 250.30

The reason for the X-O of a transformer being permitted to connect to the building ground electrode if the panel feeding the transformer is for service only, is to insure a bond is complete from the equipment ground conductor and the system neutral.

A transformer fed by a feeder panel will make for a longer distance before the equipment ground conductor is shorted(bonded) to the neutral. Anyone thinking the building steel offers a lower impedance than a direct copper conductor to the service, has lost the meat on his sandwich.