transformer grounding

[wirenut1980]

Hi all, first time posting here! I am 2 years out of college and still relatively new to the "practical" side of electrical engineering. I have a pad-mounted transformer, 11.5 kV-208Y/120 V. The grounding was installed as follows: a grounding conductor connected from the incoming primary cable shields to the enclosure grounding lug in the primary section of the transformer. In the secondary section of the transformer, the neutral is connected to a ground rod, and the transformer enclosure is also connected to this ground rod via another lug in the secondary section. My question is this installation NEC compliant? Or should a separate grounding conductor be run from the primary ground lug directly to the ground rod? Thanks!

 

[Nick]

Re: transformer grounding

I'm not sure I have all the relevant info here but from what you have described I would say no.
What kind of shielding was grounded in the primary cable? If it is tape,wire or uni-shield the grounding at the primary is to ground the Shields only. This is not a fault clearing equipment ground. If it's concentric neutral cable you are probably OK.
On the secondary you did not say if the conductor ran from the rod to the case lug to the XO lug continuously. If they are separate, where is the neutral/ground bonding jumper??

 

[wirenut1980]

Re: transformer grounding

Thanks Nick, I am not sure what type of shielding was installed on the primary, but I do have a picture that might help explain better than I can.

The ground rod is not shown in the picture, but is located as described in my above post. Thanks

Dave

By the way, this installation was done on an Air Force base powering a Chapel Center. The utility does not enter into the picture at this point of the distribution system.

[ August 27, 2004, 08:57 PM: Message edited by: wirenut1980 ]

 

[Nick]

Re: transformer grounding

I need to edit my original post a bit. Cable shields are commonly used as the sole equipment grounding conductor however I don't believe it meets the NEC. Cable manufacturers list the available fault current the various shields can handle and for how long. The values vary by size of cable, circular mills of the shielding and in the case of tape shield the amount of overlap. If protective relaying is designed so as not to exceed these values the system will function fine.
Looking at the solder block ground braids coming out of the load break elbows you don't have a concentric neutral set up. It would be one of the other methods, tape shielding being the most common. No need for a conductor from the primary ground lug to a ground rod. I am more concerned with the secondary. I still do not see a connection between the equipment ground and the neutral. Maybe it's out of site because you say the neutral is connected to ground and I don't see that either.

 

[pqtest]

Re: transformer grounding

Hello-

Sorry for the delay in responding, but it took me a while to find what I was looking for.

I'm not going to make any guesses as to your particular situation without being able to see it in person, however, I do have something that may be of assistance.

The Air Force produces a series of Engineering Technical Letters (ETL), all of them available online at this URL:

http://www.afcesa.af.mil/library/etl.asp?Category=Engineering%20Technical%20Letters

The one you want is ETL 90-6, "Electrical Grounding, Static and Lightning Protection"

This is the most current version of this Letter that I am aware of, and it was based upon the 1990 NEC- you may need to cross reference the requirements to the 2002/2005 NEC to make sure everything is still valid.

Several standards cited in the ETL as references are no longer in effect- some canceled, some superceded. I can forward this information to you if you need it.

Hope that this helps,

Mark

 

[wirenut1980]

Re: transformer grounding

Nick, Thanks for the info. I can verify that the secondary neutral is grounded. It is not apparent in the picture, but it was done.

Mark, thanks for the ETL website. I will check into that. The AF minimum requirements may very well excede that of the NEC.

Something I just came across...does 250.24(A)(2) in the NEC apply? The transformer is located outdoors.

Outdoor Transformer. Where the transformer supplying the service is located outside the building, at least one additional grounding connection shall be made from the grounded service conductor to a grounding electrode, either at the transformer or elsewhere outside the building.

(The grounding system is not a high impedance grounded neutral system, so I left out the exception from the quote.)

My question is there a typical location of this other connection to the grounding electrode other than at the transformer? Another more important question is what is the intent of 250.24(A)(2)?

Any help is appreciated. Thanks!