Confused! Grounding high voltage electrical vaults

[yanert]

Here is the problem I am having a hard time understanding.

You have a electrical substation. Out of the substation you have underground power feeding numerous electrical vaults. The power distribution into the electrical vaults is at 13KV. The vaults run around the entire site. In all the underground distribution from the substation duct bank feeding the first main distribution vault to all the duct banks that feed all the other electrical vaults you have a 4/0 ground wire. This 4/0 ground wire is cad welded to a ? x 10? ground rod at each electrical vault and a 1/0 tail is brought into the vault for grounding. The owner is requiring that all ground rods meet the 25 ohm requirement. The problem is the soil is absolutely terrible! ? x 20? ground rods are getting readings around 1000 ohms. The only way to get a 25 ohm reading is to start spacing multiple ground rods out and driving them as deep as possible. It takes over an 20 minutes with a 90LB jackhammer to drive a sectional ground rod down in this soil to a depth of 20?. Basically you are making a complete ground ring around each electrical vault. If you test a ground rod that has been bonded to all the 1000?s of feet 4/0 that has been buried throughout the site you get a good reading, but a local test of just the ground rods for that particular vault are very pore. After you get a 25 ohm or better reading, the owner wants to just take that ground ring and bond it to the grounding conductor going around the site. The contractor has not done any of this yet, but for a guy that is not really used to dealing with higher voltages and grounding it is really confusing! What is the difference between the local ground ( 25 ohm or less reading at each vault ) and the reading you get for the entire site grounding. How do these potentials come into play? I am not understanding the why the local ohm reading is needed , and to be honest the safety factor for doing it! If you get a good reading when the ground rod is bonded and in parallel with all other ground rods and 4/0 running around the site, why on earth would you need to achieve 25 ohms locally at each vault and then when you get it, just bond it again the the grounding conductor running around the site. I have asked a few smart electricians I know, and they cant help out.

Thanks folks, I am sure somebody out there can set me strait.

Glad I am not a lineman!

 

[david]

Re: Confused! Grounding high voltage electrical vaults

The utility guys on this site are going to best answer your question.
I would guess the answer is simple this is likely the personal preference of the owner. I hope some one is getting paid well for all this hard work.

The owner must be concerned that one of these vaults grounding electrodes will become isolated from the ground ring somehow.

Maybe you answered your own question when you said the soil conditions are horrible at this site.

I am wondering what he final reading of the ground ring will be when this system is complete.

 

[charlie]

Re: Confused! Grounding high voltage electrical vaults

What is the primary voltage in the substation? I am guessing the primary is 138 kV and the secondary is 13.2 or 12.47 kV.

At 138 kV, if a phase came down, was faulting to the earth, and you were reaching for the control house door, what would be the step-touch potential? The owner is trying to create the best possible return path to actuate the OC device for the primary and, at the same time lower the step-touch potential. Under the substation, the owner has created an equipotential plane and all the metal equipment has been attached to it, including the fence, metallic walls of the control house, and support beams.

This is all required in the NESC for the construction of substations and the NEC defers to the NESC for that reason (I think this is a true statement, I don't have either the NEC or NESC with me).

 

[yanert]

Re: Confused! Grounding high voltage electrical vaults

Hi Charlie
Yes, i sure do understand the step-touch potential at the substation. Actually I understand a bit more after I have surfed the net and read into it. Equipotential zones I have not read into very much, but they sound extremely important for large concrete slabs. Would you not want them in any large building with a concrete pad and wire mesh?
The Primary and sec. voltages are correct. The vaults that I am talking about are 13' x 8' and are very tall, 10' I think. They are buried into the earth so only the cast iron ring is exposed. So basically you have a extremely large chunk of concrete buried in the ground. I still dont understand the local grounding of the 25 ohms vs the ohms reading you would get if you tested the entire system however. One of the vaults I got to look at was about 7000' away from the substation. Now that 4/0 ground cable has come a long way and before it gets to that particular vault, it has been bonded to all the other vaults, all the other ground rods driven at each vault, the substation grounding ring which as I know is extremely well grounded and everything else. Also when the owner comes out of any one of these vaults and feeds a building, the ground coming into the vault has been bonded to the ground ring at each building that the vault is feeding. So do you still think you have step touch potential at the vault itself? Does that concrete slab inside that vault still have any potential at all?? Could this be why they want the local 25 ohms reading? Oh, also I found out that the owner wants 25 ohms for one ground rod, but if you install more than one, you need to get a reading of 5 ohms. Yikes!

 

[bob]

Re: Confused! Grounding high voltage electrical vaults

I think you need to strive for 25 ohms for the GROUNDING STSTEM than for one ground rod. If you are installing rods in each vault they are in parellel just as they would be by installing 3 or 4 rods out side the vault. Check the ohm reading for the system an I'll be you get a good reading

[ August 09, 2003, 03:24 PM: Message edited by: bob ]

 

[yanert]

Re: Confused! Grounding high voltage electrical vaults

Hi Bob,

Yes, If you test the system, you get a great reading, but they want to achieve 25 ohms at the ground rod itself. If you can't achieve 25 ohms, and you have to drive more ground rods, they want you to get a reading of 5 ohms. Its just plain wierd to me. So, once you get the proper reading for you local grounding at the vault, then they want you to bond it to the 4/0 running through the ductbank that is bonded to everything else. Thanks for your reply!

 

[charlie]

Re: Confused! Grounding high voltage electrical vaults

Sorry, I don't understand the reasons for the grounding resistance of better than 25 ohms unless it is for being sure of a really good fault current return path. The vaults you are describing are actually manholes and are not for equipment. You will have cable racked through the manholes. I would think there would be concrete encased duct banks connecting the manholes together and back to the substation. The step-touch potential applies to the substation, not to the manhole.

 

[yanert]

Re: Confused! Grounding high voltage electrical vaults

Yes Charlie you are completely correct. I guess they are called man holes and yes I do believe they have strut embedded in them. Hey, thanks so much for at least helping me understand a little bit about this. I honestly think the owners are not sure what they are after. Its a gov. job and the 25 ohms requirement for a ground rod is code, its just that in this instant its not going to work. Too bad it couldnt be like a house, I think the code says, 25 ohms or better, if not, drive another rod at least 6 feet apart and call it good. At least I think that is what it says, If not I am sure I will get roasted by somebody!

 

[bob]

Re: Confused! Grounding high voltage electrical vaults

Yanert
The code does not require the ground rod to read 25 ohms. Why 25 ohms was chosen is a closely guarded secret. The code requires an additional rod if the ground resistance of one rod is greater than 25 ohms. There is no magic about 25 ohms. If you have a 12.46 kv system the voltage to neutral or ground is 7200 volts. A phase to ground fault will equal 7200 volts/25 ohms will
equal 288 amps at or near you source. Depending on you phase protection this may or may not be enough to cleaar the fault. The ground rods are
not installed for this reason. They are install to lessen transient voltage disturbances such as lightning and phase to gound faults. There are some locations in the USA that you will never be able to get 25 ohms much less 5 ohms no matter how deep you drive ground rods. We had a location here that read 1000 ohms at a site and after driving 10 - 10ft rods we reduced the reading to 150 ohms. The driver could not drive any more rods. That was the best is was ever going to get.