Commercial Building Ground Ring Design

[rgeisler]

I have abuilding under constrcution. I designed my standard ground ring utilizing #4/o copper with 5/8" x 10' ground rods spaced 20' OC. qround the building. AS I was never privy to a ssoils report, and as most of the soil is sandy loam in this area I wrote a performance specification that the total system grpound be at 5 ohms or less. In addition to the copper ring I required a ufer ground at the foundation level as well as the NEC 250 requirements to bond to water service etc. My problem is this:
The building sight sets on a lot of Rock at the foundation level. TYhe contractor cannopt drive the rods adn if he did limestone is not a good media for conduction any way. I have now been directed by the oqner to assist the contractor in determining the minimum number of grounding plates (Harger is the brand I suggested), How many (I originally called for two-20') chmical ground rods and any other ground ring modificfations tha twill meet MY performance specification.
So to what I could use help with:
I am using the Tables and formulas as found in IEEE Std 142-1991 to attempt to calculate various ring topologies resistance and fault current capability.
Does any-one out there have any suggestions other than this methodology? My firm cannot support the cost of the commercial software and I found this sight via my search for information (on H.B Dwight "Calculation of resistance to Ground&quot

 

[busbar]

Testing as part of the process {initially and/or on completion} is not mentioned. Is the specified value possible with the local soil?

 

[rgeisler]

IT APPEARS SO (SEE OTHER THREAD- i POSTED IT TWICE).

 

[alehman]

Suggest you contact a supplier of grounding equipment (such as Lyncole). Some of them are willing to perform resistance to earth calculations for little or no cost.

 

[MikeDB]

I am not a grounding expert, but I wouldn't think that the grounding plates would get you into any better soil than the ground wire is in. Have you considered replacing all or most of the ground rods with one ground well? Are there any deep piles or foundations that could be grounded?

I would be tempted to check the ground grid without any rods (or whatever has already been installed) and add a well if the value is not low enough to meet the criteria.

 

[hbendillo]

I think alehman has your solution. I have worked with the Lyncole people before and they were very eager to help. If you give them soils data and site drawings they will come up with number and location of chemical ground grods needed to meet the criteria. You can drill holes in which to place the rods and if necessary drill holes at an angle if the drilling gets harder as you go deeper. Some of this just may have to be worked out in the field.

 

[cuky2000]

You could use conventional calculations or sofisticated program with finite element analysis techniques that could give you an approximate grounding requirements. However, the cost and effort associated with this will be difficult to justify.

There are other empiric methods base on experience and taking advantage of compliance with several NEC article as follow:
· Check if ground rod could be droved at 45o angle or buried horizontally in a trench per NEC Article 250-83 (3).
· Plate Electrodes per NEC 250-83(d)
· Encased in concrete electrode (Uffer Gnd)

I suggest considering the use of concrete encased electrode and installing several pigtails at each corner of the building buried diagonally. The expected resistance measurement will be most likely under 5 Ohm.

In the unlikely event that the measurement will be higher, the pigtail mentioned above could be used to install supplement grounding including the chemical rod, ground well, etc, to satisfy the minimum ground resistance.

 

[rgeisler]

March 7-
Hello all. I appologize for my neglect of this thread, but have been out of town for a few days. Here is where the project stands:
I performed a calculation based on the formula in ieee green book for a single rod and obtained a worst case value of 20.5 ohms. Utilizing the table data and indicated calculation of deviding the single rod resistance by the number of rods on the ring (I used 24) adn multiplying by the table factor of 2.15 I arrived at approximately .2 ohms. I kow that this is extremely low, but I am NOT confidednt that the rods or ring will be installed in the manner which will provide good contact with the earth at the depth I have specified. Therefore I am utilizing that quantity of rods.
As to plates, I talked with Harger and they offered to supply a suggested solution. To date they have not. The table formula for plates assumes a ROUND plate. I just haven't reduced the formula to effective area yet as there is a relation ship to burial depth and radius. It doesn't appear to be that bad, I just haven't done it yet.
I don't disagree about the Ufer ground, I just haven't been to the site to see if it is still possible.
By the way, perhaps we as a group can produce a spread sheet to offer som simple calcs for the future. I priced two software packages and they wer $7500 and $12,000 (for 15 mos. usage). NOT cost effective for me, maybe Black and Veetch.
Anyway, that is the current status. As I am not in the office I won't post my calculations until Monday.
Thanks and have a safe and fun weekend.

 

[busbar]

An interesting specialized ground installation is described at

http://electrical-contractor.net/ubb/Forum5/HTML/000203.html

 

[jbartos]

Suggestion: In some cases where there is a shallow soil layer and rock beneath, one may search for the closest area where there is not so much of rock and use that area for grounding. This may require running a ground conductor for some distance.

 

[lyledunn]

I hail from Ireland and such grounding measures would be rare. Here the power company provides earth facility. Where the supply is derived from a high voltage source owned by the user but where the HV supply is from the power co, then the two earthing arrangements are separate. Usually the neutral is employed as the earth return conductor on the LV side.Additional main equipotential bonding conductors are put in place to minimise fault voltages in the event of an earth fault of negligible impedance.
May I ask just what it is that you are grounding? Presumably this is a connection for the low voltage system? Regards,

Lyledunn

 

[mpc]

I don't have it anymore but maybe someone out there has access to the GEMI program developed by the Ga. Tech and the Steel Tube Institute and will let you model your situation.

 

[rgeisler]

March 20,
Hello all. Sorry for my absense. We never have recieved any reply from the Grounding equipment manufacturer and so have proceded as I described above. We ultimately agreed upon some 24 rods and a 3/0 copper ring. To clarify, the building is a corprate headqaurters building with Lab facilities and Worldwide Enterprise Data floor on site. Much of the severity of the grounding is driven by the Lab and Data floor.
lyledun-Yes it is a low voltage (480V-Three phaseGrounded Wye).
When it is installed, I'll follow up with the measured result.