Grounding Bed and GEM

[tin2779]

Hi,...

we are planning to use this GEM(Ground Enhancing Material) from Erico. As the location is in Northern Ontario near a mine site and is very rocky.

There will be grading done to have foundation done for structural work. I guess some soil fill up to two feet.
I am just seeking an advise, if the plan proposed seems to work.

I intend to put a trench may be 5 inches deep in the ground mat fashion and wrap the grid with GEM in trench and then same thing with ground rods wrapped in GEM within drilled holes.

My only issue is,if I have to picture this situation theoretically to find out how current will flow, I cannot picture that.

Could any body advise, having rock around and only GEM and two feet of soil, how this will work.

May be a werid question but I guess no question is stupid.

Thanks

 

[TurbineGen]

In grounding systems the ground electrodes are driven into the ground so that the earth provides an electrical return path. You are likely going to have some difficulty with the rock. I would suggest augering holes in the bedrock at least 5 feet (as opposed to 5 inches) deep and filling the holes with the GEM where you intend to drive your ground rods, and drive your rods directly into the GEM holes as you suggested above. The GEM should give a good (as good as you can get with bedrock) electrical connection.

Erico has a calculator for that on their website (I'm not sure where it is, but it is a free download). With a thin soil there might be a high step potential hazard.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[TurbineGen]

Upon rereading and evaluating your situation, I believe I misinterpreted the ground rod installation. It appears your plan is about as good as you can get. You might consider modular rods incase you need to drive your rods deeper.

I found the calaculator here

http://www.erico.com/static.asp?id=27

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If it is broken, fix it. If it isn't broken, I'll soon fix that.

 

[waross]

I am in a similar situation.
Grounding has two functions.
1> Reduce setp and touch potentials.
2> provide a return path for ground fault currents.
We have not rock but very dry, high resistance soil.
Step and touch potentials were minimized by importing soil of a higher conductivity. The soil was built up about 4 feret and the ground grid was installed near the botom of the fill. The first computer simulation showed excessive step potentials, but the proposed depth of fill was increased until an acceptable level of protection was calculated.
To provide a return path for ground fault currents, an 8 inch diameter hole was drilled about 150 ft. deep at each of the corners of the station grid. Into each hole a ten foot long ground rod was lowered on the end of a 4/0 cable. The holes were then filled with Erico GEM. When testing was complete, the cables were welded top the corners of the station ground grid.
This is a 66KV incoming, 13KV outgoing, 25MVA ONAN station. (Future 40 MVA ONAF)
respectfully

 

[cuky2000]

For rocky soil there are a few alternatives to consider to improve the overall ground grid resistance:

* Concrete electrode:
* Grounding foundations, rebars, steel structures (Uffer ground)
* Grounding plate.
* Grounding well.
* Chemical ground

The enclossed link will provide a summary of different method used in the industry to improve the ground grid resistance

http://cuky2000.250free.com/GND_Electrode1.JPG

 

[tin2779]

Hi All,

Please see the links below:

http://img514.imageshack.us/my.php?image=pic1st1.jpg

This link above shows the image that defines
GPR=Grid Resistance x I( earth current)
I kinda disagree, I think it should be

GPR= Grid Resistance x I(fault current- skywire current)

The image does not consider neutral consider in potential rise but why?
if I have to look as per Kir. current law, then you will see that I(neutral) will pass through the grid as well so why it is not considered in the potential rise calculation.

This other pic makes it more confusing:

http://img266.imageshack.us/my.php?image=pic21bo2.jpg

I would appreciate some comments.

Thanks

 

[GRP123]

Waross,


What is this soil of high conductivity....

Could you provide some detail.may be a Vendor or type.. I am using organic soil(80 ohm meter).. but looks like it will not help...
Erico GEM cannot be used for the whole yard.. I guess it will be too expensive... So, this High conductivity soil is the best bet I have...

Tin...
sorry for intruding in to your question..

 

[jghrist]

Tin2779,
The voltage at the station is the current flowing through the soil times the resistance of the soil. This follows Ohm's Law. The voltage is also the current flowing through other parallel paths (like the shield wire) times the impedance of those other paths. To oversimplify, consider a circuit of two parallel resistors (earth and shield wire) with a voltage source across the two resistors. The voltage is the same across both resistances (GPR) and is equal to the current through each resistor times its resistance.

 

[waross]

What is this soil of high conductivity....

It was not "special" soil. It was just normal soil with suitable resistivity. The civil contractor of the substation had an excavation in progress one or two miles from the substation site. The soil there was of better conductivity than the native soil at the station site.
I was not involved with the calculations but i understand that a computer simulation was run modeling a 24 inch fill of the higher conductivity soil. The results were not satisfactory, but were promising. Another simulation was modeled based on 36" of imported soil. The results were satisfactory so enough soil was trucked in to cover the substation yard to a depth of 36". The ground grid was laid in the imported soil.
In your instance, do you have any soils of different composition within trucking distance of the station? You may want to consider doing some field tests of soil resistivity in the surrounding country side.
respectfully

 

[tin2779]

Thanks Jghrist.
I guess by reading your posts about grounding on this website, I know more about grounding than any body in my office. One quick question,
If my resistance is not going down becasue it is bed rock below 4 ft. I am using this organic soil
and then there will be concrete pads. Can I reduce this resistance by increasing area.
I am just trying to find out... how resitance of grid except any other means like conductive soil or GEM.
I have no ground rods, How theortically rods help ?


Waross, GRP123 question cleared my quest about high conductivity soil as well.

Regards,

 

[jghrist]

tin2779,
The first approximation of grid resistance is:
R_g=(rho/4)·sqrt(pi/A) (IEEE std 80 eq 50)

The best way to reduce grid resistance is to decrease resistivity rho or increase area A. Ground rods help by injecting current lower down into the soil. This can be a big advantage if the soil resistivity is lower at greater depths, but will help in all soils. If you have bedrock at 4 ft, this can be an expensive solution.

 

[TurbineGen]

You can also minimize the step and touch potentials by using a low conductivity top layer 6" or more thick on top of your soil layer. At the power stations this is usually not a problem as it is often a paved surface. You might consider using gravel as it is cheap.

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If it is broken, fix it. If it isn't broken, I'll soon fix that.