designing a ground grid with a "ground well" 3

[Kamalo]

I am trying to design a ground grid, but I need to add a ground well ( a ground rod that is driven deep enough till it is in water, e.g. 300ft..). The problem is, when I do this it seems that the soil resistivity of the 2nd layer will be assumed to be 1 ohm-meter. I am following a previously designed grid and I do not know where the engineer got this assumption. I do not think this assumption is right because the calculated (modeled) results were very different from the tested results after the grid with the well was put in. The engineer is not around and I cannot seem to find much information on ground wells. Can someone shed some light on this for me. I was also wondering what was the best method for doing the calculations for a ground grid that provides an accurate model, when designing for a substation. I appreciate any suggestions. Thank you.

 

[jghrist]

I would not expect a resistivity as low as 1 ohm-meter unless the ground water is saline. You will have to measure the resistivity at different depths to develop an accurate model.

 

[Kamalo]

Thanks for your post and I agree with you. 0.1 ohm-meter for saline, 1 ohm-meter for massive sulphide, 10 ohm-meter for clay, etc...Do you know of any good software programs that can create an accurate model if I did have the resistivity at different depths?

 

[jghrist]

We use the SES CDEGS program and are quite satisfied with its ease of use and flexibility.

http://www.sestech.com/Default.aspx?tabid=56

 

[mpparent]

Kamalo,

Could you try a 3 point chemical rod system?

Mike

 

[cuky2000]

Kamalo, following blind sombody else design may not be the best advise, particularly if the previous design do not have a clear records and documentation of their design basis.

Beware that most ground grids are satisfactoryly designed without ground well particularly if the soil test results shown low resistivity (ex <100 Ohm-m).
[sub]
I cannot resist asking:
a) why you need to add a ground well ?
b) What is the impact of the well on the total cost of the grid?
c) It is possible to use other type of electrode (rods, Ufer ground, foundation, water pipe, etc.)
d) Is this the most cost effective solution?
e) Are you considering taking advantage of grounding the concrete structures and rebars in this project (Ufer Ground)?
f) Can you interconnect the grid with other existing installations?
[/sub]

This is a great opportunity to exercise your engineering curiosity challenging the previous design and show your capability to bring a new safe and cost effective solution.

If you should need further technical support, I am sure that many members in this forum will collaborate with you providing professional advise.

Good luck.

 

[mc5w]

A company named Erico in Solon, Ohio makes both chemical ground rods and a proprietary backfill for use with ground plates. They also make products for exothermically welding copper groung wires.

Your best bet in bad soil is to do what the telegraph people did and bury a 3 foot by 4 foot or larger ground plate. Then, backfill with Erico's ground improvement backfill using 6 inch deep layers and machine compact each layer.

You can also improve your existing ground mat by installing a subsurface irrigation system such as Porous Pipe(R).

 

[SierraJ]

On our experience, designing for a low ground mat resistance (lets say below 2 ohms) is not that difficult, if enough grid cover area is available. The most problematic parameter is keeping the step and touch voltages within the acceptable limits.

We have found that for problematic areas, using an "equipotential surface mesh" (installed not more than 12" below finish grade) proves to be more cost effective than adding numerous rods or using wells.

This is a pre-abricated mesh(12"x12", 24"x"24") that is installed on top of the "main" ground grid more or less the same that ground plates are provided at air break switches for personnel protection.

 

[Kamalo]

I like that idea. Where could you purchase said, pre-fabricated mesh?

 

[SierraJ]

Look at this link

http://www.erico.com/products/GroundGrids.asp

from Erico.

Jose

 

[EEJaime]

I have found also that chemical ground rods provide superior performance, and even though they are much costlier than a standard ground rod, the installation savings mort than make up for it. Look at Lyncole's XIT system at

http://www.lyncole.com.

They also have metering, measuring and design software available. Good luck with your project.

 

[Kamalo]

Thank you, EEJaime. Lyncole looks impressive. Have you ever taken one of their grounding courses? Is it worth the time or do they simple walk you through the IEE standard (which I can do on my own).

 

[EEJaime]

Kamalo,

You are welcome. Yes I have. They go through the theory, design, modeling and installation. It is a very useful course. Well worth the time and effort. Good luck with your design.