Reducing resistance to ground

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loren

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I have a ground rod that measures 242 ohms. Measured with fall of potential testing. I want to determine what size grid or how many ground rods to get my resistance below 25 ohms.
I did a ground resistivity test and found that the ground resistance is 22885 ohm-centimeters. Using my grounding Nomograph a 5/8 th in ground rod would have to be 42ft long. This is not realistic.
I have never run into this much resistance before. The companies engineer wants to use 4 additional rods tied together with a #6.
I don't know if this is right or wrong. I am just looking for the formula to how he arrived at this number.
 
If you're looking solely for NEC compliance then you only need to sink another 8' ground rod regardless of the resultant resistance. Doesn't sound like that will fly however with the engineer.
 
Even if the ground rods in parallel reacted like resistors in parallel, you wouldn't get 25 ohms. I don't see where he is going. This looks like a case where a specialized system with salts would be used. :)
 
Ground rods don't act completely like regular resistors in parallel. There is a formula that needs to be followed. What I do know is that if you have for example 4 rods with 25 ohms per rod. You would add the resistances together and divide by 4 but then you have to multiply by 1.46 as a constant for multiple rods. This may not be the exact formula but it is along these lines.
 
charlie said:
Even if the ground rods in parallel reacted like resistors in parallel, you wouldn't get 25 ohms. I don't see where he is going. This looks like a case where a specialized system with salts would be used. :)
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I agree,

I would contact someone like XIT they will do the calculations and sell you the chemically enhanced system that may be simpler that driving multiple rods.
 
I have suggested this to them.
It is all rock there. This is why for my own benefit I am looking for the formula to calculate it my self.
 
R=p(L/A)

p (Greek) = reistivity of the earch in ohms-cm
L=Length of the conducting path
A=cross sectional area of the path

This is a basic formula and assumes the reistance of the soil is unifirm in the area

Some guidelines for simple systems.

Doubling the length if the rid resuces the resistance by about 40%
Doubling the diameter of the rod reduces the resistance by about 10%
2 equal size rods will cause the total R to be reduced by about 40%
3 equal size rods will cause the total R to be reduced by about 60%
4 equal size rods will cause the total R to be reduced by about 66%

Obviously these wont help your case much, the equations for groud grids are more complex and it looks like a grid will be needed for your project. Chemical treatments work but keep in mind they will wash away over time and cause your R to increase.
 
Scores

Scores

Check out the scores book and IEEE Green book for ways to reduce resistance.
Basically go deep as you can to achive low resistance.
Reference SCORES pg 108-109 Ground Electrodes.
 
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