3 ohms resistance

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dj98hd2

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Callaway, Va
Hello, I am installing a grounding system at a new titanium plant in Va. The specs on the job call for a maximum of 3-ohms at the ground rod before connecting it to the ground loop testing it using the 3-point FOP method. We are installing 10-foot X 1-inch diameter rods with the top of the rod 3-feet below grade. My question is it even possible to get to 3-ohms in the best case scenario? I would expect it would be possible to get down to 3-ohms from the completed grid.
 
Sure depending where you are in Virginia we have sites with under 2 ohms. depends on many factors including, IS THE GROUND FROZEN, number and depth of rods, spacing of rods, size of grid, soil type and.................

Are you up in the mountains or down in the valley?

Drive a single rod and determine if you need more extensive design. If the resistance is high from a single rod contact an engineering firm that specializes in ground design.

If you have the test equipment and experience perform a 4-point test to assist the engineers and save you money from them having to perform this test.
 
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Thank you for responding so quickly. The site is in Martinsville, Va and depending on the day, it is either 30 or 60 degrees. So you think getting 3-ohms is possible without having to do any thing special.
 
You would have to have a soil resistivity of 10 ohm?m or less to get a 3 ohm resistance on a single 10 ft rod. The resistivity is more likely to be around 300 ohm?m and the single rod resistance around 90 ohms.

Do the specs say what to do if you don't measure 3 ohms? If your contract says put in a 10 ft rod, then it's someone else's problem. If your contract says get 3 ohms or less, then you're in trouble.
 
jghrist said:
You would have to have a soil resistivity of 10 ohm?m or less to get a 3 ohm resistance on a single 10 ft rod. The resistivity is more likely to be around 300 ohm?m and the single rod resistance around 90 ohms.

Do the specs say what to do if you don't measure 3 ohms? If your contract says put in a 10 ft rod, then it's someone Else's problem. If your contract says get 3 ohms or less, then you're in trouble.
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MY post regarding a single rod is to get a basic simple insight to the resistance of A SINGLE ROD. Much simpler to make some educated guesses regarding the next step once they know this.

Drive a single rod and measure 10 ohms the grid is much simpler, drive a single rod and obtain a very high reading and maybe they should consider hiring a design firm or contacting a manufacture such as XIT.
 
Installed a test rod today and hit rock about 8' down. We did a 3-point test and averaged 28.9-ohms. Not looking good. when the specs call for 3-ohm max.
 
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