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# ground grid Schwarz formula help

## ground grid Schwarz formula help

(OP)
I have an application where I want to add concrete encased ground rods on the perimeter of my plant to reduce the copmbined ground resistance. The resistivity of the ground mat or grid conductors is high about 500ohm-m. The resistivity of the concrete encased electrodes will be assumed at 20ohm -m. So my ground rods now become 70ohms instead of around 300ohms. My question is : for the combined resistance in which I use to determine the GPR, I need to use a resistivity for the mutual ground resistance between the ground grid and rod bed. Do I use som,ething between 500ohm-m and 20ohm -m ? or is this not possible due to the inherent cabibility of the formula. The total grid area is about 16,000m2. The concrete foundation cannot be used because it is "insulated" by a vinyl linerunder the entire area. This can't be punctured.

Any good advice is welcome, however I am in a bit of a rush!

### RE: ground grid Schwarz formula help

I normally don't encourage cross posting but try posting this in the Power Engineering forum. You will have good luck there.

### RE: ground grid Schwarz formula help

Why not connect you concrete incased wires to the pad ground grid, to elimate the step potenial.

### RE: ground grid Schwarz formula help

Something to consider also is that if the concrete encasements are exposed above the ground they will dry out in hot weather and the ground impedance will be affected.  I'm guessing that you already knew that tho...

### RE: ground grid Schwarz formula help

btw, could you share with us the formula you're using?
thanx

### RE: ground grid Schwarz formula help

(OP)
The formula can be found in IEEE-80-2000 and actually it is too long to show in this format. I've tried to incorporate the concrete encased formula in the original formula and this seems to help but when there is frost I've concluded it won't help the resistance at all.

Since this time I've used a factor for frost that when applied shows I'm OK after all. This information can be found in CEA 249 D 541 (www.ceatech.ca). It was written by some authors of IEEE -80-2000 and gives a simplified version of the Schwarz with tables.

Regards

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