IEEE 80-2000 Schwarz formula question - concrete ground rods

[bam55]

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 around the ground mat or grid conductors is high about 500ohm-m. The resistivity of the concrete encased electrodes will be assumed around 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 something between 500ohm-m and 20ohm -m ?
or is this not possible due to the inherent capibility of the formula. The total grid area is about 16,000m2. The concrete foundation cannot be used because it is "insulated" by a vinyl liner under the entire area which can't be punctured.
In other words the resistivity I use in the Schwarz equations will be a estimated value between the ground resistivity and the concrete rod resistivity?

Thanks

 

[jghrist]

The mutual resistance would not be influenced much by the concrete encasement of the rods, so I would use the soil resistivity in the calculation. This would be a somewhat conservative assumption, but I don't think overly conservative.

Note that Schwartz's calculation for the mutual resistance does not include the diameter of the rods. The concrete encasement reduces rod resistance by increasing the effective rod diameter.

 

[bam55]

Thanks,

This means in effect when I do my combined resistance computation that the effect of the concrete encased rods will be negligable, especially under frozen conditions. Which will in turn have negligable effect on my calculation of GPR, correct?

 

[jghrist]

The effect of the concrete encased rods is included in the rodbed resistance calculation, so the GPR of the combination will be reduced even without considering the concrete encasement in the mutual resistance.

 

[bam55]

OK, I understand this more clearly now. Appreciated.
However for frost conditions, in your experience, do we use the Lr term as the depth below frost level or will the concrete encasement act so well in sub zero conditions so that we can use the standard 3m length of rod as the Lr value?

I would tend to think that the "typical" 2m radius of dissipation of fault current from a vertically driven rod will be somewhat different in frozen soil.

Bam55

 

[jghrist]

The big effect of frozen soil will be on the horizontal grid. I've never included the effect of frozen soil in my calculations. The most I have ever done is to include ground rods in the design even if they aren't needed in normal soil. The idea being that in case of frost, there is at least some path for the fault current to flow into unfrozen soil. IEEE-80 is silent on the issue (I think) except for showing how the resistivity increases sharply at low temperatures.

I suppose the proper way to do the design would be to measure the soil resistivity when it is really, really cold out and then use a multi-layer soil model that includes the effect of the frost layer.

I've gotten around the problem by moving south where it doesn't get cold enough to worry about a deep frost level.

 

[bam55]

Thanks alot for for attention to this, Jghrist.
bam55