Ground Resistivity vs Grid Resistance

[TS06]

Hey Everyone,
I am involved in grid analysis for a number of substations. As part of this analysis I must look at the soil resistivity and grid resistance to attain step and touch potentials. I am using a program by SES called CDEGS. This program is very useful, however, I cannot seem to model everything as needed. For example, when analyizing one grid,the program used the cable lengths and soil resistivity to give me a resistance of approximately 5 ohms, however when looking at some GPR studies done on this grid in the past, they labelled the grid resistance as 0.5 ohms, due to the presence of some salt water grounding electrodes. The program will not model this scenario.
Thus, in order to get the grid resistance down to 0.5 ohms, I had to change the soil resistivity value. I do not like doing this, but could not see another way to model the lower grid resistance. Does this seem like a suitable way to model and get the required step and touch potential values?
Thanks for any help/guidance offered.

 

[jghrist]

Reducing the soil resistivity used in the program to get a target grid resistance is not acceptable.

CDEGS has the ability to model different layers of soil. If the soil is inundated with salt water below a certain depth, and the "salt water grounding electrode" are driven into this layer, they can be modelled with a multi-layer soil model.

You can also model a volume of soil that has a low resistivity if the salt water is a separate body of water.

You should call SES with your particular problem, assuming that you have a maintenance agreement with them. They are very helpful with technical issues.

The fact that the grid is labelled as 0.5 ohms doesn't mean that it is 0.5 ohms.

 

[TS06]

Thanks jghrist,
The salt water is a separate body of water, so modelling a volume of soil may be the answer, regardless I will consult SES on this problem.
Thanks again.