Minumum Earthing Resistance Value

I think it is 100 gms.

NEC 250-84 says 25 ohms.

Ohms don't weigh anything, so I think 100 gms is too high.

Are you sure you mean "minimum"?

Oops. 25 ohms, max.

Good catch. I don't have a clue as to a minimum. Maybe 0?

In my project sites, they used to maitain 5 ohms for single earthing, if it is a grid earthing, it is allowed upto 0.5 ohms.

-Vijay

for grid/mesh earthing the optimim value of net resistance has to less than 1 ohms.

You need to determine if there are any applicable local codes for your jurisdiction. There may be limits that are specified.

Aside from that, the ground grid resistance value depends on the parameters you are trying to achieve (ground potential rise, step & touch potentials, etc.). See IEEE 80 ("Guide for Safety in Substation Grounding") and IEEE 367 ("Recommended Practice for Determining the Electric Power Station Ground Potential Rise and Induced Voltage from a Power Fault").

You need to know the maximum available fault current available at your bus. This, along with the maximum limits you can have for ground potential rise, step potential, and touch potential, will determine the ground grid resistance target value. It is not as simple as saying, "Design for X ohms ground resistance."