542.2.4
The type and depth of an earth electrode shall be such that soil drying and freezing will not increase its resistance above the required value
Just food for thought,if everybody were to comply with the above regulation,the questions about stability would not be relevant
Now,If the value were low enough to operate the Rcd (be it a 30,100,300 or 500) and that value was typical of the conditions found in the area, the Ra
could be almost any figure below the operating value as long as it does indeed operate the device
My conclusion would be the maximum resistance of the earth electrode can be almost anything you want it to be, as long as the above reg is satisfied,the 100 ohms or 200 ohms or even higher may be figures that may be of little practical benefit