Di ko alam kung ano ang standard ng Oman, pero kung base sa Australian standard (AS 1768), which is derived from IEC TC 81, ang group resistance ng mga earth rods kung ang spacing between earth rods ay mahigit sa 2x Earth rod lengths is equal to the parallel resistances of all the single earth rod resistances. To quote:

AS 1758-2007
Where the desired resistance can be achieved with only a few additional earthing electrodes
and if the separation between the earthing electrodes is greater than their lengths (see Note),
then the resultant resistance may be calculated by using the ordinary equation for
resistances in parallel. In other situations the combined resistance owing to the mutual
interaction between the earthing electrodes, will be always higher than given by this
equation.
NOTE: For practical purposes, the separation between vertical earthing electrodes can be taken as
twice the length of the earthing electrode.

Kung ang spacing mo ay 120m, ito may malayong higit sa 2x3m, kaya ang formula na ito ay valid.

kailangan ko po ng Group resistance, salamat po.