Another way to lower ground resistance is to use multiple ground electrodes.
In this design, more than one electrode is driven into the ground and connected in parallel to lower the resistance. For additional electrodes to be effective, the spacing of additional rods need to be at least equal to the depth of the driven rod.
Without proper spacing of the ground electrodes, their spheres of influence will intersect and the resistance will not be lowered.
รายละเอียด
http://www.google.com/url?sa=t&rct=j&q=ground%20resistance&source=web&cd=1&ved=0CC4QFjAA&url=http%3A%2F%2Fsupport.fluke.com%2Ffind-sales%2FDownload%2FAsset%2F2633834_6115_ENG_A_W.PDF&ei=mGHPTuDHMpCnrAf4o5HkDA&usg=AFQjCNEsFAMAlDwRTDanHDjUxyCQURshPg
ระยะห่าง ระหว่าง Ground Rod
When ground rods are installed, the correct distance between the rods must be determined for proper placement. Traditionally, the minimum separation between rods should be greater than the sum of the lengths of two adjacent rods. The "sphere of influence" (Figure 5) of a rod is the amount of soil used in dissipating the charge from one rod. The area of the "sphere of influence" has a radius and depth equivalent to the length of the rod. For example, the sphere of influence of a 10-foot ground rod would have a diameter of 20 feet around the rod and would be 10 feet deep.
It is essential to determine the correct separation distance between rods. When rods discharge they will saturate the soil in their immediate area. Inefficiency will result if a rod tries to dissipate charge in soil already saturated by another rod. If the charge being dissipated by a ground rod is too great for the soil to absorb, the rod could actually fuse into glass. A glass ground rod makes a great insulator, and a poor conductor of charges.
Credit http://www.sbe24.org/archive/ltrs2001/c24sep01.asp#five
ข้อสงสัยเกี่ยวกับสายดิน
แท่งเดียวดีกว่า 
แวะมาขอโทษ