grounding formula

[elect-ok]

dear all,

i am reviewing my consultant report for grounding.

i have formula grounding like below :



R= \frac{\rho}{2.\pi.L} . \left\{ln\frac{4.L}{r}-1

and for 2 electrodes, the formula becomes below :

R(2) = R/2 . [0.377ln(2) + 0.89)

for 3 electrodes, the formula becomes below :

R(3) = R/3 . [0.377ln(3) + 0.89)

etc.......


can any body explain where the formula for 2 electrodes or more comes from ??

thank you...