In this paper, using an intelligent modeling approach called fuzzy inference systems (FIS), predicting formula for effective length of different arrangements of grounding vertical rods is extracted in such a way that for the first t ime a comrehensively predicting formulae for effective lengths with and without considering nonlinear phenomeno of soil ionization is introduced (linear soil and nonlinear soil). In creating the model, training data are computed from multiconductor transmission line (MTL). As a result, firstly in the proposed model, in despite of previous models ignoring ionization, the ionization effect is effectively included. Secondly, the results are in excellent agreement with the MTL. The achieved results for effective lengths show that considering ionization results in decreasing the effective length with respect to situation where it is ignored. It is well known that this is financially of importance .
