The equivalent impedance of NGR is quit holmic but we face inductive impedance in grounding transformers approximately. One of important difference between resistive and inductive impedance is their behavior against through fault current harmonics. Therefore when the grounding system is chosen with zero sequence harmonic current considerations (telecommunications interference and so on), the type of impedance, resistive or inductive, will be important. For example the equivalent impedance of an inductor with 5 ohm (in 50 HZ) will be 15 ohm for third harmonics (X= L.?). Also the type of grounding impedance is important in transient stability study of network when an earth fault current occurs, indeed the system shall be stable against through earth fault current losses which can occurred in neutral point of power system. Meanwhile in impedance grounding system particularly, the peak of overvoltage due to an earth fault depends to X0/R0 of earth fault path, therefore it shall be considered in best chosen.