In this thesis, the Croatian standard HRN EN 60909-3 is described in detail, and examples of calculations in the NEPLAN program are given in accordance with the used standard. The third part of the HRN EN 60909 standard describes the procedures for calculating currents during two separate simultaneous earth faults in systems with an isolated or resonantly grounded constellation, as well as the calculation of partial currents through the ground in case of a single earth fault in rigidly grounded or low-ohmic grounded systems. In the second chapter, the currently valid Croatian norms HRN EN 60909 are listed. Definitions of the sizes used for the purposes of this standard are described and the corresponding formulas are given. In the third chapter, the initial symmetrical short-circuit current, the shock (peak) short-circuit current, the interrupting symmetrical short-circuit current and the permanent short-circuit current are described. The distribution of earth fault currents during two separate simultaneous earth faults is described and relevant examples are given. The fourth chapter describes the partial currents through the ground in case of an earth fault in the substation, outside the substation and in the vicinity of the substation. The calculation of the reduction factor for overhead lines with protective ropes is also described. The fifth chapter describes the calculation of the reduction factor and current distribution in the case of a cable with a metal jacket (shield) grounded at both ends. The case of a three-core cable and three single-core cables was considered. In the sixth chapter, the NEPLAN program is described, which was used for three examples where two separate simultaneous single-phase short circuits were performed. Examples of the calculation of partial currents through the ground in the event of an earth fault in a substation and an example of the calculation of the reduction factor and distribution of currents through the ground in the event of an earth fault in a substation powered by a three-wire cable are also presented, taken from the HRN EN 60909-3 standard.