The thesis describes the influence that earthing transformer has on the values of the currents, temporary overvoltages with power frequency and harmonics of zero sequence current during single phase earth faults in the middle voltage side. The main goal was to develop an algorithm based on fuzzy logic for high ohmic fault detection in the middle voltage networks with installed earthing transformer. In order to describe the behaviour of the network with installed earthing transformer during single phase earth faults in the middle voltage side simulation model made using MATLAB SimPowerSystem software. With the intention to verify simulation model a mathematical model for calculations of single line to earth short circuit currents in case when an earthing transformer is installed was developed, as well. Mathematical model was developed using Wolfram Language Mathematica. In this thesis both simulation and mathematical model are detaily described. Comparison of simulation and mathematical model show satisfying match. The main disadvantage of the existing high impedance fault detection is that normal switching events (such as feeder switching, capacitor banks switching) generate similar components as high impedance faults which results in false tripping of the protective devices. The aim was to give an algorithm which discriminates high impedance faults against regular switching and switching of shunt capacitors. This thesis presents a new approach for HIF detection using fuzzy logic. The model described in this thesis observes substation during fault and the influence of earthing transformer; various neutral earthing methods; the influence of the length of middle voltage power lines; the moment when the fault occurred and various values of fault impedance. The goal was to achieve set of rules for protection devices for determining whether the high impedance fault occurred or not. The set of rules was based on simulation results and was developed using a Fuzzy Logic Design app in MATLAB software.