Fault resistance and intermediate in-feed in the power supply system have got substantial influence on the accurate measurement of distance to the fault location, as well as functioning of distance relay protection. Distance protection device obtains information about measurements only from one side of the protected line, and determines conditions for functioning based on them. External influences, intermediate in-feed, fault resistance, as well as additional current from the opposite end, cause inaccuracies in measurement and fault impedance determination, which can result in a distance relay failure. Analysis of influence of intermediate in-feed and fault resistance at the fault location on the functioning of distance relays has been made by using the program DIgSILENT PowerFactory, and it has been tested in one part of the electric power supply system of the Transmission Area Rijeka. Functioning of distance relays has been analysed by simulating familiar and real types of faults, with different resistance values at a fault location. The degree of change of analogous values measured by the distance relay has been investigated by changing the conditions at the fault location. The influence of the above-mentioned changes on the determination of fault conditions and distance protection operation on the tripping of a high-voltage line has been taken into consideration. It follows from the analysis that distance relays are subject to the influence of fault resistance and intermediate in-feed. Some usages of numerical relays and modern communication systems for the improvement of relay protection system have been described.