The task of this thesis was to describe limited earth fault protection and to make its application on a concrete example using the RET670 relay from ABB. The 16/121 kV power transformer block in the Zakucac HPP was chosen as an example of setting up the protection. The first chapter of the paper contains an introduction, the second chapter describes the principle of operation of the power transformer, its parts and basic data. The third chapter describes current transformers for protection as well as their basic data through which the transformer for a particular protection is selected. The fourth chapter describes limited earth fault protection (REF, 87N) which is of a differential nature. The principle of operation of the protection and the connection schemes for two-winding and three-winding transformers, autotransformers and chokes are generally presented. The fifth chapter lists the functions of the RET670 numerical relay, its appearance and dimensions, and shows tables through which REF protection is set, as well as the algorithm of limited earth fault protection used by ABB relays. In the last, sixth chapter, the calculation through which the current transformers for protection and the REF protection settings were selected is presented. The attachment at the end of the work shows the REF protection settings from the PCM600 program.