This thesis consist of 6 chapter: "Electrical power systems", "Faults and malfunctions", "Protection in power system", "ISA T1000 Plus", "Relay REF 630", "Overcurrent relay testing REF 630". In the chapter "Electric power system" we briefly describe the basic whole of the power system since the aim of the overcurrent protection is to protect the elements of the power system. The following chapter "Fauls and malfunctions" it cites and describes faults and malfunctions on elements of electric power system. The chapter "Protection in power system" describes briefly the type of protection and their task with an emphasis on overcurrent protection where under subheading 4.3 is overwhelmed by the protection of the elements of the electric power separately explained. The chapter "ISA T1000 Plus" describes the material and its main characteristics and they are described in the table of the commands of this powerful multifunctional device with which the REF 630 overcurrent relay is tested. REF 630 describes the preconfiguration and protective function of REF 630 as it was described in and the material and its current and voltage inputs and outputs. The REF 630 is described locally and remotely (via the Web HMI), the device command is the same as for the ISA T1000 Plus described in the table. The "REF 630 Overload Relay Test" section shows the connection scheme of both devices, setting both devices before testing. The overload relay test is performed for an overcurrent term and time term, these results are memorized and written down.