The topic of this final thesis was the description of relay protection. Through the work, we have shown how relay protection, with its development over the last half century, has greatly contributed to the safety and stability of our power system. We have presented its development through all three generations, and explained their disadvantages and advantages in more detail. Furthermore, the types of basic relays are described and their functioning is explained. Having familiarized ourselves with the most important types of protection and their principles of operation, we come to numerical relays, which are represented in the paper by the described Siemens SIPROTEC 7SJ82. We find its application in the renovation of the 110/35 kV Meterize substation, where the relay itself was introduced as the main protection of the plant by replacing the 35 kV plant. By presenting the plant and its technical data, we see what this relay actually protects. It contains a huge variety of protection, and is parameterized by the user to those protections that it needs. This is processed by the DIGSI 5 engineering package, which was produced to make the user’s job easier, and its capabilities are also extensive. The Omicron CMC 256 relay test device, its main parts, and its characteristics are described.