The basics of the theory of protection used in electrical networks with voltages of 6-35 kV are briefly outlined. Current and differential protection installed on power lines and transformers are considered. Detailed examples of calculating the characteristics of relay protection and selecting the operation parameters of individual protections are presented. A methodology for solving the complex problem of coordinating protection in a distribution network containing interconnected power lines, transformers and electrical loads is presented. The book is intended for an in-depth study of the theoretical and practical aspects of relay protection and can serve as a practical guide when calculating the parameters of existing protection, as well as when designing new systems power supply. For specialists from design organizations and enterprises operating electrical networks and systems, teachers and students of higher educational institutions in the electrical power industry.