This course reviews the methods of calculation of fault currents under different types of fault. Circuit breakers, current transformers, potential transformers and relays are all integral parts of the overall overcurrent protective system. Therefore, the course discusses the characteristics, advantages and disadvantages of different types of circuit breakers which are applied to isolate the faults. It also discusses the various types and connections of current and potential transformers. As the grounding system of an electrical network significantly affects the fault current and the relay characteristics, a discussion of power system grounding is included. The latter half of the course is devoted to the basic principles of various types of relays, and also to the applications of relays in the protection of generator, transformer, bus and line.