This book deals with the basics of protection theories of electric circuits as it explains the methods and tools of protection using all types of relays such as electrodynamics, static, and digital relays and timers. The discrimination principles for relays or protective circuits or for protective systems and gears for a united power network are considered and the development of protection begins at small element passing by the complete circuits for a main element in the power system and ending at the adaptation of all circuits together for a whole power system. the motors and cables are accounted as parts of the work while the protection title is tailored for over current or voltage - directional power and distance types tending to find the best performance for each. the digital relaying and circuit simplicity are introduced in a chapter to be suitable for the modern technology. A lot of examples are given with a detailed solution in different points and the base theory is indicated if needed. Finally, some problems may be found.