The book is a thoroughly revised and updated second edition of a successful text. It incorporates the latest developments in semiconductor technology and its applications to power system protection. A new chapter on Microprocessor Applications to Protection has been added. New developments in commercial relay manufacture are also included. With its wide and up-to-date coverage, the book would be indispensable to engineers in the relay industry, field engineers, and research and development personnel. It would also be useful as a reference text for students of electrical engineering.