Relays and instrument transformers are the building blocks of protection systems for generators, busses, transformers, motors, and power lines. Yet, it appears that the subject is a mystery to most. The author greatly simplified the approach in presenting the fundamental ideas of protection systems. After showing the basics of a fault, the author presented the inverse-time characteristics of overcurrent relays, their pick-up currents, and their operating times. When possible, Boolean algebra was used in simplifying relay logic. Several chapters describe the basics of relays and instrument transformers. Finally, the design project, in the last chapter, encapsulates the fundamental ideas in the book. Control system engineers, facility power engineers, distribution engineers, and transmission engineers should benefit from the book.