The purpose of this unit is to teach the basic principles of protective relays and to introduce directional and non-directional relays. The unit begins with the basic theory of protective relays, commonly used types of relays, and a brief explanation of how these relays are used. Additional details and examples of applications are provided for directional and non-directional relays. At the conclusion of this unit, the trainees should have a basic understanding of how protective relays work. They should be able to explain the need for protective relays and to list commonly used types of relays and their functions. They should also be able to explain how directional and non-directional relays work and give examples of situations in which they are used.