It covers the protection methods for generators, transformers, buses, and transmission lines using various relay types to detect and isolate faults efficiently. Protective relays and devices have been developed over 100 years ago to provide “lastline”of defense for the electrical systems. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. The selection and applications of. In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected.