Definition of Protective Relay A protective relay is an automatic device that detects abnormalities in an electrical circuit and closes its contacts. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system. Static Relays: Use electronic components without moving parts. Numerical Relays: Digital relays that use microprocessors, offering advanced protection and monitoring features. Based on Function Overcurrent Relay: Operates when current exceeds a preset limit. 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.