Relay Picture Relays are small very fast acting automatic switches designed to protect an electrical system from faults and overloads. Most substations have many relays, each with a specific purpose. When a relay senses a problem it quickly sends a signal to one or many circuit breakers to open, or trip, thus protecting it as well as human life from damage or injury. Relays can also be activated using communication lines to open or close circuit breakers on command.