29, each line has an overcurrent relay that protects the line. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system reliability. What controls it: Relay performance depends on the protected zone, CT/PT inputs, pickup settings, time delay, breaker clearing time, trip. Generally, the protection given by the protective devices can be divided in to two categories Let see the full detailed explanation about the categories. It is designed to detect faults within its own protected. The main purpose of a protection and control relay is to recognize any abnormal power system condition (s), or abnormally operating system component (s).