A printable reference covering protection relay device numbers, CT ratios, pickup settings, time-current coordination, and selectivity margins for college engineering. For example, unselective protection operation during a medium voltage network fault will cause an outage for an unnecessarily large number of consumers. They are intended to quickly identify a fault and isolate it so the balance of the system continue to run under normal conditions. The protective philosophy is fundamentally grounded on the understanding that faults or abnormal operating. Engineering use: Relays are used on feeders, transformers, buses, motors, generators, and transmission lines to protect equipment and improve system reliability. Also principles of various protective relays and schemes including special protection.