The use of overcurrent relays is the simplest and cheapest type of line protection. Three types of overcurrent relays are used: inverse time delay overcurrent (TDOC) relays, instantaneous overcurrent relays, and directional overcurrent relays. The main use of TDOC relays is on a radial system where they are used for both phase and ground protection. Basic complements of such relays are two phase and one ground relays. When it is important to limit tripping for faults in only one direction in multiple-source circuits, the use of directional overcurrent relays becomes necessary.