Directional relays are protective devices that isolate faults in power systems by detecting the direction of fault currents. As an essential. Each Cahier Technique provides an in-depth study of a precise subject in the fields of electrical networks, protection devices, monitoring and control and industrial automation systems. The latest publications can be downloaded on Internet from the Schneider server. Unlike traditional protection systems, which may treat all fault conditions similarly irrespective of fault direction, directional relays are designed to respond differently based. t and secure protection throughout the power system. The PR123/P and the PR333/P units carry out excludable directional protection (“D”) against short-circuit with.