The sensitivity angle of relay protection refers to
The sensitivity of a relay is mentioned as a ratio of the minimum value of short circuit current to the minimum value of the quantity for the operation. Directional protection requires the setting of an appropriate Relay Characteristic Angle (RCA) to define what direction the relay is "looking" to define half of the plane as the operating zone and the other half as the blocking zone. The relay in a protection system should be sensitive enough to operate when a fault occurs.
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