A) Primary and Backup Relay Constraints A primary or main protective device is a relay that is in
the nearest position to the fault and must respond to the fault as fast as possible. To achieve a reliable protection system backup, relays are devices which will be initiated within a certain amount of time after the main relay fails to break the fault. An amount of delay time, called time grading margin, must be added to the main relay operating time. This can be explained by Fig. 1 [7]. Relay m and b are the main and the backup relays, respectively. F/ and F2 are two fault cases seen by both relays. The operating time of the backup relay must be at least the operating time of the main relay plus the time grading margin for every fault case.