•Consider the following diagram. If there is an obstruction to the regular flow due to fault F1 or F2, the traffic police (relay R1) can sense both F1 and F2 obstructions because of resulting abnormality in traffic (power flow). If the obstruction is on road AB, it is in the jurisdiction of traffic police at R1; else if it is at F2, it is in the jurisdiction of R2. R1 should act for fault F2, if and only if, R2 fails to act. We say that relay R1 backs up relay R2. Standard way to obtain backup action is to use time discrimination i.e., delay operation of relay R1 in case of doubt to provide R2 first chance to clear the fault.
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