The shown two-generator system has six overcurrent relays R1–R6, using an appropriate mix of directional and nondirectional relays. The remote backup relays for R4 are
R4 protects the lower left parallel line from the middle bus. If it fails to clear a fault on that line, current can arrive at that bus from both remote source paths.
From S1 the path uses the upper parallel line, whose remote source-end relay is R1. From S2 the source-end relay on the right line is R6. Both must remove their respective feeds to back up the failed R4.
Thus the remote backup pair is **R1 and R6, D**.
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