For the three-bus power system shown in the figure, the trip signals to the circuit breakers $B_{1$ to $B_{9}$ are provided by overcurrent relays $R_{1}$ to $R_{9}$, respectively, some of which have directional properties also. The necessary condition for the system to be protected for short circuit fault at any part of the system between bus 1 and the R--L loads with isolation of minimum portion of the network using minimum number of directional relays is:}
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