For the three-bus power system shown in the figure, the trip signals to the circuit breakers $\text{B}_1$ to $\text{B}_9$ are provided by overcurrent relays $\text{R}_1$ to $\text{R}_9$, 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 $\text{R-L}$ loads with isolation of minimum portion of the network using minimum number of directional relays is
- $\mathrm{R}_3$ and $\mathrm{R}_4$ are directional overcurrent relays blocking faults towards bus $2$
- $\text{R}_3$ and $\text{R}_4$ are directional overcurrent relays blocking faults towards bus $2$ and $\text{R}_7$ is directional overcurrent relay blocking faults towards bus $3$
- $\mathrm{R}_3$ and $\mathrm{R}_4$ are directional overcurrent relays blocking faults towards Line $1$ and Line $2,$ respectively, $\text{R}_7$ is directional overcurrent relay blocking faults towards Line $3$ and $\mathrm{R}_5$ is directional overcurrent relay blocking faults towards bus $2$
- $\text{R}_3$ and $\text{R}_4$ are directional overcurrent relays blocking faults towards Line $1$ and Line $2,$ respectively.