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For the three-bus power system shown in the figure, the trip signals to the circuit breakers 
to 
are provided by overcurrent relays 
to 
, 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 
- 
loads with isolation of minimum portion of the network using minimum number of directional relays is
and 
are directional overcurrent relays blocking faults towards bus 2.
and are directional overcurrent relays blocking faults towards bus 2 and 
is directional overcurrent relay blocking faults towards bus 3.
and are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively, is directional overcurrent relay blocking faults towards ine 3 and 
is directional overcurrent relay blocking faults towards bus 2.
and are directional overcurrent relays blocking faults towards Line 1 and Line 2, respectively.
Directional relays are used at load end of interconnected line or ring mains.
We must always connect directional relay at location where current may reverse in event of a fault.
Based on above concept, the position of directional & non-directional relays are shown below.
↔ : non-directional relay
← : directional relay
& 
are directional relays that trip for faults towards Line-1 & Line-2 respectively.
Also, 
& block fault towards Bus-2.