GATE 2016 EE -SET 2 Question 13

If a fault takes place between 3 and 4 then relays R_{3} and R_{4} must operate. But when a relay fails, there are two currents that need to be in check, I_{1} and I_{2}. To limit I_{1}, R_{1} must operate. To limit I_{2}, R_{6} must operate. So the back up relays are R_{1} and R_{6}