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 $$R_{6}$$