Question 1 The operating time-current characteristic of an over-current relay controlling a circuit breaker Aina radial distribution system is given below Operating time (s) 0.45 0.39 0.265 Fault current (A) 3000 3750 7500 15000 0.2 1.5 1125 The plug setting of this relay at Ais 7.5 A and its time leaver setting is 0.1 The relay also has plug settings 2.5, 3.75, 5.00, 6.25, 7.5, 8.75 and 10 A and the time leaver is continuously adjustable from 0.1 to 1.0 A circuit breaker B operated by an identical relay is located such that it is able to provide remote back- up protection to the relay at A. The maximum load currents though A and B is 510 A. What should be the minimum fault currents through A and B? II. If the system has a minimum fault current through A and B greater than this value, determine a suitable pick-up setting and time leaver setting for the relay at B, provided that the maximum fault current through both A and B is 8000 A. What would be the discriminating margin between the two relays under these conditions? (You may consider 500/5 current transformer being used to feed the relays at both A and B)