The operating time current characteristic of an over-current relay controlling a circuit breaker A in a radial distribution system is given below

operating time (s)	1.5	0.45	0.39	0.265	0.2
Fault current (A)	1125	3000	3750	7500	15000

The plug setting of this relay at A is 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 backup protection to the relay at A. The maximum load currents though A and B is 510 A.

I. 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.

III. 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)