3-phase load current = 730kW/[ 1.732 x 415V x 0.85] = 1194.8 A
Circuit breaker : 1200A 43kA 4-pole Air Circuit Breaker (ACB)
Current transformer: 1200/5 A 15VA CT

For relay setting, let us set continuous current to carry (100%)=1200 A
Current to trip in 5 minutes (120%) = 1.2 x 1200A =1440 A
Current to trip in 1 second (150%) = 1.5 x 1200A =1800 A
Earth Fault (10%) = 0.1 x 1200 A = 120A

Let us now find Time Multiplier Setting (TMS)

In the selection of fault current to be used in calculation of relay operating time, 3-phase symmetrical fault is selected because it is the highest compared with line-line or line-ground fault. If we select either one of the unbalanced faults, and a 3-phase fault occurs, may be relays being used will be unable to correctly coordinate because the fault is so big that cause relay to operate too fast.

By setting based on 3-phase symmetrical fault, and if any unbalanced fault fault occurs, operating time of the relay will be a bit slower but reasonable to protect system.

From calculation of fault above, 3-phase symmetrical fault = 20.3kA (Refer page on fault current)
Multiple pickup, m = symmetrical fault /setting current =20.3kA/1200A = 16.9
Now, let us find operating time of the OCEF relay
There are 3 types of time-current characteristic curves - Normal inverse, Very inverse and Extremely inverse
Operating time of relay for TMS 1 is given by , t_op = a /[ m^b -1] where a nd b are given in the table below

Type of curve	a	b
Normal inverse	0.14	0.02
Very inverse	13.5	1
Extremely inverse	80	2

From the table, a=0.14 and b=0.02
Operating time at TMS 1 = 0.14/[16.9^0.02 -1] = 2.4 second

Alternatively, operating time of the idmt relay based on TMS ranging from TMS 1 to TMS 0.1 can be determined for given multiple pickup.

If there is another switchboard named Switchborad B in the upstream of the line, its relay operation should have time discrimination of 0.4s or 0.5s
Therefore, OCEF relay of this Switchboard shall have operating time of =0.4s + 0.24s =0.64s

And so on for the next Switchborad C which is in the upstream of Switchborad B, its relay operating time will be set at = 0.4s + 0.64s =1.04second.