In order to calculate the actual relay operating time, the following things must be known:
Time / PSM curve
Current transformation ratio
The procedure for calculating the actual relay operating time is as follows:
Convert the fault current into the relay coil current by using the current transformation ratio.
Express the relay current as a multiple of current setting i.e. calculate the PSM.
From the Time / PSM curve of the relay, read off the time of operation for the calculated PSM.
Determine the actual time of operation by multiplying the above time of the relay by the time setting multiplier in use.