Determine the time of operation of a 5-amp, 3-second over current relay having a current setting of 125% and a time setting multiplier of 0.6 connected to supply circuit through a 400/5 current transformer when the circuit carries a fault current of 4000 A. Use the curve shown below:
Rated secondary current of C.T. = 5 A
Pickup current = 5 1.25 = 6.25 A
Fault current in relay coil = 50A
∴ Plug-setting multiplier (P.S.M.)
Corresponding to the plug-setting multiplier of 8
(See Fig.), the time of operation is 3.5 seconds.
∴ Actual relay operating time
= 3.5 Time-setting
= 3.5 0.6
= 2.1 seconds
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