determine the time of operations of a 5 amp,3sec overcurrent relay having a current setting of1 25% and time multi0ler of 0.6 connected to the supply circuit through a 400/5 c.t when the ckt carries a foult current of 400A
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Answer:The pick up value of the relay is 5 amp but since the relay setting is 125%,
Operating current of the relay
= 5 × 1.25 = 6.25 Amp.
PSM =
Secondary current
Relay current setting
=
Primary current (fault current)
Relay current setting × CT ratio
=
4000
= 8
6.25 × 80
From the standard 2.2 sec curve, the operating time for PSM = 8 is 3.2 sec.
Since the TMS is 0.6, the actual operating time of the relay is 1.92 sec.
Explanation:
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