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What is the time of operation of a relay of rating 5 amp 2.2 sec, IDMT and having a relay setting of 125% Tms = 0.6 ? It is connected to a supply circuit through a CT, 400 ratio. The fault current is 4000 amp.
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- 10.0 sec
- 1.92 sec
- 4.5 sec
- 2.4 sec
Correct Option: B
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 = | ||
| Relay current setting |
| = | ||
| Relay current setting × CT ratio |
| = | = 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.
