Correct Answer - Option 4 : 1.92 s

**Concept:**

The plug setting multiplier of a relay is defined as the ratio of secondary fault current to the relay current setting.

PSM = Secondary fault current/Relay current setting

**Calculation:**

Relay CT ratio = 400/5

Therefore, pickup value of relay = 5 A

Since the relay setting is 125%

The operating current of relay = 5 × 1.25 = 6.25 A

Given that, primary fault current, I_{pf} = 4000 A

Therefore, the secondary fault current is

\({I_{sf}} = {I_{pf}} \times \frac{1}{{CT.\;Ratio}} = \frac{{4000}}{{80}} = 50\)

\(PSM = \frac{{4000}}{{6.25\; \times \;80}}\)

⇒ PSM = 8

For the standard IDMT curve of 2.2 sec, the operating time for PSM 8 is 3.2 sec

Since, TMS = 0.6

Therefore, actual operating time = 0.6 × 3.2 = 1.92 sec