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An earth fault relay has a setting of 150% and a current rating of 5 A. It is connected to a current transformer of ratio 200:5. The current in the primary for which the relay picks up is
1. 7.5 A
2. 20 A
3. 300 A
4. 1500 A

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Correct Answer - Option 3 : 300 A


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

PSM = Secondary fault current/Relay current setting

Pick up current = (Rated secondary current in CT) x (Current setting)


Given that,

Current setting = 150% = 1.5

Relay CT ratio = 200/5

Rated secondary current = 5 A

Therefore, pickup value current of relay = 5 x 1.5 = 7.5 A

For 7.5 A, the current in the primary of the CT is \( = \frac{{200}}{5} \times 7.5 = 300\;A\)

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