When the fault current is 2000 A, for a relay setting of 50% with CT ratio 500/5, the plug setting multiplier will be
Right Answer is:
Plug setting multiplier of the relay is referred to as the ratio of the fault current in the relay to its pick-up current.
The current transformer ratio is 500 / 5. So, the CT can be provided a rated current of 5 Amps to the relay coil.
Now the relay setting is 50%, then the relay can be operated for 5 × ( 50 / 100 ) = 2.5 Amps.
This is the pick-up current of that relay.
We know that the fault current is 2000 Amps. Hence, the fault current in the secondary of the CT is 5 × ( 2000 / 500) = 20 Amps.
Therefore PSM of the relay is,
PSM = 20 / 2.5 = 8
That means the fault current in the secondary of the CT is 8 times greater than the operating current of the relay coil.