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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 of the setting of 125% Tms = 0.6 It is connected to a supply circuit through a CT, 400 ratio. The fault current is 4000 amp.

The time of operation of an IDMT relay can be calculated using the following formula:

t = K * Tms / (Kp * Is)

Where,

K = a constant based on the type of IDMT characteristic. For Standard Inverse characteristic, K = 0.14.
Tms = time multiplier setting of the relay = 0.6 s
Kp = plug setting multiplier = 1
Is = the current seen by the relay at the fault location, in amperes.

The current seen by the relay is given by:

If = Ip / CT ratio

Where,

Ip = primary current at the fault location = 4000 A
CT ratio = 400:5 = 80

Therefore, If = 4000 / 80 = 50 A

Now, calculating the time of operation:

t = K * Tms / (Kp * Is)
= 0.14 * 0.6 / (1 * 125% * 50 A)
= 1.92 sec

Therefore, the time of operation of the relay is 1.92 seconds.