**Calculation of Relay Operation Time**

For calculating actual
relay operating time, we need to know these following operation.

1. Current setting.

4. Time / PSM curve.

5. Time setting.

**Step – 1**

From CT ratio, we
first see the rated secondary current of CT. Say the CT ratio is 100 / 1 A, i.e.
secondary current of CT is 1 A.

**Step – 2**

From current setting we calculate the trick current of the relay. Say current setting of the relay is 150% therefore pick
up current of the relay is 1 × 150% = 1.5 A.

**Step – 3**

Now we have to
calculate PSM for the specified faulty current level. For that, we have to first divide
primary faulty current by CT ratio to get relay faulty current. Say
the faulty current level is 1500 A, in the CT primary, hence
secondary equivalent of faulty current is 1500/(100/1) = 15 A

**Step – 4**

Now, after calculating
PSM, we have to find out the total time of operation of the relay from Time /
PSM curve. From the curve, say we found the time of operation of relay is 3
second for PSM = 10.

**Step – 5**

Finally that operating
time of relay would be multiplied with time setting multiplier, in order to get
actual time of operation of relay. Hence say time setting of the relay is 0.1.

Therefore actual time
of operation of the relay for PSM 10, is 3 × 0.1 = 0.3 sec or 300 ms.