In order to calculate the actual relay operating time, the following things must be known:

(a) Time/ p.s.m curve

(b) Current setting

(c) Time setting

(d) Fault current

(e) Current transformer ratio

The procedure for calculating the actual relay operating time is as follows

(i) Convert the fault current into the relay coil current by using the current transformer ratio.

(ii) Express the relay current as a multiple of current setting i.e. calculate the P.S.M.

(iii) From the Time P.S.M. curve of the relay, read off the time of operation for the calculated P.S.M.

(iv) Determine the actual time of operation by multiplying the above time of the relay by time setting multiplier in use.

Problem 1: – Determine the time of operation of a 5-ampere, 3-second over current relay having a current setting of 125% and a time setting, multiplier of 0.6 connected to supply circuit through a 400/5 current transformer when the circuit carries a fault current of 4000A. The time of operation is 3.5 for a P.S.M value of 8.

Solution

Rated secondary current of CT = 5 A

Pickup current = 5 x 1.25 = 6.25 A

Fault current in relay coil = 4000 x 5/400 = 50 A

Plug-setting multiplier (P.S.M.) = 50/6.25 = 8

Given when P.S.M is 8 time operation is 3.5 Sec.

Actual relay operating time = 3.5 x Time-setting = 3.5 x 0.6 = 2.1 seconds

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