Protection Engineering Basics and Schemes Protection Engineering | Page 114
5.3.2.1 Normal or Standard Inverse TCC Relay:
This type of time current characteristics follow below define formula for time of interruption( pick up time)
t =
0. 14 ∗ TMS PSM 0. 02 − 1
PSM is ratio of actual current to CT Secondary current.
Remember at abnormal stage, PSM will be greater then 1 TMS is time multiplier setting, now in modern digital relay it can be as minimum as 12.5milli sec, remember TMS is minimum Time a relay take to initiate operation( Pick up)