TIME DELAY:

In order to achieve selectivity and back-up protection, it is essential to ensure a delay in the relay operation. A time-graded system needs to be applied in these schemes. Time-current grading implies that the operation time is inversely proportional to the fault current. If the fault current is higher, the relay should operate quickly to protect the system by minimizing time delay and damage.

Time delay realised :   T = K/{(PSM)n - 1}

T= Operation time of the relay
PSM = Plug Setting Multiplier
         = (Fault current/Pick up current)

https://electricalenergyworld.blogspot.com/2019/05/what-is-time-delayed-in-over-current.html


*where,  K & n is Constant determine the operating characteristics of the relay.