# Re: how to find time?

#12465
Spir Georges GHALI
Participant

Dear ;

Normally, we use the formula ” I² x t ≤ K² x S² ” to know if the cable can withstand the short circuit current during the tripping time of the protection device, and as ” I² x t ” should be equal or smaller than ” K² x S² “, we have 2 possibilities :

– Either, by knowing the value of the short circuit current and by using the ” Time /  Current Curve ” we can define the value of ” t ” then do the calculation to know if ” I² x t ” is equal or smaller than ” K² x S² ” then decide if the section cable is enough or should be oversized.

– Or, by knowing the value of the short circuit current and the section of cable & the value of K, we can do the calculation to find the value of ” t “, then compare the value with the tripping time of the protection device that ca be obtained by ” Time / Current Curve “, if the ” t ”  calculated value is bigger than the tripping time, the cable's section is enough, if not the cable's section should be oversized.