What factors determine the FCT, for example i have a cable protected by a main breaker. Just after the breaker and before the cable the FCT is about1.65sec and just after the cable it is 35 sec. Why the difference in FCT? What effect does the cable have on the calculations?

A long cable run can have enough Z to lower the fault current enough that it won't activate the INST or ST trip, sounds to me like you are seeing a ST trip before the cable and a LT after the cable. Adjusting your pickups can fix that.

the breaker only has a magnetic trip and i have varied it from low to high but it does not effect the FCT

Then you have a problem, I wonder if the breaker was sixed properly in the first place, in the meantime, use the IEEE max 2 second clearing time exception, should be built into your sofware.

okay, thank you for your help

Be sure to check back, there are people on here that have done a lot more studies than I have and may have some better advice.