High voltage cable sizing

Two seconds is a long time for a fault to exist. Most OC devices will operate in cycles. 2 or 3 cycles is common. Check your fuse or breaker and determine the time to interrupt the fault of 50 KA. Let us know. You can then check it against the time for 70 sq mm.

This link may help (the conductor sizes are in AWG/Kcmil though):

http://www.okonite.com/engineering/short-circuit-currents.html

This chart helps to find relation between AWG/Kcmil and sq mm:

http://www.generalcable.com/NR/rdonlyres/2971C51A-C344-4B41-A03F-272F190B51BA/0/82_SPEC_A185.pdf

- MS

That could be because you inserted fuse instead of circuit-breaker as protective device in ETAP

i agree with the 2 second withstand time be too long...two Fault Current Level are imbedded into the breaker...one is the 5th/8th cycle level which is the Interrupting Rating and the other is the 1st-to-5th cycle level which corresponds to the Close-and-latch rating of your breaker...close and latch refers to the capability to withstand that huge amount of current for a short period of time...ussually 1.6 times the KAIC Rating...

the 300 sq. mm cable is rated to withstand the fault current and noted that the CB will interrupt the fault at the 5th/8th cycle..

and your 70 sq. mm corrrespond to a fuse application that will interrupt the fault on the first cycle time...