Cable sizing calculation : short circuit withstand time selection

[arbil]

For determining the cable size we are using the following formula
S= I*SQRT(t)/K , where
S= nominal cross sectional area of the conductor in mm2
I= Fault current in Amps
k= constant based on type of cable
t= short circuit withstand time in sec

My question is , whether there is any standard for deciding the value of 't' to be considered in the calculation . Normal practice for us is 1.0 sec for Incomer to HV switchboards , 0.6 sec for HV switchboards to plant feeder, and 0.2 sec for motor/transformer breaker feeder.But I don't have any backup for the same .

 

[jghrist]

"t" is the fault duration. How long does it take for the cable's overcurrent protective device to clear a fault. It depends on the protection settings and the fault current. You might want to consider backup fault clearing time as well. If the overcurrent protection immediately upstream of the cable fails to clear the fault, how long does the next device take?

 

[ijl]

The application guide of SKM at

http://www.skm.com/Equipment Damage Curves Cables.shtml

and the IEEE Std 242, IEEE Recommended Practice... discuss the relation between time and current, but mainly from another point of view: Cable is given, what is the time?

 

[7anoter4]

This criterion is called "thermal stability to short circuit" of the cable.
The conductor heating is considered to be adiabatic so no heat dissipation is taken into
consideration. As the short circuit current is not constant during the process a decrement factor has to be applied according to circuit impedance and the fault time as jghrist explained. Also, even this is a short time, the conductor resistance will increase very fast so you have to integrate the conductor losses in order to find the reached temperature and to compare it with permitted insulation temperature [for short time ]. A good relation may be found in IEEE80 for Grounding Grid conductor. Usually, for medium and high voltage that will be the minimum conductor cross section area. If this is the only single criterion I think 1 sec. will be fair.
But you have to check perhaps, other criteria as cable heating for the steady state [rated] current. This depends on cable construction as well on the way from one end to another: cable tray, trench, buried, underground duct, exposed or exposed conduits and may be if exposed to sunshine to add this kind of heating. For all this it is better to use the suitable standards.
Some time for a long distance you have to check voltage drop, also, and namely if it is for a motor provided only with DOL.