Cable sizing calculation : short circuit withstand time selection
Cable sizing calculation : short circuit withstand time selection
(OP)
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 .
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 .






RE: Cable sizing calculation : short circuit withstand time selection
RE: Cable sizing calculation : short circuit withstand time selection
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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?
RE: Cable sizing calculation : short circuit withstand time selection
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.