Feeder calculation - IEC Short Circuit 1

[ananeira]

Hello,

I would like to know how to take into account IEC 60909 short circuit currents (I"k, Ik, Ip and Ith) when calculating/defining the size of a cable. My guess is that I should use the Ith value considering the opening time of the associated protective device.

Of course, I know I should also take into account other issues such as ampacity, steady state voltage drop, starting voltage drop, raceway de-rating, etc.

Regards,
Ana

 

[7anoter4]

Hi ananeira
You are right. In IEC 865-1 Short-circuit currents-Calculation of effects
Ith=Ik"*sqrt (m+n)
Where m is heat dissipation to dc component factor and n is ac component factor. But when a number of short-circuits occur a short time interval between them the Ith is more complicate Ith=sqrt (sum (Ithi^2*Tki)/Tk) where Tk=sum (Tki). Tki =duration of short-circuit number i.
So even if you may calculate one Ith is difficult to appreciate how many short-circuits will be sometime. So, I should recommend to take only Ik" into consideration and the Tk =1sec. in order to calculate the minimum cable cross-section. From here you may increase this cross-section according to rated current [ampacity] and use the derating factor for ambient temperature, the heat dissipation on cable raceway and so on.
Regards