Busbar Calculation

http://cr4.globalspec.com/search/sitesearch?do=show&sort=textmatchrank&srch=busbar%20sizing&order=asc

It's one of CR4's favourite topics (the other is cable sizing, for which the answer is British Standard 7671).

The formula is:

K2S2 = I2t (K square S square = I square t)

Where,

S = Cross section of the conductor, in sq.mm

I = Short circuit current in Amperes

t = With stand time, in seconds

K = Constant, depending upon the material of the conductor, insulation & type of installtion, etc. (Defined in IEC)

Expert65,

why in somecase, there is still written value of peak s/c current ?

for example :

we have got value of rms Short Circuit (s/c) = 25 kA

the peak value is 2.7*25 kA = 68 kA

the peak value is to determine breaking capacity of CB.

the rms s/c value is to determine thermal withstand of busbar and CB.

then, question in my head, is there another choice for CB selection , if we have got rms value 25 kA then there is not automatically we chooce CB with breaking capacity 68kA ?

pls help me to explain it.