Circuit Breaker Ratings

Hi,

I am student electrical engineer and I should probably know this stuff but I am trying to work out how to determine the required short circuit current rating of circuit breaker, I have seen in the Schneider Electric catalog 'determination of short-circuit current" using cable data and upstream short-circuit current information (which I cant find - Im assuming since this a standard commercial installation there is a Australian standard upstream SC rating for 22kV/415V transformer), there is also cascading(backup) and enhanced discirmination. I'm also concerned with all the 'other' protection terminology and requirements, earth fault loop impedance, overload current, disconnection times, RCDs, abnormal voltages, equipotential bonding etc.

I have tried to go through the Australian Standards to determine how to determine the ratings for a standard commercial installation, but was hoping someone could enlighten me with a more succinct answer please!

Circuit Breaker Sizing = current flowing through the circuit multiply by 1.25 .

For example : current flowing through the circuit is 15 Amps . then 15*1.25=18.75 . so go for higher value 20 amps C.B.

coming to short circuit .

permit unit = Actual value/base value

select the base voltage is equal to supply voltage

Base current =base power/ base voltage

v=iz base impedance = base voltage/base current *1/Root 3
Actual impedance = R cos Φ + x sin Φ

pu =actual impedance/base impedance

fault power= Actual power/ P.U

fault current= fault power/root 3* base voltage

Fault current is also know as shor circuit current