Short Circuit Current Protection Question (kA ratings)

[wadeengineer]

Hi ENG-Tips,
I am designing a cascaded distribution board for a factory.
I want to make sure I am completely understand the importance of selecting the correct short circuit rating for either my cables and circuit breakers.

It seems that cables generally have a kA rating which it can only handle for 1 second,
and circuit breakers generally have a kA rating which it can only handle for 2 seconds.

Is there a reason for that? I am getting alot of my ratings and specifications from manufactures and my countries (South Africa) electrical regulations.

I understand that a circuit breaker possibly has a 2 second delay which is can only react on due to the mechanical demand of the unit. But then wouldn't it be smarter to design your system so that your cables can handle that kA rating for lets say 2 or 3 seconds aswell so that the cable isnt damage before the breaker reacts.

I clearly have something confused here.

Any help, would be appreciated.

Kind regards,

Wade

 

[7anoter4]

The rated short-circuit current and clearing fault time rated are the limits permitted for the breaker. The protection setting has to be less than these limits and suitable to the maximum short-circuit in the actual circuit. One has to calculate the maximum possible short-circuit level and the trip time selectivity-for the closer breaker and for reserve protection breaker.
For these conditions one has to calculate the cable connected with minimum cross section area.