[bstsa]

I don't understand the 65ka in a moulded circuit breaker the breaker current is 400 ampees

[hfcarson]

Bstsa:
The 65kA is short circuit interrupting rating of the device. The available short circuit current at the location where you might be using that device depends on a few things... the available utility short circuit current, the size of the utility cabling, the size of the utility transformer... etc.
That is just the utility side of your electric service, the available short circuit current is required to be considered all the way to the final device... that is a switch or receptacle.

Do you have a more specific question?