It's an interesting point. Here in the U.K. we can use fuses or breakers with a rating less than the prospective short-circuit current so long as there is another overcurrent device "upstream" which is suitably rated to break the fault current. So if you had a supply capable of sourcing 20kA, you could still use 6kA circuit breakers in the panel if there was an HRC fuse first which can break 20kA.
Conveniently, all of our supplies have a PoCo-owned cartridge fuse at the service entrance, just ahead of the meter.
