NICK, In order to be safe all switches and/or MCBs must be fully rated to carry the full load current AND be rated to make/break the maximum fault current if a fault develops anywhere down stream.
In your case 400 amps is full load but the fault current could easily be 40,000 amps assuming the incoming transformers/generators each have a 1% impedance when run separately. We normally try to run the 11kV system fault level at under 350 MVA by opening the bus section circuit breaker your auto change-over switch and limit fault current to 25kA.
