Just a pedantic clarification.
Bus bracing is not to withstand the CURRENT, it's actually to withstand the MECHANICAL FORCES that will be created by the extreme magnetic field repulsion between bus bars under fault current conditions. But to the original points, it is usually expressed as a current value because the magnetic forces are pretty well understood and that's what really matters. So when a piece of gear says the "Bus Bracing = 42kA", that is saying that the insulating support structure for the bus bars will survive the mechanical repulsion forces expressed between the bus bars during a fault with a magnitude of 42,000 amperes.
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