More specifically it must be braced to withstand the initial peak short circuit current. If it survives the peak when the magnetic forces are most powerful it will almost always survive the sustained short.
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There are not a general bus bracing rating as far as I know.
This need to be calculated for specific cases and design configuration.
In addition to SC, other horizontal forces should be considered (ie max. wind speed, seismic forces, etc)
Often insulators strength is the critical factor to withstand a particular loading scenario.
Check the IEEE STD 605 or similar guide for bus bracing calc.
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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