Withstand ratings have to do with the costruction standards asked for, not the basic design. We use Metal Enclosed FLIS (Fused Load Interruptor Switch) gear that is rated 31.5kA.
dpc's definition is correct, Metal-Clad is circuit breakers, Metal Enclosed can technically be either, but tends to be FLIS because of a key issue with vacuum circuit breakers; they cannot be used for safety disconnect without being withdrawn from the bus as in Metal Clad, or isolated with an additional switch that provides an air gap of sufficient dielectric strength.
In the event of a fault, vaporization and deposition of contact material inside the vaccum bottle can allow leakage after they interrupt. That leakage can be lethal, so when circuit breakers are used, you must have an air gap isolation for worker's safety. By withdrawing from the bus and shuttering the opening, Metal Clad gear provides that air gap without requiring an additional switch mechanism.
Metal Enclosed FLIS gear uses a Fault-Make / Load-Break rated disconnect switch that provides the necessary air gap, but short circuit protection is provided by the fuses. It tends to be much less expensive that MC gear and shallower, 36 - 44 inches compared to 84 - 96 inches for MC gear. But unlike MC gear, you can't stack one unit on top of another, so the gear length can get out of hand in large systems. If you were to use vacuum circuit breakers in Metal Enclosed gear, you must still add an isolator switch, which adds even more to the size and too much cost compared to just using Metal Clad. You used to be able to get air circuit breakers in ME design gear years ago, but I don't think anyone does that any longer.
So all the classic debate points of fuses vs circuit breakers must be decided first, then the gear you use follows that decision.
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