It is best to consider the safety issue as a whole and not focus on just one aspect unless that aspect requires top priority in a specific instance.
Example:
If an MCCB were directly fed from a huge low impedance transformer, close to the MCCB, and capable of delivering 100,000 amps of fault current, then the AIC rating of the MCCB immediately climbs to the top of the priority ladder.
Whereas, if that same situation existed except that the transformer was hundreds of feet away and the MCCB was not fed directly from the transformer but instead was fed through another protector, then the AIC rating would not be so important because the fault current is going to be much less and in reality the devices upstream are responsible for the total AIC.
Circuit breakers provide greater safety in our applications (pumps) due to their ability to fully disconnect the load under fault conditions, which fuses almost never accomplish. The vast majority of faults are low level, not high level, so very rarely will the full available fault current occur. So in the vast majority of situations, circuit breakers fully disconnect any fault whereas fuses do not provide that.
There are two policies we have to protect ourselves:
1
We never allow our equipment to be connected directly to a main supply transformer, the owner must always provide his own high level AIC protectors near or at his service entry if the potential for high level faults exists. This seems sensible as it puts the onerous requirement on the engineers designing the service to ensure that if a possible high level fault occurs the protective devices nearest the service are going to intervene.
2
We standardized on a higher level of AIC rating for our circuit breakers, and we do not sell any product without that higher AIC rating.
We realize that if the worst scenario possible develops we may still have a problem. But lets all admit one thing, it is exceedingly difficult for a bolted fault to occur, and then add that to a possibility of that type of fault occurring in a situation where the fault current can even reach the strength required to cause a problem.
A tremendous number of things have to line up just so in order for the worst thing to happen: Bolted fault occurs, and at just the wrong point in the sine wave, and the device is very close to large low impedance transformer, and the wire size from transformer is huge (not usually the case for economic reasons), and there are no other devices upstream to cause resistance, and the cause of the bolted fault must be huge, like a solid bar of 1” steel falling across a huge buss (the initial cause of the short usually blows up and disintegrates before the current can even begin to climb too high).
Does this ever occur – Yes.
However people are hurt every year messing with fuses and being shocked by partially disconnected loads, probably far more than are ever hurt by explosions from bolted faults.
I realize this is just my opinion, but I face this every day and when people default to fuses because it makes them feel safe I cringe because they are not taking all factors into account and they do not really accomplish higher safety but lower safety in many situations.
PUMPDESIGNER
