Switchgear is tested to UL 1558 which has a 30 cycle withstand test. Switchboards are tested to UL 891 which has a 3 cycle withstand test. There is a similar problem with automatic transfer switches which are tested under UL 1008 to either 1 or 3 cycle withstand ratings.

Effectively when it comes to panelboards and automatic transfer switches you need to have instantaneous tripping. You might be able to get away with a very short instantaneous with delay (I know some GE electronic trip units can do this). You are probably going to end up in the world of "forget the TCC's". When it comes to series devices, you are often forced to coordinate through the dynamic resistance of the devices, something that power system analysis software and TCC's cannot measure. If you absolutely, positively have to have some delay, I know that at least with some of the older GE Spectra RMS breakers they had the capability of putting in rating plugs with very short delays on their "instantaneous" trips.

As to your standalone device...provided that you have provided sufficient mechanical strength, you'd be OK. However any time that you have breakers over fuses (usually not so bad) or fuses over breakers (usually bad), you most likely will have a miscoordination problem to decide how to deal with. Case in point. I have several portable substations which have E-rated fuses on the 25 kV side of the transformer. On the secondary side, it is either R-rated fuses and vacuum contactors or VCB's. Either way, in the event of a bolted fault (dead short scenario), often the short will pass through and pop the E-rated fuses. I cannot size the E-rated fuses large enough without losing protection, and the alternative devices (25 kV rated switches/breakers) get to be very expensive, and don't hold up all that well in an outdoor, dusty environment. So we recognize the miscoordination problem and recognize that when we troubleshoot a situation where the E-rated fuses open, we check everything downstream, not just the transformer primary/secondary area.

All UL 489 devices incorporate some type of instantaneous protection, regardless of what settings are actually available to the user. So if your low voltage power breaker is listed to UL489 standards you should not have a concern about downstream UL891 equipment.

However low voltage power breakers built to ANSI C37.13 and C37.16 or UL1066 are not required to have instantaneous tripping so their use with UL489 equipment needs to evaluated.