I agree constant voltage for big UPS and small faults - where the fault loop resistance is high enough that the UPS just sees the fault as a load that is within its capability. BUT, as Andy alludes above,  unless the UPS is massive and supplies a great many very long thin circuits, that will not cover the case of most credible faults. The problem is that at some overload level, be that 120%,. 150% or even 200% of the maximum load, the UPS will go into current limit, and the voltage will collapse, perhaps maintaining a constant current, perhaps folding the voltage back more steeply than that. At this point you do not have the energy to operate the fuse or breaker promptly, and the other connected equipment sees a brown out - and if SMPS based, may even try and draw more current to compensate for the falling voltage, leading to even sharper voltage collapse and even less power available to perform the disconnection. This is a state to be avoided if at all possible, being bad for the UPS and bad for the other connected equipment as well.
So unless all the instant trip currents of all the breakers or fuses are well within the UPS range of normal loads, you need to consider how the UPS system can recover from this. As above, the earth fault relay or on smaller systems the humble RCD is your friend, as this can be set to a small fraction of the load current of the circuit it protects, typically 0.1 % to 1% depending on the load type and operates promptly even when the UPS is in collapse, allowing UPS power to be restored to all but the faulty branch, hopefully.

Mike

I agree constant voltage for big UPS and small faults - where the fault loop resistance is high enough that the UPS just sees the fault as a load that is within its capability. BUT, as Andy alludes above,  unless the UPS is massive and supplies a great many very long thin circuits, that will not cover the case of most credible faults. The problem is that at some overload level, be that 120%,. 150% or even 200% of the maximum load, the UPS will go into current limit, and the voltage will collapse, perhaps maintaining a constant current, perhaps folding the voltage back more steeply than that. At this point you do not have the energy to operate the fuse or breaker promptly, and the other connected equipment sees a brown out - and if SMPS based, may even try and draw more current to compensate for the falling voltage, leading to even sharper voltage collapse and even less power available to perform the disconnection. This is a state to be avoided if at all possible, being bad for the UPS and bad for the other connected equipment as well.
So unless all the instant trip currents of all the breakers or fuses are well within the UPS range of normal loads, you need to consider how the UPS system can recover from this. As above, the earth fault relay or on smaller systems the humble RCD is your friend, as this can be set to a small fraction of the load current of the circuit it protects, typically 0.1 % to 1% depending on the load type and operates promptly even when the UPS is in collapse, allowing UPS power to be restored to all but the faulty branch, hopefully.

Mike