I'm not sure its very easy to get MCBs to discriminate, bit like RCDs in the way they work, if you get a fault above the level at which they operatae, then they will operate quickly
If the fault that occured was above the level at which the upstream breaker operated at, then chances are they'd both trip, if the Z's of the final circuit was high enough that the fault currents were too low to operate the upstream breaker, then obviously thats one way of forcing discriminination.
Fuses are easier,just make sure the pre-arcing I²t of the upstream one is lower than the total I²t of the downstream one (ie. make sure that the let through energy of the smaller fuselink after the arc has cleared and the circuit opened, is insufficent to melt upstream fuselink)
Fuse-> mcb discrimination is tad harder than fuse->fuse, similar situation, the total let through energy is available for MCBs (but it varies in relation to fault level*)
*so does a fuse, but in a different way
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