Note - this considers overload protection only. Other factors such as voltage drop and disconnection times need to be considered when designing a circuit. Also, it only quotes the example tripping/non-tripping currents for Type B MCBs, not cartridge or rewirable fuses, which are not the same.

Protective devices have 3 values of interest here:

In: its nominal rating (e.g. 32A) I1: its non-fusing/non-tripping current. For a Type B MCB this is 1.13In I2: its fusing/tripping current. For a Type B MCB this is 1.45In, and it must trip within 1 hour

So if we have a circuit whose design current (i.e. the current that it is expected to carry in normal use) is Ib, then this should not be greater than the rating of the protective device:

Ib ≤ In

If Iz is the current-carrying capacity of the cable for continuous service allowing for the way that it has been installed, then this must be no less than the rating of the device:

In ≤ Iz

Finally, 1.45 times the current carrying capacity Iz of the cable must be no less than I2, the current causing effective operation of the device:

1.45Iz ≥ I2

So Ib ≤ In ≤ Iz

and

I2 ≤ 1.45Iz.

Or if you prefer pictures:

Running an MCB at or near its limit for long periods will cause it to overheat, and eventually weaken. But you shouldn't do that - look at the diagram, Ib is lower than all the rest.