MCCBs

That is true - fuses get faster as the fault current rises, being more or less constant energy devices - the number of watt-seconds to vapourise a length of fuse wire is constant, regardless of if it happens in a second or 1/100th of a second. (so as I squared R times T is more or less constant, for the purposes of a log-log graph of breaking time vs fault current, so is I squared t) This is related to the idea of a constant let through energy, or energy limiting action of the fuse.

So 2 fuses will discriminate  together over all fault currents, so that the thinner one blows first, and to be sure the larger one is not weakened, then a ratio of perhaps 2.5 or 3 to 1 between steps is recommended.


Breakers are mechanical, and beyond a certain point, they stop getting faster with increasing fault current, as the contacts do not move any quicker, and even if they could, the arc is not extinguished, and the magnetic core of the actuator coil saturates, so there is no further rise in magnetic attraction beyond a certain current.


The upshot is, at large fault currents, cascaded breakers may well not trip in the right order, or may both trip, and the let through energy is not a constant, but a varying function of supply impedance rising with PSSC..

Indeed for  large (5kA plus) faults, a fuse may well be faster than the breaker, and it is worth backing up an MCB or MCCB with a large "death or glory" fuse - often correct fuse choice allows a smaller MCCB to be specified, as well as mitigating for a welded contact, which is rare but not utterly unknown.

Note that despite the text books, real faults are rarely true  zero-resistance - if they did there would be no burning of the end of the wire or big flash - after all these represent lost energy, so some resistance to the current flow,  so the fault current is rarely quite as big as estimated, and so the big back-up fuse is rarely called upon.

That is true - fuses get faster as the fault current rises, being more or less constant energy devices - the number of watt-seconds to vapourise a length of fuse wire is constant, regardless of if it happens in a second or 1/100th of a second. (so as I squared R times T is more or less constant, for the purposes of a log-log graph of breaking time vs fault current, so is I squared t) This is related to the idea of a constant let through energy, or energy limiting action of the fuse.

So 2 fuses will discriminate  together over all fault currents, so that the thinner one blows first, and to be sure the larger one is not weakened, then a ratio of perhaps 2.5 or 3 to 1 between steps is recommended.


Breakers are mechanical, and beyond a certain point, they stop getting faster with increasing fault current, as the contacts do not move any quicker, and even if they could, the arc is not extinguished, and the magnetic core of the actuator coil saturates, so there is no further rise in magnetic attraction beyond a certain current.


The upshot is, at large fault currents, cascaded breakers may well not trip in the right order, or may both trip, and the let through energy is not a constant, but a varying function of supply impedance rising with PSSC..

Indeed for  large (5kA plus) faults, a fuse may well be faster than the breaker, and it is worth backing up an MCB or MCCB with a large "death or glory" fuse - often correct fuse choice allows a smaller MCCB to be specified, as well as mitigating for a welded contact, which is rare but not utterly unknown.

Note that despite the text books, real faults are rarely true  zero-resistance - if they did there would be no burning of the end of the wire or big flash - after all these represent lost energy, so some resistance to the current flow,  so the fault current is rarely quite as big as estimated, and so the big back-up fuse is rarely called upon.