kA^2s

mapj1:  not suitable for mechanical ADS with moving contacts ....  stops getting faster at higher current beyond a certain point.

Forgive me if my rather vague initial question made you search for a deeper technical issue in my question. I wanted to leave a bit of puzzle about what the issue could be, requiring some thought about what I2t values made sense in view of the other details about peak and implied duration. But my main point was only the pedantic one about units, now explained in my recent posting.


Back to your more interesting technical point: yes, although MCBs and larger breakers have become much better at their current limiting over the past few decades, they can't match fuses as the fault currents increase. In case anyone would like quick examples, curves of let-through I2t versus prospective current are good at showing this. Page 24 of Sentron MCB has an example of what looks a good MCB - I've seen cases where the increase of I2t with I_prospective is steeper.  In contrast, page 59 (Fig.4.4.5) of Sentron Fuse, although of miserable graphical quality, shows the classic flat curve for a fuse.


A few years ago I was at an event where some manufacturers explained the modern world to users of LV switchgear etc. One theme was that fuse backup is no longer needed (provided that ratings aren't exceeded), but is just a folly of dinosaurs (ok, my exaggeration). We could get into the usual issues of statistics, etc, which might so far do little to support a need of fuses.  But I always feel more confident to know there's a good-quality fuse there as well. 

 

mapj1:  not suitable for mechanical ADS with moving contacts ....  stops getting faster at higher current beyond a certain point.

Forgive me if my rather vague initial question made you search for a deeper technical issue in my question. I wanted to leave a bit of puzzle about what the issue could be, requiring some thought about what I2t values made sense in view of the other details about peak and implied duration. But my main point was only the pedantic one about units, now explained in my recent posting.


Back to your more interesting technical point: yes, although MCBs and larger breakers have become much better at their current limiting over the past few decades, they can't match fuses as the fault currents increase. In case anyone would like quick examples, curves of let-through I2t versus prospective current are good at showing this. Page 24 of Sentron MCB has an example of what looks a good MCB - I've seen cases where the increase of I2t with I_prospective is steeper.  In contrast, page 59 (Fig.4.4.5) of Sentron Fuse, although of miserable graphical quality, shows the classic flat curve for a fuse.


A few years ago I was at an event where some manufacturers explained the modern world to users of LV switchgear etc. One theme was that fuse backup is no longer needed (provided that ratings aren't exceeded), but is just a folly of dinosaurs (ok, my exaggeration). We could get into the usual issues of statistics, etc, which might so far do little to support a need of fuses.  But I always feel more confident to know there's a good-quality fuse there as well.