Will Dean wrote:

At the time I made the original post, I only had a 15th edition to hand. I
have since bought a 16th (I have waited long enough in between for both my
15th and 16th to be brown-cover!), and it actually has explicit notes on the
MCB graphs about checking with mfg data for very short times.

Indeed so.

As you rightly point out, the calculations for very small fractions of a
cycle *are* different in nature anyway.

Firstly, the reason that the fault calculation is usually done at the
furthest point of the circuit is that that is /usually/ the worst case.
When we're not at the bottom of the graph, as you put it, a lower fault
current leads to higher I^2*t let-through and hence more risk of cable
damage.

Hmm, I'm not sure I'm convinced about this by the kinds of calculations I've
done. Current is falling linearly with cable length, which brings I^2 down
very quickly. Because one almost always seems to be on the magnetic part of
the breaker curve, it's not clear to me that 't' is rising so quickly.

You're right. For MCBs I^2*t increases with /falling/ current below the
'instantaneous' trip point (i.e. on the thermal-trip part of the
characteristic) but increases with /rising/ current on the magnetic-trip
part of the characteristic. There's a graph of I^2*t against I on page
130 of the /commentary/.

[...] I was merely trying to understand the proper way to do the
calculations.

Me too... You would find the commentary interesting (well I did):
http://www.iee.org/Publish/Books/Wir...?book=NS%20031

--
Andy