"Andy Wade" wrote in message
...

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/.

Interesting. That suggests that one should be very careful about where one
does the adiabatic calculations - it's certainly a valuable footnote when
demonstrating compliance by applying that calculation only at the far end of
the circuit, if you've assumed that the magnetic + limited-let-through saves
you at the near end.

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

Grrrr. As I've just updated both my wiring regs and OSG, and bought BS7909
( GBP4.30 per page!) in the last couple of weeks, my standards budget is
rather blown at the moment. I'd like GN3, as well.

But it's useful to have a recommendation for the book - some of the 'helper'
books that go with the regs are poor, IMO. Paul Cook seems pretty good, in
general, though. And I do get an IEE discount.

I'll just about get the set up to date, and they'll issue a '17th edition',
no doubt.

Cheers,

Will