Zikki Malambo wrote:

When my friendly local Sparky was round a couple of nights ago I asked
him if he thought the 30A MCB which had been feeding the previous
setup was man enough for Christmas Day, and he said no and fitted a
40A. I suppose he works on the basis he doesn't want to get called
away from his Christmas Lunch!

I think your assessment of his motives is just about right ;-) The whole
business we're arguing through here is a little bit marginal. On the one
hand, you'd find it really quite hard in practice to pull enough current
from the hobnoven to make the 30A breaker trip - the closest you could
get would be by having the appliance isolator off, turn all the controls
to demand maximum heat (i.e. turn on all rings, + heaviest-draw setting
of oven - prolly 'oven+browning-elements' if your Miele does that), and
then turn on the isolator. That gets you all elements drawing their cold
inrush currents at once - but they very soon get warm, settle down to
"merely" their rated draw, and a little while after that start dropping
in and out of the load as their thermostats, simmerstats, or fancy
PWM-based regulators - hey, you did say this was top-of-the-line Miele
kit! - start to do their stuff. As shewn in previous response, that
"maximum sustained" draw would be 48A, barely 1.6 times the 30A MCB's
nominal current. Yer typical Type B/type " domestickle MCB will take
15-30 minutes to trip on so "small" an overload - and it's really,
really, hard to believe that your hobnoven will keep drawing their full
rated load for that long, even when you do the turkey, sprouts,
parboiling-of-the-roasties, sosij-meat-balls, resteam-the-pud, and the
nut roast in the second oven for the non-carnivores.

On the other hand, though it's correct to worry about the thermal
behaviour of the cables/flexes supplying your hobnoven in the case of a
hefty short, the conditions needed in practice for the larger trip (your
40A) to pass fault current for long enough to cause irreversible damage
- or even notably shorter cable/flex life - are also unlikely to occur.
Kitchens are typically not a huge distance from the CU; with say 10m of
6mmsq and a couple of "wimpy" 2.5mmsq flex at the end, total loop
resistance down the 'worst case' path of L-to-E (so as to put the fault
current down the thinner protective conductor of the 6mmsq) will be in
the range of 0.1 ohms. Let's quadruple that to allow for increased
contact resistance in a couple of connections along the way: that's
still a rather low 0.4ohms resisting the 240V, giving us a
rather-MCB-persuasive current of 600A. That's 15 times the nominal
rating of the 40A MCB, which takes you into the fast solenoid-based
'gross overload' part of the MCB's operating regime (rather than the
slow thermal-based 'smaller overload' part), so you'll get disconnection
within 0.1s or so - not nearly long enough for your 'wimpy' cable to
heat up towards the 140 or so degrees which is the 'don't go there' temp
for PVC. From which semi-quantitative argument, we conclude that your
sparky's uprating of the MCB from 30 to 40A - while not best practice -
doesn't cause a keep-you-awake-at-night level of risk.

HTH - Stefek