tony sayer wrote:

['Lurch' wrote:]
That may be but the instantaneous current that flows immediately prior
to a circuit protective device operating is many thousands of amps.

Well they are always looking to disprove this and that. Such as is
progress made. When I see him again I'll ask

It's very simple you know. The short circuit current depends on the
voltage (EMF) of the source and the impedance of the circuit between the
distribution transformer [1] and the point where you have elected to
apply a short-circuit fault. I = V / Z.

For single phase mains the EMF is nominally 230 V, 240-250 V in
practice. The impedance is highly dependent on the length of cable
between the transformer and the fault.

For public mains the supply industry will tell you that the maximum
prospective fault current at the supply terminals is 16 kA [2] -
implying an impedance of about 15 milliohms - most of which is the
leakage reactance of the transformer. Such high fault levels are
actually pretty rare and only arise when you are very close to the
substation.

For the other end of the scale we can use the maximum external earth
fault loop impedance value quoted by the industry for PME supplies.
This is 0.35 ohm - implying a minimum fault level of about 700 A.
Again, this is the relatively uncommon other end of the scale - where
you are on the end of a long line out in the sticks.

None of this depends on the rating of any fuses in the way, except to
the extent that each fuse adds a tiny bit of resistance to the circuit.
Obviously, lower rated fuses will blow more quickly at any
particular fault level, but the full current will flow until the fusible
elements have melted and the ensuing arc has been quenched. An
exception can arise at high fault level where the fuse can clear the
fault within less than half a mains cycle, so that peak value of the
current waveform is never reached; the fuse is then said to exhibit
'current limiting' behaviour.


[1] This ignores the rest of the network on the supply side of the
transformer, which has near-enough negligible impedance for the matter
under consideration.

[2] Except in parts of the old LEB area where larger transformers and
cables may be in use.

--
Andy