Andy Wade wrote:
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.
snip
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.

For reference, I'm in the middle of a village, about 100m from the nearest
substation, and get about 0.5 ohms.

If you've got a voltmeter and enough clue to connect it to the mains without
killing yourself, this is easy to measure.
Take a known load, for example the immersion heater or an electric shower.
Measure the voltage with this off and on.
You have to do this a few times, as the mains voltage varies a bit normally.

This gives you a measure of the impedance from the busbars of your
consumer unit back to the nuclear/gas/solar/wind/... power station.

Meausuring and using a load on the same circuit isn't a good idea as this
adds that resistance in.