lyledunn said:

The impedance difference was only 0.07 ohms but the resulting current difference was substantial.

Not in relative terms.

I am inclined to wonder how often the PFC gets anywhere near the rating of the devices in a DB (as opposed to heftier panels).

But on the load side of a suitable 'death or glory' fuse to limit the energy let-through, even that 25.9kA becomes rather more manageable, as the  maximum damage is reduced to being the equvalent of a much lower PSSC.

Which is how 10kA breakers are fine downstream of almost any supply that a 100A B1361 can protect, almost regardless of the pre-fuse pssc.

Of course the loop tester  does not 'see' the fuse and has no means to allow for it's  energy limiting action so may well over-read the danger.

(that cast iron box by the incomer does something.)

Mike

In the National Grid link I posted above 19.6 kA is given as the stated PSCC for a single phase supply,  new domestic consumer units should have a conditional rating of 16 kA when protected by the DNO fuse as Mike indicated and as explained by BEAMA in this article, although the stated PSCC is 19.6 kA the 16 kA conditionally rated consumer unit is okay. 

www.google.com/url

But, that doesn’t hold true for distribution boards and fuse boxes with semi-enclosed fuses.

electrical.theiet.org/.../semi-enclosed-fuses.pdf

So it may be better to actually take a measurement and record the test result, rather than just writing down the DNO figures, to be realistic.

There is a note at the bottom of the BEAMA article I linked to about certain parts of England and Wales having LV Interconnected Pairs, not a generally an issue. Which I presume is part of these projects, presumably with the local network being fed from multiple points rather than just one transformer PSCC readings are going to be very variable.

https://www.enwl.co.uk/globalassets/innovation/smart-street/smart-street-key-docs/smart-street-design-and-operation-of-lv-interconnected-networks-study.pdf

and

innovation.ukpowernetworks.co.uk/.../lv-interconnected-pairs

That study includes some odd ideas - almost everything else we know about losses says higher voltages are better.

"Conservation voltage reduction (CVR)
Electrical equipment made for the European market, including household appliances and
lighting, is designed to operate most efficiently in the region of 220 to 230 volts. This
equipment can, however, operate adequately at voltages in the region of 200 volts. If power
is delivered at voltages higher than these optimum levels, energy is consequently wasted "

Boiling a kettle or heating a room faster is not a loss it is a saving, as is obvious by thinking about boiling it very slowly, the as then it never gets warm. A 1kW kettle is a lot slower than 1/3 of a 3kW one,  as heat is lost during the process, and a 100W kettle never boils at all. So, boiling adiabatically in a fraction of a second with megawatts would be ideal but is sadly impractical.
Anything electronic with a switch mode regulator will draw more current at lower line voltage to compensate, so the wiring losses are higher.
I do now wonder now about the rest of the thinking.

Mike

mapj1 

My take on it all is we have got to stop thinking in terms of the simple line diagrams we have been shown for Ze, PSCC and Zs with a single transformer and its earth with simple circuits from it in the DNO and consumers installations, because the grid is getting so complex with many connections to generators and lots of interconnections of earthing.

Though having said that we have lost many interconnections of earthing through gas and water pipes over the last twenty years.

Don't forget the original questions:

Why should we waste time with prospective earth fault current?

Are there cases where PEFC is higher than PSSC?