Fair enough. Used the wrong terminology. Obviously not a Zs value between L and N but a line impedance value which is used to derive the PSSC.
On a three core SWA the CPC core and the armouring are connected at the 'supply' side of the circuit but not at the 'load' end. Therefore they are deemed 'earthed'. At the 'load' end there is a 16mm2 CPC to an earth rod. Most Zs results at the CU at the 'load' end of the circuit have been around 12 - 14ohms with a PFC of about 14A.
Although under earth fault conditions the 14A would be enough to operate the 100mA RCD, it would not operate the disconnection device at the 'supply' end, ie, 40A MCB in time / at all.
However, the measured line impedance of say 0.39ohms with a PSSC of 0.594kA would.
The question I was trying to ask in a very roundabout way is that although I can see in theory that this would all stand up to scrutiny (I thought), is it correct in your opinions?