Sorry sorry I'm confusing zs and pfc again!
If you measure PFC in both cases it will be differenct. Zs by definition must be corrected for voltage, so should be the same in all cases. But the point is it's your fault current that trips the MCB so the zs isn't quite enough to know it'll trip.
Yes, my tests indicate that my meter is measuring true (unchanging) Zs:
No load. V=248.6V. Zs = 0.68Ω. Calculated PFC = 365.6A.
2kw fan heater. V=247.4V. Zs = 0.68Ω. Calculated PFC = 363.8A
2 x 2kw fan heaters. V = 246.2V. Zs = 0.68Ω. Calculated PFC = 362.1A
[all of the following again assuming TN-C-S for simplicity]As I've said before, one problem which arises is that, during the duration of a negligible impedance fault, my 'supply voltage' will fall to a very low level. My current Ze is about 0.32Ω. With the Zs of 0.68Ω, that means that R1+R2 is about 0.36Ω. That means that with a negligle impedance fault, and assuming a no-load voltage of 248.6V, the 'supply voltage' (across R1+R2, and the fault) should fall to about 131.6V, giving a calculated PFC of 193.5A - albeit still enough to magnetically trip a B32. However, if the Ze was just a bit higher, it would not be enough toguarantee magnetic tripping the B32, even though the Zs was well below the specified maximum for a B32. For example, if the Ze were, say, 0.5Ω, making Zs 0.86Ω (still well 'within limits'), the during-fault voltage ought to be about 104V, leading to a calculated PFC of about 121A - far too low to guarantee a magnetic trip of a B32.
Given the answers that the meter gives, I can but presume that it uses the 'before fault' (not 'during fault') voltage to calculate PFC. However, as above, as far as I can make out, the actual current during a fault would be much lower than that figure (quite possibly too low for a guaranteed magnetic trip), since the voltage would be so much lower. Yet again, I suspect I'm missing something, or making some silly error!
Kind Regards, John