@ Alan Belson
TT-system: transformer neutral grounded, Consumer neutral grounded, no conductor but ground/earth between both electrodes. Voltage against ground equal voltage against neutral.
Quite a lot of power grids use this system all over Europe, particularly in rural areas.
IT-system(ideal): transformer neutral not grounded at all. No voltage against ground (at least in small systems without capacitive connections). First fault allows to continue service.
Typical for hospital emergency sections, sensitive industrial processes, isolation(insulation?) checking device (continuously, with alarming) mandatory.
Conclusion: Without a grounded transformer neutral there will no reasonable current on PE conductor to ground at all in case of a fault.
In France maximum resistance of ground electrode is limited to 100 Ohms by Normes Francaises. They typically use tiny, handy 1m long rods and a 6mm2(?) conductor to connect the PE of a domestic installation.
I got a small house in the Gard (30). There I measured the loop impedance of all the houses around by means of my Amprobe Genius 5080E installation tester:
My house has got a loop impedance of 314 Ohms or a maximum current of 0.8A on PE in case of short to PE! This is really above limit, but protected by a second 30mA RCD.
My neighbours got a very new installation and about 57 ohms. A friend downhill 143 Ohms.
Okay this region is rather dry, but I made the measurements at Easter after a wet winter. As it is down there, nobody bothers as CONSUEL has passed already.
For the 100 Ohms limit I have a very simple explanation:
Voltage of the bodies of faulty devices against ground shall be limited to 50VAC by harmonized norms. 50 Volts/100 Ohms equal 0.5 Ampere and zhis is exactly what you find on the EDF-RCD in the "branchement", also in yours, as you told us.
To achieve 5 Ohms on a ground electrode is not that simple at all. More difficult furthermore, to guarantee this value all over the year. Even in much wetter Germany no one expects that value for much longer and thicker electrodes than used in France.
