(4) The distributor shall not connect his combined neutral and protective conductor to any metalwork in a caravan or boat.
As to mixing of earth systems. I can’t find the document at this time. It is because with a mixture of earth systems one could have two houses with exposed metalwork for example iron soil pipes which, under fault conditions, could have lethal voltage between them.
I did queried this at the time of reading as for example where a caravan is parked next to the house and supplied with power to ensure batteries etc are kept topped up you could if supplying it with a TT supply as required with regulations get the same problem.
Which brings me to next point Yes I was in error in saying has to be TT and yes 704.411.3.1 says must not use TN-C-S and TN-S is permitted and the supply from house to caravan would be TN-S.
However with a boat ref fig 709.3 Unless you have an isolating transformer
fitted on board to isolate the electrical system on your craft from the shore supply
system, corrosion through electrolysis could damage your craft or surrounding craft. So with a boat a TN-S supply would not normally be used. I still feel the whole idea of connecting the earth through diodes does not seem safe but it is a common method with boat supplies and at £300 for an isolation transformer I can see why.
I have also questioned in the past of the problems where one house on a PME supply has for some reason a good earth independent of the boards earth and under fault conditions could end up being the earth for a whole group of houses and have quite a high current travelling through the earth cables. I should think this is why 10mm cable is required. I have seen the result of using smaller cable.
A radio ham employed an electrician to put a supply into his shack. The electrician did not use the armour of the SWA for earth but ran a 4mm cable under the SWA (2.5mm) with earth and bonded all metal work in the shed. The radio ham also connected his earth to all metal work and his earth was 4 sets of rods in each corner of the garden with earth tape between them.
Workmen down the road hit the supply cable. The radio ham called me when the plastic started to drip off his earth wire in spite of turning off the main supply switch.
By time I got there the earth cable had fused. Removing the cover off the CU to measure I found voltage floating between 10 and 390 volts. His was only house not to have electrical equipment blow up that day.
After that we changed his house to a TT supply but were told that we should not be doing this by the supply company but under the circumstances they turned a blind eye. Hence why I know it is officially up to the supply company to say what supply you have. I was shown the regulation at the time but didn’t have any way to make a copy.
Maybe the rules have now changed this was back in 1994 or there about.