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If the proposed CU change is likely to happen before the hot tub installation you are best simply to put in one big enough CU for the lot and keep the house on PME. Single SPD, single isolation switch, everything consistent.
If you can run SWA out to the hot tub you have the option to simply feed it from a MCB and put the RCD out there in an IP65 or similar enclosure (as for caravan feed, etc) and have all that is down-stream of the RCD on a local TT earth rod.
Or if it is easier and your CU choice has DP RCBO then all you need at the hot tub end is the isolator and to separate the SWA earth (from the PME supply) from the outdoor circuits' TT earth. So use a plastic gland, etc, and not the usual metal type for connecting the armour up.
If the sockets are not expected to be used for large loads or extended in the future, then just put them on the same 32A supply as the hot tub. That way you don't risk a PME-derived earth to a lamp or similar being next to a good true-Earth TT supply.
Others might have some better ideas!
The customer is just wanting an additional consumer unit fitting. There is an isolator on the meter tails so I was planning on henley blocking the tails and installing the new consumer unit. this gives 1 isolation point to the property.
so going the TT way at the hot tub if I take the the supply down to the isolator with the PME connected is it just a normal stuffing gland used and the armour cut back ? or is there a special plastic gland made for this situation ?
The double socket they have asked for it to be able to be switched from inside the house. so I was going to run a separate 2.5mm SWA for the socket but again its difficult to earth rod it and keep the PME separate.
I had planned to use a 3 core SWA for the hot tub as originally I was going to transport the PME. But could just change this to 2 core. If however I keep the 3 core and use the 3rd core as a switched feed for the socket. I have a 4 pole rotary isolator I can take it through that so switching the hot tub off isolates the lot. Then just take a loop a 4mm SWA from the isolator to the socket using the TT earth and a shared neutral.
Inside the house fit the new consumer and double pole RCBO feed the Hot tub supply and also a supply to a 20amp DP switch from that RCBO so it keeps it all on the one supply. then join the DP switch switched supply onto the 3rd core of the swa.
originally I had planned to run a separate 2.5mm SWA back to the house but this then means id have a PME earth near the hot tub.
the socket and Hot tub supply are in separate areas as the isolator is 2 meters from the TUB and I want the socket to be at least 3 meters away.