UK Consumer unit TT system far from DNO fuse

[Sib_elec]

Evening all, I’d appreciate your opinions here:
I’m looking at an old (150 years+) stone house. Complete rewire and renovation. DNO fuse is currently where the master bedroom will be (upstairs). Client wants it to be moved to a more accessible location downstairs about 15m away depending on the routing of cable.
The timescale for DNO to move their supply and make my life easier is too great so to be avoided if theres any other option.
So the plan is DNO fuse -> meter tails (1m long) -> 100mA Type S RCD (100A) -> SWA (15m x 3c25mm)-> new CU.
Perhaps I can get away with a main switch instead of Type S RCD if tails are properly protected.
Conveniently (or not) there is an earth rod pit on the other side of the wall to where he wants the new CU.
This means I can fairly easily install the MET next to the new CU and bond the CU and incoming SWA to that.
All ccts from CU will be RCBO protected radials.
Any thoughts on this?
What about fitting backboxes to old stone? Its going to be an absolute nightmare to drill, thankfully he plans to dot and dab so no chasing needed.
Thanks

 

[pc1966]

Hmm, usually you would look at a switched-fuse on the SWA in case of a fault, but on TT then it would need a (delay) RCD for clearing L-E faults. Relying on the DNO fuse for L-N fault clearance is slightly suspect, but that is less likely than L-E in the event of someone nailing the SWA by mistake. As it is a TT system you don't need 3C cable as the armour on 2C will do for CPC since its impedance will be several orders of magnitude less than your earth rod(s). However, if it is ever "upgraded" to PME the installation would need the 10mm main bond conductors back to the DNO cut-out point as even 3C 25mm SWA is only 7.7mm Cu equivalent in the armour (though obviously with 3C you have your >= 10mm CPC internally).

Just to note that 25mm SWA in duct is marginal for 100A incidentally. Meets it for 90C insulation types but you would need to check accessories are up for that. In the 100A case I would be tempted to go for 35mm 2C SWA, but if 80A or less 25mm has no worries.

Going all-RCBO is certainly the best plan, and I will avoid commenting on RFC-versus-radial to keep out of those wars here. However, I would suggest you get DP switching RCBOs such as Wylex. Crabtree, or Fusebox, so a N-E fault is not going to take out the 100mA delay RCD feeding the lot.

 

[snowhead]

Drilling stone isn't a problem with SDS.
What can be a problem is if the walls are 600mm + thick, twin leaf with loose infill, as the fill drops when you pull the drill bit out.
The solution is to remove the drill and push the drill bit through with a sleeve, as I had to with my Son's house in Northumberland.

If the DNO head (and meter) are upstairs presumably it's overhead supply?
If so why not run SWA back outside for now and in to the new CU postition, then when the DNO move their cable and head, do away with the SWA and switch fuse.