Thank you all for the thoughts, amazing. To clarify things, the house is TT (not TN-C-S) and outbuilding/shed also TT. House is currently on a re-wireable fuse board with a standalone upsteam 30mA RCD protecting the whole installation, so no selectivity. Outbuilding is old skool metal Wylex with power and lighting on MCB's. New DB's are planned for both
OK. But if you have a separate 30mA RCD/RCBO feed for the outbuilding that Wylex board could stay.
house and outbuilding. The house is currently destined for a full rcbo board with SPD. It will have double pole RCBO's and a built-in Type 1+2 SPD as required to cover the overhead TT supply. For a belt and braces approach the main switch will also be a time delayed 100mA RCD to protect any non-RCD wiring such as the wiring/MCB in the db for the SPD.
That is a sound approach. Personally I would use an up-front RCD as you suggest since it means there is no single point of failure in terms of electronics for earth-fault disconnection. OK you might lose the 30mA 'additional' protection if an RCBO fails but you don't risk the whole CPC system becoming live since the incomer will trip.
The outbuilding currently only has power and lighting circuits, but there is potential to expand in the future. To enable selectivity, the main switch on the new shed DB was going to be a 30mA RCD with individual MCB's for the shed's circuits. The 2-core SWA supply to the shed DB was going to be put on it's own MCB on the house DB, rather than an RCBO. House and shed are only about ten meters apart if it makes a difference.
If the out building needs are really simply just feed the sub-main from a 20A RCBO for the sockets and take the lights off with a 3A FCU. No need for a 2nd CU then!
If you plan on using it as a workshop or similar where loss of lights is a safety issue then consider putting in one or more light that has built-in emergency power (often available for baton lights and the similar anti-corrosive lights).
If you do seek selectivity then your proposal of taking it off the house CU's 100mA protected supply is fine, but then use RCBO in the outbuilding CU otherwise you might as well go with the RCBO+FCU suggestion as trip due to sockets also takes out lights!
Getting over-current selectivity is hard when fed from a MCB, you can get some of the way there by using a MCB with as high a rating as the SWA supports and going for a D-curve to push up the "instant" trip point, since your earth fault disconnection is on the delay RCD anyway.
Personally unless you need anything special I would just go with the RCBO feed and FCU approach. You can use an adaptable box for the SWA gland and then some plastic glands for T&E out of there, a few companies make ones with oval slots for that job, or if using metalclad sockets then gland directly to one of them, etc.
The diameter of the SWA is only 20mm, so a stuffing gland in an enclosure with armour wrapped/heat-shrunk would not be too taxing, but as mentioned, cable needs to be properly secured.
OK.
The reason for divorcing the earth was to avoid a potential difference between the outbuilding and the house. Maybe this isn't required... Maybe the shed doesn't need it's own rod and as suggested the outbuilding just use the house rod through the SWA armour. I guess I'm keen to aim to make the house and shed installs as safe as possible. Maybe the optimum is a rod at the shed in addition to the house and leave both DB's connected to both rods through the SWA armour. Your thoughts are appreciated, aiming for safe as possible especially given pets in vicinity...?
Having more than one rod for TT is a good plan, more reliable and lower Ra so no risk of failing to disconnect if fault during drought spell when Ra rises. I would simply link both via the armour but include a 2nd rod. In fact linking the earths is the best way to reduce any difference!
Probably the biggest thing you can do towards greater safety beyond the delay RCD incomer is regular trip-testing of the RCD & RCBO!
