Sub main protection, views please.

[polo1]

Domestic, three phase supply, one phase to each of three consumer units. Two consumer units are local to the intake, the third CU is on the first floor.
Currently the first floor CU is fed via a fused isolator at the intake. The replacement CU will be rcbos. However, we thought this submain was SWA, but discovered today that it is split concentric. Soo, looks like we now need to apply additional protection to the submain as it is buried in the building fabric.
We want to achieve discrimination with the 30mA rcbos in the new board, so is a 30mA time delayed rcd replacing the switch fuse acceptable in this situation?
Also, we would prefer to provide overcurrent/short circuit protection via a fuse, as opposed to an mcb. I recall there are fuse holders which fit on a DIN rail, but not sure of make/spec - anyone point me in the right direction?
Oh, it’s TNS, Ze of 0.34.
TIA

 

[suffolkspark]

Ohh the last stuff i came accross had a single inner core, then a layer of insulation, then the neutral strands, then the outer black sheath.... That was it...

 

[DPG]

Isn't that dual-con rather than split-con though?

 

[123]

That sounds more like concentric cable rather than split concentric. Split concentric will have the neural insulated and also have bare copper earth strands

 

[suffolkspark]

Oh I've not come accross that then haha well i dont see an issue then as long as its booted properly at each end

 

[stevels4]

Split concentric should be terminated with a 3 finger boot and heat shrink on the conductors.