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

 

[multimick]

yep i get that polo ,just asking to further my knowledge , could come in handy in future

 

[Risteard]

I am certain it was okay under the 16th Ed but disappeared from the 17th Ed.

That's correct. And in answer to the OP, a time-delayed 30mA RCD (if you could source one) would not meet the requirements for additional protection anyway.

 

[Risteard]

I am certain it was okay under the 16th Ed but disappeared from the 17th Ed.

That's correct. And in answer to the OP, a time-delayed 30mA RCD (if you could source one) would not meet the requirements for additional protection anyway.

 

[Wilko]

... 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? ...

Perhaps something like this?
125A MS Modular Fuse Holder Grey (Sold in 1's) | CEF - http://www.cef.co.uk/catalogue/products/1780321-125a-ms-modular-fuse-holder-grey-sold-in-1-s

 

[suffolkspark]

if its split concentric then the neutral is only really actually single insulated isn't it..... and we normally give single insulated conductors that arent in trunking etc a C2. Imo I wouldn't want to leave it personally

 

[Risteard]

if its split concentric then the neutral is only really actually single insulated isn't it..... and we normally give single insulated conductors that arent in trunking etc a C2. Imo I wouldn't want to leave it personally

The conductors will only be insulated once regardless - the sheath is there to provide mechanical protection to protect the insulation - not to afford an additional layer of insulation. (That's why Pyro can get away with having an earthed copper sheath - the magnesium oxide only provides one layer of insulation.)

However the neutral conductors in split concentric cable will be both insulated and sheathed. The problem is that there is no guarantee you won't contact neutral (a live conductor) rather than the protective conductor when piercing the cable unlike with SWA.

 

[suffolkspark]

I'm sure the last split concentric I came across only had the 1 layer of sheath around the whole thing, I could be wrong it was a while ago

 

[Risteard]

I'm sure the last split concentric I came across only had the 1 layer of sheath around the whole thing, I could be wrong it was a while ago

There only will be one sheath. But all live conductors will also be insulated.

 

[suffolkspark]

yea but the neutral wont be will it? I am assuming this is single core stuff

 

[DPG]

Inner phase and outer neutrals will be insulated. Earth strands will be bare. I think - someone will hopefully confirm this!

 

[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.