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

 

[westward10]

If you are only replacing the first floor CU why alter circuit protection for the supply to it.

 

[Leesparkykent]

As above...you are only replacing the board so no need to provide additional protection to the distribution circuit. And a time delayed RCD wouldn't comply to the intent of the regulation in your situation.

 

[polo1]

Fair point Westy, but our view was to have the submain meet current standards of protection.
You think it’s ok to leave as is?

 

[westward10]

I would leave as it is.

 

[Leesparkykent]

Same here...the only way of realistically providing additional protection would be to have a 30mA RCD at the switch fuse. It would probably be more of a risk doing this if someone had to walk a couple of flights of stairs to go and reset it in the event of a fault....imo you are better of not providing additional protection to the distribution circuit and having the RCD at the far end.

 

[polo1]

Thanks guys, helpful.
Supplementary Q.... if you were providing o/c & additional protection to this submain, what setup would you use?

 

[westward10]

I would keep the switch-fuse and you would have to fit a 30ma rcd.

 

[Leesparkykent]

Thanks guys, helpful.
Supplementary Q.... if you were providing o/c & additional protection to this submain, what setup would you use?

You aren't left with a lot of choices unless the customer is willing to have the fabric of the building disturbed. The only real option you have is to have a 30mA RCD at the switch fuse end and a main switch board and circuit breakers at the end of the distribution circuit. If you were installing the distribution circuit from new you wouldn't even consider this option. If you assess the situation and take all risks in to account ...How likely is someone to nail/screw through the distribution circuit? Is it acceptable that the end user has to travel down a few flights of stairs to reset an RCD? Not to mention the fact you aren't working on the distribution circuit and only replacing the board at the end of it so there is no requirement for you to provide additional protection to the distribution circuit.

 

[davesparks]

I’m my opinion there is an interesting question here.
Does a split concentric cable with its outer neutral conductor provide equivalent protection, if penetrated by a conductive object, to that of a cable with an earthed metallic screen or armour?
Particularly in the case of TNCS where neutral is reliably connected to earth at the cutout.

My train of thought with this is heading towards whether this would be a justifiable departure.

 

[westward10]

I believe although I maybe incorrect that these types of cable were deemed to provide protection when concealed in walls but it was later amended and removed. I am certain it was okay under the 16th Ed but disappeared from the 17th Ed.

 

[telectrix]

also, if cable is buried, is it < 50mm from surface?

 

[multimick]

that's an interesting one, a fuse carrier that sits on a din rail anyone know of this ? particularly for a 3 phase board ?

 

[polo1]

that's an interesting one, a fuse carrier that sits on a din rail anyone know of this ? particularly for a 3 phase board ?

In this case, not a 3 phase board, but three single phase boards, each served by one phase of a three phase supply.

 

[Toneyz]

I would try to see if the distribution circuit cable is in the wall further than the 50mm eg in boxing/void as you are only changing C.U as said there is no need to RCD protect the cable if you did it would have to be a 30mA RCD which will give no discrimination if you want to provide some sort of RCD protection then recommend 100mA time delayed.