TNCS Dist CCT. to TT installation - front end protection question.

[slipnduck]

Good Morning everyone.

Quick question regarding a sub main from a TNCS supply feeding an outbuilding on TT.
This is a domestic installation not agricultural.

TNCS supply to house feeds High Int. CU from which 4mmsq 2 core SWA feeds an non attached rear building.
THE SWA of the sub main is glanded and connected to the TNCS earth at the front end and R1 + R2 measured between Line and SWA at the extremity is 0.31Ohms. (measured before final termination).

At the outbuilding end of the sub main the armour has been stripped flush to outer sheath, taped and then glanded through a nylon stuffing gland via a WISKA box which is itself nylon glanded hard up to the metal 2 way garage type CU which is TT earthed. In effect the two earthing systems are completely separated by 100mm or so of nylon enclosure.
Within the 2 way CU the live conductors of the sub main are terminated in the 40A RCCB main switch.

My question regards front end protection of the sub main. My plan was, in fact I have, fed it via a 20A mcb from a non rcd protected way of the upstream CU. It being earthed SWA it doesn't require RCD protection and 30mA protection of the finals is via the RCCB at the 2 way unit.
Max disconnection time for TN distribution ccts. is 5 seconds but the EFLI of this cct. is eaily low enough to cause instant activation of the 20A mcb.

So all good except.... the last 8 inches or so of the sub main after the armour is stripped where the two earthing systems are separated and inside the 2 way CU.
An earth fault here would have a never ending duration. I have measured Ra at 48.5 Ohms so such a fault would be less than 5 amps which the 20A mcb would supply forever.
Now I don't envisage that such a fault would ever occur... But, my question is - Does this present arrangement comply with regs in terms of disconnection of the Dist. Cct.?

In effect the issue is exactly the same as when supply/meter tails enter a post 17th ed. 3rd. ammend. CU on almost any TT install.

Thanks in advance for your considerations.

 

[slipnduck]

Just wanted to add.
I know there are other ways the sub main could be protected front end.
RCBO and swap garage RCCB for an Isolator

Move the MCB to one of the RCCBs of the house CU and as above.

My reason for wanting to feed via MCB was to be able to have the 30mA device local to the final ccts.

 

[Rockingit]

I think what you are asking is do the L & N tails from the SWA need protection inside the CU? If so then the answer is no.

 

[slipnduck]

Thanks.
Yes that was my question.
I did call Elecsa/Nic tech line - he said as much but also added that i should ensure the L & N supply tails are double insulated within the CU right up to termination at the switch.
Thanks again.

 

[westward10]

Did they explain how to apply a layer of insulation so it becomes "double insulated" because it is a feat beyond possibility.

 

[pc1966]

You are quite right to be wary of a non-clearing L-E fault in the CU but normally the cable in there should be adequately protected and not likely to short out.

If you were worried about any point where it is likely to come in contact with any metalwork you could put additional sleeving over them. That is what Hager and Wylex do with the ~1mm cables for the meter kits that are wired direct to the incomer and go to a DIN rail fuse arrangement so are not adequately fault protected by the 100-250A upstream feed.

 

[slipnduck]

Did they explain how to apply a layer of insulation so it becomes "double insulated" because it is a feat beyond possibility.

The gist of it was to maintain double insulation as far as possible inside the CU and up to the termination point.
The suggestion was to cut the live conductors of the supply inside the adaptable box (in my case Wiska 308) and connect them at this point to double insulated singles (6181Y) which can then be taken through into the CU with the outer sheath left in tact as far as possible.
I haven't done this as I haven't got any double insulated singles in 4mmsq. But I have run the supply conductors together in some 12mm mini flexible conduit, which I did have for some reason, starting inside the wiska box and covering to within 15mm or so above the switch - therefore double ins. almost the entire internal length.
Any way I'm happy, it would take something astronomical freakish to create an earth fault there now - not that it was likely in the first place.

 

[pc1966]

The suggestion was to cut the live conductors of the supply inside the adaptable box (in my case Wiska 308) and connect them at this point to double insulated singles (6181Y) which can then be taken through into the CU with the outer sheath left in tact as far as possible.

Sleeving is easier!

But some plastic conduit as you did is exactly the same protection against outside metalwork, etc.