TNC-S Supply but combined again at DB?

[JosephandAmber]

Took a look at a block of flats today that has a TNC-S supply. Each flat is fed by a 2-core mineral insulated cable. The earth and neutral are then combined again using a henley block prior to entering the consumer unit. Has anyone come across this before?

 

[westward10]

Are these DNO cables.

 

[JosephandAmber]

I'm going with no. The 3 phase supply head goes into a ryefield board where it is then split into 60a fuses that feed each flat. See photo below.

 

[pc1966]

Interesting. I can see why it might be done and that is could be considered safe, assuming the Ryfield board is TN-C-S, but I don't know where the line on ESQCR prohibition of TN-C use is drawn as far as (possible) BNO operations are concerned.

MICC is fantastic stuff, but the high CCC is achieved by running the conductors hot as the 'MI' part can safely and reliably do that, so often VD is the limiting factor on an otherwise unremarkable length. Perhaps it was upgrading flats from historic 40A to modern 60A and someone realised that using the outer copper CPC in parallel with the N conductor would achieve acceptable VD and CCC dissipation limits?

As far as safety is concerned, I see it very unlikely that both the outer CPC and the neutral conductor could become disconnected to present a risk. However, that is still not really dealing with the specific limits on ESQCR & wiring regs on TN-C use.

 

[westward10]

Can you show the other end of those cables.

 

[Julie.]

Interesting. I can see why it might be done and that is could be considered safe, assuming the Ryfield board is TN-C-S, but I don't know where the line on ESQCR prohibition of TN-C use is drawn as far as (possible) BNO operations are concerned.

MICC is fantastic stuff, but the high CCC is achieved by running the conductors hot as the 'MI' part can safely and reliably do that, so often VD is the limiting factor on an otherwise unremarkable length. Perhaps it was upgrading flats from historic 40A to modern 60A and someone realised that using the outer copper CPC in parallel with the N conductor would achieve acceptable VD and CCC dissipation limits?

As far as safety is concerned, I see it very unlikely that both the outer CPC and the neutral conductor could become disconnected to present a risk. However, that is still not really dealing with the specific limits on ESQCR & wiring regs on TN-C use.

ESQCR regulation 8 only dictates that a customer may not combine the functions of a live conductor and earth in a single conductor.

It doesn't prevent two seperate conductors being connected together (though I think it should), and it only talks about the "customer" - essentially predating BNOs etc.

I would expect the interconnection of N & E to be prohibited in the connection agreement with the DNO/supplier.

It also breaches 543.4.3 of course.

 

[pc1966]

ESQCR regulation 8 only dictates that a customer may not combine the functions of a live conductor and earth in a single conductor.

Ah, an interesting and important distinction between having a a PEN conductor and more generic TN-C

It doesn't prevent two separate conductors being connected together (though I think it should), and it only talks about the "customer" - essentially predating BNOs etc.

I would expect the interconnection of N & E to be prohibited in the connection agreement with the DNO/supplier.

Would be interesting to know when, why, and by whom the linking was done.

It also breaches 543.4.3 of course.

Presumably then the BNO side is under the wiring regs rules and not DNO rules?

 

[Lister1987]

Could the linking be for PNB purposes and not PME?

 

[JosephandAmber]

Can you show the other end of those cables.

Unfortunatly I don't have a photo with the trunking off but this is what was going on:

The MICC is terminated onto metal trunking via a gland. The trunking then had a bolt going through it with a 16mm2 earth fly lead into the consumer unit and another into the henley block containing the incoming neutral.

The main supply prior to the Ryefield is TNC-S.

So the question is... what to put on the EICR?