TNC I presume. Where is the earth coming from?

'7671 is not keen on buried cables where the armour is  not earth, but it is done.

M

outer copper wire screen in one cable as neutral and earth in the other. 

Sounds like an attempt to keep it TN-S (in compliance with the ESQCR for consumer's installations) - but not having N in the same wiring system or mulitcore cable as the corresponding Ls sounds like a contravention of BS 7671 to me.

   - Andy.

Split con would have been better. I'm not sure that wavecon is really suited to use the 's' part of a TNC-s installation - being firmly aimed at the 'C' part,  and this proposal has my 'here be dragons' antennea twitching.

When you say " The conductors are configured as three lines in parallel" are these 3 cores lashed as single phase ? If so I'd re-jig to L-L N and use the armour as E..

Or is it 3 phases, each phase 2 cores in parallel - which is very awkward as the N current will not be in the right place to ensure a good share.

How close to the knuckle is the new proposed load, and how easy is it or not to get another line in the duct.

No chance to 'TT' at the load end I suppose or to run a supplemental earth ?

It may be that the best re-purpose of the DNO cable is as draw line for something with the right no of cores.

Mike.

Mapj; "Or is it 3 phases, each phase 2 cores in parallel - which is very awkward as the N current will not be in the right place to ensure a good share"

Correct, Mike. It is a TPN, TN-S. I cannot blame the contractor for wanting to use the DNO cables when they become redundant. they are 300mm2 x 2 and about 70m long so a few quids worth to replace!

BS7671 perspective

I think 521.7 is going to supper your plans even if 521.5 doesn't for a TN-S approach.

BS 7671 doesn't prohibit PEN conductors (see 543.4 - a lot of BNO installations would be in trouble if it did) - the stumbling block is presumably the ESQCR prohibiting PENs in consumer's installations. So one potential way out is to create a separate legal entity (company) to own and maintain the cables - they wouldn't then be the consumer's installation (merely something like a BNO).

   - Andy.

I am interested in how come the cables were configured as carrying the earth in one and neutral in the other. Why would a DNO do that? Is there a reason for it as this would be very unusual. 

I suspect that when used for the  DNO originally they only need 4 cores for TPN and E as to them, N and E are the same,  a combined PEN - hence my comments about the 'C' = combined part of TNC-s . If this in the UK then combined NE is not permitted in consumer side installations.  I am less sure about other places using BS7671 as  a basis of their regs, and in may EU countries TNC-c is permitted.

The slightly better way is to run both cables LLL-N and lay in only an external CPC. Still cheaper than 5 core.

On the theme of why  PME, I do recall a rather uncharitable comment made once about a particularly thoughtlessly  made off temporary building site supply where not only had N and E been recombined part way along the temporary feed, but the cables came out in such way that the cabinet could not be properly closed and cable ties had been deployed , to the effect that perhaps the reason the DNOs prefer TNCS was that  otherwise  they would need to employ contractors that could plan and count beyond four.

However the real reason is to allow the use of less copper, and as plastic coat has replaced the older lead covered paper insulated, the self earthing kilometer long electrode from the outer of the distribution cable itself has had to be replaced with a small no of deliberately placed electrodes.

The problem with all earthing systems is that in general without a fault, or at least a fault emulator to pass some test current occasionaly, you cannot tell if the earth path has corroded away.

In a PME system you can tell more or less at once, because you lose the neutral as well. Which is also quite a nasty condition. To that end in PME done to the UK rules, all neutral joints in overhead singles are double crimped,  the wave-cons are designed so that if you pull them to failure (think subsiding buildings etc) the outer wavy path has some slack and breaks second, and connectors are designed so that a lost neutral is mercifully rare. 

Mike.

Thanks for this Mike, must admit that I presumed that this is in the UK. I've come across this issue of adopting DNO cables in the past and my take is that it is never a good idea. In this case, if one of the cables is carrying neutral and the other earth, I wonder what will happen if there is ever a cable strike or a fault that occurs. I can see an unsuspecting repair team being confused, not knowing the design intentions or the history, with the result that the neutral gets earthed anyway. Then there's the issue of communication of design intentions to every every inspector into the future. It all looks feasible but I would be rather uneasy about putting my name to it. Maybe it goes in the category of not complying to industry norms? Mike

lyledunn said:

but I was just wondering about the BS7671 perspective.

Lyle, I thought you'd be all over this one. Here's my take on how it could be viewed (hopefully not in a Court):

Not permitted by BS 7671, not safe at the very least for maintainers, possible breach of legislation... and nothing to do with the protective conductor which is a more tenuous argument. A neutral that is not a PEN conductor (i.e. in a TN-S system, or a TN-C-S system downstream of the "separation") is a live conductor and is required to be insulated. The sheath of this cable is not insulation.

Regulation 131.2.1 is the fundamental principle. Regulations 411.1, Sections 416 and 417 are the primary Part 4 protection for safety) references.

Regulation 7 of EAWR 1989 is of course the legislation in question that may be breached, along with, possibly, Designer's Duties under CDM Regulations (safe maintenance if only skilled or instructed persons have access to touch the Neutral, otherwise, safety for all employees in a place of work).

4c waveform is available ...