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

mapj1 said:

he slightly better way is to run both cables LLL-N

Neutral is required by BS 7671 to be insulated, unless it's a PEN conductor, so that would not comply.

mapj1 said:

he slightly better way is to run both cables LLL-N

Neutral is required by BS 7671 to be insulated, unless it's a PEN conductor, so that would not comply.