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.

Graham, your take was much the same as my own, at least initially. As I said in the OP, the cable cannot be touched. In fact, it is in two separate 160mm PVCu pipes underground for the full length, which is nearly 150m and not the 70m I had indicated in an earlier post.

Any way, as I rummaged through the 7671 stuff, I just couldn't see a specific reference that would lead me to be concerned that the regulations were being breached in a fundamental way. 

131.2.1 is satisfied by the live part being in a PVC duct for its full length. 411.1 indent (i) is similarly satisfied. 

416 is also similarly met and 417 does not apply. 

So I guess the question is does BS7671 permit a live part, albeit one with a plastic sheath, to be totally enclosed in a plastic pipe that is effectively a sewer pipe such that basic protection is satisfied?

In that regard, I cannot see how it doesn't. It may be that 511 might have something to say about the use of a sewer pipe but that is nuanced to some extent.

By the way, I am not trying to argue a case for doing so, just trying to be 7671 specific in not doing so.

131.2.1 is satisfied by the live part being in a PVC duct for its full length. 411.1 indent (i) is similarly satisfied. 

Are you claiming that the duct provides basic insulation then? Does the product standard provide the necessary data for that? (e.g. insulating properties, voltage withstand). If the duct is basic insulation what's the sheath/conduit (521.10.1)? As it's underground are you claiming the duct provides mechanical protection sufficient to avoid the need for a concentric c.p.c?

  - Andy..

No, preventing a current passing through a body can be achieved by having the live part within an enclosure. The Waveform cable is in PVC ducting and GK is adamant that the sheath would not provide basic insulation, presumably as per the relevant cable standard and therefore the neutral can be considered as bare.
Just looking for something specific to demonstrate that the arrangement of the waveform cable is unacceptable as far as 7671 is concerned. I am not trying to argue a case one way or the other.

lyledunn said:

So I guess the question is does BS7671 permit a live part, albeit one with a plastic sheath, to be totally enclosed in a plastic pipe that is effectively a sewer pipe such that basic protection is satisfied?

522.8.10 is not so clear.

lyledunn said:

416 is also similarly met

Hmmm ... not it would seem according to 521.1? (Bare conductors ... in this case, not insulated I'm taking as bare) are only permitted on insulators according to Table 4A1. Insulated conductors are permitted in ducts, though.)

lyledunn said:

In that regard, I cannot see how it doesn't. It may be that 511 might have something to say about the use of a sewer pipe but that is nuanced to some extent.

Well, even if the sewer pipe were classed as a proper underground conduit or duct, 521.1 would say it's not acceptable for the neutral to be uninsulated ... unless it's a PEN conductor (i.e. protective conductor).

lyledunn said:

No, preventing a current passing through a body can be achieved by having the live part within an enclosure.

Table 4A1 (a "shall" reference from Regulation 521.1) indicates that perhaps a duct is not an enclosure.

lyledunn said:

No, preventing a current passing through a body can be achieved by having the live part within an enclosure.

I also thought ... what happens when the cables come out of the ducts? Are the cables not accessible then (I did in an earlier post say that we'd at least need to protect skilled and instructed persons)?

You are right, Graham, there is a short section entering the trough below the intake panel. As I understand it, the cable is staying or the project stands a good chance of being shelved.

It is important to look after the safety of skilled persons. Whilst the proposal may not fully comply, I would think that reasonably robust mitigation could be put in place.

There are some countries that are not so cautious with dangers posed to skilled persons: 

Actually there are a still a few 'skeleton frame' substations in the UK that have the LV bars exposed. 

But that is OK as it is on the DNO side where electricity is safer, and neutral is considered to be earth-like.

Actually more seriously they are being upgraded, and meanwhile rubber sheet mats are used to reduce the risks from direct contact when folk have to be in there, which is organized to be minimally.

Mike.