522.8.10 Does all concentric cable satisfy BS7671 for underground use?

[FatBob]

Following a query in another post with a respected member about the suitability of pvc concentric cable for underground wiring, I wrote yesterday to the respected author John Whitfield, who has written widely on the subject of the IEE Wiring regulations, his most notable work probably The Electricians Guide by EPA Press.

I noted that both his work and an NICEIC C&G 17th guide listed concentric cable as being suitable for underground use, without constraints, simply by being concentric in construction.

I asked:
1 Does the acceptability of concentric use, lie in the interpretation of 'metal sheath suitable for use as a protective conductor' in 522.8.10?
2 Does Concentric layer: Neutral conductor & an uninsulated CPC count, or does the outer layer need to be a continuous sheath?

522.8.10 a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor.

522.8.10 does not specify that concentric construction alone would satisfy BS7671

Mr Whitfield has kindly replied by return (I have paraphrased the reply):

Having examined your note and BS7671, I am sorry to say that there appears
to be no definitive answer to your question.
BS7671 is, as you have said, silent on the subject. It is really down to
whoever signs the final certificate, but by then it is too late!

For what it’s worth, in my own opinion, a concentric outer cover made up
partly of neutral and partly of protective conductor will comply. BUT I am
not an accepted authority.
I hope that this helps.


If anyone else would like to comment, I would welcome the input, and again I would like to thank Mr Whitfield for taking the trouble to respond.

 

[Engineer54]

Shame he didn't explain ''Why'' in his opinion, that he considered the copper neutral and earthing conductors constituted ''Mechanical Protection'' for this type of cable!!

The main criteria for a cable being suitable for use in a underground ''Direct burial'' installation is the metallic mechanical protection afforded. Which would normally be either SWA or STA, both of which can be used as a CPC....

 

[johnboy6083]

i suppose, that the concentric neutral and earth conductors will offer ADS, either as an earth fault, or short circuit. Regarding the possbuilty of exposing the neutral conductors, and not causing a short, nopt resuling in ADSm, then that is no different than the Wavecon cable that the DNO's bury not very deep. if BS7671 does indeed have nothing to say, then i think that the designers professional opinion and reasoning have to be used.

 

[Engineer54]

i suppose, that the concentric neutral and earth conductors will offer ADS, either as an earth fault, or short circuit. Regarding the possbuilty of exposing the neutral conductors, and not causing a short, nopt resuling in ADSm, then that is no different than the Wavecon cable that the DNO's bury not very deep. if BS7671 does indeed have nothing to say, then i think that the designers professional opinion and reasoning have to be used.

One very big difference, the DNO's don't work in any way to BS7671, they have their own set of codes/rules. The SWA and STA are there for a purpose, that they both provide suitable primary mechanical protection. Basically that's what BS7671 has always called for where direct burial of cables has been involved!!

I can't think of any direct burial situation off hand, where a concentric cable would be a suitable choice of cable for permanent or long term installations...

 

[MarkieSparkie]

Following a query in another post with a respected member about the suitability of pvc concentric cable for underground wiring, I wrote yesterday to the respected author John Whitfield, who has written widely on the subject of the IEE Wiring regulations, his most notable work probably The Electricians Guide by EPA Press.

I noted that both his work and an NICEIC C&G 17th guide listed concentric cable as being suitable for underground use, without constraints, simply by being concentric in construction.

I asked:
1 Does the acceptability of concentric use, lie in the interpretation of 'metal sheath suitable for use as a protective conductor' in 522.8.10?
2 Does Concentric layer: Neutral conductor & an uninsulated CPC count, or does the outer layer need to be a continuous sheath?

522.8.10 a cable buried in the ground shall incorporate an earthed armour or metal sheath or both, suitable for use as a protective conductor.

522.8.10 does not specify that concentric construction alone would satisfy BS7671

Mr Whitfield has kindly replied by return (I have paraphrased the reply):

Having examined your note and BS7671, I am sorry to say that there appears
to be no definitive answer to your question.
BS7671 is, as you have said, silent on the subject. It is really down to
whoever signs the final certificate, but by then it is too late!

For what it’s worth, in my own opinion, a concentric outer cover made up
partly of neutral and partly of protective conductor will comply. BUT I am
not an accepted authority.
I hope that this helps.


If anyone else would like to comment, I would welcome the input, and again I would like to thank Mr Whitfield for taking the trouble to respond.

I wrote to EPA Press the publishers, flagging up this error and about a dozen others when the first edition of the book came out. I offered to proof read the second edition before publication, this offer was ignored and lo and behold virtually all the original errors were repeated again complete with 16th Ed references. The third edition is due soon, reflecting the 17th Amendment 1 changes, it will be interesting to see if they get it right this time. It's a shame this otherwise good and once respected book is spoiled by lack of attention to detail. At least it's not as unbelievably error strewn as Chris Kitcher's Inspection and Testing (2nd Ed) book where about 10% of the pages in the book have errors and a huge rewriting exercise is required for his third edition due soon.

 

[johnboy6083]

I agree ENG 54, regarding the fact that the DNO's work to completely different regs ect, im just playing devils advocate. I agree with your sentiments, but would you code it on an EICR, and if so, what reg would you use to back it up?

 

[Guest55]

Concentric cable is NOT considered to incorporate mechanical protection and additional measures should be used if buried.
And to answer jonnyboys question , yes i would record it as a code 2 defect if it was used without rcd / conduit protection