Use of Armour Wires for earth CPC

[Rick Hughes]

About 18 months or so a thread was posted and some emails swapped regarding armoured cable for an outside project.

At that time the information given ( and a pointer to a good document) was that it was no longer considered good wiring practise to rely solely on the steel armour wires for a CPC (earth connection)

I therefore for my work used 3 core throughout ......... the armour wires are connected to earth, but the earth conductor actually providing the CPC.

I have just been picked up on this and told that this is NOT correct and that it is still fully permissible under the regs to rely solely on the steel wires armours for CPC portion.

Anybody care to comment further on this.

The quote made was " .........In fact regulation 543-02-02 (v) explicitly states that cable armouring may be used as a CPC. "

Is this a question of the Regs allowing something but good wiring practise advise something else ?

It makes no difference to the job I did as the only issue is that it may be 'over engineered, but I would like to get to the bottom of this.

Rick

[Andy Hall]

The actual wording is:

"A protective conductor may consist of one or more of the following:

(i) A single core cable

(ii) a conductor in a cable

(iii) an insulated or base conductor in a common enclosure with insulated live conductors

(iv) a fixed bare or insulated conductor

(v) a metal covering, for example, the sheath, screen or armouring of a cable

(vi) a metal conduit or other enclosure or electrically continuous support system for conductors

(vii) an extraneous-conductive-part complying with Regulation

543-02-06"

The operative words appear to be *may* (not *must*) and *one or more* not just *one*.

Given that situation, I don't see why what you have done should be criticised.

The problem is that if somebody inspecting goes down the route of "good practice" and this is at variance with a specific aspect of the Standard, then it opens Pandora's box on what is allowed and what is not, making it arbitrary, It may please some inspectors to have a perceived sense of power (so to speak), but the "good practice" aspect should be a fall back to situations that are not explicitly covered in the standard.

Not really because you can use one or more.

I have yet to find anything in BS7671 which treats something that is over-engineered as non-compliant.

I would suggest asking whoever did the inspection to justify their comment. If it is simply what NICEIC recommends or something like that, it is comment only and does not carry the authority of the standard.

One point is that if you used three core three phase cable and the third conductor is yellow, then it should be fully sleeved in green/yellow.

.andy

To email, substitute .nospam with .gl

[G&M]

Who is picking you up ? Electrical work is not yet in the building regs (unless you are n Scotland) though I know a few BCOs have voiced opinions on the right way to do this. Both ways of course !

[Christian McArdle]

I was under the impression that the problem when using the armour is being able to ensure that the earth loop impedence is adequete and will remain so, even when the steel corrodes at the terminals.

Christian.

[Ed Sirett]

When I was doing some industrial electircal work many years ago. A new length of thick SWA was being installed. (It might have been around 200mm^2 x3). This had been made with several of the steel armour wires replaced with copper to mke sure the CPC had at least 50% of the conductivity of the core conductors.

I suspect therefore that manufacturers make sure that armourings are always adequate for use as a CPC.

[Dave Stanton]

AS Christain has said, its the long term corrosion factor which is the main worry and I would have thought steel and copper in intimate contact would be a surefire winner on the corrosion front.

Dave

[Andy Hall]

Don't forget that the terminating glands are normally brass, so you don't have steel wires in contact with copper.

I was reterminating the house end of one of my outside supplies last weekend - I needed to move it. The gland is fitted to an exterior box and is exposed to the elements apart from a rubber johnny pulled up over the cable and the gland. On disassembly there was no sign of corrosion at all. The installation has been there for 20 years.

.andy

To email, substitute .nospam with .gl

[Dave Stanton]

Fair comment, but I was more concerned with the guys comments about copper and steel strands forming the armour.

Cheers

Dave

[Andy Hall]

OK, I see what you are saying.

.andy

To email, substitute .nospam with .gl

[Andrew Gabriel]

I have a piece of 11kV 150mm^2 aluminium cable. All the armour on that is copper, possibly because it's a single conductor with unbalanced current flow where magnetic armour could cause a problem.

[Ed Sirett]

The armourings were steeped in pitch possibly for this very reason.