What derating table do I use for multicore armoured thermosetting cable in insulation.

AJJewsbury said:

Other thought, is rather than use 16mm² 3-core, use 25mm² 2-core (it's about the same diameter) and if you don't trust the armour as a PE for downstream,

Why would you not trust the armour of a 2-c 25 mm2 ?

I can see that there may be circumstances where the armour is not up to it (although)

I am also aware that not all courses provided the appropriate "tools" to work out the loop impedances, but if it's safe, you can always test that to verify at an appropriate socket-outlet.

"Corrosion" or "can't get a good connection" is NOT an answer ... if it is, then one of the following is awry, and is, of course, a non-conformity to BS 7671:

• poor workmanship
• equipment not suitable for external influences

AJJewsbury said:

add a 16mm² G/Y along side (all permitted these days).

Doesn't mean it's always valid do to that and everything is covered for all fault conditions. In particular, if the cable is direct-buried, BS 7671 requires the armour to be suitable as the protective conductor (Reg 522.8.10) ... but a designer might, in other circumstances, be advised to take into account that there are other conditions in which the cable could be completely severed, with or without the parallel conductor being severed, rendering the parallel protective conductor useless.

gkenyon said:

Why would you not trust the armour of a 2-c 25 mm2 ?

Just a common preference not to (similar to drawing in a g/y into steel conduit) - the OP had spec'd 3C 16mm² after all. I was just forestalling a possible objection.

   - Andy.

AJJewsbury said:

Just a common preference

Yes, this has, for some reason become a 'preference', but the only issue I can find that drives this decision, is that glands are not made off properly, or the wrong type of gland is selected for a particular application.

The wider problem comes when:

• the parallel cpc becomes damaged (it is not protected in the same way as the armour or armoured cable), or
• the terminations for the parallel cpc are not adequately protected against corrosion, or
• there is a situation as I alluded to in my previous post, where the parallel cpc becomes ineffective in the specific conditions.

An internal conductor used as a cpc is not quite as "frail", but perhaps the issue of severing a buried cable brings the same issue.

A straight "parallel conductor", where either a conductor inside the armour, or parallel cpc, are each separately adequate to act as the cpc is the safest approach.

gkenyon said:

Yes, this has, for some reason become a 'preference', but the only issue I can find that drives this decision, is that glands are not made off properly, or the wrong type of gland is selected for a particular application.

I do wonder where it had its origins but it was very common on specifications from consultants back in the 1980s, at least those presenting designs for the Department of Education. Back then, my team did school after school, all with the SWA cables and separate cpc. Cables were often clipped along corridors. 
Similarly, the steel conduits all had separate earths for each circuit. Interestingly, the plethora of mims used the copper sheath without question. 

Not sure if the influence was Table 54.7 (or what preceded) being interpreted incorrectly and as the only option.

I am changing the CU on the supply and load side, therefore believe the circuit needs to comply with current regulations.

I'm not sure that's actually a requirement - to take that demand literally would mean replacing or re-marking at all positions all red/black wiring with brown/blue when replacing a CU - as red/black doesn't comply with current regs, or re-clipping cables that are only held up with plastic clips or plastic trunking, or 101 other changes the regs have brought in over the years. Certainly you shouldn't do anything that would reduce the safety of the existing (e.g. replacing a 15A fuse with a 16A MCB where Zs was good enough only for the previously acceptable 5s disconnection times), and upgrades where sensible/affordable/agreeable-to-the-customer are never to be discouraged, but to my mind it's only your new work (and the direct consequences of your changes) that needs to comply, not parts of the exiting installation that happen to abut it. Certainly draw the inadequacy to the customer's attention - that's what the 'Comments on exiting installation' box on the cert is for.

   - Andy.

There is a kind of precedent for not replying on metallic parts of wiring systems for downstream PE purposes - 543.2.3 prohibits flexible or pliable metallic conduit to be used as a protective conductor, even though it still normally needs to be Earthed as an exposed-conductive-part - even if the flexible/pliable conduit isn't subject to movement once installed. I gather the Irish regs don't allow reliance on the armour either (the Irish wholesalers are full of 3-core SWA in brown/blue/green-yellow colours).

Belt and braces wise, glanded at both ends and connected to a separate c.p.c at both ends, with both the armour/conduit and the extra core being rated to be able to handle the full fault current alone, does seem like a good scenario, as everything thing remains earthed even if there's a break at either gland or in the armour/conduit. Well above and beyond minimum BS 7671 requirements of course, and with extra costs, but some may feel the extra precaution is worth while.

   - Andy.

AJJewsbury said:

543.2.3 prohibits flexible or pliable metallic conduit to be used as a protective conductor, even though it still normally needs to be Earthed as an exposed-conductive-part - even if the flexible/pliable conduit isn't subject to movement once installed. I gather the Irish regs don't allow reliance on the armour either (the Irish wholesalers are full of 3-core SWA in brown/blue/green-yellow colours).

That's very different from 522.8.10 of BS 7671 which requires the armour [or earthed metallic screen ... of direct-buried cable] to be suitable for use as a cpc ... if you couldn't rely on it on its own, the requirement could never be achieved because the armour would never be suitable as a cpc.

There are definitely issues with flexible/pliable conductive conduit becoming "unravelled" for some reason ... again, one could question suitability for external influences isn't really achieved?

Thanks all for the responses.

Having lifted some floors and stuck a camera in I can see the submain cable is running in the soffit space, I think sitting on the soffit board with other cables. Not ideal, but ok at the moment because the entire installation is protected by a single 30mA RCD. But as soon as I replace the 30mA RCD with a 100mA time delayed RCD it becomes a problem.

I can route a cable so that it's well above the insulation. Looking at 25mm or 16mm swa the difference in diameter between 2 core and 3 core is less than 1mm, although there is some oval 2 core available. I wonder how that would work in round glands?

I have an alternative to the section going through the soffit, which is through 6 inch floor joists. As I can keep the cable more than 50mm from floor and ceiling I can avoid using armoured cable and the challenging minimum bend radius of about 168mm, 6.7 inches.