RCD & cable mechanical protection

Use of the LSX is sufficient to negate the requirement for RCD protection due to installation at a depth of less than 50mm. However, BS 8436 cable is limited in terms of available conductor sizes and the suitability of certain MCB types/ratings.

I wouldn't directly compare BS 8436 and SWA cabling as they are tested to different standards.


P

Pww235 I agree. It actually states it that the nail penetration tests happened for 1.5 and 2.5mm2.

And the conductor sizes ARE limited. Correct.

However I am led to believe that for the 1.5 and 2.5mm2 the cable is considered mechanically protected for the purposes of an RCD application.

522.6.204 requires that a cable (buried in a wall or partition less than 50mm deep) shall incorporate an earthed metallic covering which complies with the requirements of these regulations for a protective conductor of the circuit concerned, the cable complying with BS 5467, BS 6724, BS 7846, BS 8436 or BS  EN 60702-1.


or be installed in earthed metallic conduit complying with BS EN 61386-21,


or be installed in earthed trunking or ducting complying with BS EN 50085-2-1


or be provided with mechanical protection against damage sufficient to prevent penetration of the cable by nails, screws and the like,


or form part of a SELV or PELV circuit.....


The whole idea of earthing the armour strands of a S.W.A. cable is to provide automatic disconnection if a screw or nail enters the cable and also hits a line conductor.

https://www.elandcables.com/electrical-cable-and-accessories/cables-by-standard/bs-8436-cable


Z.

In order to get away from an RCD (assuming that the circuit does not feed a socket...) you need mechanical protection for cables chased in the wall at less than 50mm.

There are two different approaches - either earthed protection (i.e. where the live conductors are surrounded by something that can act as a c.p.c.) and mechanical protection (which is purely mechanical and so doesn't require any earthing).


Earthed steel conduit/trunking, armoured cables, BS 8436 cables etc all normally fall into the first (earthed) category (reg 522.6.204 options (i) to (iii)), not mechanical protection.


Mechanical protection is reg 522.6.204 option (iv). In these days of nail guns etc it's very difficult to be sure that mechanical protection can work reliably. Some offer "safe plates" for protecting cables/pipes passing through joists/studs - but as far as I'm aware there's nothing official to say they're adequare or acceptable to BS 7671. In some limited circumstances (e.g. where site procedures or H&S rules would prevent the use of power tools) then perhaps mechnical protection could be justified - but in general the safest option is to go down the 'earthed' route.


   - Andy.

Thanks Zoomup.

So the mechanical protection on its own is not enough, it needs to be earthed if it is metallic.

That is what I thought as well but I remember someone telling me that once you have mechanical protetion that's all you need since the nail cannot penetrate it.

Well....I think that it is perfectly possible a nail gun to penetrate a galvanized conduit...

P.S. The earthing of the SWA refers to the cases where you use the SWA as mechanical protection I pressume...Becuase when I was working in the Water Industry, there was a rule for VFDs not to earth the SWA and always provide the earth through a separate green yellow CPC. he SWA would not be earthed at both ends...

Thanks for your answer!

Armour, as in the sort of stuff which covers military hardware as opposed to the strands of SWA, which could easily be penetrated by any sharp pointy object, may not be tough enough to resist a nail gun, but let's consider the risk. Any contact between a nail and it's gun on the one hand, and a live conductor on the other is almost instantaneous. The nail is then usually buried beneath the surface, so casual contact is limited. In any event the damage has been done - it has not been prevented!


I would like to think that a nail gun would be used by a tradesman who has some idea as to what lies beneath the surface. Now think of an old fashioned nail and hammer wielded by an ordinary person who has never given a moment's thought as to how the wires get to sockets. There is a very real risk of holding the nail whilst it makes contact with the live conductor. So simple mechanical protection may not only prevent a householder from getting a shock, it may also prevent significant damage to the building. I would not discount it in the domestic environment.

543.2.1 (v) allows the armour of a cable to be used as a protective conductor, if suitable sized. I have always earthed the armour of steel wire armoured cables. It affords greater safety if the cable is mechanically damaged or penetrated.



Z.