S.W.A. Armour Earthing.

gkenyon:

AJJewsbury:


We are a bit double standarded (is that a word?) though - on one hand we claim the sheath isn't an adequate insulator, but then when we create a TT island we typically rely on the sheath (and boot over the gland) to prevent contact between two different earthing systems (which under some conditions can differ by full mains voltage).

I don't agree that a "boot" over the gland does the job in the case that the armour is not earthed as an exposed-conductive-part or protective conductor. Suitable insulation such as cold shrink sleeving (that can only be removed by destruction - Regulation 416.1), is far more appropriate, and indeed the actual requirement of BS 7671.


Think about the fact that the person you are trying to protect isn't just someone who is neither skilled nor instructed, but perhaps the "next spark in the queue" ... and the fact that the EAWR and CDM Regs apply in almost all circumstances to the work being undertaken by that person, or the designer or both ... and if you decided to take that approach, you're the designer.

I was thinking more of the situation where the armour/gland is Earthed - but to a different Earthing system to the one in area it is located in. As far as I recall guidance metions the possibility of glanding into an insulating enclosure but with no mention of replacing the usual boot with adhesive lined heat shrink.


Indeed the whole subject of insulation between Earthing systems seems rather lacking.


  - Andy.

In this case, an undetected fault to armour might leave the armour "live" and wouldn't necessarily be detected by the tests prescribed in BS 7671 (insulation resistance test to cpc or earth won't pick it up) - you'd specifically need to test to armour. What if the outer sheath becomes damaged at some point?

AJJewsbury:

In this case, an undetected fault to armour might leave the armour "live" and wouldn't necessarily be detected by the tests prescribed in BS 7671 (insulation resistance test to cpc or earth won't pick it up) - you'd specifically need to test to armour. What if the outer sheath becomes damaged at some point?

Could the same argument be used with metal clips on ordinary insulated & sheathed cables?


Are we left with the argument that a SWA cable isn't deemed to meet double/reinforced insulation requirements - hence it needs to be protected by ADS (presuming ELV, separation etc options aren't applicable).


I'm starting to think that the exposed-conductive-part concept is getting to be a bit outdated - it might be simpler to think about shock protection the other way around as it were - requiring hazardous live conductors to be surrounded by either additional insulation (section 412) or by a protective-conductive-part (connected to the c.p.c.) to initiate ADS (section 411) (or some combination of the two) (all presuming some alternative method of shock protection - e.g. separation - don't apply).


   - Andy.

Andy, that is precisely the point. Currently, there is no construction of SWA where there is a sheath, or another layer of basic protection, between the insulation and armour.


Unfortunately, whilst there is what appears to be a "sheath" around the conductors, it is classed as a "filler" in the cable constructional standards.


As a very real example of a problem this causes us, this precludes SWA being used on the DC side of a solar PV system - meaning there's no way of burying DC cables between frames and inverters at present, except for laying concrete-encased ducting or similar (for "equivalent mechanical protection").



In terms of the point regarding the use of a weather protection boot is suitable for insulation ... well, I've always known it was for weather protection, and not insulation. In the railway industry, its common to have "two earthing system" issue, and "gapping" (you can get insulated adaptors specifically for this purpose)along with cold-shrink or heat-shrink insulation over the gland, is necessary. If the designer feels it's necessary to access the armour for testing, an insulated tail can be brought out to a separate marked insulated IP2X terminal in the enclosure, so no-one [no skilled or instructed person, that is]  is inadvertently exposed to accidental contact with two earths.

lyledunn:

I am confused! So this “filler” that surrounds the wire armour is of insufficient construction to regard the wire armour as not being an exposed conductive part. OK, let’s accept that argument, so it is an exposed conductive part. Does that mean that we have an issue with all those early EVSE points wired in SWA with the wire armour connected to the PME terminal but separated from the TT earth at the EVSE as we are breaching 411.3.1.1? 

I think the  "filler" surrounds the insulated live conductors of S.W.A. cable. Recent comments appear to be considering the possibility of an INTERNAL cable breakdown between live conductors and armouring,  rather than the obvious EXTERNAL causes.


The "filler" is the cream/grey coloured insulation shown in the image below covering the insulated  live conductors. The cables are though rated at 600/1000 Volts which says something.

https://www.electricalcounter.co.uk/custom/upload/attachments/products/1/cab_6943x_1.pdf


Z.