S.W.A. Armour Earthing.

davezawadi (David Stone):

The filler or bedding layer is outside the conductors to provide a smooth surface for the armour to lay on and has nothing to do with the sheath. If it were not there the armour would be variously corrugated and might damage the conductor insulation if the cable is moved. It ensures that all the armour strands are exactly the same length and thus can move freely. I pointed out why sheaths are not considered insulating above, it is because cables pass through a cooling water bath after extrusion which is used to also test that the insulation is complete with an insulation test. The sheath cannot be tested separately and so cannot be guaranteed to be complete, although it is rarely defective. Simple. I suppose the SWA sheath could be tested in the same way but I believe that it is not.


There is another point about the armour too, why would you not use this as the CPC? In sizes up to I thtink 95mm always has at least as much corrected cross section as the conductors, so is very suitable. I know that SWA in 3 core is often cheaper than the 2 core version, but this is not a reason to use the spare core as CPC, you would bet better off paralleling one of the live conductors fro reduced volt drop.

The outer black sheath is robust enough to withstand abrasion and penetration  by sharp stones and pebbles in most cases, although a suitable sand layer is better for a buried S.W.A. cable. The outer sheath keeps out water thus preventing the cable from becoming a water pipe thus transporting water from a higher to a lower level enclosure. It also prevents water from eventually damaging the steel wire armour, despite the galvanisation, it will still rot over time I have found. I am happy to consider the tough black outer P.V.C. sheath as an insulator.


Z.

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? 

Should be installed with gapping and insulation as I have suggested.


I've seen a number of situations like this, were metal glands are used to terminate TN-S or TN-C-S cable, that then is used for TT. The glands must not be simultaneously accessible with exposed-conductive-parts of the TT system, as you point out. The application of appropriate insulation, such as cold shrink sleeving, and the use of gland separators and plastic enclosures, is entirely reasonable, feasible, and meets the requirements.

But personally I like a large plastic stuffing gland into a plastic box at the load end if creating a TT island, rather than trying to fit heatshink over cut back armour.

AJJewsbury:

But personally I like a large plastic stuffing gland into a plastic box at the load end if creating a TT island, rather than trying to fit heatshink over cut back armour.

But you need an accessible connection to the end of the armour in order to test Zs or R2.


At least the brass gland into a plastic enclosure arrangement (with a boot or heatshrink over the gland outside of the enclosure) gives access with the enclosure lid removed.


   - Andy.

As I've said in an earlier post, all this does is store up the problem for the next electrician ... or a member of the public.


Connect an insulated fly-lead to the SWA gland before you put cold-shrink over it. Use a "gapping adaptor" ... and take the insulated fly-lead into the enclosure via a separate stuffing gland - connect it into a separate DIN-rail terminal (one used for a LIVE conductor), and appropriately label it "ONLY FOR TEST PURPOSES, DO NOT CONNECT TO EARTH".


And in this manner, if you heat-shrink over the insulated live conductors so they are "double insulated" you can even terminate into a metal enclosure without a problem.

AJJewsbury:

But personally I like a large plastic stuffing gland into a plastic box at the load end if creating a TT island, rather than trying to fit heatshink over cut back armour.

But you need an accessible connection to the end of the armour in order to test Zs or R2.


At least the brass gland into a plastic enclosure arrangement (with a boot or heatshrink over the gland outside of the enclosure) gives access with the enclosure lid removed.


   - Andy.

slightly at cross purpose with my explanation. Select a stuffing gland that goes over the whole lot, and push enough of the cable jacket and armour inside the plastic box so you have probe access to the armour end for your  meter as required, just not exposed to touch when the box is shut.  I have been known to then treat the cores and bedding like a rather wilful flex and route it  as one  into the next enclosure before breaking the cores out, but strictly as others have noted the bedding is not supposed to be adequate to count as insulation, so it should be in trunking, conduit or other protection. In reality it is at least as good an insulator as the outer sheath on the average lawnmower flex after a few seasons so though I do not do it now, I do not feel that bad about having done it.


M.

mapj1:

AJJewsbury:

But personally I like a large plastic stuffing gland into a plastic box at the load end if creating a TT island, rather than trying to fit heatshink over cut back armour.

But you need an accessible connection to the end of the armour in order to test Zs or R2.


At least the brass gland into a plastic enclosure arrangement (with a boot or heatshrink over the gland outside of the enclosure) gives access with the enclosure lid removed.


   - Andy.

slightly at cross purpose with my explanation. Select a stuffing gland that goes over the whole lot, and push enough of the cable jacket and armour inside the plastic box so you have probe access to the armour end for your  meter as required, just not exposed to touch when the box is shut.  I have been known to then treat the cores and bedding like a rather wilful flex and route it  as one  into the next enclosure before breaking the cores out, but strictly as others have noted the bedding is not supposed to be adequate to count as insulation, so it should be in trunking, conduit or other protection. In reality it is at least as good an insulator as the outer sheath on the average lawnmower flex after a few seasons so though I do not do it now, I do not feel that bad about having done it.


M.

... and potentially put electricians at risk?


The approach I'm describing reduces the risk (esp. longer cable run, of the order 0.5 to 1 km, where the risk of differing potentials increases):