S.W.A. Armour Earthing.

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I'm starting to think that the exposed-conductive-part concept is getting to be a bit outdated - it might be simpler to think about shock protection the other way around as it were - requiring hazardous live conductors to be surrounded by either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.

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.