There is no specification givenue in the standard for this 'bedding' material, unlike the outer sheath of DI cables like twin and earth. I'm therefore inclined to agree.
earthing the armour on SWA
- Thread starter SparkyTris
- Start date
There is no specification givenue in the standard for this 'bedding' material, unlike the outer sheath of DI cables like twin and earth. I'm therefore inclined to agree.
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That sounds reasonable /credible. However, as I've said, for those who are, for whatever reason, hell-bent on finding a reg-compliant justification for not earthing the armour of SWA (when armour is not actually required), they can probably achieve that so long as the armour(or anything connected to it) is not 'exposed', whether the cores count as single- or double-insulated ... not that I can understand why anyone would want to do that.
Kind Regards, John
Page 119 of GN3:
"Even where the armouring is not serving as a cpc, it will still need to be earthed as it is an exposed-conductive part "
I don't really understand that bit as the sheath is properly specced in BS5436 and as you say it's not an exposed part of the equipment. Ignoring the product standard's guidance though is a bit of a poor show.
"Even where the armouring is not serving as a cpc, it will still need to be earthed as it is an exposed-conductive part "
I don't really understand that bit as the sheath is properly specced in BS5436 and as you say it's not an exposed part of the equipment. Ignoring the product standard's guidance though is a bit of a poor show.
As I have said, for those who wish to pursue this seemingly silly line of thinking, if one ensures that the armour is not 'exposed' anywhere (e.g. by sending all of the SWA through a stuffing gland into an enclosure at both ends), it surely cannot possibly be an exposed-conductive-part, since the first requirement to be that must surely be that it has to 'exposed'?
As I've also said, I think this is a silly and unnecessary discussion!
Kind Regards, John
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The armour of an SWA cable is always considered to be an exposed conductive part, and therefore must be earthed.
Furthermore, these cables are NOT double insulated - they are insulated and sheathed. This is simply for mechanical protection of the cables.
Furthermore, these cables are NOT double insulated - they are insulated and sheathed. This is simply for mechanical protection of the cables.
That's true but it's this additional mechanical protection which allows the cable to meet the definition of the protective measure to which I was referring.
Where does that come from? As I've said, for those who wish to pursue this rather pointless line of thought, I really don't see how something which is not 'exposed' can ever be considered to be an exposed-conductive-part.
"Double insulated" is a somewhat misleading term. T+E, meter tails, most flexible cables etc. are also, technically speaking, 'insulated and sheathed', but they are considered safe enough to touch etc. - i.e. they are what we colloquially describe as 'DI'.
SWA cable as a whole is surely at least 'double insulated'? There are three layers of insulating material between live conductors and the outside world, the inner and outer ones of which are at least as 'good' as the ones one has with, say, T+E cable. I actually suspect that the outer sheath of SWA is sufficiently tough that it would qualify as 'reinforced insulation'. The issue being raised is about whether or not there are two layers of insulation between live conductors and the armour - but, as I've said above, if the armour is not exposed, I don't think that really matters (regs-wise).
Kind Regards, John
As I've just written, I think there are some terminology issues here. SWA as a whole is no less 'double insulated' than things like T+E and flexible cables, which are also 'insulated and sheathed'. Furthermore, as I think you're implying, the armour could be considered to be reinforcing the insulation.
I still don't really understand why we are bothering to discuss this hypothetical issue! I can think of no situation in which there would be a rational reason for not wanting to earth SWA armour, even if it could be argued that this was 'not required'.
Kind Regards, John
Regs-wise or common sense and logic ?
Common sense strongly suggests that if the SWA is damaged there is a significant risk of the armour coming in contact with a Live conductor. If the armouring is not connected to ground then this damage could go un-noticed leaving the armour at Live potential. This presents a hazard to anyone who comes in contact with the ends of the SWA where the armour strands are exposed or anywhere along the cable if the outer sheath is punctured or removed for any reason.
For that reason in my opinion it is both negligent and stupid to leave the armour of SWA un-earthed.
As I keep saying, this entire ('silly') discussion is about "regs-wise", not about "common sense or logical". As I also keep saying, I can think of no "common sense or logical" (or rational) reason for not earthing SWA armour, whatever wriggling through the regulations people might think of doing.
Given the above, I can but play Devil's Advocate here. On that basis, I can see some people arguing that what you say is conceptually true of any 'double-insulated' item - if (as a result of one or two 'incidents'), both of the layers of insulation on which one is relying are breached, then there clearly is a hazard. As for 'exposed armour strands at the ends', as I've said, they could get around that by having them exposed only within enclosures - so that the none of the armour would be 'exposed' unless the outer sheath were damaged.
As above, I cannot disagree.
Kind Regards, John
I put it to you that you do not know what the insulating properties of the mechanical protection are. After all they are not tested for their insulating properties as this is not the purpose.
Likewise with other insulated and sheathed cables. The manufacturers are not claiming any insulating property for the sheath which is simply provided for mechanical protection.
You may be right that no claims are made, or tests required in Standards, in relation to the insulating properties of the sheaths of cables (SWA, T+E, 'insulated+sleeved' singles, flexible cables or whatever) - I don't know (even though you and I know that, in reality, the insulating properties of those sheaths would be 'fine'!).
However, this doesn't really matter in terms of BS7671, since, although this phrase "double insulated" gets thrown about a lot (including in BS7671), it seems apparent that "insulated and sheathed" is considered acceptable as providing "double or reinforced insulation" ....
As you say, the sheath provides (presumably 'adequate') mechanical protection, and it seems that this is an acceptable means of satisfying 412.1 as regards "double or reinforced insulation".412.2.4.1 said:
Kind Regards, John
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