522.8.10 (cables buried in the ground to have an earthed armour/sheath) never did sit very well with unearthed or non-ADS circuits (I've felt for a while that this regulation might be better re-written for each individual chapter 41 section). Rather going down the concentric earth route, the other option of a 'conduit or duct giving equivalent protection against mechanical damage' might be more appropriate. Presumably if you're burying d.c. cables it's in a somewhat controlled environment, so it should be possible to put suitable procedures in place - e.g. prohibiting mechanical excavators in the vicinity.

   - Andy.

I think the answer is tied up in Chapter 41, Section 412.

Two layers of basic insulation is not necessarily considered to be equivalent to 'double insulation' - but the examples cited in 712.521.101 definitely are.

Regulation 412.2.4.1 tells you what is acceptable for wiring systems, and the examples cited in 712.521.101 align with this. You can't just put insulating sleeving over a single insulated conductor and say it's equivalent to double or reinforced insulation.

Note that in all of these examples, both Regulation 412.2.4.1 and Regulation 712.521.101, consist of basic insulation, plus non-metallic mechanical protection.

Further, Regulation 522.8.10, as Andy points out, means that the armour ought to be earthed, and double insulation from earth is required.

A suitable alternative for burying may well be a cable equivalent to BS EN 50618, in a suitable plastic underground conduit, such as 750N twin-walled ducting - provided it's appropriately sealed of course, and at the ends to prevent entry of flora and fauna.

Hi both,

Thanks for your thoughts. I don't disagree with either of you in principle.

However I would point out that it the presence of an armour / screen may be warranted for reasons other than ADS. For example, the current (7y.o.) IET CoP for PV says that for protection against lightning:

Where a system features longer d.c. cables (e.g. over 50 m) consideration should be given to the use of a screened cable (for example, armored cable) or installing the cable in earthed metal conduit/trunking

Worth noting that the CoP also specifically lists SWA as being suitable for this measure... Is this in conflict with the more recent BS7671?

Other potential reasons might include EMR, water ingress defence (tape), mechanical protection during installation, IMS functionality etc. Conversely if armour should never be used on DI systems surely neither should steel trunking?

I absolutely agree any armour would need to be earthed (to be honest I took that as a given).

Jam said:

Worth noting that the CoP also specifically lists SWA as being suitable for this measure... Is this in conflict with the more recent BS7671?

I would be inclined to rely on BS 7671 for this, because of the difference in legal position between Standards (BS 7671) and Industry Guidance (the CoP).

IET CoP for Solar PV is a number of years old, and the IET is working on a Second Edition. The Draft of the 2nd Edition was out for public comment between 30 May and 27 June 2022. The Draft for Public Comment (although not the final version, and things may change between draft and final) was indicating removal of SWA as an option.

Jam said:

Conversely if armour should never be used on DI systems surely neither should steel trunking?

Well, I don't 100 % agree - if the cable inside the metallic containment is 'insulated and sheathed', the requirements of BS 7671 for Class II wiring systems may be met, but if insulated (unsheathed) cables are used, the containment would have to be non-conductive.

The issue with armoured cable, is that there is only insulation between conductor and armour, not conductor+sheath (or conductor+non-metallic protection)

Thanks Graham.

Appreciate that there is a difference between bedding and sheath, although presumably this depends on the constructon and material of both.

But what I am trying to angle at is not whether AWA to BS5467/6724 is specifcally acceptable (though your suggestion that it is not is noted), but whether the presence of a concentric metallic component in the cable is prohibited dull-stop, or if one has a cable specifically built with an "inner sheath" twixt the conductor and the metallic component that then equivalent protection is provided and it can be used where required as a potential solution.

I am thinking along the lines of TUV Rheinland 2PfG2642/11.17. I appreciate that that's not a BS or even EN but we might not need to cross that bridge if the principle is verboten.

Jam said:

or if one has a cable specifically built with an "inner sheath" twixt the conductor and the metallic component that then equivalent protection is provided and it can be used where required as a potential solution.

That might work .... but like yourself, I'm not aware of a cable constructional standard that covers such a cable ... so agree we would be going down the "non-standard cables" discussion, of having extra "due diligence" and confirming on certification its "at least as safe" etc.

Hi Graham,

I'm curious if there is any more news on whether the use of SWA for solar DC will be (or is) removed from the CoP? 

mow22 said:

I'm curious if there is any more news on whether the use of SWA for solar DC will be (or is) removed from the CoP? 

Well, the 2nd Edition of the CoP is due to publish on 29 November.

But ... I hope to be able to share a link to a presentation I gave at a public event a couple of weeks back ... which covers this question.

Please bear with me.

Thanks. It seems to me that there are legitimate safety and maintenance concerns with running DC from PV long distances in SWA . It's a tempting option considering cost especially with ground mount arrays long distance from the intake. So it will be good to have it covered in the CoP.

MCS advice to keep DC cabling to a minimum can be interpreted many ways. But in the end you are weighing cost against safety. The latter better be the one that wins.

I promised to get back when I could share a link to a presentation.

Please see the following presentation on IET TV, which was given at the recent Solar & Storage Live event.

Wiring systems on the DC side are discussed at around 5:59.