Wylex 100A dp isolators in plastic enclosures.

Alcomax:

Did you really believe that? 



It is TT.   411.3.1.1, 1st paragraph:  "ECP shall be connected  to a protective conductor under the specific conditions for each type of earthing system as spec'd in regs 411.4 to 411.6, of which one is TT.

Where ADS is used, a cpc has to be run and terminated at each point in wiring (Reg 411.3.1.1).

Is a selective quote from the last paragraph of the reg  

So, if you provide the RCD because it's TT, you provide a CPC ... and terminate it ...

Not so if that equipment does not need one because it is class II.    It should, or would, not be altered by anyone else [an ordinary person] to a class 1 enclosure unless the earthing system was changed to TN or the person doing the alteration decided to take advantage of 531.3.5.3.2.201 and have a class 1 [ steel ] enclosure for the upfront RCD and make a claim on the tails being double or reinforced insulation

Yes, I'm still sticking with this one, because this piece of equipment may well be Class II, but supplies equipment which is not Class II, hence requires the cpc to each point in wiring on the way to the Class I equipment.

But it is TT.

531.3.5.3.2.201 slightly bent changed the usual accepted rules for ADS . A kind of patch, if you like. We are told that a condition covered by 412 group of regs [ double or re-inforced insulation] is "allowed" for single  insulated and sheathed tails in TT up to a demarcation point. It "deems them to comply" with 412 group. It is quite an exception.

411.3.1.1, 1st paragraph:  "ECP shall be connected  to a protective conductor under the specific conditions for each type of earthing system as spec'd in regs 411.4 to 411.6. Those specific conditions include TT and the new reg 531.3.5.3.2.201.

If you consider the tails to the consumers equipment in TT as a circuit- I assume you do from earlier comments- that circuit has double or re-inforced insulation as the "sole protective measure" [ dictated by 531.3.5.3.2.201] and 412.1.2  tells us, in general terms, this measure shall not  be used to any circuit with an earthing contact. So you cannot have an earthing contact for termination.

This is likely an unintended consequence of the 531.3.5.3.2.201 "get out".  We are told something quite exceptional for expediency, similar perhaps to the note for 421.1.201


Away from the issue of the material of the outboard box, there is still a limitation with the class 1 consumer unit for TT, if you are tailing directly in with no upstream RCD, as many are being encouraged to do. The new reg "get out" is flimsy as, however way anyone wants to spin it, the box is Class 1 and has earth terminals. The consumer unit is not class II.  The protective measures outlined in 412 group have defined limits of use, but for TT purposes the demarcation point dividing the "unearthed " enclosure from the "earthed" enclosure does not exist in reality.


From experience, the REC2 is not really suitable in most instances, but they are tried and tested. They are designed for their own isolator and that is fine. You may find most, if not all, makes of RCD are not really a good fit, particularly with respect to the limited cable clamping and the way terminal entry  of the enclosure [does not]  lines up with the RCD terminals. These REC2's are cheap. There are far better non metallic enclosures, with the benefit of a lid, that are much more adaptable and fit for purpose, though you will be paying in the region of £20 + VAT for these, plus of course, the cost of the RCD.

The regs in their current form can be read in a way that seems to recommend some  highly suspect practice.

That's partly what I'm trying to illustrate. Let's be clear, I don't want to make a serious objection to the plastic REC2 isolator, and, if individual designers or installers believe that a plastic box with RCD meets the regs and is safe, then they should, of course follow their experience.


It's worth considering that, if BS 7671 (and its interpretation) were too prescriptive, some installations would not be practicable. I guess it's easier to be prescriptive in the tighter parameters of small houses, for example, but other installations need thinking "outside the box".

 

 

 

I agree with the above. People need to consider why they are doing something, how it can be done effectively and what it is going to achieve. However, we seem to have a bit of a change in direction in the regs [ for domestic , mostly at the moment ] influenced more by carelessness . That is fine for carelessness by users, but when that is maybe influenced by the habits of installers, some of the new "rules" appear, on face value, to be prescriptive. In any event I would say that is how they are applied by the various membership schemes, "deemed to comply" for low risk for compliance with membership rules - a tick box approach. So, for example, all new work has to be RCD protected [ generally]. But does the installer then not pay enough attention to fundamental principles of Chpt 13, the RCD being a bit of a safety net?  Same for metal Cu's on TT, you are encouraged to deviate from usual principles of shock protection, encouraged, instead, to applying a patch of a glorified grommet , which in reality is  no more than a gesture?

Other makes exist, but Wylex is happy for their enclosure to be used for their Type S RCD - I have enquired of them.

I did ask the same question some time ago, so I bought one on the strength of that. But they are not quite the same, in the way the terminal tunnels line up with the enclosure clamp. The whole thing is cramped and could cause more problems than you are trying to solve.  Also be aware there are "differing" Wylex REC2's and REC4's. For example there is a REC 2 and a REC2S, the latter being the "slimline" and far inferior to the former. You may find that "the better ones" are only sold to the DNO.

Not always, depends surely where the origin of that part of the installation is declared.

Zs or Zdb and all that.

A large installation on a farm, or indeed a BN in a block of flats can be in all parts to bs7671, but you can have islands tat are different installations where different rules apply.


The origin of the installation in the stables may well be a pair of bare singles  on insulators and gallows brackets fed overhead, but the metering may be at the farmhouse where it comes from, and there it may be pme. You may not want to put that in now, but plenty remain in use and have many years to go.


Similarly in flats, each flat may have a TNS origin at its redlink, but  its meter may be there or in the basement, and the basement may be tnc-s.. Bonding to the water main will be done both at the origin in the basement, and again in the flat, to the standards of pme and TN-S respectively.

don't forget, sheathed cables (e.g. T&E) are considered to have "double insulation".

Could we pick an example other than T&E please? (say a nice round 3-core flex - or singles in insulating conduit - or even the new Irish style of T&E).


I think there's an issue with T&E and providing protection by double or reinforced insulation. 412 allows protective conductors to be present, but requires them to be insulated as if they were live conductors. I think there's a good reason for this - which T&E illustrates nicely.


Say we had a breakdown of the basic insulation in a T&E cable - do we have protection from electric shock? Well yes and no. Yes, you're protected by the insulating sheath from getting a shock if you touch the cable itself. But what you're not protected from is getting a shock from something else in the system that's connected to the c.p.c. Remember that in T&E they're only one layer of basic insulation between the live conductors and the c.p.c. - so while there's protection from getting a shock from the cable, there's no protection from getting a shock as a result of a fault in the cable. Unless of course a fault to the c.p.c. triggers ADS. 99% of the time that's exactly what would happen of course - so no problem. But occasionally we don't have that luxury (like the supply to a TT system upstream of the first tier of RCDs) - then we really do need 412 to fully provide protection against shocks, so the last thing we want is just a single layer of basic insulation between live parts and protective conductors (or exposed-conductive-parts). Internationally almost all insulation sheathed cables have an insulated c.p.c. - thus providing a 2nd layer of insulation between live parts and protective conductors, so maintaining the usual double fault to danger approach. The UK's tradition of bare c.p.c.s is very unusual in that respect.


My preference when it comes to pre-RCD live conductors would be to keep protective conductors (and exposed-conductive-parts) well away (not in the same enclosure say), but occasionally that's not possible. Take for instance the supply to a touring caravan (where we can't make many assumptions about the 'shore' supply, especially when abroad) - so I must admit that the insulation on the c.p.c. of the flex as well as the line conductor does provide the required two-faults-to-danger.


Having T&E (with the c.p.c. connected to the installation's earthing system) on the supply side of the first RCD in a TT installation is something I wouldn't be at all happy with (regardless of the exact wording of 412.2.4.1 which I suspect was copied from IEC documents which in turn presume that all 'class II' cables have an insulated c.p.c. if they have a c.p.c. at all).



  - Andy.

Perhaps not "double" insulation in the sense of two layers, but the sheath is thicker around the CPC, to fill in the gaps. Might be considered to be reinforced? 

Edit: ah just re-read your post, wasn't thinking about between cpc and live conductors. there is some, but not double the thickness