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Modifying Ring with no RCD Present on BS3036 CU & MEIWC Question

Joseph Day

Hi all. Second year apprentice still learning the ropes so just a few things to clarify.

I've got a two part question today, but first some context: I've just modified the ring final circuit in my brother's house. There is a ring where the previous house owner had cut into the ring and spurred off a double socket. Something (I think) he's quite entitled to do assuming he only does it the once on the entire ring (although this would be different if he'd spurred from a socket). Anyway, I've since removed that spur and extended the ring into two double sockets, effectively removing the JB for the spur and just having a singular joint extending the new leg into the outgoing leg to the rest of the living room. I hate joints all together, but this was an unavoidable one.

Question time -

  1. Whenever I've been working on a job under my supervisor, to my knowledge he's never issued a MEIWC for alterations to a circuit, including swapping over an RCD (when required e.g. if the function test failed). I was under the impression this was a mandatory requirement. Am I wrong? If it is a mandatory requirement, I plan on issuing said certificate with the approval of my supervisor once it's had a secondary opinion on my test results which I've recorded.
  2. This second question is a bit more problematic. The installation isn't 18th edition which is quite alright, but the CU is BS3036 fuses with no RCD present on any circuit, nor incoming to protect the full board. Now obviously I've modified the circuit, plus being the last man in the board, I'm not sure what the legal requirements are here. I'm hearing mixed sources saying because it's not an 18th edition install, RCDs are not a requirement but because I've altered the circuit, then it is? What's the stance of the community on this one? I've recommended (stressed) that he should upgrade the board, but I don't think he fully grasps the importance of it which is fine as I don't expect him to.

Any advice is appreciated. Thanks all.

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  • 0

    To my mind your responsibilities are to make sure your new work is to current standards and any alteration to existing doesn't bring it any lower compared with current standards (e.g. extending a ring would increase it's overall Zs - so you'd need to make sure that the existing furthest point still has an adequate Zs (or at least no worse than it was) as well as checking Zs at your new work (which might be a lot nearer the origin) was OK. Ditto for other constraints such as voltage drop or fault protection.

    So a lot depends on exactly what your modifications were. If you added a new socket, that would need 30mA RCD additional protection, if you concealed new cables in walls (without an earthed concentric c.p.c.. of some kind) that would need RCD protection.  If on the other hand you were just re-routing existing cables on the surface or removing an existing socket, then you probably wouldn't have that constraint.

    If you did decide that RCD protection was needed for your new work, there are a number of options to achieving that. You might use an RCD-socket (e.g. if the new cables supplying it didn't require additional protection) or RCD fused connection unit at the start of your new addition (OK for spur perhaps, but not if extending the ring). Better still introduce RCD protection at the start of the ring - that could be by adding a standalone RCCB straight after the fusebox, upgrading the CU or even by adding a 30mA RCD in the tails before the fusebox. There are pros and cons to every option - some give wider protection, some cost less. Some look prettier (if the CU is somewhere very obvious adding extra boxes next to it might be entirely unacceptable to the customer), other require more space (which may or may not be available). Some carry a higher risk of unwanted tripping, other options might be easier for the customer to reset. There's no one size fits all answer - it all depends on the circumstances (including the depth of your customer's pockets).

       - Andy.

  • 0 in reply to AJJewsbury

    Hi Andy. Apologies, I would've answered sooner but having some issues with EngX loading.

    Zs measured as 0.55 when I tested the ring, so I was quite happy with that result. They've just had a media wall installed by a DIYer, so I'm not sure what the condition of the overall final circuit is other than the point of work which I altered.

    AJJewsbury said:
    So a lot depends on exactly what your modifications were. If you added a new socket, that would need 30mA RCD additional protection, if you concealed new cables in walls (without an earthed concentric c.p.c.. of some kind) that would need RCD protection.  If on the other hand you were just re-routing existing cables on the surface or removing an existing socket, then you probably wouldn't have that constraint.

    The work involved removing an old JB and moving one leg of the ring into a double socket, and then looping to a new double socket, and then taking the leg back out into the existing ring. Cables were installed at less than 50mm into the wall, clipped direct. No capping. Cable was 6242Y 2.5mm complete with CPC, which was tested. I believe in this instance, the constraint of requiring RCD protection is needed in this instance, but unfortunately I can only advise my brother.

    Thank you for the options, too. Just on the note of the RCCB after the CU, how would one achieve that? Would that be a case of taking the feed from the rewireable fuse into the RCCB then out into the ring and back?

    Thanks for your reply and helpful guidance.

  • 0 in reply to Joseph Day

    Just on the note of the RCCB after the CU, how would one achieve that?...

    Yup - typically 4mm² or 6mm² radial from the 30A fuse to the RCCB, then start the ring proper from the load terminals of the RCCB (or if the ends of the existing ring can't be moved out of the original CU, 4/6mm² back into the CU and use suitable connectors to join to the original the ring conductors), or any other similar variants...

      - Andy.

Reply
  • 0 in reply to Joseph Day

    Just on the note of the RCCB after the CU, how would one achieve that?...

    Yup - typically 4mm² or 6mm² radial from the 30A fuse to the RCCB, then start the ring proper from the load terminals of the RCCB (or if the ends of the existing ring can't be moved out of the original CU, 4/6mm² back into the CU and use suitable connectors to join to the original the ring conductors), or any other similar variants...

      - Andy.

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