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.

well there is nothing stopping you having a ring that is all junction boxes and  a spur from each of these per socket. Indeed a similar  arrangement is not that uncommon with houses with a solid floor, so that all the downstairs sockets only have one cable in the back each one being a spur from a ring of sockets upstairs.  It's not an arrangement I like but in practice it gives no problems and saves some cable, and side-steps a cable grouping issue there might be if there were two cables coming down the wall side-by side to keep the ring complete but up and down.

MW certs are not a legal requirement ,but it is probably an insurance  requirement of work done under an niciec or similar scheme, otherwise who knows what was done and is supposed to be guaranteed. I'd strongly recommend recording the same info whether on an MW cert or not, as when you do a lot of stuff, and a few years later someone tries to blame you for a change that you are pretty sure is not one of your own, then it is a good thing to have some paper - or electronic form- to look back on to say 'ah yes, as you can see we added one socket in the bedroom to existing ring, and here are the Zs and insulation checks we did, - the spur you refer to, that set fire to the loft, is not my work at all.... '

As regards RCDs, in practice of course the extra risk of one more socket sans RCD is diddly squat, and the building regs requirement is only to not make things worse, so no laws broken. However without an RCD anywhere it does not comply to current regs, so cannot be signed off as a proper 18th edn. compliant install, and anyone doing that under any of the  part P schemes certainly  would not be quite right.

Cheapest compliant fix is an RCD in a small DIN box in the tails into or out of the fuseboard if you are doing much more than adding a couple of sockets with integral RCDs - RCD sockets are not cheap compared to normal ones

Longer term a new consumer unit is indeed the preferred.

But be aware that before the big fuseboard swap, a very thorough check to eliminate any chance there are crossed neutrals, neutral earth shorts etc that fuses alone do not detect. or the first thing after fitting the new board is a call back saying 'it never used to trip like that before you came '....

There is plenty of 'what have RCDs done for us' literature to show to the unconvinced.

https://www.ddfire.gov.uk/residual-current-devices-can-save-lives Worth having something like that to help the explanation.

M.

Joseph Day said:

I was under the impression this was a mandatory requirement. Am I wrong?

There is no statutory (i.e. legal) requirement, but it is good practice. An MEIWC can be issued for anything less than a new circuit - see the top of page 514 in the Big Brown Book.

Joseph Day said:

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?

I am not sure that I entirely agree with Andy. Your responsibility is to ensure that your work is to an appropriate standard. One way of doing that is to demonstrate that it complies with the current edition of BS 7671. However, where does it end? If you replace a broken socket outlet in this installation, does it have to have RCD protection? I think not. The replacement must be safer than the existing socket.

Joseph Day said:

I've just modified the ring final circuit in my brother's house.

Joseph Day said:

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.

I have never lived in a house with an 18th Edition installation and I have lived long enough to collect my State Pension. If the ancient installation is in good condition, all well and good.

Joseph, your approach fills me with confidence. You seem to be thoughtful and have asked the right questions. Well done!

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.

Hi Chris, thank you for your reply. Apologies in the late reply back, EngX has been really playing up this past week.

Chris Pearson said:

There is no statutory (i.e. legal) requirement, but it is good practice

I'll bare that in mind for my work moving forward. I suppose it's up to my supervisor in that case, but I've always been told that MW is important.

Chris Pearson said:

However, where does it end?

This is part of my concern, too. Because of my addition of an extra socket, I've therefore modified the circuit being the last person to work on the CU and on that circuit, therefore the legal requirement falls to me. The installation has now become a mix of 18th edition and whenever-the-house-was-built edition. I've advised the "customer" (my brother) that he needs to do something about upgrading the CU or at the very least, install an RCCB/RCD but I know full well that won't get done. It'll just be another expense to him, but can you blame him (or anyone for that matter)?

Chris Pearson said:

I have never lived in a house with an 18th Edition installation and I have lived long enough to collect my State Pension. If the ancient installation is in good condition, all well and good.

Joseph, your approach fills me with confidence. You seem to be thoughtful and have asked the right questions. Well done!

Well that's good to know. Maybe I'll be able to retire when I'm 80... maybe. :P

And thanks for the compliment, Chris. To me this trade is just about being proactive, asking the right questions, and giving a damn about your workmanship.

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.