Is it permissible to join 2.5mm cable in a ring main using insulated crimped butt connectors and a ratchet crimp tool, providing the connectors are of the correct rating. If so, is it permissible to plaster over if covered by sheathing?
Crimping is an accepted method of joining cables. However the joint will need some form of protection. This can vary from a junction box, to shrink sleeve, it could also be epoxy resin encapsulated or even just Self Amalgamating Tape and normal insulating tape.
In a wall one would want it to be damp proof and normal insulating tape is really not good enough. but this is a matter of judgement by the electrician doing the job and it is hard to condemn a method without seeing it.
Capping is today a problem. The plastic stuff flexes and the plasters don't like it. The metal stuff conducts and alters the status of the wall. We look at 522.6.8 "(i) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective conductor of the circuit concerned." which is followed by a string of BS numbers showing what is permitted. This does not include metal capping.
What the capping does mean is because of phrase. "the internal construction of which includes metallic part, other than metallic fixings such as nails, screws and the like" it makes RCD protection necessary. The thoughts are a fault will not only make a spot live but the metal in the wall can transmit that fault elsewhere making the chance of touching a live part much increased.
Using the plastic oval conduit likely any joints would not even in the most damp wall cause a problem. However the standard blue pre-insulated crimp does not give any strength to a cable and to draw a cable through conduit which has a crimp on it is really asking for problems.
It is a bit of common sense. I have when stuck without a suitable JB used silicon sealant to moisture curing type. Its just as good as Self Amalgamating Tape. Short lenght of oval conduit slipped over joint again sealed with silicon sealant is likely as good as any junction box.
But one have to use common sense and also test it after. And I can't say test it enough times. I have made joints which I though were A1 but when I measured the loop impedance I realised there was something wrong. So had to cut out and start again. Many times I look at a job and it looks OK but when I test I realise with it's proximity to the consumer unit the readings don't match what is expected. And it's not a case of pass and fail. If you measure at the consumer unit and read 0.35 then 2 meters from consumer unit you measure 1.2 ohms clearly there is something wrong. But if the socket was 20 meters from consumer unit with same reading then likely all is A1. The problem is I will look at the reading and as a skilled man in seconds decide if OK or not. An on looker will likely not even realise I have done the test.
You don't say if you want to join cables or if worried about some one else joining cables. I have jumped in with both feet in the past to say you shouldn't do what ever only to find some thing was missed in the tail and there was in that case no problem. So I am now wary of these questions as all too easy to say you shouldn't only to find some house holder is trying to tell off an electrician when because of something missed in the tail he had done nothing wrong.
Or the reverse and one finds crimped cable connectors buried in plaster with no protection and damp is causing the RCD to trip. So it needs some common sense not just following rule book.
Question has to be at the mid point (closest to centre of ring) what is the loop impedance?
There are two loop impedances one line to earth and one line to neutral. With non RCD protected circuits the line to earth (ELI) is very important. This tells you if with a fault the fuse/MCB will open or not within the prescribed time. With RCD protection and all new sockets should be RCD protected the earth loop impedance (ELI) is not quite so important although it does give indication of a fault. However the line neutral impedance is still important. Although unlikely if too low it could mean the fuse/MCB will be unable to handle the short circuit current. If too high it will mean to large of a volt drop and also the fuse/MCB may not disconnect the supply in time.
In the old days of fuses it was on a sliding scale so being slightly out would mean it took slightly longer for fuse to blow. However the MCB is very different it is in fact two devices built into one. The thermal trip is slow acting and with a B32 MCB will after maybe an hour trip if overloaded the time of course will vary according to how much of an overload. This means at centre of ring main even a full short circuit could take a few seconds before the thermal part would trip. So it also has a magnetic part. The "B" in front of the 32 means the magnetic part is set to 5 times rated value. C = 10 times and D = 20 times. So for the trip to work within 0.1 seconds with a B32 a current of 160 amp has to flow. The meters will normally show either prospective fault current or at flick of switch loop impedance which for a B32 = 230/160 = 1.4375 (1.44) ohms.
Adding cable to a ring will mean the resistance will increase. So step one is measure what it is now so you know if there is a margin available to add extra. This will also help latter as it will highlight any errors.
The 13A fuse is considered to be OK with a loop impedance of 2.42 ohms so taking a fused spur from a ring one would assume you could have up to 0.98 ohms in the cable and still be within limits. With 2.5mm rated at 0.018 ohms per meter that's over 50 meters so the fuse will likely offer the protection required. However that will not help with volt drop, but at least it will be only the new sockets affected not all sockets on the ring.
Normally to extend a ring one will select two sockets and replace the wire between them to include the new sockets. So there will be no cable joined. When extending from a single socket one cable may be crimped but that would be within the socket box so your questions make me think you are doing something wrong. To cut a cable found under the floor can cause many problems including selecting the wrong cable.
You of course must insure it is a ring. I have found in the past where even electricians have assumed sockets were part of a ring and extended but in fact it was a 4mm radial and they used 2.5mm cable to extend thinking the cable they found was an old imperial cable and still part of a ring.
Their problem was they assumed. And it is dangerous to assume. You need to test and unless you can test then I would advise don't do the job. OK you may be lucky and have a test certificate which gives you the loop impedance. But once completed do test to ensure it's still a ring.