I am in the process of fitting a new double socket in a property built 1993 and have located a nicely accessible cable which runs between two sockets and which are both on the 63A 30Ma upstairs ring main (two wires in each). I am proposing to use a junction box on this cable to run the new 2.5mm cable to the new socket (only 6 feet or so). I can either use a single junction and run the new socket as a stud or I can use two junctions and run the new socket as part of the ring. Is either more preferable, in terms of safety or otherwise ?
The permitted use of a ring main is dependent on following some special rules and these include the limiting of the supply to 32A not 63A I will assume the 63A is the rating of the RCD and not the MCB but seems prudent to mention.
Spurs are protected by the fuse in the plug where as the main ring main is protected by the 32A MCB so the cable to a spur has to be where it is unlikely to be damaged and no longer than 3 meters. 433.2.2 although some feel 433.4 means there is no length limit it is still better to keep spurs short.
Making a socket part of the ring would therefore seem at first to be a better option however it is not quite that simple.
The B32 MCB is in fact two devices in one and has both thermal and magnetic trips and although the thermal will after some time trip at 33A the magnet is 5 times the thermal (if a C type than 10 times and D type 20 times) so for it to trip on a short circuit using ohms law 230/160 then the resistance must be less than 1.4375 ohms. Some of this is taken up with the supply typically about 0.35 ohms so the total ohms is near to 1 ohm. Cable has resistance with 2.5mm that is about 0.018 ohms per meter to be exact one needs to correct for temperature.
Now if one was to run a ring around a building with spurs dropping to each socket the resistance to each socket would be less than taking the ring to each socket however we then have another problem. Screws! It seems that in time screws can work lose. Vibration and heat (Expansion and contraction) both play a part so all junction boxes using screws must be accessible. Although one may write on the floor board "JB below" as soon as one lays carpet then really it is not accessible so we where ever possible use the sockets as junction boxes so ring goes right to the socket.
In exceptional circumstances we do need to hide the JB and there are special maintenance free junction boxes with spring clips instead of screws.
However returning to the question to answer we need to know the loop impedance (Resistance is called impedance on AC circuits) before we can answer the question. Likely to extend the ring will be best option but to do the job one will need to complete the "Minor works certificate" and on that you will need to enter:-
System earthing arrangement
Method of fault protection
Earth fault loop impedance
RCD time to trip
To measure these you will need what is referred to as a 17th Edition test set which is often three meters. RCD tester. Low and High ohms ohm meter using 200ma on low ohms and 500 volts on high ohms. Loop tester which will often read in either amps or ohms. The set costs around £750 and even to hire not cheap.
As a result in real terms following the regulations the DIY man can't do the work as cheap as getting an electrician in to do the work. Of course many take no notice of the regulations and do the work anyway without testing.
Hence why one is suppose to have your house tested every 10 years of change of occupier which ever is sooner.
As with most trades when one watches the expert it seems easy but once one looks deeper not quite so easy. I don't say don't DIY but do be aware of all you are not doing.
The worry with ring mains is that the ring is broken and then the cable can be overloaded and ringing the two cables to ensure it is a ring can as the name suggests be done with a simple door bell. Not to do the test does give a high fire risk.
I don't like ring mains. In USA and many other countries they are banned. I prefer radials and 20A MCB instead of ring with 32A MCB although I think our plug and socket with fuse in plug is second to non. The ring main was invented to save copper after the last war and if maintained nothing wrong with it. But not really good when the DIY'er has been let loose as a missing wire does not stop it working it only makes it unsafe.
Spurs from ring mains are not protected by the fuse in the plug. if a short circuit occurs on a spur the fuse in the plug will never blow. only the protective device for the circuit will operate. the ring circuit is the only permitted circuit where the CCC of the cable is LESS than that of the protective device. if ring continuity is lost on either line or neutral, possible when mr diy swaps a socket but doesn't test continuity of ring final circuit, you then have 2 x 2.5mm radials protected by a 30 or 23A protective device- warm cables as It <In I install A2 and A3 radials- every conductor always protected as It > In
Just to expand on Russell Fields valid comments on the drawbacks of ring circuits and to clarify ericmark's ref. to spurs protected by fuse in plug. Ring circuits are not the only time cable CCC (Current Carrying Capacity) are less than the protective device since it also applies to 4mm radial on 32A mcb which can also have 2.5mm spurs. Ref. fusing, regs allow for device to be other than at source (of spur) if short run etc as Ericmark says but I would suggest only to a single outlet so spur can draw no more than 13A. The prefered method of doing what the OP wants is to remove the link between the 2 sockets and replace with a length from each to his new socket, however as this may mean disruption to decorations etc then in this case one spurred J.B. is preferable to 2 J.B.'s. As said though reg 526.3 in old red book states J.B.'s must either be accessible later or use maintenance free terminal type.
Spurs from ring mains are not protected by the fuse in the plug.
Correct but the item plugged into the spur is. 434.2.1 restricts the lenght of a spur to 3 meters because the cable is unprotected.
If a short circuit occurs on a spur the fuse in the plug will never blow.
Again correct but we are not really worried about the spur 434.2.1 says "(ii) be installed in such a manner as to reduce the risk of fault to a minimum" so we would hope there will be not problem with the spur its self only things plugged into the spur. Although what you say is correct it is likely to be miss interpreted.
Only the protective device for the circuit will operate.
Again yes you have a cable rated at 21A being protected by a 32A MCB but unless more than a single outlet is connected to the spur this should not be a problem.
The ring circuit is the only permitted circuit where the CCC of the cable is LESS than that of the protective device.
Not correct there are a few exceptions for example current transformers and the CCC because the cable is doubled up is not exceeded you have for a balanced load 42A but it is protected at 32A. You have been pedantic up to now so you must expect the answer to be the same.
If ring continuity is lost on either line or neutral, possible when mr diy swaps a socket but doesn't test continuity of ring final circuit, you then have 2 x 2.5mm radials protected by a 30 or 23A protective device- warm cables as It <In I install A2 and A3 radials- every conductor always protected as It > In
Yes this is the big problem with all DIY lack of testing. Even simple testing will highlight mistakes. If before adding a socket the ELI is 0.98 and after it's 1.2 then alarm bells ring. And one opens up the socket again and re-tests. However if one does not test the earth loop impedance mistakes will go unnoticed.
However there is no requirement to compare before and after results it's just electrical sense. Unfortunately not that common with the DIY guy. It's the same with the ELI (earth loop impedance) in general we know with a RCD see TABLE 41.5 NOTE 2:* The resistance of the installation earth electrode should be as low as practicable. A value exceeding 200 ohms may not be stable. Refer to Regulation 542.2.2. So 200 ohms is a pass. But in real terms one has to work out what would be expected. So if the ELI at the CU was for example 60 ohms then anything above 62 ohms should be investigated.
We as electricians understand this but for the DIY guy even if he does measure the ELI will it mean anything to him? They in general seem to think if it works then it's OK. However we know that's not the case. But how do we tell the DIY guy in a way he can understand that not to test is potentially lethal I don't know. Maybe you can put it in a way they can understand?