I think you must be talking about a broken ring - which, of course, reproduces the problem seen with a very asymetrrically-loaded complete ring.
Kind Regards, John
Alternative to fused switched sockets above kitchen worktops
- Thread starter fuzzytrifle
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Kind Regards, John
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No (unless you are prepared to seriously distort and reshape the solid 2.5mm² conductors). As I said it's all to do with the geometry. See this illustration of the smallest-sized circular hole into which you can get four undistorted 2.5mm² (~1.78mm diameter) conductors:
D = (2d/√2) + d
For d= 1.78mm, D = 4.30mm, which gives a CSA of the minimum sized circular hole as about 14.3mm². You therefore need a circular hole with a capacity of at least 14.3mm² (not 10mm²) to accommodate four undistorted 2.5mm² conductors. In contrast, because they are stranded, you can 'rearrange the shape' of 4mm² or 6mm² conductors, and therefore get three of the former or two of the latter into a hole not much larger than the total combined CSA of 12mm². To get four 2.5mm² solid conductors into a circular hole of 12mm² CSA would require considerable distortion of the conductors. To get them to fit into a circular hole of 10mm² CSA would require them to be totally reshaped into 'quadrant' (not circular) cross-sectional shape.
Does that make sense?
Kind Regards, John
You can get 4no. 2.5mm² wires into a MK socket. C'mon we've all done it 
I know we have (even though it appears to be non-compliant with the MIs, hence also presumably the regs) - but that must mean that they have quite generous holes. As I've illustrated, you could not get four of them into a 12mm² (let alone 10mm²) circular hole without hammering at least some of them first!You can get 4no. 2.5mm² wires into a MK socket. C'mon we've all done it
Kind Regards, John
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I am not sure what we are discussing.
You have just worked out the absolute minimum diameter for four 2.5mm² conductors.
The fact that it is possible to insert two 2.5mm² with folded ends, albeit pinched tightly, or three 4mm² conductors into the terminal easily without force means that the terminals are larger than this.
Some are square.
I have an MK socket here which has terminals which have a diameter of a good 5mm, i.e csa of approximately 20mm² - 6 is 28.
You have just worked out the absolute minimum diameter for four 2.5mm² conductors.
The fact that it is possible to insert two 2.5mm² with folded ends, albeit pinched tightly, or three 4mm² conductors into the terminal easily without force means that the terminals are larger than this.
Some are square.
I have an MK socket here which has terminals which have a diameter of a good 5mm, i.e csa of approximately 20mm² - 6 is 28.
Well, primarily I was trying to illustrate the flaw in your assertion (complete with dictionary definitions
) that if a terminal has the capacity to take 3x4mm² conductors or 2x6mm² conductors then it necessarily must have the capacity to accommodate four (or maybe even five) 2.5mm² conductors. As I hope I've illustrated, if the terminal hole is circular, and if the (stranded) 3x4mm² conductors or 2x6mm² conductors can be made to 'just fit', the four 2.5mm² ones almost certainly would not fit. As I said, the crucial difference is that the 2.5mm² conductor is solid, whereas the others are stranded (hence their 'shape' can be changed).However, as a secondary motive, I was also trying to see if I could find a way in which the MK MIs made sense, in saying that the maximum capacity was only 3x2.5mm², whereas it had the capacity for 3x4mm² conductors or 2x6mm² conductors. Qualitatively, I've certainly found a possible type of explanation. As below, maybe they feel that, because they are solid, four 2.5mm² conductors would be 'too tight' in their terminal.
That is theoretically enough, if the conductors are dead straight. As I illustrated, you need a minimum diameter of about 4.3mm to be able to just squeeze 4x2.5mm² conductors in. However, in practice, one is going to need a bit more than 4.3mm if it is going to be anything like an easy task to get them in (particularly if the conductors are not perfectly straight), so maybe MK feel that even ~5mm diameter is not quite enough? Who knows?!!
Anyway, regardless of their reasoning, it is clear that putting four 2.5mm² conductors (and I assume that includes 'two doubled') into one of those terminals is non-compliant with the word of the MIs
Kind Regards, John
So is the general consensus that it would be better to fit a radials instead of rings? If I did fit a radial, then I'm guessing that it should start at the socket where we intend to put the kettle - and just hope that we don't change our minds. With the current plan for a ring with three sockets on it the kettle is at one end and the toaster at the other. It makes me wonder how future proof any installation is.
On another note, as I've mentioned earlier I've had four electricians work on the barn over the passed six years, five if you include the one who looked at the job for about an hour before deciding it was too much for him.
Each one has had his own method of wiring a socket, one bent the ends of the wires and stacked them in the terminal, one twisted the single wires together and the last just inserted parallel straight wires - is there a standard way of connecting a socket? The current electrician hasn't wired any sockets for me yet, so I've no idea what he does.
On another note, as I've mentioned earlier I've had four electricians work on the barn over the passed six years, five if you include the one who looked at the job for about an hour before deciding it was too much for him.
Each one has had his own method of wiring a socket, one bent the ends of the wires and stacked them in the terminal, one twisted the single wires together and the last just inserted parallel straight wires - is there a standard way of connecting a socket? The current electrician hasn't wired any sockets for me yet, so I've no idea what he does.
Two Cables per terminal, straight in. Single cable doubled over. Mr Twisty man should be taken outside and shot! Bet he sleeved all the cpc's in a single sleeve as well!
What you need to understand is that the discussions you've been seeing in more recent parts of this thread are essentially academic. Ring final circuits are fully compliant with the regulations, have 'stood the test of time', represent a very high proportion of socket circuits which exist in domestic installations and I'm pretty sure you would find that a high proportion of electricians still routinely install socket circuits in domestic premises as rings, probably without even thinking of doing anything different.
In other words, if you used a ring circuit, you would be in extremely good company, and probably in line with 'the most common current practice'. It can, as you've seen, be questioned whether rings offer any advantage over radials, and it's even possible to cite some potential downsides (particularly under fault conditions) of rings, but they are still very widely used.
Having said that, if you do not have a particular reason for using a ring, then the "why not just use a radial?" argument shows its face. If you did decide to use a radial, then you would obviously have to decide whether to have a single 4mm² radial which could supply a total of 32A (about 7.4kW) or two 2.5mm² radials (using the cable you already have!), each able to supply 20A. That decision would depend on (amongst other things) a judgement as to whether either circuit would ever be likely to be required to supply more than 20A (about 4.6kW).
Kind Regards, John
Re: Chief Devil's Advocate
Only one, only two or four 2.5mm² conductors would be non-compliant.
Only one or only two 4mm² conductors would be non-compliant.
Only one 6mm² conductors would be non-compliant.
If you consider this ridiculous, which it obviously is, then the fact that the ends of two 2.5mm² conductors may be folded makes no difference to compliance as there are still only two conductors.
Applying logic to the assumption that the above is considered to be 'manufacturer's instructions' and not following them would render the installation non-compliant.
Only one, only two or four 2.5mm² conductors would be non-compliant.
Only one or only two 4mm² conductors would be non-compliant.
Only one 6mm² conductors would be non-compliant.
If you consider this ridiculous, which it obviously is, then the fact that the ends of two 2.5mm² conductors may be folded makes no difference to compliance as there are still only two conductors.
I think you may need to re-visit your dictionary definitions of 'capacity', or even just your commonsense understanding of what the word means. In the sort of context we are talking about, 'capacity' indicates the maximum ability of something to accomodate something else, without any implication that all of that capacity has to be utilised. MIs saying that a CU has the 'capacity' to accomodate 10 single-module MCBs/RCBOs would not mean that it is non-compliant to only install 8!
As above, I think it's your (tongue-in-cheek) interpretation which is 'ridiculous', not what the 'MI' says. Given the context (of 'terminal capacity'), I think that 'any reasonable man' would regard a doubled conductor as being equivalent to two conductors.
There has to be some reason why MK have written what they did, and I've offered my suggestion as to what they may be. I don't have an MK socket to had, but you clearly do - how easily can you slide four 2.5mm² conductors (with proper degree of insulation stripping) into one of the terminals? One issue we have not discussed is that, unless one strips 'too much' insulation (leaving appreciable bare conductor exposed), the presence of insulation can make it very hard to get 'a lot' of conductors into a terminal, even if they would go in easily as just bare conductors.
Whatever, and for whatever reason, MK clearly are saying that one should not put more than three 2.5mm² conductors into the terminal. I admit that I'm citingthe 'Technical Datasheet' but I imagine/presume that the same is probably said in whatever 'MI' comes with the product.
Kind Regards, John
Fair enough.
However, It's not necessarily obvious (at least, not to me) which 'wins' in risk-assessment terms. A break in either L or N is obviously twice as probable as a break in the CPC. However, particularly given the apparently generous 'safety margins' in terms of CCC and the fact that overload is very rare (an L or N break would probably rarely result in inadequate fault protection), the actual risk associated with such a break is probably less than that associated with a loss of CPC.
Kind Regards, John
Another issue with "too many" conductors in a terminal is it increases the risk that the terminal gets into a state where different conductors experiance vastly different levels of clamping force from the terminal depending on where they sit in relation to each other.
Indeed so. In fact, the moment one has more than one conductor in a terminal, particularly a 'screw' terminal with a round hole (or more than two conductors in a 'clamp' sort of terminal), there has got to be some uncertainty as to what will happen when the screw is tightened.
I don't really understand why 'multiple terminals' are not much more common. It's years since I've seen one like this - does anyone still make anything like this? ...
Kind Regards, John
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