Why does the UK use rings for sockets?

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Discuss Why does the UK use rings for sockets? in the Electrical Wiring, Theories and Regulations area at ElectriciansForums.net

Oh this will open a can of worms when it comes to the arguments for and against ring final circuits!

Yes, you can use radial circuits and in the UK case with any number of sockets, subject the maximum typically of a 32A MCB feeding cable of adequate size.

Similar for a ring, but there you can use less cable and/or longer route, again no limit to sockets as such, but total is limited to typically 32A (some older systems would have a 20A fuse or MCB).

The key difference with the UK are really the fused plugs. That came from the idea of having a standard plug/socket for all normal loads but a choice of fuse, instead of the old arrangement of radials with different feed fuses each for a single socket, and different plug sizes so you could not plug a 2A appliance in to a 15A feed.
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Advantages of the ring:
  • Less copper for a given load / area
  • No single point of failure for protective earth
Advantages of radial:
  • Simpler idea
  • Faults show up immediately
While for some it almost gets to a religious argument, in general if you only need a few sockets then a radial is best choice, if you are covering all rooms in one floor then a ring is probably better able to do it.
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For the history try a search for "THE ORIGIN OF THE BS 1363 PLUG AND SOCKET OUTLET SYSTEM" as there is an IET publication from 2006 with details.

For a quick idea of what you can do with ring or radial in practice then the IET's On-Site Guide book has "Table 7.1(i)" giving the typical cable length limits for ring final circuits on page 65, and for radial final circuits on pages 68-73
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In case you don't have that book, some numbers are:
  • Ring final 2.5mm cable, 32A B-curve MCB, length 106m on TN-C-S
  • Ring final 4mm cable, 32A B-curve MCB, length 171m
  • Radial final 2.5mm cable, 25A B-curve MCB, length 33m
  • Radial final 4mm cable, 32A B-curve MCB, length 43m
 
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bjouis said:
How does a ring use less copper?

A radial is basically a ring without the return cables. Confused!
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because to crate a 32A radial, you need to use 4mm or 6mm cable. and a radialcould well end up with the last socket alomost back to the board, so the extra leg of a ring might be only a few yards.
 
bjouis said:
How does a ring use less copper?

A radial is basically a ring without the return cables. Confused!
Click to expand...
As already mentioned by @telectrix because the ring can use smaller cable for the same overall rating. Almost all final ring circuits in the UK use 2.5mm cross sectional area cables for a 32A rating, to do the same total current as a radial you need to use 4mm (saving in copper of typically over 30%) or even 6mm if the length is great.

The other thing you might not have realised is the UK difference due to fused plugs, here we can put on as many sockets as we want to either type of final circuit. Where as in most of the rest of the world the plugs are unfused so the supplying breaker has to be small enough to protect the final cable against fire, so often that makes for more expensive final cables as well as more radials per house as there is a limit on the number of sockets per radial.

Now the fact we can put on as many as we want per circuit does not mean we should! The higher total capacity (typically 32A) means better load diversity is possible, but even then in the UK you would often plan for more than one final circuit for several reasons:
  • Fault resilience
  • Expected load distribution
  • Expected total load
As a "rule of thumb" you would be using in the UK one ring final per floor in a multi-story property, and even with a single floor flat you might plan for a separate ring for the kitchen where the majority of power-hungry appliances tend to live (washing machine, tumble dryer, dishwasher, etc).

Finally in the UK radial circuits are still very much alive and well for large fixed loads (such as cookers or larger air conditioning units), and for cases when you really don't want an unrelated fault to deny power to somthing critical (e.g. fridge-freezer might have its own breaker, or computer UPS supply in a home/office arrangement, etc).
 
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I suppose we could go with the French method of radials. But hang on, it's a nightmare testing such circuits where there are branches and additions of various kinds and you end up with a radial circuit that is a tangle and very hard to determine what is connected to what. Hence the British like ring circuits because even where mistakes are made you have a beautifully mathematical way of deducing what has happened. The logic intrinsic in the ring circuit and testing methods are second to none throughout the world.
 
@Vortigern also raises the issue of circuit testing. A common complaint by folk looking at testing for the first time is the test procedure for ring final circuits looks unusual, and they think it is more complicated and time-consuming.

It is, but that misses the point! The ring test procedure also provides much more comprehensive fault coverage than it is easy to do with a radial circuit.

The loop impedances (end to end for L-L, N-N, and E-E) give you a good indication of bad joints if you see they are not matching (L & N should be around 0.05 ohm or less difference) or in ratio (typically E/L is 1.67 due to the 1.5mm earth versus 2.5mm phase size used with our T&E cable). This is also easy to do for an inspection as you can do it all at the consumer unit (fusebox).

The "figure of eight" test (L1 joined to E2 and L2 to E1) not only allows socket polarity to be verified (i.e. L and N are correct way round) but is also quite a good test of socket quality as each socket should have practically identical resistance (L to E in this case), and any showing an increase of 0.05 ohm or so might be poorly connected or tarnished contacts, etc. Now 0.05 ohm may not sound like much, but at 10A that is 5W dissipated!
 
No we are populating DB's with RCBO's do you think that radials will become the norm, what do the team think? :cool: can't find a smily that sit's on the fence. :innocent:
 
Mike Johnson said:
Do you think that manufactures will standardise with the rest of the world and produce dual module RCBO's and get rid of the flying lead?
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Schneider produce rcbos with no neutral fly lead on their acti 9 isobar P DB’s, think the rcbo’s are only single pole tho.
DP rcbo’s are not necessarily required tho, be it a TN or TT arrangement, there’s no requirement to disconnect or switch the neutral during overcurrent conditions apart from those requirements detailed in part 7 of bs7671.
 
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I don't see any need for them to be double-width, but for sure we should be moving to double-pole switching to make testing easier, and to have a neutral busbar arrangement in place so less effort and uncertainty in wiring the flying leads up.

There is the same problem for 3-phase where RCBO are rare and often need a 2nd slot for an add-one trip unit. Again, why not a neutral bar to all positions on the 3-phase system so you can have 3*single phase RCBO or 1 * three-phase, etc, per slot?

I have not played with the Schneider system to see how well it works, or if it addresses all of these things.
 
telectrix said:
no neutral or no earth?
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No fly leads on either

Isobar P is the first LV distribution board to have a plug-on RCBO (residual-current circuit breaker with overcurrent protection) capability. RCBO neutral and functional earth connections are usually wired into the neutral and earth terminals inside the distribution board, costing significant time by introducing additional wiring. With the new Acti9 Isobar P, installers can simply plug their RCBO directly into the board. This eliminates the need to cut, straighten or dress any excess wiring therefore reducing installation time by half.
 
Ian1981 said:
No fly leads on either

Isobar P is the first LV distribution board to have a plug-on RCBO (residual-current circuit breaker with overcurrent protection) capability. RCBO neutral and functional earth connections are usually wired into the neutral and earth terminals inside the distribution board, costing significant time by introducing additional wiring. With the new Acti9 Isobar P, installers can simply plug their RCBO directly into the board. This eliminates the need to cut, straighten or dress any excess wiring therefore reducing installation time by half.
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not seen these as yet.
 
telectrix said:
not seen these as yet.
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See link above
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Mike Johnson said:
Contactum make dual pole RCBO's to international standard which in France, Switzerland and Germany it is a requirement to disconnect the neutral on activation: 40 Amp 30mA RCBO - 2 Module - Contactum - https://www.tlc-direct.co.uk/Products/CPBR402.html
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2 modules is Far too big for some of the domestic CU’s and won’t fit in 3phase DB’s
Wylex and Crabtree do a single module type which is DP
 
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Ian1981 said:
2 modules is Far too big for some of the domestic CU’s
Wylex and Crabtree do a single module type which is DP
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I went with Wylex for that reason, but in my case it is more about ease of testing.

Another difference is in the UK a TT supply with a RCD incomer is relatively rare so you don't usually have to worry about a N-E fault tripping the RCBO and that, if single pole, will not isolate the N fault so the main incomer RCD might also trip.
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Unfortunately the Wylex DP RCBOs are not approved for use in their 3-phase boards. Doh!
 
pc1966 said:
I went with Wylex for that reason, but in my case it is more about ease of testing.

Another difference is in the UK a TT supply with a RCD incomer is relatively rare so you don't usually have to worry about a N-E fault tripping the RCBO and that, if single pole, will not isolate the N fault so the main incomer RCD might also trip.
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DP rcbos is definitely a consideration and necessary for that reason but you can have a domestic CU with just a mains switch isolator giving you your DP isolation and just single pole rcbo’s for the final circuits. Same principle for 3 phase, as long as the tails are subject to non movement and enter a ferrous material in a suitable gland etc, and are the insulated and sheathed type
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[Unfortunately the Wylex DP RCBOs are not approved for use in their 3-phase boards. Doh!
[/QUOTE]
Why not?
If it’s because of them being only 6Ka then they don’t necessarily need to be greater than 6Ka, all would depend on the PFC and if there’s sufficient back up protection/ coordination if the breaker can’t break the fault current.
 
Ian1981 said:
Why not?
If it’s because of them being only 6Ka then they don’t necessarily need to be greater than 6Ka
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Not the current, though 6kA is low for a TPN board. It might be the phase barriers are not up to 400V or something, but whatever the reason they are not listed by Wylex as approved.
 
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The problem is they are not true double pole switching in a single module, only the double module units have double pole switching, its the physical size of the internals that dictate the width of the module two contacts at a distance of 3mm width can't fit in a single module.
 
Mike Johnson said:
The problem is they are not true double pole switching in a single module, only the double module units have double pole switching, its the physical size of the internals that dictate the width of the module two contacts at a distance of 3mm width can't fit in a single module.
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How can they claim to be DP but are not?
Is it not 3mm between contacts?
 
Because they do not provide a 3mm air gap when triggered on both, look at the side of any of the modules and you will see the circuit diagram.
 
I'v obviously lead a sheltered life, especially over the last four weeks, cost seem a bit prohibitive, but then if you take into account the three modules it replaces, perhaps not that expensive, thanks for that.
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Anyone found one without the flying lead?

Also is anyone able to confirm that they are truly dual pole and break the connection by 3mm on both the phase and neutral? Been looking for a wiring diagram, but without success so far on any of the specification sheets.
 
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Wylex are pretty rubbish in the documentation area, but most companies are these days. I doubt you will easily find one without a flying lead though, as virtually all UK domestic CU only have provision for a live bus-bar, and not neutral.

You can get the Wylex single module RCBO for around £25 so worst case you buy one to sacrifice and see what is inside it!
 
I doubt that Wylex do anything more than Hager's RCBO who I have already investigated and found they are not true dual pole in the UK in single module width, but in dual module width in the rest of Europe they are true dual pole, I have come to the conclusion that you can't get true separation of 3mm on each pole without the extra width, which seems logical when you consider the doubling up of the number of mechanical components needed, obviously flying leads are not required.

All of the above makes me suspect that the current ARC/RCBO offerings with the flying leads are not true dual pole either, but it is only a suspicion.
 
Mike Johnson said:
I'v obviously lead a sheltered life, especially over the last four weeks, cost seem a bit prohibitive, but then if you take into account the three modules it replaces, perhaps not that expensive, thanks for that.
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With AFDD you are looking at something in the £1-2k region for a full CU at trade price, so it is something that is a very difficult to sell to most folk.

It is also a little unclear as to how effective they are in real terms, and so far no test equipment that I know of so for the 'AFDD' aspect you are trusting the self-test, though obviously you can verify the RCD action which provides some confidence on the electronics and mechanical trip reaction.

I suspect they will come down in price though but no idea how long for that to show.
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Mike Johnson said:
I have come to the conclusion that you can't get true separation of 3mm on each pole without the extra width, which seems logical when you consider the doubling up of the number of mechanical components needed, obviously flying leads are not required.
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I can't see a problem in getting 3mm clearance when open, but I can see a problem in having an arc-quench chamber in a single-module size device if you were interrupting a N-E fault at high current.

Do you have a specific reason for needing this?
 
I think you'r right, the problem is IMO, domestic electrical instals are to a greater extent very reliable, so it's not seen as a priority to improve what is already working well, the old adage "If it ain't broke......." seems to be the majorities mantra.
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@pc1966 the only reason for needing this is the French regulations which stipulate dual pole 3mm gap, they are a bit behind in not using RCBO's in the main, but the availability of true dual pole units is getting better, I wont go into what I think they do better as it will start a very long debate. :yum: I am still looking for the "sat on the fence" smily.
 
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If you really need dual-pole tripping because you might have a L-N polarity reversal, etc, then it would need to be really a 2-P breaker (i.e. thermal/magnetic trips on both poles).

Is that a real concern? I guess you always have a high capacity dual-pole RCD incomer anyway due to them all being TT supplies?
 
The single module units offered in the UK do not have the thermal/magnetic trips on both poles, hence they do not comply with the French Regulations.

The incoming RCD is in reality set up to stop the consumer exceeding the tariff they are paying, there used to be a bit of head room to this, but with the installation of smart meters this head room has been drastically reduced, from what I have been told from 10% to 2%, most consumers who where on the limit prior to the change over have to upgrade their tariff at a cost.
 
Mike Johnson said:
Interesting to watch Nagy at 2:18 and how much pressure he is putting on his torque screwdriver to get it to click over, seems to fly in the face of those that told me that 2.1N is only just over hand tight.
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He may be putting more downwards pressure to stop it camming out the screw head.

I have found that 4Nm is about as much as I can sensibly do single-handed, but that is not while worrying about a pozi screw head (e.g. when using a hex socket, etc).
 
Mike Johnson said:
Interesting to watch Nagy at 2:18 and how much pressure he is putting on his torque screwdriver to get it to click over, seems to fly in the face of those that told me that 2.1N is only just over hand tight.
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I’ve got the same torque screwdriver, you do have to apply a bit of pressure on the 2.8Nm for the Hager mcb,s , Schneider rcbos are from memory around 2Nm so not so much pressure
 
Of course he is trying not to cam out the screw head, what I was alluding too is the comment made on a different thread that 2.1N is only just over hand tight, you would not need both hands for that, it was a very miss-leading comment when I was trying to find out how difficult turning a torque screwdriver was, I will try to find the thread and post on here, I eventually set up a rig to test my Armeg and found it was way over calibrated and sent it back.
 
Trying to decribe force is very difficult to do as you need some sort of reference that people have in common, and screwdriver use varies a lot even among engineers. I find 2.1Nm is not that hard to do single-handed, will try to film something later but never posted a video so far so it might take a bit of faffing about.
 
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