Ring main.

[JKMRK1]

Question I have is, is it ok to have x2 ring main circuits on one 32amp type B MCB??

If so does this meet the current regs.

TIA.

 

[SparkyChick]

And this is why it's a good topic for discussion because we can all agree we wouldn't do it, but if you had to say code it up... what regs would you cite? Personally for me, I think it would be a C3 with reference to 314.1 (items (i), (iii) and (iv)).

My gut feeling is, as has already been mentioned, it was a temporary fix that's become permanent following an MCB failure, but before separating them again I would want to be 100% sure they are entirely electrically separate. Could be someone did some work and found there was a DIY bodge that linked the two and the home owner didn't want to pay to put it right so they combined the circuits on a single OCPD.

Who knows, but it's a good discussion

 

[Mark Wright]

I know something similar has been said but At the Consumer Unit you could take 1 Wire of One and 1 wire of another using a connector, connect them the other 2 wires into the MCB - There one circuit no more questions.

 

[pc1966]

... before separating them again I would want to be 100% sure they are entirely electrically separate. Could be someone did some work and found there was a DIY bodge that linked the two and the home owner didn't want to pay to put it right so they combined the circuits on a single OCPD.

Ohh! Now I never though it might be an M. C. Escher final circuit!

 

[Mike Johnson]

The inspired Labyrinth, I miss Bowie.

 

[radiohead]

Post #88 is quite possibly the best post I've ever seen on this forum. Nothing else to discuss as far as I'm concerned.

 

[SparkyChick]

Post #88 is quite possibly the best post I've ever seen on this forum. Nothing else to discuss as far as I'm concerned.

I wouldn't quite go that far Marconi and Lucien Nunes (to name two) are much more eloquent and technically capable than I, but thanks

 

[7029 dave]

Post #88 is quite possibly the best post I've ever seen on this forum. Nothing else to discuss as far as I'm concerned.

Agreed very good post.
IMO good topic for discussion, however no good practising Electrician would leave/create a circuit like that.
That is why we are taught electrical craft principles at college/ on site mentoring so there is a definitive standard across the industry, adjacent the current regulations.

 

[Pete999]

So I figured I'd go through the regulations and look at things that specifically talk about ring final circuits and give my thoughts on them in regards to the current situation and then give some thoughts about possible breaches.

Discussions like these are, in my humble opinion, vital learning tools. They also serve to reinforce the importance of being able to think beyond the regs and your own past experience and to that ends, the regs include this statement...

"While the publisher and contributors believe that the information and guidance given in this work is correct, all parties must rely upon their skill and judgement when making use of it."

This case is a good example I feel where skill and judgement come into play. The regulations aren't the be all and end all, they are the starting point.

"2 Definitions - Ring final circuit. A final circuit arranged in the form of a ring and connected to a single point of supply."

So by definition, the circuit in the OP is organised as two rings. Unorthodox? Yes. Unsafe? Assuming that in all other ways it complies, then probably not.

"433.1.204 Accessories to BS 1363 may be supplied through a ring final circuit, with or without unfused spurs, protected by a 30 A or 32 A protective device complying with BS 88 series, BS 3036, BS EN 60898, BS EN 60947-2 or BS EN 61009-1 (RCBO). The circuit shall be wired with copper conductors having line and neutral conductors with a minimum cross-sectional area of 2.5 mm2 except for two-core mineral insulated cables complying with BS EN 60702-1, for which the minimum cross-sectional area is 1.5 mm2. Such circuits are deemed to meet the requirements of Regulation 433.1.1 if the current-carrying capacity (Iz) of the cable is not less than 20 A and if, under the intended conditions of use, the load current in any part of the circuit is unlikely to exceed for long periods the current-carrying capacity (Iz) of the cable."

That regulation is the only real technical specification for a ring final circuit and you'll no doubt notice it's focused for the most part on the current carrying capacity of the cable.

Since we don't have access to the circuit in question we have to make some assumptions, so if we assume each individual ring is designed and installed correctly and is compliant with this regulation in it's own right, by extension if they are both connected to the same OCPD they will still be compliant.

"433.4 Overload protection of conductors in parallel
Where a single protective device protects two or more conductors in parallel there shall be no branch circuits or devices for isolation or switching in the parallel conductors.
This regulation does not preclude the use of ring final circuits with or without spur connections."

"433.4.1 Equal current sharing between parallel conductors
Except for a ring final circuit, where spurs are permitted, where a single device protects conductors in parallel and the conductors are sharing currents equally, the value of Iz to be used in Regulation 433.1.1 is the sum of the current carrying capacities of the parallel conductors. It is deemed that current sharing is equal if the requirements of the first indent of Regulation 523.7(i) are satisfied."

These two excerpts are really about conductors in parallel and the mention of ring final circuits is to exclude them from the provisions of 433 clauses.

"523.7 Conductors in parallel
Where two or more live conductors or PEN conductors are connected in parallel in a system, either:
(i) measures shall be taken to achieve equal load current sharing between them
This requirement is considered to be fulfilled if the conductors are of the same material, have the same cross sectional area, are approximately the same length and have no branch circuits along their length, and either:
(a) the conductors in parallel are multicore cables or twisted single-core cables or non-sheathed cables, or
(b) the conductors in parallel are non-twisted single-core cables or non-sheathed cables in trefoil or flat formation and where the cross-sectional area is greater than 50 mm2 in copper or 70 mm2 in aluminium, the special configuration necessary for such formations is adopted. These configurations consist of suitable groupings and spacings of the different lines or poles or
(ii) special consideration shall be given to the load current sharing to meet the requirements of Regulation 523.1.

This regulation does not preclude the use of ring final circuits with or without spur connections.

Where adequate current sharing is not possible or where four or more conductors have to be connected in parallel consideration shall be given to the use of busbar trunking."

As with 433, this section is excluding ring finals from the erection guidelines around conductors in parallel.

"543.2.9 Except where the circuit protective conductor is formed by a metal covering or enclosure containing all of the conductors of the ring, the circuit protective conductor of every ring final circuit shall also be run in the form of a ring having both ends connected to the earthing terminal at the origin of the circuit."

As this is domestic, it's likely to be wired in twin and earth (another assumption), but this is just clarifying the installation requirements of the CPC. We all know the conductors of a ring final circuit should be continuous rings (as evidenced by the testing we are required to carry out). So, assuming in this situation the CPC is continuous, it's compliant.

"543.7.2 Socket-outlet final circuits
543.7.2.201 For a final circuit with a number of socket-outlets or connection units intended to supply two or more items of equipment, where it is known or reasonably to be expected that the total protective conductor current in normal service will exceed 10 mA, the circuit shall be provided with a high integrity protective conductor connection complying with the requirements of Regulation 543.7.1. The following arrangements of the final circuit are acceptable:
(i) A ring final circuit with a ring protective conductor. Spurs, if provided, require high integrity protective conductor connections complying with the requirements of Regulation 543.7.1
(ii) A radial final circuit with a single protective conductor:
(a) the protective conductor being connected as a ring, or
(b) a separate protective conductor being provided at the final socket-outlet by connection to the metal conduit or ducting, or
(c) where two or more similar radial circuits supply socket-outlets in adjacent areas and are fed from the same distribution board, have identical means of short-circuit and overcurrent protection and circuit protective conductors of the same cross-sectional area, then a second protective conductor may be provided at the final socket-outlet on one circuit by connection to the protective conductor of the adjacent circuit
(iii) Other circuits complying with the requirements of Regulation 543.7.1."

Since this is about the implementation of high integrity earthing, we once again have to make an assumption. In a domestic situation it's unlikely high integrity earthing will have been installed.

"643.2 Continuity of conductors
643.2.1 The continuity of conductors and connections to exposed-conductive-parts and extraneousconductive-parts, if any, shall be verified by a measurement of resistance on:
(i) protective conductors, including protective bonding conductors, and
(ii) in the case of ring final circuits, live conductors."

Testing... we are required to conduct specific tests on ring final circuits. These can easily be done but the results sheets are not designed to record multiple ring result sets for a single 'circuit'. I would perhaps use two lines of the schedule of results to ensure I captured all the relevant information, with clear labelling applied to the circuits within the consumer unit to allow easy identification later (and thus allow easy cross referencing of results).

So, now lets look at some possible areas this installation could breach the regulations.

@Pete999 has already provided one in the form of 134.1.1. This is however somewhat subjective. If we assume the cable is sized correctly, it's properly supported along it's run, it's properly installed (protected against mechanical damage for example, all conductors are properly identified etc.) and the connected accessories are compliant with the required standards, the only possible reason we may be in breach is if the manufacturers instructions provide guidance on the number of conductors in a particular termination at the consumer unit. With a 3036 rewireable board this could be a problem, with early MCBs it could be a problem, with 60898 compliant MCBs it's not likely to be an issue (4 x 2.5mm sq. conductors) but with 61009 compliant RCBOs it may be a problem due to the reduced size of the terminals. So, an assessment of the overall installation and the terminations should be made to determine if there are any issues (this is no different to any other circuit).

"314 DIVISION OF INSTALLATION
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid danger and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Chapter 46 and Section 537)
(iii) take account of hazards that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor (PE) currents not due to a fault
(v) mitigate the effects of electromagnetic disturbances (see also Chapter 44)
(vi) prevent the indirect energizing of a circuit intended to be isolated.

314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled, in such a way that those circuits are not affected by the failure of other circuits, and due account shall be taken of the consequences of the operation of any single protective device.
314.3 The number of final circuits required, and the number of points supplied by any final circuit, shall be such as to facilitate compliance with the requirements of Chapter 43 for overcurrent protection, Chapter 46 and Section 537 for isolation and switching and Chapter 52 as regards current-carrying capacities of conductors.
314.4 Where an installation comprises more than one final circuit, each final circuit shall be connected to a separate way in a distribution board. The wiring of each final circuit shall be electrically separate from that of every other final circuit, so as to prevent the indirect energizing of a final circuit intended to be isolated."

This is the section I think we are most likely to be in breach of when considering this arrangement.

314.1 (i) as this arrangement has the potential to take out all the sockets in the event of a fault, (ii) arguably this is a case of inconvenience for the installation user whilst we are working because whilst we can facilitate safe inspection, testing and maintenance, we have to shut off all sockets which may result in incovenience for the user, (iv) nuisance RCD tripping could be a problem, but no more than two ring circuits connected to different breakers on the same RCD (a situation that occurs quite frequently with split load boards).

314.2 ordinarily this may not be a problem, sure it's inconvenient but what happens if say someone has a new requirement for some medical equipment? Having all the sockets on a single circuit wouldn't be a good plan in this case as a fault could take out the supply to life support equipment. Arguably this should be considered and addressed when the equipment is installed, but it may not be and thus we could have a dangerous situation.

314.4 this point has resulted in a lengthy discussion in the past in relation to a similar topic (is it ok to combine radial circuits on one MCB). It could be argued that each ring is a final circuit in it's own right and thus should be supplied by a separate way but as the definition of circuit is somewhat wishy washy, it can be argued that 'a circuit' is defined as whatever is connected to a particular way in the distribution board.

"411 Protective Measure: Automatic Disconnection of Supply"
If both rings are installed correctly and are capable individually of meeting the requirements for this, when connected to the same protective device they should continue to meet these requirements. We would of course need to check the earth fault loop impedance and as good practice check the line-neutral loop impedance to ensure it doesn't exceed the maximum EFLI for the circuit breaker (to ensure we can meet our disconnection time for LN faults). Clearly we need to conduct more testing, but essentially this is no different to any other circuit.

These are my thoughts about it. I don't believe there are any clear contraventions of the regulations with such an arrangement. Much of it is down to one's interpretation/views. From a safety perspective, assuming the circuit is compliant in every other way, there should no increase every installation shall be divided into circuits tod risk of danger from this arrangement as the conductors will be adequately protected (they would be if they were installed on their own MCB).

Would I install this way? No because I don't believe it adequately splits the installation to minimise inconvenience. I would however do this to restore supply in the event of a fault. I agree we should be doing our utmost to ensure people don't believe this is acceptable, but from a regs perspective as stated, I don't believe it breaches them but I am happy to be proven wrong. From a danger perspective I don't believe it is any more dangerous than a ring final connected to it's own MCB.

Discuss

314.1 Does however state that every installation shall be divided into circuits to avoid
(1) DANGER and minimise any inconvenience in the event of a fault, so to my mind there could be an element of danger involved.

 

[stevethesparks]

Not good practice - No.

However, my main concern is that the mcb can safely accommodate the four conductors whilst maintaining a good connection. If it can then not dangerous.

How many small houses have we seen with one final ring for all outlets on older installations?
Had the two rings been made into one big ring within the CU and all readings are ok, whats the difference - apart from the two rings will give a far better zs.

I reiterate, not good practice - C3 at most if connections are sound.

 

[pirate]

I have read this thread from start to finish, twice. For an amateur like me i have found the debate fascinating and informative. I won't wear out my "like" button but just say that I am better informed now, and very impressed by the high quality of the debate and the lengths many have gone to in order to explore this question. That the debate has been constructive and courteous as well is a real bonus, and I applaud all who have contributed.
Clearly I have nothing to contribute, albeit I fall into the camp of "It just doesn't seem right, and would be better done properly"...but now I feel as if I have learned a lot from this thread, so I thank you all for your contributions.

 

[davesparks]

314.1 Does however state that every installation shall be divided into circuits to avoid
(1) DANGER and minimise any inconvenience in the event of a fault, so to my mind there could be an element of danger involved.

But you still can't describe what the danger is.

It's quite simple, you think something is dangerous so you must know what the danger is that you think is there.

This is the difference between cowboys and tradesmen,
For example you have a class1 ceiling light with no cpc.
Cowboy: that's dangerous you have to pay me £--- to fix it.
Tradesman: this is dangerous because it presents an electric shock risk under fault conditions, I could rewire the circuit, run a cpc to this point or replace the light with a class 2 fitting etc etc

 

[Vortigern]

I flagged up 314 etc. as clearly this requires separation of circuits for the reasons stated. The question is really, is this two circuits or one. Thinking of the definition of a ring circuit this is clearly two circuits combined into one ocpd/mcb. I therefore think this is not compliant with regs, which we are required to comply with. There may also be a labelling issue. i.e. does the MCB clearly indicate two ring circuits are on the same MCB. The definition of a ring circuit refers to "a ring main" "final sub circuit" as in singular item in the case of the noun after the indefinite article. I think that it is unambiguous enough to say that this is not a ring circuit as defined but two ring circuits and there is no provision in the regs to support with regs, this type of installation. It is bad workmanship and I would not be happy if anyone of you came into my house and did this kind of work. I'm gonna ring my scheme see what they say. Finally as has been asked (not answered by OP) do we know this is two rings? Have tests shown this to be so. Or is it as is often seen a couple of radials poked into the ring/MCB at the CU? I seem to remember no-one seems to think several light circuits are ok in one MCB in previous discussions of this nature.

 

[pc1966]

Finally as has been asked (not answered by OP) do we know this is two rings? Have tests shown this to be so.

Yes according to post #69

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[radiohead]

I flagged up 314 etc. as clearly this requires separation of circuits for the reasons stated. The question is really, is this two circuits or one. Thinking of the definition of a ring circuit this is clearly two circuits combined into one ocpd/mcb. I therefore think this is not compliant with regs, which we are required to comply with. There may also be a labelling issue. i.e. does the MCB clearly indicate two ring circuits are on the same MCB. The definition of a ring circuit refers to "a ring main" "final sub circuit" as in singular item in the case of the noun after the indefinite article. I think that it is unambiguous enough to say that this is not a ring circuit as defined but two ring circuits and there is no provision in the regs to support with regs, this type of installation. It is bad workmanship and I would not be happy if anyone of you came into my house and did this kind of work. I'm gonna ring my scheme see what they say. Finally as has been asked (not answered by OP) do we know this is two rings? Have tests shown this to be so. Or is it as is often seen a couple of radials poked into the ring/MCB at the CU? I seem to remember no-one seems to think several light circuits are ok in one MCB in previous discussions of this nature.

If the 'two circuits' are connected to a single device then by definition it is one circuit.
If you are going to apply that argument then a ring circuit with a spur connected at the origin, (an arrangement nobody questions), would be classed as a ring and a radial connected together, two circuits in the same device. But it is not, it is one circuit.

 

[Pete999]

If the 'two circuits' are connected to a single device then by definition it is one circuit.
If you are going to apply that argument then a ring circuit with a spur connected at the origin, (an arrangement nobody questions), would be classed as a ring and a radial connected together, two circuits in the same device. But it is not, it is one circuit.

It would be an RFC end of, you can take a spur anywhere off of an RFC CU MCB included, one circuit I agree with