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.

 

[pc1966]

(A) has lots of potential for issues, (B) does not, and therefore (A) does not comply with the regs

As we seem to be in the mood to endlessly debate this now-fixed case, lets move on to the issues with the case of (A) and why it would not be acceptable.

As for the OP's initial discovery there is nothing intrinsically wrong with (A) that presents an operational safety hazard here (assuming all cables are rated OK for ring final use, blah, blah) but it would be a problem for testing or future work as it is so unconventional and unexpected.

So how would it be coded any why?

The answer is probably heading towards the answer for (B), save for the "how many cables can my MCB take?" question.

Discuss

 

[SparkyChick]

I would code situation (A) as a C2.

My reasoning...

(i) it's not a ring
(ii) it could be difficult (depending on cable lengths) to establish it exists
(iii) it could be difficult (depending on cable lengths) to establish there isn't a break in the interconnect
(iv) if we can't be certain it exists and we can't be certain it's continuous, we cannot be certain the cable won't be overloaded under normal use
(v) because we cannot be certain it's continuous it's feasible we may not test the entire circuit during IR testing

And that's the crux of it. It's mostly about our inability to properly test it and thus our inability to be certain it's safe.

 

[Lucien Nunes]

Excellent thread! Ordinarily by post #122 I'd have a long list in my mind of various points not yet addressed or imperfectly answered and still apparently worth tackling. Here, there's little I can add.

Although, one point that I can't recall being drawn attention to explicitly is to limit the scrutiny of the bow-tie circuit to the cable configuration. Consider a normal ring that is fully compliant in every way, that is rewired into a bow-tie. The only thing that has changed is the wiring layout; the number of points, total load, the area served and subdivision of the load into separate circuits have not changed. Comparing these two situations is the heart of the matter.

Clearly, if a situation requires two separate circuits or >32A, and yet is served by one 32A bow-tie circuit, it's inadequate, but a conventional single ring would be inadequate in the same way. This does potentially apply in the OP's scenario because there is evidence that the installation was originally two circuits totalling 64A OCPD, therefore it is likely that the present bow-tie is inadequate for reasons other than the way it's connected. Such problems could be equally important for both safety and compliance reasons, but they are not about bow-ties specifically.

The same is true for four conductors in a terminal - it could be a risk but it's not specific to bow-ties. One could imagine three conductors being securely clamped in a trefoil, and the fourth one not receiving its share of the force. Or, all four initially being clamped but not lying snugly side by side, so that thermal movement causes them to give into a slacker configuration. (I was pulled up by an NIC inspector for having put three cables into an MCB terminal!)

I agree that in operation, specifically, if the same points connected in a ring would be both safe and compliant, then the bow-tie will be equally safe or more so, due to the likely lower fault loop impedances at some points. However, during maintenance and testing I agree that it is a potential trap for the unwary that could reasonably be expected to result in confusion or incorrect methods being applied. It is a circuit layout that is not explicitly defined, cannot be simply tested by the method that is defined for RFCs, and does not compensate for these abnormalities by a significantly improved level of safety in some other way. That would make it a C3 IMHO.

 

[ChipFanDave]

Excellent thread! Ordinarily by post #122 I'd have a long list in my mind of various points not yet addressed or imperfectly answered and still apparently worth tackling. Here, there's little I can add.

Although, one point that I can't recall being drawn attention to explicitly is to limit the scrutiny of the bow-tie circuit to the cable configuration. Consider a normal ring that is fully compliant in every way, that is rewired into a bow-tie. The only thing that has changed is the wiring layout; the number of points, total load, the area served and subdivision of the load into separate circuits have not changed. Comparing these two situations is the heart of the matter.

Clearly, if a situation requires two separate circuits or >32A, and yet is served by one 32A bow-tie circuit, it's inadequate, but a conventional single ring would be inadequate in the same way. This does potentially apply in the OP's scenario because there is evidence that the installation was originally two circuits totalling 64A OCPD, therefore it is likely that the present bow-tie is inadequate for reasons other than the way it's connected. Such problems could be equally important for both safety and compliance reasons, but they are not about bow-ties specifically.

The same is true for four conductors in a terminal - it could be a risk but it's not specific to bow-ties. One could imagine three conductors being securely clamped in a trefoil, and the fourth one not receiving its share of the force. Or, all four initially being clamped but not lying snugly side by side, so that thermal movement causes them to give into a slacker configuration. (I was pulled up by an NIC inspector for having put three cables into an MCB terminal!)

I agree that in operation, specifically, if the same points connected in a ring would be both safe and compliant, then the bow-tie will be equally safe or more so, due to the likely lower fault loop impedances at some points. However, during maintenance and testing I agree that it is a potential trap for the unwary that could reasonably be expected to result in confusion or incorrect methods being applied. It is a circuit layout that is not explicitly defined, cannot be simply tested by the method that is defined for RFCs, and does not compensate for these abnormalities by a significantly improved level of safety in some other way. That would make it a C3 IMHO.

Glad to see I’m gaining some traction with the introduction of the new “bow-tie” circuits (oops, should say circuit as protected by one OCPD) Look out 20th edition !!!

 

[pc1966]

And that's the crux of it. It's mostly about our inability to properly test it and thus our inability to be certain it's safe.

There was another thread that brought up the "lollipop circuit" where something like an unused cooker feed has a kitchen ring final circuit added to it, and my issues with that were not the concept (as a garage CU with ring is much the same) but this issue of testability and documentation (or the likely lack of) to determine its existence and to have access to the ring ends for testing.

Now to me this is serious issue so I can understand @SparkyChick grading it as a C2 but equally it is not explicitly against any regulation that springs to mind (beyond good workmanship) and it is not "potentially dangerous" in the strict sense so it could easily be coded C3 as a result.

In software engineering there is something known as the "principle of least astonishment" which has it that anything behaving in an unexpected or unconventional manner is bad practice. We really need something equivalent in the wiring regulations:

Principle of least astonishment - Wikipedia

en.wikipedia.org

I think what this thread has thrown up is there really ought to be some regulation to cover issues of unconventional arrangements, inadequate documentation, or insufficienct access to cicuits junctions that present an issues for unexpected behaviour or inadequate fault coverage when testing.

 

[littlespark]

I can’t remember who said what now, but this is my opinion.

It is 2 circuits in one OCPD.
It has been designed, from the start, as 2 circuits in 2 separate OCPDs.... why they have ended up together is anyone’s guess... (although the spare way and a damaged mcb suggests it’s been a temp repair)
It should have been sorted properly soon after with a new breaker, but sometimes it’s difficult to get parts for older boards

Dangerous? No...
Against regs? Yes... for the reasons given in previous posts.

We are (mostly) all professionals here giving our take on a situation. We KNOW it shouldn’t be done like that, and it sets off alarm bells thinking what else could be wrong here.

Let’s agree to disagree in this situation and hope the OP takes the advice of getting someone to reinstate the missing breaker, and test the 2 circuits to prove there is indeed 2 rfc’s and no interconnection between the 2.

Apart from maybe the odd nuisance trip, I think this is not “emergency” work and can wait until after lockdown.

 

[westward10]

So called lollipop circuits were common in schools whereby a radial or to complicate matters a pair of 2.5s were taken to a 45A switch where the ring final to the socket outlets was connected from, teacher had overall control of having them on or off. This practice seems to have stopped probably due to increased use of additional rcd protection.

 

[pc1966]

I would expect schools or industrial places to have things documented though, with homes you really expect to find only the few "standard circuits" which is the issue!

What is interesting is pretty much everyone is unanimous these unconventional arrangements are a bad thing, but pinning it down to a specific test / regulation is proving harder. A bit like the "I know it when I see it" test from USA obscenity law
[automerge]1588233888[/automerge]
Interestingly the Best Practice Guide #4 has this C2 coding example:

• A ring final circuit cross-connected with another circuit (including live and circuit protective conductors)

Now I suspect they are really talking about ring 1 linked to ring 2 where each has OCPD with the result they are in parallel, but if we read that in a more general sense it covers these examples (bow-tie and paralleled rings).

 

[SparkyChick]

There was another thread that brought up the "lollipop circuit" where something like an unused cooker feed has a kitchen ring final circuit added to it, and my issues with that were not the concept (as a garage CU with ring is much the same) but this issue of testability and documentation (or the likely lack of) to determine its existence and to have access to the ring ends for testing.

Testing this actually isn't as bad as it might first appear.... you can break into a ring for performing the continuity and fig8 checks at any point on the ring, so you can measure and record r1, rn and r2. Using the L-CPC cross link stage of the fig8 test you can establish an R1+R2 value for the ring portion of the circuit, and if you can find it, you can carry out an R1+R2 check on the feeder cable. If you can't find it, you can at least confirm R1+R2 from any point on the ring fed by the cable, sum those two and you have the R1+R2 for the circuit (you can also get someway to locating the feed into the ring - link L+CPC of the feedcable at the CU and then look for the lowest R1+R2 values on the ring), thus you can carry out the required continuity tests. IR is easy as is Zs. So, I don't particularly see testing a lollipop as the issue. I would say the documentation is the key, in particular making a note of where the transition point is so it can be accessed if required. Problems arise here if it's not a maintenance free junction box and it's say under the bathroom floor, so if I was going to do this, I might bring them up into a double box with a blank plate on and use some MF connectors like the Wago 221-613 lever connectors which can accept cables 2.5-6mm, then make a note of this on the certificate for the works, or even leave a sticker on the inside of the CU.

@SparkyChick grading it as a C2 but equally it is not explicitly against any regulation that springs to mind (beyond good workmanship) and it is not "potentially dangerous" in the strict sense so it could easily be coded C3 as a result.

Just to be clear, I would code a ring final with any form of interconnect (either between the legs or different circuits) as a C2 and that scenario is against the regulations. The situation in the original post with two well formed rings connected to a single breaker, that would be a C3 for me, possibly an FI if I wasn't easily able to establish separation etc. In reality however, if I came across it on an EICR and I was able to prove they were both separate and safe and I had a breaker to hand, I'd probably just put a new breaker in the board and move one of the rings to the new MCB and record what I found and what I did on the certificate.

In software engineering there is something known as the "principle of least astonishment" which has it that anything behaving in an unexpected or unconventional manner is bad practice. We really need something equivalent in the wiring regulations:

Principle of least astonishment - Wikipedia

en.wikipedia.org

Having been a software engineer for over 20 years, I must admit that's a new one on me. The problem with trying to have the regs cover all eventualities is it would be a significantly larger volume and navigating it would be a nightmare.

The regs set out the basic rules and you have to use your experience and judgement to make an informed decision.

Technically, 433.1.204 is pretty much the entire definition of how a ring final circuit should be constructed (it goes without saying you must factor in other elements of the regulations). If your ring circuit meets that, it's hard to argue it's unsafe electrically. From a convenience perspective it's a different matter and it could result in an unsafe situation in the event of a power loss, but electrically as I say, I think it's hard to argue it's dangerous.

I think what this thread has thrown up is there really ought to be some regulation to cover issues of unconventional arrangements, inadequate documentation, or insufficienct access to cicuits junctions that present an issues for unexpected behaviour or inadequate fault coverage when testing.

The regulations do cover all of these things, just not directly, so as above... it's down to your skill and judgement to determine whether what you're presented with is acceptable or not. We're all agreed we'd never install like this, not because it's electrically unsafe but because it could present a danger in other ways and because it is unconventional.

On the subject of documentation, I just look at it like this... if I got called out to one of my jobs, what information beyond the test results would help me and I generally stick whatever I think of on a generic continuation page and/or produce diagrams. I also try and think ahead a little during the install and maybe have designated areas for junction boxes (Typically Wagoboxes) and I record those.

 

[pc1966]

So, I don't particularly see testing a lollipop as the issue. I would say the documentation is the key, in particular making a note of where the transition point is so it can be accessed if required. Problems arise here if it's not a maintenance free junction box and it's say under the bathroom floor

Exactly!

If there is a blank cover over the former cooker switch and terminals behind it to gain access fine, but not if it has been tiled over.

Having been a software engineer for over 20 years, I must admit that's a new one on me.

A former colleague who is a software engineer told me about that one, I was astonished.

On the subject of documentation, I just look at it like this... if I got called out to one of my jobs, what information beyond the test results would help me and I generally stick whatever I think of on a generic continuation page and/or produce diagrams. I also try and think ahead a little during the install and maybe have designated areas for junction boxes (Typically Wagoboxes) and I record those.

If everyone was as competent and conscientious as you would not be debating this now! Thanks again for your detailed inputs.

 

[Midwest]

You lot are obviously all working from home.

 

[Mike Johnson]

No us lot are not working at all, but practicing our keyboard skills.

 

[Midwest]

Think some’s keyboards must be worn out by now.

 

[ebee]

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.

What makes having 4 conductors wired as two rings on one fuseway dangerous? I have not seen an answer. There isn`t one

 

[SparkyChick]

What makes having 4 conductors wired as two rings on one fuseway dangerous? I have not seen an answer. There isn`t one

The short answer is electrically, if the rings comply in their own right with the regulations, they should be safe, but there are possibly implications with the installation not being split to minimise inconvenience and possibly danger from other factors (such as a loss of power to say life supporting equipment). It's unconventional and I don't think any of us would install like it, but certainly I would do it at a pinch to restore supply as a temporary measure.