two ring mains joined

[visionset]

I've recently moved to a house that seems to have a separate ring downstairs in an extension that is joined to the upstairs ring. It doesn't seem to be extended as typical ie breaking the loops and inserting more sockets, rather it is a separate ring but joined to the upstairs ring at two places (sockets). So two sockets upstairs have been chosen with 3 cables each and two in the extension with 3 cables each. IYSWIM

Is this okay, acceptable, legal?

I am going to extend further and likely include those downstairs sockets (of upstairs ring) in the downstairs circuit in the more typical extending a ring fashion. Or that's what I expect the electrician will do?

 

[ericmark]

We are told to test for figure of 8 wiring and list it as a fault. However I have failed to find any thing in the regulations which says you can't other than "to provide reasonable sharing of the load around the ring" ref 433.1.5 which in turn directs to 433.1.1 which talks about overload. Kirchhoff's laws become complex to work out, I did in in University, but as to working out on back of fag packet forget it. I would have to get the text books out, or use electronic workbench, which is not installed on my PDA, so I would not be doing in in a private house.

Extending a ring can result with excessive volt drop or loop impedance without those reading no one can say yes or no. This has been talked about before, the volt drop is not easy to be certain as to results, because the meters are not that accurate, and much is up to the electrician doing the job. It would be nice to say the limit is 0.96Ω, but the loop impedance of incoming supply also has a bearing.

Before 2004 there were few installation certificates or minor works certificates completed for domestic work. Part P changed this, in the main to protect the electrician so he can say "here's my reading clearly some one played after I left".

In 2008 we got a new book, which has resulted in RCD protection for near every circuit. This has caused a problem with extending ring finals. For all new sockets we have to provide RCD protection. In the main it means a new consumer unit before it is extended. The problem for house holder is problems like yours are highlighted and costs can go up as a result. We can talk about it until the cows come home, but at the end of the day it's the guy would signs the paperwork who calls the tune.

 

[winston1]

Is this okay, acceptable, legal?

Yes.

 

[Jackrae]

Because of what is in effect a partial parallelling of the loop, the loop impedances of L, N and CPC, as measured at the CU, will be lower than it was before the extra "loop" was installed. This should, if anything, improve the capability of your circuit protection facilities.
However, if the two "pick-up" points in the original loop were adjacent to each other, then the new loop is just about equivalent to a spur - irrespective of whether there's one or two cables feeding it.
As to whether it is legal or acceptable, that's for others to comment.

 

[JohnW2]

We are told to test for figure of 8 wiring and list it as a fault. However I have failed to find any thing in the regulations which says you can't other than "to provide reasonable sharing of the load around the ring" ref 433.1.5 which in turn directs to 433.1.1 which talks about overload.

What is described is not a true 'figure-of-8' circuit in the normal sense. If it were, then I think it's true to say that converting a ring into a (true) 'figure-of-8' will always tend to improve 'sharing of the load around the ring', as well as potentially reducing EFLI and VD, and increasing redundancy. Hence, AFAICS, a true 'figure-of-8' is, in most senses, 'better'/'safer' than a conventional single ring, the only real 'problems' being in relation to more complex testing and fault-finding.

In the OP's case, if the two connection points between the 'primary' ring and the 'secondary' one are a reasonable distance apart (on the 'primary' ring) and/or are fairly distant from the ends of the primary ring, then the situation will be much the same as described above for a true 'figure-of-8'. As Jackrae has said, a potential issue would only really arise if the two points of connection to the primary ring were both very close to one another and also close to an end of the primary ring - but the same would be true if there were a lot of sockets installed very close to one end of the ring.

Kirchhoff's laws become complex to work out, I did in in University, but as to working out on back of fag packet forget it. I would have to get the text books out, or use electronic workbench, which is not installed on my PDA, so I would not be doing in in a private house.

As mentioned above, any such calculations would inevitably demonstrate that any sort of 'cross-connections' in a ring will (for any given arrangment of loads) improve current-sharing in the conductors of the circuit, as well as reducing EFLI and VD at many points in the circuit.

To actually get any absolute numbers out of such calculations would require wild guesses as to 'what loads might be plugged in where'. If, even with a simple ring, you considered the 'worst case scenario' (with a a full 32A load plugged in very close to one end of the ring), you would very often conclude that the CCC of a cable with CCC=20A could be exceeded (although much less likely if, as often, CCC=27A).

Kind Regards, John

 

[ericmark]

Is this okay,

No

acceptable,

No

legal?

Yes

There is very little which breaks the law with electrics in the home. Although there is a lot which does not comply with regulations. There are two main laws, electricity at work act, and Part P building regulations in England and Wales. To break the law, in the main, one has to show reasonable care was not taken, or the person doing the work did not have the warranty of skill required. To simply say BS7671 was not followed is not enough in its self, although showing BS7671 was followed is normally considered as showing a warranty of skill.

With a figure of 8 arrangement what you need to show is no cable will be overloaded. It is accepted if you follow the standard ring final design it is unlikely overload will ever result in normal use. However once you depart from the standard design it is up to the person designing the system to show that overloading is unlikely.

The problem with the ring final is if a lot of load is placed at the start/finish point i.e. near the consumer unit one could get 28 amp on one leg and 4 amp on the other so overloading the leg with 28 amp on it. This is why the appendix warns not to use fixed appliance over 2kW from the ring final. So where on the ring would make a big difference. The other point is testing. We will normally test a ring to see it is still a ring, and fig of 8 connections will hide faults so the tester would not know if there was a potential of overloading or not.

As a result all text books say you should not use a figure of 8 design. However the regulations have to allow for all sorts of systems not only those in the home. So the regulations only warn against over loading they don't actually outlaw figure of 8.

I personally would not extend the ring final until fig of 8 had been removed, I would expect all other electricians would do the same. If one can't test then one can't say it's safe and with the fig of 8 you can't really test.

 

[JohnW2]

With a figure of 8 arrangement what you need to show is no cable will be overloaded. It is accepted if you follow the standard ring final design it is unlikely overload will ever result in normal use. However once you depart from the standard design it is up to the person designing the system to show that overloading is unlikely.

As I just wrote, if you start with a (any) 'standard ring final design' and add a cross-connection to turn it into a (true) 'figure-of-8' circuit, you will inevitably reduce the likelihood of overloading of any of the cables ever occurring. Hence, if 't's 'OK' (cable current wise) as a simple ring, then it will be 'even more 'OK' if converted to a (true) 'figure-of-8'.

The other point is testing. We will normally test a ring to see it is still a ring, and fig of 8 connections will hide faults so the tester would not know if there was a potential of overloading or not. ... If one can't test then one can't say it's safe and with the fig of 8 you can't really test.

I'm not sure that one can really test in order to ascertain "if there was a potential of overloading or not". As for routine testing, and testing to find faults, it is perfectly possible for someone who understands electrical principles to undertake perfectly adequate set of tests. It is those whose knowledge/understanding of testing (and/or electrical principles) is limited to what they have read in 'cookbooks' who might find themselves out of their depth.

Kind Regards, John

 

[JohnW2]

Deleted: quoted instead of edited - again!

 

[ericmark]

It is those whose knowledge/understanding of testing (and electrical principles) is limited to what they have read in 'cookbooks' who might find themselves out of their depth.

This is a point which my son and I often argue. Should the electrician expect to find normal wiring? So standard book test, open consumer unit and remove ring cables and use two connector block to form a ring. Test every socket for low resistance and all on the ring should have same reading and all on spurs will have a higher reading standard R1 + R2 test. With a figure of 8 this test would need repeating in the next ring and to do that means having a detailed plan of how the cables run. In a standard EICR one would not remove every socket and it would show sockets on the second loop as being spurs on spurs unless you actually did connect to test.

When for example wiring test benches one could wire like a ladder with cross links everywhere but it would be near impossible to test to ensure no overload. Some one needs to sign "I being responsible for the design, and/or installation, and/or inspection and testing." and to be frank I would not sign that paperwork unless there was a detailed plan.

Some one will need to extend on the existing and the question is how many people would be willing to sign the paperwork with a figure of 8 system unless they have the original paper work? When I did my C&G 2391 I was told to reject any fig of 8 wiring and I am sure many others were given the same instructions. So I would say most will want it correcting before they work on it.

I see what you are saying and yes it could comply. But for any work to comply there must be paperwork, and I very much doubt if anyone would raise paperwork for a fig of 8.

Be it a shared neutral, or two supplies to same system, electricians can and will be caught out with non standard. Yes they should test everything first. I have put my clamp meter around the neutral cable before disconnecting and still ended up with sparks where when tested that item was not in use. We should not need the labels saying two versions of BS7671 etc. But in the real world we do. Without those extra safe guards electricians will make errors.

 

[JohnW2]

I see what you are saying and yes it could comply. But for any work to comply there must be paperwork, and I very much doubt if anyone would raise paperwork for a fig of 8. .... Some one will need to extend on the existing and the question is how many people would be willing to sign the paperwork with a figure of 8 system unless they have the original paper work? When I did my C&G 2391 I was told to reject any fig of 8 wiring and I am sure many others were given the same instructions. So I would say most will want it correcting before they work on it.

Anyone claiming (by action or inaction) that a circuit is 'non-compliant' has to be prepared and able to cite what regulation it is 'non-compliant' with. The fact that someone does not have the knowledge or competence to properly test the circuit surely cannot be taken to mean that the circuit is 'non-compliant'.

When for example wiring test benches one could wire like a ladder with cross links everywhere but it would be near impossible to test to ensure no overload.

As I said before, I don't really understand this. Firstly, I'm not sure what you mean by "testing to ensure no overload". Whatever, as I've said, adding any "cross-links" to a ring is bound to reduce the likelihood of any cables in the circuit becoming overloaded.

We should not need the labels saying two versions of BS7671 etc. But in the real world we do. Without those extra safe guards electricians will make errors.

I don't really understand that, either. Surely anyone who has any right to go anywhere near an electrical installation, and who has some colour vision, will immediately see that there are two different wiring colours in use, without needing a label to tell him/her so?! If where they are working they only see red/black or blue/brown, the fact that there are other colours elsewhere in the installation in the installation is surely irrelevant?

Kind Regards, John

 

[PBC_1966]

This is another instance where I see a difference between what I would regard as a true electrician and somebody who is, for want of a better term, nothing more than a BS7671-trained cable installer. The latter sometimes has a shaky grasp of basic electrical principles and is unable to think outside the confines of BS7671 and the "standard" circuit arrangements. The former has a true understanding of what's going on at the amps/volts/ohms level and will be able to work out any circuit from those fundamental principles, even though for the most convoluted of cross connections it might take a fair amount of time isolating and testing to figure out what's been done.

For another example, as we discussed in the international area, there are those "qualified electricians" who seem incapable of understanding what will happen with a local earth rod added to a TN-C-S system and who seem to think that it's not allowed and/or actually dangerous, but have no problems with multiple tens of feet of buried metallic water pipe bonded to the same system. And some of whom seem to think that current won't flow through that pipework because "BS7671 doesn't allow the water pipe to be used for earthing." Again - a blind following of wiring regulations without any real understanding of what goes on at the electrons' level.

Surely anyone who has any right to go anywhere near an electrical installation, and who has some colour vision, will immediately see that there are two different wiring colours in use, without needing a label to tell him/her so?! If where they are working they only see red/black or blue/brown, the fact that there are other colours elsewhere in the installation in the installation is surely irrelevant?

Similarly with the whole voltage warning issue when the rules stated that a label must be provided for voltages exceeding 250V between conductors within an enclosure (or between two enclosures in close proximity) "where not normally expected." Normally expected by whom?

 

[JohnW2]

This is another instance where I see a difference between what I would regard as a true electrician and somebody who is, for want of a better term, nothing more than a BS7671-trained cable installer. The latter sometimes has a shaky grasp of basic electrical principles and is unable to think outside the confines of BS7671 and the "standard" circuit arrangements. The former has a true understanding of what's going on at the amps/volts/ohms level and will be able to work out any circuit from those fundamental principles, even though for the most convoluted of cross connections it might take a fair amount of time isolating and testing to figure out what's been done.

Exactly!

Kind Regards, John

 

[scousespark]

Before 2004 there were few installation certificates or minor works certificates completed for domestic work. Part P changed this, in the main to protect the electrician so he can say "here's my reading clearly some one played after I left".


Read more: //www.diynot.com/diy/threads/two-ring-mains-joined.443000/#ixzz3nL3YoRsh

Part P has not changed the requirements for an electrician to issue a certificate to cover the work done. I do the same as I did in December 2004, with the addition of LABC notification for the appropraite types of job.

 

[scousespark]

If there is a break in the second ring, the cable could be overloaded. I don't know any 'BS7671-trained cable installer' who would install this circuit. It must have been done by a 'real' electrician.

 

[PBC_1966]

If there is a break in the second ring, the cable could be overloaded.

The same could happen with a normal, single ring.