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Topic Title: Ring final on lighting
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Created On: 10 August 2017 10:20 PM
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 10 August 2017 10:20 PM
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Ruddy

Posts: 89
Joined: 16 September 2014

got call to a fault in a rented terrace house both RCD anf type B 6 amp MCB tripping alternately on groundfloor lights disconed at the C U and found the lights were wired as a ring final confirmed by disconnecting one leg expecting some of lights on the circuit wouldn't work but all ground floor worked and the discon leg was live!! who does this and why in a two up two down terrace house.
not sorted the fault yet as lights wired from joint boxes so need to get under floor to split circ up but typical rental in the Lincs area occupied by multi persons cant see the floor let alone take it up the fault is intermittent either RCD or MCB trips after 2 to 15 minutes !!!
 10 August 2017 10:30 PM
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westfield6

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Joined: 12 October 2007

Originally posted by: Ruddy

who does this and why in a two up two down terrace house.



An idiot who doesn't know what he/she is doing. Not a spark.
 10 August 2017 10:33 PM
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mapj1

Posts: 9107
Joined: 22 July 2004

A 6A lighting circuit would have to be extremely long to really justify a ring, or perhaps lots of fittings with high earth leakage.
Equally I have seen it done, but was never entirely sure why.
Try using a small clamp meter around L and N to look at the leakage - it may be just very close to tripping all the time.
last time there was one like this it took some resolving ..

-------------------------
regards Mike
 10 August 2017 10:43 PM
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dustydazzler

Posts: 1177
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Originally posted by: westfield6

Originally posted by: Ruddy



who does this and why in a two up two down terrace house.







An idiot who doesn't know what he/she is doing. Not a spark.



A Badger


On a serious note , I have come across domestic lighting wired in a ring formation and simply can't see any justification for doing so in a regular sized house

Blooming .BADgers at it again
 10 August 2017 11:02 PM
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broadgage

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Joined: 07 August 2007

I too have seen lighting circuits wired as a ring, and for no good reason.
It might of course be the correct approach if unusually high CPC currents are expected, but that is very rare.
I have wired lighting circuits as a ring when the supply is ELV, and consider this to be a good way of minimising voltage drop. A very rare situation though and arguably becoming still rarer.

Perhaps, long ago, someone modified the lighting circuit, or connected together two previously independent lighting circuits, so as to form a ring and increase the load capacity to 10 or 15 amps, maybe for some of the once popular "heat an light units" with a loading of 800 watts each.
Then decades later the premises might have been re-wired by someone who simply replicated the original arrangement, without thinking.
 10 August 2017 11:26 PM
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potential

Posts: 1616
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If ring final circuits are acceptable I can see no reason to not apply it to a lighting circuit.

I'm puzzled how a lighting ring circuit might be more appropriate where a high CPC current is found.
 11 August 2017 01:10 AM
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broadgage

Posts: 2317
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Originally posted by: potential
If ring final circuits are acceptable I can see no reason to not apply it to a lighting circuit.
I'm puzzled how a lighting ring circuit might be more appropriate where a high CPC current is found.


I see no direct objection to a ring circuit for lighting, but unless some exceptional circumstance requires this, it is a very odd thing to do.

If high protective conductor currents are expected, then it is always good practice, and often a requirement to take special precautions against an open circuit CPC. A common of way achieving this is a ring circuit with the two CPCs in separate earth terminals.
More commonly applicable to power circuits with a lot of IT equipment, but possibly applicable to a lighting circuit.
 11 August 2017 03:46 AM
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leckie

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I'm sure you have checked but, are you sure it's a ring? Have you measured continuity across both line and neutral?

It might be that two circuits have had a link between lines in a switch. As often seen at the bottom af stairs in halls of a house. Then the board gets replaced, the RCD trips if they are wired to separate MCB's if they are on separate RCD protective devices, so they get put in the one MCB. Just a thought. And if that were the case, the circuit could be split while you look for the fault, it might be helpful in narrowing down where the fault actually is.

Regarding it bring wired as a ring. If it is, what is the problem? It might not be required, but what is the actual problem? It's just a tad unconventional and may lead you to conclude that it is the work of someone not fully aware of standard circuit arrangements, so you might find other non-standard bits of work. Or possibly badgery.
 11 August 2017 03:08 PM
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ToniSM

Posts: 318
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The few houses I've wired (four in forty years) I always did the lighting as rings. For a contractor the down side would be cutting in to profits. I didn't have such financial constraints, the cable was virtually free and my profit margin nil.

-------------------------
Could there be a better way?

In theory yes, but in practice?
 12 August 2017 10:50 PM
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alanblaby

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Originally posted by: potential
If high protective conductor currents are expected, then it is always good practice, and often a requirement to take special precautions against an open circuit CPC. A common of way achieving this is a ring circuit with the two CPCs in separate earth terminals.


Would someone be kind enough to explain to me the theory that 2 earth terminals should be used, rather than both wires going into the one terminal?
I'd expect the impedance between the 2 cables to be very slightly lower (OK, it'd be hard to measure) if they were in the same terminal, touching each other. What does the separate terminals and bar between them achieve?
Thanks
Alan.
 12 August 2017 11:49 PM
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MWalker86

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Halving the probability of losing earth from a terminal coming loose?
 13 August 2017 06:42 AM
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alanblaby

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I cant see that being a reason, I've been running a job recently, and having to inspect some less than capable individuals work. There were many loose terminals.
2 terminals means double the chance of getting one wrong. I know it shouldnt happen, but in the real world when you put apprentices and subbies on a job, they do make mistakes(as do all people occasionally)
 13 August 2017 08:18 AM
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dustydazzler

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In my experience solid cores into a screw terminal prove an 'issue' for many junior electricians and subbies in a hurry.
Even I have managed to over tighten a 1mm in a terminal snapping it off.
I did a big 2nd fix on a housing site (me and the company apprentice) he was doing all the sockets and swi spurs at low level.
He was told to leave all the face plates hanging so I could check the terminations , I reckon 50% we either too loose and I could pull the cables out with a gentle tug or he had snapped the core in the terminal by over tightening. Getting the feel for the correct tightness is not an exact science and takes practice.
 13 August 2017 08:27 AM
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AJJewsbury

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The requirement for each conductor of a duplicated pair to be separately terminated is to maintain the 'double fault to danger' policy. If both conductors were in the same terminal then a single loose screw could result in the circuit loosing its earth connection and so exposed-conductive-parts downstream of the break being immediately pulled up to a hazardous voltage by leakage currents without a 2nd fault being required.
- Andy.
 13 August 2017 08:13 PM
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MWalker86

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Originally posted by: alanblaby


2 terminals means double the chance of getting one wrong.


Not when they are the same terminal essentially.
 13 August 2017 08:16 PM
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MWalker86

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So can anyone actually say why a ring lighting circuit is so bad? (Copper cost excluded)
 13 August 2017 08:33 PM
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leckie

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No, because it's not.
 13 August 2017 09:48 PM
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mapj1

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Really only that it is non-standard - and like "lollipop" circuits, "Christmas tree" radial layouts and houses where lights are split front to back instead of upstairs and downstairs, non-standard may lead to confusion and delay during repair and maintenance, as well as perhaps being more complex to install.

Oh, and it can strike terror into the hearts of those who think ' I would never do it that way, so it must be bad ', and their more easily influenced customers.

As a related example, some time ago I fitted some C6 MCBs to a couple of lighting circuits to avoid tripping due to inrush and blown lamps for a family friend, and later they have been told by AN Other electrician that ' C type breakers are commercial and not safe in a domestic setting.' In that case I got in touch with said AN Other, and after we gently discussed the subtleties of breaking times, R1 +R2, PSSC, fuses and so forth, and a few other concepts that needed revising, the objections evaporated, as really what he meant was 'I don't see many C type breakers, and I don't fit them, and never have in the last 30 years, so they must be very bad...'

-------------------------
regards Mike
 14 August 2017 06:43 PM
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Ruddy

Posts: 89
Joined: 16 September 2014

thanks for all the replies gents. it defo was a RF circ fault found in bathroom ceiling ground floor flat roof vermin chewed through cable !!!!!
 14 August 2017 07:48 PM
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geoffsd

Posts: 1601
Joined: 15 June 2010

Originally posted by: Ruddy
who does this and why in a two up two down terrace house.


Perhaps someone who was told to install an RF circ so he installed what he thought was an RF circ instead of the intended RF circ.

Ring Final circ or Radial Final circ.
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