Final ring circuits with radial CPC

[bwm730]

Have justed looked at a job to replace the consumer unit and it appears that both the final ring circuits ( Kitchen and the rest of the Flat ) have Radial CPC's. The flat is wired in multicore Singles and Plastic Conduit , has anyone come across this type of design before. Both "ring circuits" are currently on 32A MCB's , Singles appear to be 2.5mm and CPC is 1.5mm. Did a quick continuity test and LL and NN meter out fine and all sockets have a good Earth . There is no way of adding the return CPC ( concrete floors and ceiling ), what are my options? Thanks

 

[MarkieSparkie]

If you have a radial CPC then you do not have a Ring Final Circuit which obviously requires a ring CPC. If you are certain that you are unable to pull in another CPC, your options are limited in the circumstances, the easiest fix is probably to convert the final circuits to radials on 20A MCBs (or 16A if you need to derate) as appropriate.

 

[bwm730]

Can i just leave them as is ( Ring L & N and Radial CPC ) on put them both on 20A MCB's or do i have to break the ring ? Strange design though probably about 25 years old.

 

[trev]

If you have plastic conduit how can you not use one of the existing conductors to pull through a new one plus the cpc between the two points where it is missing?
If not then Markie's post is the correct way forward

 

[markc123]

not really, how you gonna sucessfully achieve a ring final continuity without a cpc?

 

[Guest55]

yes you can fit a lower size mcb as an easy fix ,
but will the kitchen run on only 20A ? maybe not with washer , dryer , fridge , dishwasher etc.
but why cant you pull an extra cpc in if its conduit ?

 

[MarkieSparkie]

Can i just leave them as is ( Ring L & N and Radial CPC ) on put them both on 20A MCB's or do i have to break the ring ? Strange design though probably about 25 years old.

I would not do this as it just passes the buck to any future electrician and is unprofessional. It should be easy for you to disconnect the ring return legs of the L & N from the last socket and from the MCB in the consumer unit.
You will need to consider the floor area served, loading and thermal insulation constraints before converting any final circuit. The final circuits, particularly serving the kitchen, may require the 'ring(s)' being split in to several radials to meet the loading conditions.

 

[bwm730]

Trev, Have tried this in the past but not always that simple. Some conduit takes some strange routes and twisted cables can be a bugger to pull.

 

[trev]

Trev, Have tried this in the past but not always that simple. Some conduit takes some strange routes and twisted cables can be a bugger to pull.

True but it has to be worth a try mate.

 

[Engineer54]

Perfectly acceptable to have a radial earth on a ring circuit, depending on it's use etc. But the radial earth would need to be 4mm on a 2.5mm wired ring.... lol!!

 

[GLENNSPARK]

Can i just leave them as is ( Ring L & N and Radial CPC ) on put them both on 20A MCB's or do i have to break the ring ? Strange design though probably about 25 years old.

eh?...you mean feed both ends of the ring (live conductors) with individual 20A OPDs?....
in other words have 2 seperate supplies to the ring ?..
come on, what do you think?

 

[captainkev]

surely the ring has to be made complete, you cant split into two radials as one of them would be left with cpc not connected to cu

 

[KAS1]

Could you explain that in a bit more depth please, I must be missing something here??

Perfectly acceptable to have a radial earth on a ring circuit, depending on it's use etc. But the radial earth would need to be 4mm on a 2.5mm wired ringQUOTE]

 

[Engineer54]

Could you explain that in a bit more depth please, I must be missing something here??

Perfectly acceptable to have a radial earth on a ring circuit, depending on it's use etc. But the radial earth would need to be 4mm on a 2.5mm wired ring QUOTE]

Well, you say how experienced you are, ...so you tell me. Is it too hard for you to think outside of the box??

 

[Deleted member 26818]

Whilst in the past it was accepatable, to just earth any exposed-conductive-parts, there is now this requirement:
543.29 "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."

 

[Engineer54]

Whilst in the past it was accepatable, to just earth any exposed-conductive-parts, there is now this requirement:
543.29 "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."

Doe's it say anything about ''Lolipop Ring Circuits'' in BGB?? lol!!!

 

[KAS1]

Didn't he say it was plastic conduit and if so how can he do as you said (Perfectly acceptable to have a radial earth on a ring circuit) are you misreading 543.29???

Doe's it say anything about ''Lolipop Ring Circuits'' in BGB?? lol!!!

 

[Engineer54]

Didn't he say it was plastic conduit and if so how can he do as you said (Perfectly acceptable to have a radial earth on a ring circuit) are you misreading 543.29???

What i'm saying is, that it will safely comply to the circuits Zs values etc... So as far as i'm concerned, is perfectly acceptable if unconventional!! If the BGB now says differently, so be it, but then BS7671 is not the definitive last word on circuit configuration.

Which is why i asked if anything is stated about lollipop rings, which if configured/designed correctly, is again a perfectly acceptable conforming circuit!!

 

[KAS1]

The thing is, if the circuit he's referring to is in a domestic environment shouldn't he comply with the building regs that refers to BS7671 I'm just curious as to your thoughts

 

[Robert Turnbull]

I'd drop all the sockets and try and see which ones lead to the fuse board. And rewire from the board till to you can meet the earth coming the other way. If your just missing 1 return earth leg then this shouldn't be to hard.

Usually a bit of pulling on cables can establish where they go.

I love rewiring these flats 2 of you can have one buttoned in 6-7 hours.

If all else fails you'll have to go surface