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

 

[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

 

[Engineer54]

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

It's always, always better to stick to conventional circuit configurations, ...in any area of the industry. So yes, in this case i would advise that he comply to BS7671. After all, he has had to ask for guidance, where perhaps you or I wouldn't need too... lol!!

 

[KAS1]

Point taken

 

[rattlehead85]

This subject has always been a shady area. Yes the regs require the cpc to form a ring unless it is conduit or trunking used as the protective conductor. However, in the cold light of day what legitimate argument can you possibly raise against this set up if
A) The circuit is RCD protected
B ) The L&N conductors do form a ring.
C) Earth is present at each socket.
Any feasible answers apart from the fact the BGB says so!

 

[KAS1]

Isn't one of the reasons for having 2 x earth terminals in sockets and the cpc ring connecting to one terminal in and the other terminal out, to and from the CU to ensure in the event of a cpc being damaged / broken. The socket would still have an earth from the other leg of the cpc. I know that if only one leg of the cpc ring is still intact it'll probably be only a 1.5mm but better than no earth at all at some of the sockets on the ring in the case of a broken cpc, Then the situation of only one leg of the cpc connect should be picked up on the next EICR. This of course is not the case with conduit/trunk as any spark doing any sort of alteration etc would make sure the earth is continuous via the metal work.

 

[MDJ]

Isn't one of the reasons for having 2 x earth terminals in sockets and the cpc ring connecting to one terminal in and the other terminal out, to and from the CU to ensure in the event of a cpc being damaged / broken. The socket would still have an earth from the other leg of the cpc..

Yup for computer systems, can't confirm your right in a domestic situation though, that won't wear there as sockets which cost 99 pence and have one earth are allowed then LMAO, not arguing at all with your point of view, I welcome it, but alas it isn't the case I'm afraid.

 

[Deleted member 26818]

Well you could argue, that while for instance a 1.5mm² CPC is acceptable with 2.5mm² circuit conductors, that a 1.5mm² CPC would not be acceptable whith 5mm² circuit conductors.
If run in singles, then the minimum CSA for the CPC would be 2.5mm², which would most likely be sufficient.
That said, it should also be considered, that with all the conductors being in a ring, a break in the CPC would not necessarily be catastophic.
However two breaks in the CPC could well mean that one or more accessory is now no longer earthed.
Whilst that again would not be catastrophic as far as earthing exposed-conductive-parts are concerned (most accessories are plastic), it could be as far as an earth fault ocuuring in whatever item of equipment is connected to the circuit.

As far as lolipop circuits are concerned, no there is no mention in BS7671 of these particular circuits or indeed any othe non-standard circuit.
However, I would still axpect that the requirement to run the CPC as a ring would apply to the ring part of the lolipop circuit.

As for circuits whith high earth leakage.
Whilst all the guides show that it is acceptable to wire using just one CPC, each leg being connected to a separate earth terminal at each accessory.
My understanding of the requirements is that this is incorrect.
The requirements call for two CPCs, running one in the form of a ring does not equal two CPCs, it is still one albeit with two ends.
It is also a requirement that each individual CPC complies with the requirements of Section 543, and that both be suitably sized to act as the individual CPC for the circuit.
One of the Requirements of Section 543, is that the CPC of every RFC be run as a ring.
I really cannot see how running a single CPC as a ring could become two CPCs both run as a ring, simply by using separate earth terminals at each accessory.

 

[rattlehead85]

The argument of the breaks in the cpc on a ring leaving some sockets unearthed would be the same if the earthing continuity of a radial was also broken. The outcome would be exactly the same. So the fact of a ring having only one cpc is exactly matched to a radial. Strictly speaking no, 1.5mm earth with a line size equating to 5mm isn't sufficient but if the circuit origin incorporates a 30mA RCD......?

 

[Deleted member 26818]

The argument of the breaks in the cpc on a ring leaving some sockets unearthed would be the same if the earthing continuity of a radial was also broken. The outcome would be exactly the same. So the fact of a ring having only one cpc is exactly matched to a radial. Strictly speaking no, 1.5mm earth with a line size equating to 5mm isn't sufficient but if the circuit origin incorporates a 30mA RCD......?

You should take on board, that there is no requirement in BS7671 for a circuit (unless it is of a special location) to be provided with 30mA RCD protection.
As such the requirements for CPCs on such circuits do not take into account that the circuit may perhaps be provided 30mA RCD protection.
Now if an installer does provide the circuit with 30mA RCD protection, they are quite within their rights to note the fact that the CPC is not run as a ring as a departure.
Obviously they would be attesting that the departure provides the same degree of safety as would be achieved by compliance with BS7671.

 

[Engineer54]

God forbid that i would advise anyone to go down the non-standard circuit route, especially those that can just about get conventional circuits right. I'm not just talking about those that have been mentioned here either. I'm basically just making a point that non-standard circuits are not out and out forbidden and not available to those that know what they are doing, and can substantiate and verify/vindicate their use.

 

[rattlehead85]

You should take on board, that there is no requirement in BS7671 for a circuit (unless it is of a special location) to be provided with 30mA RCD protection.
As such the requirements for CPCs on such circuits do not take into account that the circuit may perhaps be provided 30mA RCD protection.
Now if an installer does provide the circuit with 30mA RCD protection, they are quite within their rights to note the fact that the CPC is not run as a ring as a departure.
Obviously they would be attesting that the departure provides the same degree of safety as would be achieved by compliance with BS7671.

No requirement in BS7671 for a ring to have RCD protection?? There is if a socket is used by unskilled or likely to be used outdoors, or if the Zs tolerance of the CPD is exceeded which may be the case if the cpc is broken. Or is it i have totally misinterpreted the regs all these years lol.