Is a borrowed earth allowed?

[a-z electrics]

Done an EICR yday on an old house and all ok apart from 2 issues..
1) 32A cooker circuit had high resistance pec continuity (over2kohms) ALTHO it did buzz out on my voltage probes. Hence not able to get a Zs reading and no rcd tripping. The dual rcd consumer unit was fitted 10 yrs ago and those test readings all available and fine. Kitchen was refitted 3 yrs ago and no docs available so they extended original cooker cable prob from the switch which is now tiled over somewhere.
.. I've put in an earth wire link from an immediately adjacent rfc socket into the cooker switch and this solves this problem.
Question .. is this ok as a permanent solution if i label up what ive done?
2) lowish IR readings 1.75M L-E and N-E on light circuits and one rfc but possibly in hindsight due to some bulbs, fan and appliances still plugged in. Could this account for it?

 

[TJ Anderson]

how can it be a different . are you saying its ok to borrow a earth knowing if their is fault to earth It want clear quicker. IMO the op needs to ring his scam and get clarification.

He doesn't need to ring a scam Buzz, he is an Electrician who can think on his feet.

Come on......A borrowed neutral situation is nothing like and the implications are totally different.

The OP has made it far safer by linking neighbouring cpc, and has now actually ensured satisfactory disconnection by MCB with RCD in addition.

Good on you OP. Nice to see a spark thinking outside the OSG standard circuits lol and using their head to resolve a problem. Proudly make a note on your EICR of what you have done and explain what would be required to the customer to make it 100% again. You have done them a favour and made it safer than you found it.

 

[johnduffell]

543.6.1 says you can't share a cpc unless it's part of the same wiring system or in immediate proximity. So if it's part of another cable, good luck.
I had this discussion in reverse that's how i know.

 

[SparkyChick]

543.6.1 says you can't share a cpc unless it's part of the same wiring system or in immediate proximity. So if it's part of another cable, good luck.
I had this discussion in reverse that's how i know.

I would say you've misinterpreted that.

I read 543.6.1 to mean you can't run the line/neutral down one side of the building and the CPC down the other. And I believe this is the regulation that requires you to run the earth along the route of the original cables if you add a CPC to say a lighting circuit wired in twin.

It doesn't preclude sharing a CPC with another circuit because the CPC of both circuits is still within the same wiring system as the rest of the circuit elements, they are just interlinked somewhere other than the consumer unit.

I'm sure I'll be corrected if I'm wrong.

 

[johnduffell]

I agree @SparkyChick assuming it really is routed with the live conductors all the way but, it's one of those things where everyone knows it's weird like over sleeving green and yellow cores of a flex with brown, and you wouldn't want to have to justify it in court (hence the good luck) but the regs seem to allow it but not totally explicitly.

 

[Electric Magic]

I wouldn't have done the remedial work when doing the EICR (CPC issue potentially dangerous in the event of a fault not immediately dangerous so shouldn't be addressed if you are just there to do an EICR IMO). I would have coded it C2 (not FI as suggested elsewhere) as there is a known issue (CPC resistance unacceptably high) rather than just a suspected issue. I would also have considered an additional FI if I had good reason to think an oven isolator may have been tiled over. To echo Westward10 and others, no reason you can't borrow a CPC provided it meets the requirements size wise. I would however (as had also been said above) want to find that high resistance connection because of the high probably that other aspects of the workmanship may be poor (had a similar situation with a high resistance CPC on an extended electric shower circuit once where there ended up being 2 junction boxes and 2.5mm2 twin and earth connecting them - 8.5kw shower).

 

[Wilko]

Hmmpff bad spelling, that's all I get

Be grateful- it could've been a slap in the face with a wet fish .

 

[SparkyChick]

I agree @SparkyChick assuming it really is routed with the live conductors all the way but, it's one of those things where everyone knows it's weird like over sleeving green and yellow cores of a flex with brown, and you wouldn't want to have to justify it in court (hence the good luck) but the regs seem to allow it but not totally explicitly.

The definition of wiring system...

Wiring system. An assembly made up of cable or busbars and parts which secure and, if necessary, enclose the cable or busbars.

Given it's a domestic situation and is likely to be wired in twin and earth (the wiring system), the CPC for the circuit will be routed with the live conductors (or in close proximity - in back boxes) along it's entire length.

All that's different in this case is the CPC is sourced at two points. One at the origin of the circuit and then another at the end to provide a better connection to the earthing system. The CPC for the circuit (bearing in mind the circuit ends where the live conductors terminate) is contained in the same wiring system (the cable) along it's entire length (valid assumption).

There's nothing weird about that, the ideal solution is to find the bad connection and fix it but sometimes ripping a customers house apart is not an option so you have to go for a less than ideal solution.

The only real point of concern is the current carrying capacity of the CPC used as the source of earth. Total CSA in a typical ring final is 3 sq. mm, so unless the cooker circuit is 10mm, this is more than ample to handle fault currents.

I don't think good luck is required, the OP made an educated decision and has improved the safety of the installation. I struggle to see how such a move could come back and bite him in the bum later.

As for sleeving yellow/green with brown in a flex... that's not weird, it's plain wrong. If you need three live cores you should be using an appropriate 4 core flex.

I'm also curious about which part @buzzlightyear disagrees with in my post

 

[Wilko]

All that's different in this case is the CPC is sourced at two points. One at the origin of the circuit and then another at the end to provide a better connection to the earthing system. The CPC for the circuit (bearing in mind the circuit ends where the live conductors terminate) is contained in the same wiring system (the cable) along it's entire length (valid assumption).

My thought is - there are not 2 cpc as one is high resistance due to an unknown fault. And the one that works does not run in the immediate vicinity. Therefore some of the cooker circuit has no functional cpc.

 

[SparkyChick]

My thought is - there are not 2 cpc as one is high resistance due to an unknown fault. And the one that works does not run in the immediate vicinity. Therefore some of the cooker circuit has no functional cpc.

So I guess it all depends on how you got about defining what a CPC is.

Using the BBB...

Circuit Protective Conductor (cpc). A protective conductor connecting exposed-conductive-parts of equipment to the main earthing terminal.

So by that definition, is it one or two... or does it actually matter at all how many segments there are as long as they fulfill their designated function?

I'm obviously in the it doesn't matter camp as long as they work and are sufficient to handle prospective fault conditions.

 

[Midwest]

This is an interesting discussion, and I think could go on for some time. I have googled the phrase ' borrowed circuit protective conductor', and its interesting a lot of the searches relate to older threads on this forum. Its also interesting to note, that the few so such threads that I've read, members, some are even trusted members, opinion is that you cannot borrow an earth.

I'm in the camp, that if the circuit supplying the cpc is altered, the cpc for the cooker could be lost, regardless of any notice. There is that reg I think refers to this, its down to ones interpretation.

Not knocking the OP, but I would replace the final circuit cable, which sounds as if its an easy task in any case.

 

[Wilko]

Yes, very interesting discussion
To me - the conductor used as cpc for the cooker circuit was selected and installed to have less than 1 Ohm (say) impedance. So at 2k Ohms it's faulty (surely?). So there is only the "recently added" conductor which provides protection to the appliances and the circuit cpc is to be assumed lost.

 

[SparkyChick]

I think it's one of those interesting debates because the differences in opinion seem to be very much based on peoples interpretations of the regulations.

To be clear... this would be the absolute last resort solution for me personally, I would always look to ensure the continuity of the CPC along the length of the circuit, but if that's not achievable then I believe this is a valid option that doesn't break the regs.

@Wilko so I'm looking at this particular situation as the original cable to the original switch location is fine and thus the CPC is good for that segment. The fact that there is a connection at the end of the new segment suggests they are in fact connected, just the connection is bad. This suggests the CPC itself is intact (without opening walls it's impossible to verify, but it's a reasonable assumption since there doesn't appear to be any other faults) along it's length. So at one end, it's connected to earth (from the CU at the circuit origin), there is a bad joint but the CPC is continuous, and then at the other end it's connected to earth via another circuit's CPC.

 

[Baddegg]

This is a really interesting thread especially to us newbies, with good arguments from very good sparks on both sides (I love these ones) seems to me that there is a fair degree of interpretation within the BB (yellow or blue) here, for what my opinion is worth op I think it needs coding and noting on the eicr as that is why you were there primarily and after that whatever allows you to sleep at night with a clear conscience that you have done all you can to make the installation safer be it using the other cpc and labeling appropriately with a view to full repair or further investigation at a later date or stressing to the customer that you need to isolate the circuit untill a repair or FI can be done.
Personally I’d go with option 2 but that is just me as I’m not confident enough to argue interpretation of regs at any level yet.....

 

[telectrix]

my main concern is that, although the bypass cpc provides protection to the point of use, the original cable with it's damaged cpc is not, in itself, protected, so I lean towards the find and fix option.

 

[Midwest]

This could go on all day, and I've got a real kids birthday party to go to

To simplify things, if the cooker circuit had no cpc whatsoever, would taking a connection from the RFC to provide a cpc be correct, according to the regs mentioned?