Earthing problem: CPC broken mid-circuit, inaccessibly

[rnews]

Is it ever acceptable to run one or more additional CPCs to the middle of a radial circuit, to restore the integrity of the earth until the whole circuit can be replaced?

Friend asked me to check why she gets a "tingle" when touching both her stainless steel kitchen sink and an adjacent metal electrical appliance.

Inspection of her house - originally a small 2-up 2-down, extended backwards and sideways about 15 years ago - revealed many bodges. There is one lighting circuit, one radial to the boiler/CH controls and just one ring main, all basically OK. There is also a fourth circuit. This appears to have originally been a radial to a single socket in the kitchen, or possibly to a small oven (it seems to be 4 or 6mm sq at the CU end). It appears to have been extended with the house to serve 11 sockets (including washing machine, dishwasher, microwave and an electric heater) as well as various spurs to lighting circuits via FCUs. Very difficult to check the layout: most cabling is buried in the concrete floor or behind three layers of tiling or numerous fixed cabinets and half the sockets are now inaccessible (you can plug things in but not remove the faceplate). Surprisingly enough the 32A MCB never trips - seems the heavy power consumers are never on together.

CU is old 4-way Wylex now fitted with MCBs; no RCD anywhere. No main equipotential bonding of any sort in place.

What I think is the first socket on the radial reads 240v phase to neutral and the CPC has continuity back to the main earthing point at the CU. Others read between 115v and 175v phase-neutral with no load; and their CPCs between about 50v and (at worst, near the sink) 115v to the main earthing point (tested with a flying lead and multimeter). Or in the latter case to the sink itself, hence the tingle.

Connections at all accessible points OK (for what it's worth bearing in mind the circuit "design"). Haven't meggered anything yet - not sure I've found all the parts of the installation - but hardly need to to show there's a problem.

Temporarily bonding the sink to the CPC of the nearest socket solves the "tingle" problem (of course) and doesn't trip the MCB but I don't want to create a worse problem elsewhere eg in the bathroom (given the lack of main bonding).

I can install main equipotential bonding reasonable easily and will do so soon. But tracing and fixing the leakage to earth and broken CPC - i.e. replacing the broken and overloaded radial with the new ring it needs - will be disruptive and time-consuming. I won't have time to do it for ages. Friend (single mother, two kids, failing job) is smart but short of cash. I know she won't call in professionals unless I convince her her life is in danger: the problem has existed for at least 13 years and hasn't killed her (yet). Nor, amazingly, do any of the appliances on the affected circuit seem to have under-performed. Until I can do the job properly - probably several months - I plan to:

(a) install main equipotential bonding to the gas and water services

and

(b) extend it directly to as many accessible CPC connections on the affected circuit as necessary to restore proper earthing at all points I can test.

Yes it's a temporary bodge and no substitute for ripping out and replacing the dodgy circuit - not least because I find it hard to believe that this 50-115v potential in the CPC is not coming from some phase-to-earth fault somewhere (though as I say above, temp bonding doesn't trip the MCB). But is there any reason why in the circumstances I should not do it?

[David Pashley]

Hopefully having taken the time to write this considered summary and read it back, you will realise that there is only one solution.

That is to *immediately* disconnect the faulty circuit and replace it (hopefully with an appropriate ring). If you can't replace it right away, even extension leads to vital appliances would be preferable to the existing situation in the very short term.

The fact that the dangerous situation has existed for some time will be no consolation to you should a tragedy happen now that you have uncovered it. Remember it will only take one failure on the unearthed section for *all* the appliances on that section to potentially become live - and there are signs that a latent fault is already present.

I feel for your dilemma, but I think in this situation safety must come first.

David

[rrh]

David,

Thanks - you are right of course but reality is that either I install some sort of fix which will improve safety and not make things worse, or nothing at all will be done for possibly many months. Hence my question.

[David Pashley]

Well, my original advice stands, but if you intend to go for a "fix", one way to approach this would be to turn the radial circuit into a ring by connecting the far end of the radial back to the originating MCB.

However, that in itself is not a solution, you must then ensure continuity of the CPC throughout the newly-created ring by finding the break and replacing that portion. And of course the whole ring must comply with the regs.

You should also find and fix the earth leakage problem, install an RCD, and install main equipotential bonding and supplementary bonding.

This type of approach could ultimately be more work than installing a new circuit. David

[Andy Wade]

Yes, but... (read on).

That's not surprising, it takes over 45 A to trip a 32 A MCB "quickly".

If it's old enough to have no bonding then the Wylex CU will probably be the type with a partly wooden frame. Put replacing that on your longer term remedial actions list.

This is odd, you haven't mentioned things downstream of this point not working. What is the P-N voltage on-load? You have either loose connections (immediate fire risk), or a dodgy meter, or you really meant P-E perhaps?

Is this wired in twin-and-earth? Have you checked for a broken or loose CPC connection at the first and second points along the cct. Most often such breaks will be visible at points of termination, not buried in concrete. The evidence may be buried under the earth sleeving, so give everything a good tug.

You'll need to install an RCD at the origin of the circuit concerned. On a circuit with no RCD the CPC must be run in the same wiring system (i.e. same route) as the P & N conductors, or in the immediate proximity [Reg. 544-01-01]. With an RCD in place this restriction is removed and you can run the CPC wherever you like, within reason. A separate CPC with no mechanical protection (i.e not in conduit or trunking) should be at least 4 mm^2.

[John Rumm]

That does not really make sense - at that level you would not expect appliances plugged into sockets there (or downstream) to perform correctly.

(If you are talking about phase to earth then that I could understand!)

You could observe that with a disconnected and floating CPC. Mains RFI filters on many appliances will tend to pull the CPC toward 115V if it is not connected.

As a temporary fix I would be inclinded to prove the first couple of sockets are OK, and disconnect the rest. Use extension leads from the good sockets in the mean time.

[rrh]

Sorry yes, the measurements are P-E. Meter is recently calibrated so I think OK. Cabling is T&E and I have checked all accessible runs and points of termination (though many aren't accessible). Thanks for alerting me to that reg - though I know the proper solution is a full new circuit.

[rrh]

Thanks yes it's actually P-E as also pointed out by Andy. Helpful to know that these readings could be found with a disconnected and floating CPC without there necessarily being other problems present.

[Jim Alexander]

Given the time and money constraints you pose I suggest a different focus than the main bonding issue which is not really relevant to the problem. You have a suspicion of phase to earth breakdown but I would suggest an initial theory of only a break in the CPC. You should perform a fuller survey of which accessible outlet has CPC continuity to which accessible outlet. Continuity is a much better test that voltage which can be misleading. Identify a section of the circuit which is sound and as you suggested connect that back to the installation earth. Place all other sockets out of use. It may even become clear where the break is.

Finally you say some outlets are accessible but the faceplate can't be removed. Care to share the reason since its an issue an electrician would need to address. In fact while not the "problem" its the fundamental issue.

Jim A

[rrh]

Thanks Jim. Main bonding is indeed not the key issue. Where socket faceplates can't be removed, it seems to be because they were installed and cabinets or other fitted furniture were then put in over them, leaving just enough room (eg at the backs of cupboards) to squeeze a plug in but none to remove faceplates or allow work on the cabling. Of course all this junk will have to come out for a proper fix, however it's done.

[Roger R]

CPC's. ?

Circuit Protective Conductors Combined Precision Components Certificate of Professional Competance Chemical Protective Clothing. etc, etc.

Take your pick.

Roger R

[Andy Burns]

alternatively apply common sense in selecting the one appropriate for the given context ...

[John Rumm]

This is also particularly true when using a digital meter to make your readings. These place very little load on the circuit under test (usually a good thing), but it does mean that completely disconnected wires can appear live just from the inductive coupling resulting from their running in close proximity to wires which are live.

[John Rumm]

Sounds like a job for a Fein Multimaster - that will be able to chop out the surrounding wood etc without risking damage to anything else.

[Roger R]

The Original Poster was addressing those who would immediately know what he meant, but I couldn't initially get it because two others in my list somehow blocked out further thought. It would help ordinary readers like me if acronyms were expanded on first use.

Roger R

[rrh]

I apologise if I wasn't clear, but I doubt that anyone who did not immediately know what I meant by CPC, CU, RCD and MCB in the given context would have been able to offer me sound advice.

[ARWadsworth]

LOL. Anyone bar an electrician then :)

Enjoy doing a partial rewire or installing a new circuit for free. That is what friends are for is it not?

It looks like it will be just as difficult to bodge this one as rewire the circuit. I hope s/he is a good friend and will repay the favour.

Adam

[Andy Burns]

Ditto broken PCs in my case, substitute day-job here ...

[rrh]

Indeed, that's what friends are for.

Her cooking is out of this world and that's the payback!