missing lighting cpc

[gts860]

Good Evening,
I have failed an EIC test, C1 lighting circuit has induced voltage on light switch screws.
The lighting circuit is on a split load CU non RCD side only MCB fitted.
All switches and pendants have a connected R2 however the CPC conductor present does not connect to any Earth, its floating.
The break or disconnection is probably under the floorboards.
Is there any reason why one cannot take from a tested radial socket cct, on 20A RCD, which is 1.5M distant from one of the light pendants in this failed cct, run it through the loft and connect it to the pendant ? I am told one cannot do this and the electrician wants to drill holes through the walls, run an extra CPC from the CU, up a wall surface, within trunking and tap it into one of the light switches on this circuit.
He tells me my suggestion cannot be done "the cpc must come from the same cct" Surely the many and various cpc on the other circuits are all connected to the same Earthing point so i do not see the logic in his explanation
The entire house is rewired to 16th Edition

 

[loz2754]

On the question of which regulation states that you cannot use a CPC from one circuit to add a CPC to a different circuit, you will not find any such regulation. On the contrary 543.1.2 and onward shows the requirements for where two or more circuits share the same CPC. So in some circumstances it might be perfectly acceptable.

However, as Lucien and others have pointed out, this would not be an appropriate solution to the "missing" CPC in your situation. You would only be masking the symptoms, not correcting the fault.

A bit like taking painkillers to treat a bleeding artery. You need to find the problem and repair it.

 

[Lucien Nunes]

If the conductor has been partially severed and is now live would an insulation test L/E indicate >500Meg

The CPC is completely severed. I'm not so worried what that might or might not be in contact with, as one side is already earthed and the other side is going to get earthed. It's the other bits that might be involved. What was the insulation reading from line to shelf bracket?How about line to shower-curtain rail? (we had one of those some years back, the rail was solidly live from where a fixing screw had penetrated a cable.)

I won't bother trying to convince you any further.

 

[gts860]

On the question of which regulation states that you cannot use a CPC from one circuit to add a CPC to a different circuit, you will not find any such regulation. On the contrary 543.1.2 and onward shows the requirements for where two or more circuits share the same CPC. So in some circumstances it might be perfectly acceptable.

However, as Lucien and others have pointed out, this would not be an appropriate solution to the "missing" CPC in your situation. You would only be masking the symptoms, not correcting the fault.

A bit like taking painkillers to treat a bleeding artery. You need to find the problem and repair it.

Thanks for that reference

 

[gts860]

The CPC is completely severed. I'm not so worried what that might or might not be in contact with, as one side is already earthed and the other side is going to get earthed. It's the other bits that might be involved. What was the insulation reading from line to shelf bracket?How about line to shower-curtain rail? (we had one of those some years back, the rail was solidly live from where a fixing screw had penetrated a cable.)

I won't bother trying to convince you any further.

I appreciate your engagement. Thanks.

 

[Lucien Nunes]

Having pressed the point about not leaving a damaged joint / cable in service, perhaps I should give my approach to locating one.

1. Walk the course. Is there any evidence of work carried out since the installation in areas where the cable is likely to be? Sometimes it's a no-brainer.

2. Inject tone into cable and search with tracer along likely routes. If it can be found at least at a few waypoints, estimate total length along that route.

3. Ping cable from both ends (CU and first lighting point) with a TDR or capacitance meter to discover distance to the break. Compare sum of lengths with measured route. Take average, measure along assumed route, look carefully around epicentre for likely causes of damage.

4. If possible, make access hole behind some other fitting or fixture that will conceal it later. Insert borescope. If nothing interesting visible, return to step 2.

etc.

 

[Mike Johnson]

2. Inject tone into cable and search with tracer along likely routes. If it can be found at least at a few waypoints, estimate total length along that route.

Lucian what do you use to Inject a tone into the cable?

 

[Lucien Nunes]

Three choices:

The normal AF warbler, ideal for close-up work, finding a core or pair in a cable, following it around a panel etc, but range limited to about 12".

801K | Tempo 200FP Tone Generator, 2 Tone, 1100Hz Tone Frequency | RS

uk.rs-online.com

Then there's one of these around the place somewhere . Ironically, when it's lost, you can't locate it by waving a piece of cable around.

RS PRO Cable Tracer Kit, Cable Detection Depth 2m CAT III 300 V, Maximum Safe Working Voltage 400V | RS

uk.rs-online.com

Many moons ago I made one myself out of the guts of a couple of transistor radios. Depending on how one were to do that, one might end up with a device that radiates on bands where one is supposed to keep quiet. One would need to use discretion as it might have significant range.

 

[Mike Johnson]

Been looking for one of these for a while: Socket and See FFCB100UKA Fuse Finder | ISSWWW.CO.UK - https://isswww.co.uk/socket-and-see-ffcb100uka-fuse-finder?gclid=Cj0KCQjwutaCBhDfARIsAJHWnHtzziofqm8WHz1L4ITtnx4ylS8TlwtITySmtjAXp1RnOctjGdMYPWgaAnNTEALw_wcB ideal for me as it has a removable cable so can be configured for UK and EU via what looks like a standard appliance plug.

 

[timhoward]

All depends on the accuracy required ! I once used a bit of wire wrapped around a lawnmowers HT lead and and attached to a cable conductor , and an AM radio to track a cable run underground. (Not that I’m suggesting that for a domestic lighting circuit of course!)

 

[magnoliafan89]

Personally I think the OP was only looking for someone to say it was OK to take a CPC from somewhere else , not wanting to entertain any other sound advice and now we won't hear from him now someone has said it is OK.

 

[Mike Johnson]

Thread drift does not annoy me, but obviously does others.

 

[gts860]

Personally I think the OP was only looking for someone to say it was OK to take a CPC from somewhere else , not wanting to entertain any other sound advice and now we won't hear from him now someone has said it is OK.

Sorry but I am back to haunt you !
No I did not want to just get an OK, I want to really know the theory behind why such an arrangement is against the regulations. This topic has also appeared on other forums like the IET and it appears to be an issue that some are absolutely against on principle and others see no problem, with the caveat that CSA is considered and appropriately selected and that the work is documented in the system.
As a non electrician i cannot see why the route a fail current takes to get to earth is important the main thing is that it gets there. I appreciate that someone inspecting the system at a later date might assume that the cpc bound to a particular cable does indeed go to the same place as the line and neutral.
At the end of the day the electrician tasked to resolve this issue has to put his name to it and if he is not prepared to do that then I respect this. As another responder here noted it seems that my guy has not actually done any troubleshooting, and that is the main problem here. If you cannot identify where that cable is physically connected then the status of its associated cpc is simply unknown, it could just be loose in the first ceiling rose. If it is identified and the cpc is then measured and found to be open cct then you know what the problem is. If that cable is routed under floors in trunking through walls etc then the question is whether it is reasonable to raise those boards, remove buried trunking out of walls in order to run a new cable or to use an existing cpc

 

[gts860]

Having pressed the point about not leaving a damaged joint / cable in service, perhaps I should give my approach to locating one.

1. Walk the course. Is there any evidence of work carried out since the installation in areas where the cable is likely to be? Sometimes it's a no-brainer.

2. Inject tone into cable and search with tracer along likely routes. If it can be found at least at a few waypoints, estimate total length along that route.

3. Ping cable from both ends (CU and first lighting point) with a TDR or capacitance meter to discover distance to the break. Compare sum of lengths with measured route. Take average, measure along assumed route, look carefully around epicentre for likely causes of damage.

4. If possible, make access hole behind some other fitting or fixture that will conceal it later. Insert borescope. If nothing interesting visible, return to step 2.

etc.

Thanks for the suggestions. That tone generator looks like its a useful bit of kit, what kind of range does it have ?
With only six lighting points in the upstairs circuit it surely cannot be so onerous to unscrew the pendants ,access each ceiling rose, disconnect all conductors und with your wanderlead (or tone generator)measure from the CU to each rose in turn until you can ping it. Once you have located the L/N you have by definition also located the cpc.

 

[Lucien Nunes]

If you cannot identify where that cable is physically connected then the status of its associated cpc is simply unknown, it could just be loose in the first ceiling rose.

Perhaps I had jumped to this conclusion but I thought the two ends of the supply cable had been identified at the board and first point, and that inspection/testing of the ends confirmed that the CPC was O/C somewhere unknown and inaccessible between them.

If, as you imply here, the first point has not been inspected and tested then this is simply daft and he is wasting your time. As you say, there would be a high probability of the disconnect being within the rose.

 

[westward10]

Nine times out of ten it is.