missing lighting cpc

[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.