How to trace a break in a cable? In a wall!

[competant person]

Hi all,

Hoping for some advice, I ran a cable to some light fittings some time ago during refurb of my place. I've now gone (2 yrs later) to connect that light fitting... and nothing. Dead.

I wired my place all myself, then had a spark come and sign it off. All connected circuits passed with flying colours. FYI, as that circuit was not connected it was not tested.

I've rewired, reframed, plastered and fitted cornice. All of which might have brought the chance of a screw or some other damage hitting the cable after it was run.

The live feeds through the house run to all the light switches, in this case a 4-gang dimmer. Two of the gangs are connected and running. So the live feed is fine.

Then from the switch this particular run of 1.5 T&E heads up a wall, across a section of ceiling then behind the cornice to a wall light. I ran a 9v battery on the pos only then back to the battery via my multimeter with a long cable to check continuity. I then did the earth and the neg separately the same way. The earth and the neg test ok. The live is dead.

This cable must have a break somewhere, or taken a screw hit.

When I connected the switch and the light, the breaker/RCD doesn't trip
so if it is a screw it's broken the live and not disturbed the Earth or Neg. Which is a bit weird.

So hoping to get some tips. Especially to avoid making a load of holes in my lovely cornice.

Thanks in advance.

Greg

 

[Octopus]

I very much doubt you'll able to locate the issue without competence, a proper tester and making some holes.

Apologies if this isn't what you want to hear.

 

[competant person]

Hi there, thanks for your comment. Can I ask - what do you mean exactly by competence?
I can hire a tester - if that's the only way.

 

[kingeri]

But do you know how to use a tester? It's a bit pointless anyway if you've already confirmed a break in the cable. That cable needs replacing, really.

 

[Wilko]

Hi CP and Welcome to the Forum!
Cable breaks can be tough to find and it does take some experience. If you do find the break it's likely under something and how will it be fixed? Agree @kingeri - replace the cable

 

[competant person]

No I've not used one. Was looking at something like a Fluke 2042 Cable Fault Locator

I understood jointing cables inside walls/floors was allowable as long as you used a maintenance-free junction/terminal.
So not convinced the entire cable needs to be replaced.

Thanks

 

[Spoon]

So just to confirm. As you say, the live at the switch is ok. You think you have a break in the live cable between the light switch and the light.
Get yourself a tester that can test the cable is live without damaging the insulation and test the live cable in different sections. Test it where it goes into the ceiling, then you will know if that part of the cable is fine... and so on..

 

[Wilko]

... So not convinced the entire cable needs to be replaced ...

Hi - Agree it doesn't need replacing in entirety but its a guaranteed fix

 

[kingeri]

OP, with all respect, you admit you energised this in the knowledge it hadn't been tested. This in itself was unwise. By the time you have hired a tester, figured out how to use it, realised you likely will still need to punch holes in ceiling and wall anyway, it would be easier and probably quicker to replace the cable. Or get an electrician to do it (shock horror).

 

[competant person]

Just after some friendly advice is all chaps.

 

[kingeri]

Just after some friendly advice is all chaps.

And that's what you have received. But I get the impression it's not what you want to hear. It's your house, your cable, so your decision!

 

[Dave Jenkins]

How are you defining 'Dead'?
Choices are:
Line (aka Live) Open circuit (OC)
Neutral OC
Both Line and Neutral OC?
Is the CPC (aka Earth) also OC or do you have continuity from the general area of the switch to the end of the cable?
As far as screws through cables - I've generally found they cause a short circuit rather than a break in the cable. More likely to have trapped a cable behind plasterboard or something of that ilk.
Is there any junctions present in the cable run? A loose connection would be a more likely candidate IMHO.
I use a great bit of kit for finding breaks and shorts in cables but it was expensive (£150+), but as good as it is, it still requires chipping away plaster to effect a repair.
To be honest my friend, it may well be quicker and cheaper to get an electrician in for a couple of hours...

 

[competant person]

Thanks - appreciate it all, really.
I'll go with a consensus.

My issue is that cable lies behind some pretty ornate cornice. So just want to understand all the possibilities, based on people's experiences so I can make an informed decision.

Thanks again.

 

[competant person]

Live = open
Neutral = ok (continuity tested ok)
Earth = ok (continuity tested ok)
The RCD didn't trip so assume no short.

Agree that a screw would normally cause a short.

 

[Dave Jenkins]

Here's a thought - it's definitely the right cable isn't it? After two years I'm not sure I'd remember exactly what I'd done - these days have issues remembering what I did yesterday!!
Identify the cable (beyond all doubt) by end-to-end testing with both ends open-circuit...

 

[Pete999]

Here's a thought - it's definitely the right cable isn't it? After two years I'm not sure I'd remember exactly what I'd done - these days have issues remembering what I did yesterday!!
Identify the cable (beyond all doubt) by end-to-end testing with both ends open-circuit...

Is/has the cable in question been/or is connected to anything? or is it just a piece of cable buried in the building fabric?

 

[Dave Jenkins]

You'd need to test the Insulation Resistance (IR) of the cable. That will show whether there is damage to the Line, Neutral and/or cpc conductors.. My thinking here is that if the Line is oc due to crushing or screw damage, then it's likely that the others are damaged too.
You'd be looking at an IR of 100's of M-ohms. In order to conduct the test you'd need to have an appropriate (and calibrated) test instrument. Mine cost me £650 five years ago....not sure whether you can hire one.

 

[Pete999]

You'd need to test the Insulation Resistance (IR) of the cable. That will show whether there is damage to the Line, Neutral and/or cpc conductors.. My thinking here is that if the Line is oc due to crushing or screw damage, then it's likely that the others are damaged too.
You'd be looking at an IR of 100's of ohms. In order to conduct the test you'd need to have an appropriate (and calibrated) test instrument. Mine cost me £650 five years ago....not sure whether you can hire one.

In my opinion, for a Diyer to hire a MFT or IR tester will be a waste of time, he needs to hire a Spark for a few hours to rectify the fault.

 

[The Ghost]

I usually drill behind the cornice to put a cable in. So there is no need to damage the work you have done.

 

[kingeri]

Remember, to replace the cable doesn't mean you actually have to remove the old one. You can just disconnect it and forget about it, and concentrate on installing the new one.