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

 

[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.

 

[Spoon]

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.

As @Wilko said in post #8, it is a guaranteed fix.
You can also crimp or solder cables together, as long as you do a proper job. I would, whenever possible, replace the cable.

 

[David M]

I once had a break in a brand new "un damaged" cable, a leg of the RFC. It turned out to be a faulty piece. Fortunately I had installed it neatly in capping so although now newly plastered I was able to pull a new leg in.
The moral of the story being that care and attention during installation pays dividends even when least expecting.

 

[HandySparks]

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

Yes, a Fluke 2042 or similar cable tracer might well help to locate the break.

 

[MJPD29]

Hire a trained electrician save yourself time and money in a tester. DIYers are dangerous when not properly trained

 

[Spoon]

Hire a trained electrician save yourself time and money in a tester. DIYers are dangerous when not properly trained

A bit of a sweeping brush statement....

 

[MJPD29]

A bit of a sweeping brush statement....

My response to all DIYers. My gas boiler was broken down in my house 2 weeks ago and i certainly didnt try to fix it. I hired a qualified competent person to do it properly and safely. Electric is just as dangerous, so why do so many DIYers mess with it?

 

[Spoon]

My gas boiler was broken down in my house 2 weeks ago and i certainly didnt try to fix it. I hired a qualified competent person to do it properly and safely. Electric is just as dangerous..

I think not..
Gas is way more dangerous. Electricity will usually kill one person. Gas on the other hand can take down houses... killing a lot more than one..

 

[Spoon]

My response to all DIYers.

Maybe you should limit your replies to just electricians that post then.

 

[MJPD29]

I think not..
Gas is way more dangerous. Electricity will usually kill one person. Gas on the other hand can take down houses... killing a lot more than one..

Is electricity not capable of starting a fire which could burn down a building? As little as a poor connection can start a fire.

 

[Spoon]

Is electricity not capable of starting a fire which could burn down a building? As little as a poor connection can start a fire.

It is possible...
Still not as dangerous as a gas explosion..