Chasing a fault

[Chris J Dixon]

So, the kitchen lights tripped. OK, it must be another of those LED downlights blown, I'll reset and see what is out. No, still trips.

Remove all eight lights, still trips. OK, must be the movement detector, let's disconnect that, and link the feed through. Still tripping.

That's not good. Check for a L-N short at the fittings. Oh yes.

Looks like today's task is to see what I can find under the bathroom floor. I need to split the wiring to see which way the fault moves.

I am hoping it is something I can reach, not buried cables. My guess is that it must be one of the (mains) light fitting connectors, though it seems a bit strange. Maybe one of the lights did fail, and the resultant disturbance has damaged something.

Chris

[Chris J Dixon]

Well, it looks like I have found it. The first place I broke the circuit put me straight on the problem. A length of cable - PVC lighting cable about 12 years old - had failed live to earth in the middle of a run. No external signs at all, not in a location that would have experienced any unusual conditions. In the ceiling, not near anything else.

I can only guess that a transient of some form created an unusual overvoltage.

Here's hoping it was the fault, not a symptom.

Chris

[Dave Plowman (News)]

Be interesting to remove it and strip it out to see the actual failure. I'd guess more likely to be a manufacturing fault as it shouldn't have got hot enough to melt, etc.

[David]

Could be a dodgy run of cable, although IIRC most of that was many years ago.

When we knocked the house about we had to replace some runs of cable which were oozing green stuff. Some sort of reaction between the copper and the sheathing.

Cheers

Dave R

[tabbypurr]

very unlikely.

[Chris J Dixon]

This is what I found:

Looks like the fault actually severed the earth conductor.

The movement detector is no longer functional, though I don't suppose I will know if that was cause or effect.

Chris

[Brian Gaff]

I remember some time ago having a break in a cable made from stranded two core flex, that what it looked like was that the wire strands were not continuous and the whole thing was made out of about 1ft strands overlapped out of synch, so to speak to appear to be a continuous run, but at the fault site it was as if some of the strands were short or broken leving very few to connect to each other inside the sleeving or as if it had been stretched pre manufacture. There was the problem. Over time it just stopped conducting. Now on the one mentioned maybe its thinner part got so warm as to melt the pvc or something inside. Seems a little worrying that if it is the case. We trust people to make cables that are in fact continuous, some are, it seems not!

Brian

[Brian Gaff]

Really? That is a little worrying. What would be the mechanism for such a fault to be fast enough to do that, but not actually have the trip before it did it, if you get my drift.

Switch mode psu on the motion detector? Did it really have an earth? Brian

[Dave Plowman (News)]

Unless you have mixed up earths somewhere, a lighting circuit earth is never going to carry enough current to do that? Any signs of a nail etc having gone through it?

[harry]

Several decades ago, there was a big flap over a batch of faulty cable that got out. Could be some of that. Can't remember the size and manufacturer.

[tabbypurr]

and to do that the CPC would need to be both carrying live current AND there to be a short. If wired & fused correctly that would not happen. And since that spot is burnt while the rest is not, there must have been a cable fault at that point too.

Someone will check that, I'm real tired.

NT

[ARW]

To the left hand side I can see what looks like a split in the brown sheathing.

Either the cable was faulty or it has been damaged during installation.

[Terry Casey]

I was in the shop one day, many years ago when an irate customer was complaining about a length of aerial cabe he'd bought. He'd installed his aerial and the cable and it didn't work.

In the middle of the cable, there was a strange lumpy bit several inches long. When the sheath was slit open, it revealed two bits of coax joined together with a piece of string!