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captaincaveman

captaincaveman

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Hiya.

For anyone interested in domestic fault finding I had a funny one today...

Customer complained about her bs 3871 popout kept popping on her downstairs and basement sockets so I went round, did my testing and found the circuit was failing the IR tests (I will pass on telling you about the hour long conversations with the senile pensioner customers, where the ground was retrodden more than an army barracks parade ground...).

So... as usual, I split the ring at the believed half way point - to discover both legs were failing the IR test. Ok, not the first time I have had multiple faults on a job...

So... I removed all accessories and tested at each point resulting in fails at 1 leg at lounge socket, 1 leg at a dining room socket next to it, and both legs at a socket next to an electric fire place in the basement directly below. I then deduced whoever fitted out the basement obviously split the cable between the lounge and dining to extend it down to this basement socket and back. All I needed to do now was find the connections (which I'd have expected to be below the lounge/dinging sockets).

So... I ended up cutting 7 holes in the ceiling to find the terminal boxes :grimacing:. They were nowhere near any of the sockets (I couldn't pull the floor up - before you ask). The cables, it would transpire, run from the basement socket, behind the fireplace (with connection teminals enroute in the fire place), through the wall to the other side (which is a utility room), up the wall behind dot & dab where it's worth pointing out they had no protection, no rcd, not in a zone... In fact they were mm away from a picture nail. and then across the ceiling under joists to the terminal boxes 3 feet away from the wall.

IMG_20200827_152914_LI.jpg basement socket right hand side of fireplace going left and then through the wall to 2b.
IMG_20200827_152927_LI (2).jpg 2b) where they come through wall. 3) terminal boxes. 4) dining room socket above. 5) lounge socket above.

Now the interesting thing is these newer cables were only about 3-5 years old (hard to tell cos the customer got confused as to when it was done) but showed no sign of damage. I managed to pull out one bit between the fireplace and the basement socket, meggered it at 0.17, then opened the cable up to find no damage what so ever. I can only conclude it is immitation black market cable which has let condensation penetrate the insulation making it useless. I would be interested if anyone else has come across this?

It confused me how after splitting the ring I got bad readings from both legs. It was nice to realise I wasn't going crazy o_O... Although the customers made up for that in spades :tearsofjoy:
 
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pc1966

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Have you kept sections of the removed cable?

The folk here might want to try and find out its origin, etc, as that is clearly a serious safety risk and they can't be the only household impacted:
 
Lucien Nunes

Lucien Nunes

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I too would be interested to see some, especially the markings on it. There have been products insulated with unsuitable / contaminated material that is partially conductive. On the basis of the poor installation method it might not have been tested on completion.

Did you find the cause of the MCB tripping? You mention a reading of '0.17' which I am guessing is megohms. This length of cable would dissipate 0.3W of heat when subjected to 230V. If the resistivity was uniform throughout the material that would probably go unnoticed, but if there are small hotspots amongst generally insulative material, they could reach a high enough temperature to carbonise and enter a runaway condition. Pics of the actual short-circuit would be interesting.

Obviously if you meant 0.17 ohms, that's the short circuit and the only conceivable causes are the conductors touching or a piece of metal embedded in the insulation. (Always state the units, grrr.)
 
telectrix

telectrix

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i like ohms. they transcend all sysyems whether you work in cgs (centipedes/grams/seconds); MKS (meters /kellogs/seconds) or YTW (yards /tons/weeks) ohms are still ohms.
 
static zap

static zap

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i like ohms. t
Hang on a MHO !
Post automatically merged:

One thought about the cable ,is the area well ventilated ,or is it a bit like pond flex over time absorbing water .
Signs of tight rusted steel screws in wood around ...
Put your detective head on.
 
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captaincaveman

captaincaveman

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  • Thread Starter Thread Starter
  • #7
Have you kept sections of the removed cable?

The folk here might want to try and find out its origin, etc, as that is clearly a serious safety risk and they can't be the only household impacted:
Hi.
I managed to pull & keep the bit between the fireplace and socket which I opened up (see below). I have left the rest in place for now as it is embedded in walls etc. There are no markings on this section at least and the primary and outer insulation is fully intact with absolutely no signs of damage. If you want me to post some to you I can do.

There is no sign of moisture in the basement at this time (although they do have a small humidifier on!) I couldn't see any building insulation in the walls/ceiling either which may degrade it. There is the small electrical fire of course which the cable ran a couple of inches behind (with no insulation between). This is where I found the terminal connectors. Not really any signs of thermal damage although it obviously wouldn't do it much good.

Sorry Lucien, It was a long day. I did indeed mean 0.17 megohms on the IR test

I am concerned as there is more of this cable (I assume it's the same cable) in the basement and although I tested it ok that doesn't mean it could be a future hazard.

as an afterthought (I should have done it at the time - Damn it!!) I have just done insulation tests on both the live and neutral, individually, between the conductors and their insulation to get 999. Which is a bit perplexing and I can only assume they have dried out since I removed it??

IMG_20200828_191715.jpg
 
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Lucien Nunes

Lucien Nunes

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What exactly are you testing this time to get the high reading?
 
captaincaveman

captaincaveman

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  • #9
What exactly are you testing this time to get the high reading?
Insulation test at 500v between the actual live conductor and the inner insulation around the conductor. I did it at several points on the insulation along the cable length (3ft). If it failed before between live and earth (0.17) then I expected it to fail under this test but it hasn't. That could be because I am only testing 5mm surface area at a time (width of my test clip) at several positions along the length as apposed to testing the whole length surface area or it could be it has dried out. I can't think of another explanation

IMG_20200828_203521.jpg
 
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pc1966

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It is perplexing.

An IR value of 0.17M won't fire a 30mA RCD, let along a MCB, but it clearly says that circuit is seriously wrong. If you measured it on a removed section (i.e. isolated) then it has to be the cable, but I would be most surprised even with some damp to get anything that low. Unless the PVC has some other issues, which is not unbelievable :(
 
captaincaveman

captaincaveman

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  • Thread Starter Thread Starter
  • #12
It is perplexing.

An IR value of 0.17M won't fire a 30mA RCD, let along a MCB, but it clearly says that circuit is seriously wrong. If you measured it on a removed section (i.e. isolated) then it has to be the cable, but I would be most surprised even with some damp to get anything that low. Unless the PVC has some other issues, which is not unbelievable :(
I should point out, the circuit was energised and working when I got there. The customer had been complaining it kept tripping though (about several hours beforehand and the day before etc.). I checked through the usual suspects (kettle, toaster etc) and couldn't find any issues. I was unable to get much info from the customers unfortunately. I did start a few arguements between them, on what they were doing at the time etc, though :tearsofjoy:

Old Wylex board btw. No RCD
 
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