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Line -CPC Insulation resistance

Discussion in 'Electrics UK' started by studentspark, 31 Jan 2020.

  1. studentspark

    studentspark

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    Could I just ask for some advice , as I am doubting myself..., or my test results

    Dual RCD board, in an unoccupied house, (between moves)

    Recent rewire, probably a year old - No installation certificates.

    Doing insulation resistance tests.
    Global test, gives poor readings, so I separate all the lives , neutrals,
    and test to cpc - still the same so I separate cpcs , and still the same.

    One side of the RCD, these circuits test around the 200 M ohms. For a new install I would expect off the range. (Megger 1741, so 999)

    The other side, Im getting 0.2 Mohms on every circuit.
    This is with all the circuits separated.

    Testing between line and CPC's and Line, with another circuits CPC, I still get a really low reading in the 0.2 Mohms range.

    Neutrals- CPC readings in the 200 Mohms range.

    Circuits are Oven. two socket circuits and two lighting circuits.

    Lighting circuits have dimmers, transformers etc. nothing plugged in, well its an empty house.

    Am I missing something that can effect a Line to CPC IR reading.
    Thanks
     
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  3. JohnW2

    JohnW2

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    Can you explain what you mean by that - an RCD shouldn't be part of the the equation when you're IR testing a final circuit.

    Kind Regards, John
     
  4. studentspark

    studentspark

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    Yes. the RCD's have nothing to do with it really, its just the two groups of circuits.
     
  5. JohnW2

    JohnW2

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    Oh, I see, when you talked of 'one side of the RCD' and 'the other side' you had me confused!

    So are you saying that one set of circuits (those normally protected by the same RCD) all individually (when disconnected) have low L-E IRs, bit that all the circuits protected by the other IR have acceptable IRs? If so, that sounds bizarre, since it surely can but be an amazing co-incidence (if all the circuits are 'disconnected')?

    Kind Regards, John
     
  6. studentspark

    studentspark

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    Thats right.
    Wondering if that section of cables has some how been collectively damaged, passing over a hot water pipe or a sharp edge. for example, but just pure speculation.

    Normally splitting common connections on the the circuits, drops the fault on the other conductors, but in this case all the conductors still show the same very low IR readings.
    It make you doubt yourself a bit, am I doing something wrong with the testing.
    But since this is between L and E could other factors effect it? like you can with L-N, and be reading a load.

    Ive been caught out with fridge capacitors before having a link to earth.

    USB sockets, could they be an issue? But how could that effect a lighting circuit.
    Perhaps it is what it is, and the cables have been damaged.
     
  7. chivers67

    chivers67

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    Could this be a Live wire squashed against a metal earthed accessory or similar somewhere with a slightly damaged insulator? Bit perculiar
     
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  8. JohnW2

    JohnW2

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    Very odd. From what you've said, the only main alternative to an 'incredible co-incidence' would be that cables from several circuits had been simultaneously damaged - which, although unlikely, I suppose is not totally impossible (in the sort of way you describe) - although, even then, we would be back to 'incredible coincidences' if all the affected circuits had similar low IRs.

    Thinking of another thread we had (I think within the last year or so), I suppose one other possibility to consider is that all the circuits concerned were wired with cable(s) from duff batch(es) - but even that would seem unlikley given that, presumably, different sized cables are involved.

    Kind Regards, John
     
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  9. flameport

    flameport

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    Two thoughts:

    1. With the affected circuits totally disconnected from the board (L,N and CPC as well), test between L&L from two different circuits, and N&N from two different circuits. Repeat for all possible pairs of circuits. If all those results are not off the scale, there are link(s) between multiple circuits somewhere.

    2. Has everything actually been rewired completely - or did some fool put new lengths of cable from the CU to a big rack of junction boxes under the floor/in the ceiling connecting onto ancient VIR cables ? Check at a selection of sockets/switches as well.
     
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  11. JohnW2

    JohnW2

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    Yes, but to get what the OP has described would require that to have happened to cables in several totally separate circuits - which, as independent events, would be an almost unbelievable co-incidence. That's why he is trying to think of ways in which several different cables could be damaged by a 'single cause'.

    Kind \regards, John
     
  12. studentspark

    studentspark

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    Yes will try this, thank you

    I have opened lots of sockets and switches. All new cable. Can't get under the floor.
    Its a neat install, all nicely terminated, grommets in back boxes, very neat board. Good Zs readings, R2 readings, RCD readings, end to end (r1-r2-rn) all perfect, even for the 1.67 for the cpc. So perfect in fact it looks like I have made them up.
    Tried different test leads, charged the tester.

    Did consider, but as you say that would be 6, 2.5 and a 1.5mm2 rolls...

    Wondered if cables could be sat in water somehow. no visual signs of damp

    No RCD tripping.
    Circuits have been loaded as painters have been using heaters, as no boiler , and lighting has been used regularly.

    House has been cold, (but not damp) for about 3 months, so I can't imagine that would cause such low reading


    So apart from the IR its all good, and all works.
    will retest on Monday. But if still the same, what on earth do you tell the home owner??
     
  13. JohnW2

    JohnW2

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    Indeed - that's why my 'second thought' suggested that my 'first thought' probably would have been very viable!
    It shouldn't really matter if I were.

    You may remember that I have an ongoing experiment with T+E, and also "50V" alarm and 'signal', PVC cables submerged in brine. I think it's now been going for about 8 years (maybe more) and I IR all the cables every few months. So far all have remained 'off the scale' at 1000V - it seems that PVC is pretty good stuff :)

    Of course, water (or even just 'dampness') at the terminations, where the copper conductors are exposed, is an entirely different matter - and, I suppose, remains a possibility in your case.

    Kind Regards, John
     
  14. studentspark

    studentspark

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    I didn't know that, really interesting. I would have thought water would always win.... eventually
     
  15. JohnW2

    JohnW2

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    Yes, that's probably what intuition tells us all. It may even be true 'eventually', but it seems that that requires many years or decades.

    However, if you think about it, there are countless situations in which PVC (or rubber) cables are constantly (or very frequently) exposed to water, and seem to last 'for ever'.

    I've used strong brine, rather than 'clean water', for my experiment so that the slightest amount of ingress should have a noticeable effect on IR.

    Kind Regards, John
     
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  16. securespark

    securespark

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    I installed a radial circuit once on the RCD side of a "16th" board and got a call back because of RCD trips.

    I disconnected the cable at the CU and at the other end.

    I got a dead short.

    It was a relatively short piece (5m), and easy to replace, so I pulled in a new piece.

    It was soaking under the floor, so when I clambered out, I dried and cleaned off the cable, looking for the damage.

    There was a pin hole in the cable. It must have been there when I installed it!
     
  17. JohnW2

    JohnW2

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    That's a strange story, since one would not expect a 'dead short' (or, indeed, necessarily any measurable problem) as a result of a 'pin hole', even in the presence of water.

    What do you mean by 'dead short', and do I take it that the pinhole penetrated the insulation of one of the conductors?

    Kind Regards, John
     
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