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Low Resistance on IR Testing

Discussion in 'Electrics UK' started by wcavanagh, 13 Feb 2019.

  1. wcavanagh

    wcavanagh

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    Very strange one here, renovation project, full strip out and start again basically.

    I pulled in 2.5mm radials for most circuits. R1+R2 all test out great.

    But when I do IR I would expect >299 but I am getting between 50 - 80 on most. Any ideas?

    1st fix was slow so cable where in for 6 months unheated, and plastering and painting was done, painting was sprayed so a lot of moisture was in the building.

    I disconnected from CU to first outlet and run test just on that cable so no terminations at all, and get 55. So makes me think moisture in cable?

    I know regs say etc, but normally on new install I would expect off the scale

    Any ideas at all?
    Please
     
  2. JohnW2

    JohnW2

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    Have you tried with a different meter? As you are implying, the results you're getting don't make any real sense. Moisture (or even total submersion in water) would not effect the IR of PVC cable.

    Just to be clear, you're saying that (in addition to the 50-80Ω M 'in situ' measurements) just an isolated piece of new cable gives an IR of 55 MΩ - is that correct? If so, and if your readings are actually correct, then one might be left with little other than (the very rare occurrence of) a duff batch of cable. Have you tried measuring the IR of something else with your meter?

    Kind Regards, John
     
  3. RF Lighting

    RF Lighting

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    Change your batteries
     
  4. wcavanagh

    wcavanagh

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    Many Thanks, will swap out batteries and try again. I have a 4mm radial to the utility room of a different make cable, and that tests at >999

    Everything is new, and the whole batch of 2.5mm was purchased at the same time from a good supplier, so was hoping for it not to be a bad batch as it would be near impossible to replace all.
     
  5. JohnW2

    JohnW2

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    Whilst my first thought was, as you know, a meter problem (possibly batteries), if you get a >999 MΩ reading on a different cable, that does tend to point a finger at the new cable - but that would, I think, be an incredibly rare occurrence, and not something that I (in my very limited experience) have ever encountered, or heard of happening.

    A localised manufacturing problem could produce a problem with just one bit of the cable, but if you're experience the low IR on several different pieces of the cable, that would (if due to the cable) mean that the cable would have to be systematically faulty ('throughout') - which seems even more 'incredible'!

    A different meter - or, at least, new batteries in your existing one would certainly still seem to be the first step.

    Kind Regards, John
     
  6. wcavanagh

    wcavanagh

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    Many Thanks John, indeed, never thought of batteries myself so a very easy first step. Will pop a new test on tonight.

    Really got me scratching my head, as never had an issue before. I mean I know its OK on paper according to regs, but being all new I would expect better readings for sure.
     
  7. JohnW2

    JohnW2

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    Yes, it's certainly worth a try. However, if it is able to display >999 MΩ on some cable (and also, presumably, if you don't connect the test probes to anything) then, despite my initial thoughts, I'm now far less convinced that it's going to prove to be a meter issue.
    Yes, very much 'within regs', but I still would not want that situation in my homw (at least, unless there was some 'reasonable' explanation).

    Do you still have some of the new cable that has not been used (i.e. 'on a drum')? If so, what reading do you get if you measure the IR of that (first making sure that the 'other end' is not shorted!)??

    Kind Regards, John
     
  8. wcavanagh

    wcavanagh

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    Yep I still have have a whole reel and half left over, so I will be running a test on that tonight. I used about 5 reels of the 2.5 in the house, all stored the same way in the garage and then installed 6 months ago, but never got round to testing. Just left at 1st fix for 6 months over winter etc.

    I heated one room, to test and that improved the situation in that room by about 30M Ohm......

    I am REALLY hoping its not a cable issue.

    Yes with the regs, normally I like to see that >999 on a brand new install like this.
     
  9. JohnW2

    JohnW2

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    That will be interesting.
    If the place is incredibly damp, is it conceivable that what you are seeing is a consequence of moisture just around the area of the terminations (at both ends) of the cable? If it were me and I ran out of other ideas, I would probably take a hair dryer to (both ends of) the cable!

    Kind Regards, John
     
  10. DetlefSchmitz

    DetlefSchmitz

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    I'm pretty sure I remember a similar or worse problem just after our extension was plastered. Vaguely recollect it took a week or so to fix itself.
     
  11. JohnW2

    JohnW2

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    Interesting. Did you have any clues as to the cause - perhaps the 'moisture at terminations' thought I mentioned?

    Other than where the cores are exposed (i.e. at terminations), no amount of moisture should make any difference to PVC cables. I am now a good few years (6-7 years, I think) into my long-term experiment of T+E (and also '50V alarm cable') sibmerged in concentrated brine, and, at the last check, all were still 'off the scale' of IR testing at 1000V (Edit: see post #9 of this thread )

    Kind Regards, John
     
    Last edited: 13 Feb 2019
  12. DetlefSchmitz

    DetlefSchmitz

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    Even clearer in my case as all my cables were in plastic conduit. I just assumed the damp inside the back boxes was causing it.
     
  13. JohnW2

    JohnW2

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    That's obviously credible. However, in the OP's case ....
    If I understand that correctly, it sounds as if the back boxes were not really part of the equation - leaving, as I said, the only thing I could think of (other than the cable itself) being moisture around the exposed bits of conductors at the end(s) - although that doesn't sound particularly likely. Whatever, as I said, if I ran out of other ideas, I think I'd try the hair dryer!

    Kind Regards, John
     
  14. wcavanagh

    wcavanagh

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    B84A4CE2-E1EE-4150-B05B-8C98B2B9AA90.jpeg Well just grabbed a reel and done a test on it. Guess what 58.2M. If I hold the test button down the longer I do the higher it goes.
     
  15. JohnW2

    JohnW2

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    Can't argue with that!

    What reading do you get with the meter's probes connected to nothing (and well apart, and away from you!)?

    Kind Regards, John
     
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