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  1. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    anyone shed some light?

    100m run of 4mm nyy cable

    New cable and getting 135meg ohm when testing surely this should be >999 on martindale mft

    Checked cable and no damage(visual anyway)

    I know it's well above minimum permitted but still
     
  2. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    Is the cable completely isolated, or are any conductors connected to a load?
    How long was the test voltage applied for before taking the reading?
     
  3. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    Not long just usually press test and within seconds it gives me reading it's the et4000 if that helps
     
  4. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    Nothing connected just cable either side
    I got 365 from the socket before this
     
  5. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Sounds like damp or contamination.
     
  6. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    It's the same give or take between all conductors ,would the reading of 135 be considered acceptable on a new installation cert? So for so many questions I'm fairly new to testing so these results threw me a bit and just filling cert out now
     
  7. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    Last bit of info for you

    I had a 3m length left and tested that and got >999
     
  8. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    Yes it is acceptable but for a new cable of only 100m I would expect above 200 meg.
     
  9. westward10
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    westward10 In echoed steps I walk across an empty dream. Electrician's Arms

    Location:
    Northamptonshire
    So you are testing this with the conductors at both ends free and nothing connected although strictly speaking the cpc should be connected to earth.
     
  10. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    At a test voltage of 250 I'm getting >200
     
  11. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    If you carry out IR testing like that then you have to expect to get readings like this on long cables.
    The cable forms a capacitor and you need to apply the test voltage for long enough to fully charge this capacitor, otherwise you will not be measuring the resistance of the insulation but will just get a slightly random number.

    re-test the cable, but this time make sure you allow long enough to overcome the effect of the capacitance of the cable.

    Don't forget to safely discharge the stored energy in the cable.
     
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  12. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    Id say it sounds more like the capacitance of the cable since it is brand new.
     
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  13. Ian1981
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    Ian1981 Electrician's Arms

    Location:
    North east
    Compression fault in an accessory perhaps?
    Cables crushed/squashed in too tightly
     
  14. Des 56
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    Des 56 Trusted Advisor

    Location:
    Gliese 581C
    It can be worthwhile doing a longer test if a reading is not acceptable to you
    Try doing the test for a minute or two to see if the value rises
    There are a couple of reasons for doing so,I have copied and pasted some info for you

    Capacitance:
    The capacitance charging current necessary to charge the capacitance of the insulation being tested. This is a transient current which starts relatively high and falls exponentially towards a value close to zero once the circuit being tested is charged electrically. After a few seconds or tenths of seconds, this current becomes negligible compared with the current to be measured.

    Absorption:
    The absorption current, corresponding to the additional energy necessary for the molecules of the insulating material to reorient themselves under the effect of the electrical field applied. This current falls much more slowly than the capacitance charging current, sometimes requiring several minutes to reach a value close to zero. •
    Leakage current
    The leakage current or conduction current. This current characterises the quality of the insulation and is stable over time
    .
     
  15. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    So due to the length off cable this can take longer to get the expected result while testing ?

    All other tests fine and compliant as expected

    Really helpful and useful this forum

    Thanks guys

    Anymore info help on this still welcome
     
  16. davesparks
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    davesparks Trusted Advisor

    Location:
    guildford
    Yes, which is why you continue the test until the reading stabilises or a suitable length of time has elapsed.
    I have always used 2minutes for a 500V test.
    I think modern higher voltage IR testers perform a timed test (amongst other functions) automatically.
     
  17. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    From the socket I tested with the plug from the tester and got 365 and like I've said 135 from the board so wasn't making sense to me

    But I now know where I've gone wrong thank you
     
  18. anthonybragg
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    anthonybragg Electrician's Arms

    Location:
    NORTHAMPTON
    Business Name:
    CHARLTEC ELECTRICAL LTD
    The good thing is as you say you're fairly new to testing you are questioning the values you get'' is this right, should I be getting this reading ,why have I got this reading.
     
  19. Dannyg8810
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    Dannyg8810 Electrician's Arms

    Location:
    Leeds
    Yes always learning and being making sure everything I do is correct ,
     
  20. soms
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    soms Trainee Access

    Location:
    Somerset
    This is interesting, I wasn't previously aware of this. There is no explicit mention of it in the OSG, which many trainees/apprentices use as their quick reference/refresher to the testing procedures.
    Looking into it, I see it is explained in GN3 as being introduced with the 17th Edition referring reg. 612.3.1.

    Out of curiosity, how would this be done?
     
  21. Murdoch
    Online

    Murdoch Electrician's Arms

    Location:
    Woking
    ^^ you should always do IR tests on a cable with the cpc connected to the cu, along with the other circuits.....
     
  22. Hellmooth
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    Hellmooth Electrician's Arms

    Location:
    Livingston
    Business Name:
    CM Electrical
    Is this why my Megger will give a low reading when button is first pressed but increases pretty rapidly as button is held down?
     
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