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Minimum values of insulation resistance

Wombat
What is the science behind the 1 MOhm minimum insulation resistance? What is the basis for this particular value?
  • 0

    Zoomup:

    "...two electricians went to investigate two different electric shock events at different times and presumably neither of them did a global insulation test."


    They were unprofessional in their attitude and work, and just wanted to get out quickly. You are professional, dedicated, conscientious and competent Andy, and a credit to our industry.


    Z.




     

    I don’t know about that, it seems to me that quite a few guys just bang test and consider that if the RCD doesn’t trip the job’s a good one.


    Andy B
  • 0
    Well I would not suggest that a bang test has nil merits. However a pro makes all relevant dead tests plus insp before livening up. We all know that, in relity, many soley rely on OPD and RCD though
  • 0
    We can rule out a minimum insulation test result value below which a RCD will trip, because a load has to be applied before it will trip, no load no trip and then it depends on the current draw of the load.


    Andy B
  • 0
    Currnt draw and leakage to E I would say Andy, so "Earth Leakage" does have some effect here
  • 0
    So the fact that the RCD isn’t tripping is not a good indicator of the likely outcome of an insulation test,  neither is using an earth clamp meter or for that matter a thermal camera to check an installation that is not fully loaded much use.


    The insulation tester may be basic, but it is very useful.


    Andy B.
  • 0
    Is an NE fault immedately dangerous, potentially dangerous, or just extremely annoying if it causes nuisance tripping ?

    Personally, I think without another fault, such as an open CPC - which may come with the cable damage (rings are nice as you can test for that) or LN reversal, it is probably more of a pain  than an immediate  danger.
  • 0

    mapj1:

    Is an NE fault immedately dangerous, potentially dangerous, or just extremely annoying if it causes nuisance tripping ?




    I would say 'potentially dangerous' in that it needs a second fault (possibly two further faults) to be dangerous. However it is still a fault that should be rectified, particularly as it can give rise to a problem that may not be protected against..

  • 0
    Many older installations have had a N to E fault on them for years with no bad consequences. It is only when a new consumer unit is fitted with R.C.D.s that the problem becomes evident.


    Z.
  • 0

    Alasdair Anderson:




    mapj1:

    Is an NE fault immedately dangerous, potentially dangerous, or just extremely annoying if it causes nuisance tripping ?




    I would say 'potentially dangerous' in that it needs a second fault (possibly two further faults) to be dangerous. However it is still a fault that should be rectified, particularly as it can give rise to a problem that may not be protected against..


     




    Dangerous, requiring immediate attention when the users of the installation have already twice reported receiving electric shocks from an appliance and when using a mixer shower over a plastic bath.


    If it had been a TT installation rather than TNCS things could have been a lot more lively.


    Andy Betteridge 

  • 0
    The other option is FI- Further investigation required, but how appropriate is it to walk away and leave it for another day when there is a clear and obvious risk that users of the installation will receive more electric shocks in the interim?


    Andy B

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