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Minimum values of insulation resistance

What is the science behind the 1 MOhm minimum insulation resistance? What is the basis for this particular value?
  • Former Community Member
    0 Former Community Member
    I think most people dont really know much about insulation resistance and with all the myths around it gets confusing, I think this simple statement made by BICC who know a little bit about cables sums it up for electrical contractors.

    The purpose of the IR test is to check for damaged insulation, this can be mechanical damage or damage by heat, (overloaded cables), readings less than 2 Mohm indicate damaged insulation, readings of 2-50 Mohm are indicative of long circuit lengths, moisture and contamination and do not indicate the insulation quality, therefore readings of 2-50 Mohm should not be specified as a fail value.


    Rob

  • Wombat:

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




    The value was chosen because it is a nice round figure and easily remembered. 98759.3k Ohms just would not do. It was set to limit current leakage to safe levels and not waste electricity. In fact in the 14th edition wiring regs. fixed wiring was to have an insulation resistance of at least 1 Meg Ohm, but disconnected apparatus could have a minimum insulation resistance of 0.5 MegOhms if the apparatus had no British Standard. Actually our "modern" regs some years ago also specified just a 0.5MegOhm value. (16th Edition, 2001, Table 71A.


    Z.


  • Wombat:

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




     

    To give you the correct answer I need to find the old Megger insulation testing booklets I bought via Ebay some years ago.


    First off there isn't a 1 Mohm minimum insulation resistance for an existing installation, when you are preparing an Electrical Installation Condition Report you need to make an engineering judgement as to whether the insulation resistance test result is acceptable; and it may be less than 1 Mohm.


    Secondly, the insulation testing requirements are based on the standards laid down by insurance companies and the original requirement was based on a fraction of the total load the installation was to be loaded with. 


    I bet Bod knows the calculation without looking it up ?


    Andy Betteridge

  • original requirement was based on a fraction of the total load the installation was to be loaded with. 



    That sounds like the old Electricity Supply Regulations - they permitted a maximum leakage of one ten thousandth part of the installation's max demand. (e.g. 10mA for a 100A single phase supply). I'm struggling to equate that to 1 MΩ though.


    Earlier regs did have various different ways of calculating the IR limit - often based on the number of points.


       - Andy.
  • Indeed, as my very old black and white illustrated book "Electricity" by The Bennett College states, the requirements of the I.E.E. must be met, but some consulting engineers and local authorities have there own requirements for insulation resistance values.


    Z.

  • AJJewsbury:




    original requirement was based on a fraction of the total load the installation was to be loaded with. 



    That sounds like the old Electricity Supply Regulations - they permitted a maximum leakage of one ten thousandth part of the installation's max demand. (e.g. 10mA for a 100A single phase supply). I'm struggling to equate that to 1 MΩ though.


    Earlier regs did have various different ways of calculating the IR limit - often based on the number of points.


       - Andy.

     




     

    That sounds right.


    Back when I did the 16th Edition Regs course the figure was given as 1 Mohm with a recommendation for 2 Mohm, because the JPEL committee and HSE agreed to disagree. (IET 1 HSE 2)


    Andy Betteridge
  • However, if you do get 1Mohm on a new, clean, dry bit of wiring it is probably a good idea to investigate why before saying "OK, it passes". But is it is on a circuit with several old motors and contactors then no need to investigate further.

  • A local church asked me to do their PAT, they had never had it done before and I condemned several items including the bell ringers electric heater because I could see the copper conductors through the gaps in the cable insulation and also I also put an advisory note on the churchyard Christmas tree lights because they had an insulation value of around 1 Mohm despite having been on top of a cupboard in the vestry for several months.


    They have not asked me to do the PAT again.


    Andy Betteridge




  • They have not asked me to do the PAT again.


    Andy Betteridge 



     

    Obviously you did not convert them to see the light of truth. So they remain in the ignorance of darkness.


    Z.

  • If you can see the copper conductors through the cable insulation it’s a fail on visual inspection, but it doesn’t mean the cable will fail an insulation test.


    You could have a pair of uninsulated 25 mm meter tails which are absolutely lethal, but they could easily pass an insulation test. So insulation isn’t the be all and end all of determining the safety of the insulation within an electrical installation.


     Andy Betteridge