Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 66 replies
  • Subscribers 32 subscribers
  • Views 27801 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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?
Parents
  • 0

    Zoomup:




    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.


     




    Thank you for this comment. Where you mentioned "It was set to limit current leakage to safe levels.." What was the criteria for determining the safe level?


    Referring to  IEC/TR 61200-413, a touch voltage of 50V can be sustained by a person indefinitely. (dry conditions) Using an average body resistance of 2000 Ohms gives a current of 25 mA.ac


    Referring to IEC/TR 60479-1 25 mA is in the AC-3 region
    Strong involuntary muscular contractions. Difficulty in breathing. Reversible

    disturbances of heart function. Immobilization may occur. Effects increasing

    with current magnitude. Usually no organic damage to be expected

    So that is considered acceptable and presumably the rationale behind the 30mA RCD


    If we go to the parallel DC curves in the DC-2 region which is less hazardous than the DC-3
    Involuntary muscular contractions likely especially when making, breaking

    or rapidly altering current flow but usually no harmful electrical

    physiological effects

    And we use say 25mA dc to parallel the 25mA ac.


    With a 500V DC Megger and a 25mA leakage current we get 20000 Ohms insulation resistance.


    So the numbers do not stack up on what is considered safe.


     

Reply
  • 0

    Zoomup:




    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.


     




    Thank you for this comment. Where you mentioned "It was set to limit current leakage to safe levels.." What was the criteria for determining the safe level?


    Referring to  IEC/TR 61200-413, a touch voltage of 50V can be sustained by a person indefinitely. (dry conditions) Using an average body resistance of 2000 Ohms gives a current of 25 mA.ac


    Referring to IEC/TR 60479-1 25 mA is in the AC-3 region
    Strong involuntary muscular contractions. Difficulty in breathing. Reversible

    disturbances of heart function. Immobilization may occur. Effects increasing

    with current magnitude. Usually no organic damage to be expected

    So that is considered acceptable and presumably the rationale behind the 30mA RCD


    If we go to the parallel DC curves in the DC-2 region which is less hazardous than the DC-3
    Involuntary muscular contractions likely especially when making, breaking

    or rapidly altering current flow but usually no harmful electrical

    physiological effects

    And we use say 25mA dc to parallel the 25mA ac.


    With a 500V DC Megger and a 25mA leakage current we get 20000 Ohms insulation resistance.


    So the numbers do not stack up on what is considered safe.


     

Children
No Data

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines