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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
    The shocks will not be just down to an an NE short on its own.

    Something else also has to be wrong, and indeed that needed investigation - and may well have been a broken CPC to boiler or sockets given your description of the fault found and the symptoms. I presume a MET wanderlead  R2 reading or a voltage test MET to the plumbing would also have shown up a problem - it does suggest the previous sparks were at best slipshod, if not dangerously negligent.
  • 0
    There were other issues in that the plumber had drilled the cable causing more than one issue all at the same time and place.


    Andy B.
  • 0

    Wombat:

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




    It matches perfectly the permitted touch current for double insulated appliances (0.5 mA) for a supply voltage U0 of 500 V.


    For U0 = 1000 V, 1 MΩ would lead to a touch current of 1 mA (the accepted threshold of perception).


    These values align with Clause 5.2.7 in BS EN 61140 Protection against electric shock - common aspects for installation and equipment, which states the limit of steady-state touch current to be 0.5 mA AC or 2 mA DC (higher levels are permitted in some circumstances).


    In fact, there is some science behind these values, and they are based on the IEC 60479-series of standards Effect of current on human beings and livestock which is used as a reference standard for developing electrical safety standards - BS EN 61140 is itself based on this series of standards.

  • 0
    Thank you Graham. Very helpful.

  • 0

    It matches perfectly the permitted touch current for double insulated appliances (0.5 mA) for a supply voltage U0 of 500 V.



    But why would we then apply the same value to a 230V installation?


    Or indeed to only part of an installation - as I recall, the 1MΩ limit is for one DB and its outgoing circuits (with all appliances disconnected, even fixed ones).


       - Andy.
  • 0

    AJJewsbury:

    Or indeed to only part of an installation - as I recall, the 1MΩ limit is for one DB and its outgoing circuits (with all appliances disconnected, even fixed ones).




    Not quite. If you get 1 MΩ for a whole board IR test, you know that all circuits have passed. However, if it is less, then you need to test each circuit separately. As you know, each may be just over 1 MΩ, but together they will fall below that value.


    And before anybody else says it, the most likely explanation for a whole board result of less than 1 MΩ is that one of the circuits is below that value.

  • 0

    AJJewsbury:




    It matches perfectly the permitted touch current for double insulated appliances (0.5 mA) for a supply voltage U0 of 500 V.



    But why would we then apply the same value to a 230V installation?


    Or indeed to only part of an installation - as I recall, the 1MΩ limit is for one DB and its outgoing circuits (with all appliances disconnected, even fixed ones).


       - Andy.


    Does the insulation resistance test refer to the stress voltage generated at twice the nominal single phase voltage to earth? The importance being that its electrically stressing the insulation?

    Legh
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    Not quite. If you get 1 MΩ for a whole board IR test, you know that all circuits have passed. However, if it is less, then you need to test each circuit separately.



    OK, it's late and I'm usually slow on the uptake, but why? If a whole board IR comes in at less than 1MΩ - then isn't it just a simple fail? (working out which is the culprit is fault-finding rather than testing).

        - Andy.
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    It’s late and I haven’t done the calculation for a long time, 


    1/Rt = 1/R1 + 1/R2 and so on


    So the global insulation test result for the whole board is always less than the test result of the lowest reading circuit.


    Therefore each circuit can pass an insulation test, but the whole board can fail, so what do you do next?


     Andy Betteridge
  • 0

    AJJewsbury:




    Not quite. If you get 1 MΩ for a whole board IR test, you know that all circuits have passed. However, if it is less, then you need to test each circuit separately.



    OK, it's late and I'm usually slow on the uptake, but why? If a whole board IR comes in at less than 1MΩ - then isn't it just a simple fail? (working out which is the culprit is fault-finding rather than testing).


    Ok, think of it the other way around. Instead of thinking about splitting a group of circuits, ask yourself why it is that you can group them in the first place. In the case of an initial verification, each circuit is tested individually before connecting it to the CU. That avoids a big bang test. However, that is not an issue for periodic inspection and testing so you can save time and effort by testing the circuits as a group.


    So here's a question for those of you who do routine periodics day in, day out. How often is there a fail on IR testing in the absence of a reported problem. I suspect very rarely indeed.

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