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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?
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  • 0

    AJJewsbury:




    Regulation 643.2 says nothing about DBs



    It's implied - given 314.4 you've got to have a DB between the distribution circuit and the final circuits (OK I could have been clearer that it includes the sub-main as well as the DB and final circuits).


        - Andy.

     




    I don't agree. What about installations with multiple levels of distribution?


    For example, even the simple case of Main three-phase switchboard feeds a number of three-phase boards, which in turn feed some equipment plus some single-phase boards. To comply with 643.2, the minimum requirement is for each of the following to be 1 MΩ (unless of course the entire switchboard with everything connected complies):

     - The main switchboard including any final circuits it supplied

     - Each distribution circuit of the main switchboard (individually) through the DB it supplies, having all final circuits connected, along with the sub-mains this DB supplies to the single-phase DBs, along with the supplied single-phase DBs with all final circuits connected.


    So, whilst all DBs will have to meet the requirement, in the case of the three-phase DBs in the tier above, their tests must include not only their submains and final circuits, but also the submains (and associated DBs and all of their final circuits) too.

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  • 0

    AJJewsbury:




    Regulation 643.2 says nothing about DBs



    It's implied - given 314.4 you've got to have a DB between the distribution circuit and the final circuits (OK I could have been clearer that it includes the sub-main as well as the DB and final circuits).


        - Andy.

     




    I don't agree. What about installations with multiple levels of distribution?


    For example, even the simple case of Main three-phase switchboard feeds a number of three-phase boards, which in turn feed some equipment plus some single-phase boards. To comply with 643.2, the minimum requirement is for each of the following to be 1 MΩ (unless of course the entire switchboard with everything connected complies):

     - The main switchboard including any final circuits it supplied

     - Each distribution circuit of the main switchboard (individually) through the DB it supplies, having all final circuits connected, along with the sub-mains this DB supplies to the single-phase DBs, along with the supplied single-phase DBs with all final circuits connected.


    So, whilst all DBs will have to meet the requirement, in the case of the three-phase DBs in the tier above, their tests must include not only their submains and final circuits, but also the submains (and associated DBs and all of their final circuits) too.

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