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TT system Rcbos DP

Raymond Obrien

Issue where a time delayed RCCB supplying DB which has single pole rcbos tripping the time delay under fault conditions.

where as if you have double pole rcbos disconnects the rcbos under fault conditions. Why is not stated that the use of Double pole in  IET on these conditions.

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    Raymond Obrien said:
    Why is not stated that the use of Double pole in  IET on these conditions.

    It seems the powers that be are of the same mind - the DPC for AMD4 contains a new note to 536.4.1.4 ...

    "NOTE 4: To achieve selectivity between an upstream Type-S RCD and any downstream RCD with respect to both a
    line to earth and a neutral to earth fault, the downstream RCD shall switch all live conductors (including the
    neutral)."

    although to be fair it could be argued it was already implied - 531.3.1 demand that RCDs disconnect all live conductors (including N) with the exception of 531.3.1.201 that for protection against electric shock N need not be opened in TN and TT systems - which sort of makes sense as the system is safe from shock if the upstream device opens as well. But if you want discrimination (selectivity) then the consideration is not just for shock, so maybe the exception shouldn't apply. The new note is much clearer though.

       - Andy.

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  • 0
    Raymond Obrien said:
    Why is not stated that the use of Double pole in  IET on these conditions.

    It seems the powers that be are of the same mind - the DPC for AMD4 contains a new note to 536.4.1.4 ...

    "NOTE 4: To achieve selectivity between an upstream Type-S RCD and any downstream RCD with respect to both a
    line to earth and a neutral to earth fault, the downstream RCD shall switch all live conductors (including the
    neutral)."

    although to be fair it could be argued it was already implied - 531.3.1 demand that RCDs disconnect all live conductors (including N) with the exception of 531.3.1.201 that for protection against electric shock N need not be opened in TN and TT systems - which sort of makes sense as the system is safe from shock if the upstream device opens as well. But if you want discrimination (selectivity) then the consideration is not just for shock, so maybe the exception shouldn't apply. The new note is much clearer though.

       - Andy.

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