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RCDs for both Fault & Additional Protection

PG
BS7671 states for Additional Protection [415.1.2] - "The use of RCDs is not recognised as a sole means of protection and does not obviate the need to apply one of the protective measures specified in Sections 411 to 414." - These methods being methods of protection against shock - ADS, Double Insulation, Separation & SELV/PELV.


531.3.6 states " - - - - - - -RCD - - --  - - -may provide fault protection and additional protection simultaneously."


This appears to be inconsistent. Is 531.3.6 meant to apply to only one type of neutral  earthing system?


Regards

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  • 0
    There are fault types that are not L-E,  such as insulation failure between phase-neutral or phase phase.

    The key difference with the L-E fault is that they have potential to expose a dangerous voltage to touch, and the ADS operates.


    For this reason there are strict limits on either the current or the operation time of the ADS for this.

    For the other faults we may allow a longer time - almost indefinite, so long as nothing melts or burns down the building,

    though 5 seconds is a good starting point for any near short condition.

    Be aware that the phrase ADS often refers only to the L-E case.  Obviously the MCB or fuse will do either, an RCD will not detect L-L or L-N
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  • 0
    There are fault types that are not L-E,  such as insulation failure between phase-neutral or phase phase.

    The key difference with the L-E fault is that they have potential to expose a dangerous voltage to touch, and the ADS operates.


    For this reason there are strict limits on either the current or the operation time of the ADS for this.

    For the other faults we may allow a longer time - almost indefinite, so long as nothing melts or burns down the building,

    though 5 seconds is a good starting point for any near short condition.

    Be aware that the phrase ADS often refers only to the L-E case.  Obviously the MCB or fuse will do either, an RCD will not detect L-L or L-N
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