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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

Parents
  • 0
    No the 2 sections are looking at different things.


    RCDs can be used to provide additional protection on all earthing systems.


    Also you may decide /need to rely on  an RCD as the interrupting part of an ADS system that has a CPC, even if it is TN-s or TNC-s - it may be that Zs is not low enough to blow a large fault fast enough for safety of life, a 40A C type MCB needs 500A to break for certain in the 'half a  heartbeat' required, and  on a 0.8 ohms TN-s (230V /0.8 ohms = 287A ) may come unstuck for example. The solution is an earth fault relay or an RCD at the origin of the circuit to be protected.


    What you may not do however is to decide to use  the RCD as the sole means of ADS and omit the CPC, you still need the earthing and bonding.

Reply
  • 0
    No the 2 sections are looking at different things.


    RCDs can be used to provide additional protection on all earthing systems.


    Also you may decide /need to rely on  an RCD as the interrupting part of an ADS system that has a CPC, even if it is TN-s or TNC-s - it may be that Zs is not low enough to blow a large fault fast enough for safety of life, a 40A C type MCB needs 500A to break for certain in the 'half a  heartbeat' required, and  on a 0.8 ohms TN-s (230V /0.8 ohms = 287A ) may come unstuck for example. The solution is an earth fault relay or an RCD at the origin of the circuit to be protected.


    What you may not do however is to decide to use  the RCD as the sole means of ADS and omit the CPC, you still need the earthing and bonding.

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