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Omitting 30ma RCD Protection for single S/O in a domestic property

Former Community Member
Former Community Member
I installed a dedicated circuit for a hifi system for a customer last year. The customer requested a 6mm2 radial from a 16A MCB housed in its own independent consumer unit into a single, un-switched socket outlet. No problem, bit unusual but no worries.I wired it using a 3c 6mm2 armoured cable as I half anticipated the forthcoming...


The hifi equipment is causing the rcd to trip when started up. I haven't been over to have a look but I am assuming that the startup current for the many power supplies (he has told me there are ten!) coupled with electronic earth leakage is causing a CPC current that is sufficient to trip the RCD (perhaps only 16ma but enough). The earthing is high integrity having a 6mm2 cpc + armour and the Zs is sufficiently low enough that the 16A MCB can be used for fault protection. So, if this wasn't domestic I'd ditch the RCD (or replace with a 100ma) assuming that my assumptions to this point are correct.


The customer has now decided he doesn't want RCD anyway for 'reasons' but I'm still wary of removing it in a domestic situation, not because I believe the installation would become less-safe but just because it contravenes regulations.


Assuming there's no fault on the equipment and it is just a case of startup/inrush current and earth leakage, what approach would you take? Remove the RCD and write it up as a deviation from 7671 with a signed disclaimer/waiver from the customer? Install a 100ma RCD? Do nothing and walk away? Something else?


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    AJJewsbury:




    Hang on Andy,


    The change was advertised quite clearly, on Page 5 of BS 7671, which says that Chapter 53 has been completely revised.

    Regulations 531.3.4.1 and 531.3.6 are definitely included in that.



    But no-one went around saying "Big change in the 18th - RCD sockets/FCUs no longer permitted" - so I still reckon it was easy to overlook that particular implication (unlike SPDs or AFDDs).




    Why would a change be advertised? BS 7671 never made reference to BS 7288 at all, in the 17th Edition Amendment 3 (2015) or earlier, so it's not been removed.


    Further, the change has (apprently) been made to the product standard in 2016 (between 17th AMD3 and 18th), removing the requirement for the RCD in the product to provide 3 mm isolation, and a resulting number of changes to the Scope - i.e. a MAJOR change to the use-case. Where was that advertised to the industry?

     


    the installer must ensure that such devices can provide isolation in accordance with Regulation 531.1.1






    I've been wondering about what exactly was meant by "Devices for protection against electric shock by automatic disconnection of supply shall be suitable for isolation..."


    Do they mean that if ADS is activated that the circuit must be automatically and immediately isolated (so 3mm+ contact gaps, also disconnecting N in TT systems etc) or just that the same device used for ADS can also be used for manual isolation.


    For instance, with fuses I can easily ensure a 3mm gap by pulling the fuse out, but I'm not sure I can if the fuse blows of its own accord (thinking of rewireable fuses, while the fusewire often vapourises entirely, sometimes you just end up with a small break).


    And (depending on what we decide 'low resistance' of 461.2 means) is this the end of SP switching RCBOs (and possibly single pole MCBs too).


    If it's just manual isolation is needed, then things get a lot simpler.


      - Andy.



    Well, I absolutely agree these are good discussion points and could do with some further clarification.

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  • 0

    AJJewsbury:




    Hang on Andy,


    The change was advertised quite clearly, on Page 5 of BS 7671, which says that Chapter 53 has been completely revised.

    Regulations 531.3.4.1 and 531.3.6 are definitely included in that.



    But no-one went around saying "Big change in the 18th - RCD sockets/FCUs no longer permitted" - so I still reckon it was easy to overlook that particular implication (unlike SPDs or AFDDs).




    Why would a change be advertised? BS 7671 never made reference to BS 7288 at all, in the 17th Edition Amendment 3 (2015) or earlier, so it's not been removed.


    Further, the change has (apprently) been made to the product standard in 2016 (between 17th AMD3 and 18th), removing the requirement for the RCD in the product to provide 3 mm isolation, and a resulting number of changes to the Scope - i.e. a MAJOR change to the use-case. Where was that advertised to the industry?

     


    the installer must ensure that such devices can provide isolation in accordance with Regulation 531.1.1






    I've been wondering about what exactly was meant by "Devices for protection against electric shock by automatic disconnection of supply shall be suitable for isolation..."


    Do they mean that if ADS is activated that the circuit must be automatically and immediately isolated (so 3mm+ contact gaps, also disconnecting N in TT systems etc) or just that the same device used for ADS can also be used for manual isolation.


    For instance, with fuses I can easily ensure a 3mm gap by pulling the fuse out, but I'm not sure I can if the fuse blows of its own accord (thinking of rewireable fuses, while the fusewire often vapourises entirely, sometimes you just end up with a small break).


    And (depending on what we decide 'low resistance' of 461.2 means) is this the end of SP switching RCBOs (and possibly single pole MCBs too).


    If it's just manual isolation is needed, then things get a lot simpler.


      - Andy.



    Well, I absolutely agree these are good discussion points and could do with some further clarification.

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