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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?


  • 0
    Previously the IET described SCRD devices as:

    "installed to provide additional protection for users of the socket-outlet when it may not be advantageous to protect the entire circuit with an RCD"  

    A BEAMA press release informs us Timeguard as now manufacturing SRCDs to the new edition of the British Standard but it's being to sounds like they might as well be making chocolate teapots.


    So the older devices  which are still on sale  may be better engineered than the latest versions.


    When I contacted the IET earlier this year to ask about SRCD and FCURCD devices I was told to phone BEAMA,  I'm being to see why now. 


    Andy Betteridge
  • 0
    Over and above a requirement for 30 mA RCD protection only makes sense if you are installing 10 mA RCD devices.
  • 0
    An interesting debate.


    I suggest that BS 7671:2018 has made BS 7288 obsolete for new circuits.


    BS 7288:2016 Scope:

    This British Standard applies to residual current-operated devices (RCD) incorporated in, or specifically intended for use with, single pole and neutral and single pole and switched neutral and double pole socket-outlets, with provision of earthing of the socket-outlet for household and similar uses (SRCD: socket-outlet residual current devices). SRCDs, according to this standard, are intended to be used in single phase systems such as phase to neutral. SRCDs are only intended to provide supplementary protection downstream of the SRCD. SRCDs are intended for use in circuits where the fault protection and additional protection are already assured upstream of the SRCD. (My emphasis)


    Wires in walls,sockets, and now lights all need RCD protection at the origin of the circuit, so why provide it at a socket?


    As for isolation, BS 7288:2016 introduction:

    The isolation function is not necessary since pulling out the plug from the socket-outlet is recognized as providing effective isolation. The absence of permanently connected long conductors downstream of the RCD, together with a limited number of powered appliances, justifies reduced EMC levels. Residual current devices covered by this standard are intended for additional protection in case of direct contact only. These particular features having been considered, it was recognized that a dedicated standard for socket-outlet residual current devices (SRCDs) was necessary.


    Then we have the reason for SRCDs (back to scope):

    SRCDs are only intended to provide supplementary protection downstream of the SRCD.


    Supplementary protection is not defined in either BS 7288 or BS7671, but may have been in previous editions.


    What is clear to me is that there is no requirement for a chain of RCDs.


    So why make them?


    It seems that MK no longer make RCD FCUs (Sentryspur).


    I suggest that the answer is (1) like for like replacement in older installations; (2) when a customer (or DIYer) wants RCD protection without going to the bother and expense of fitting RCD protection at the origin of the circuit; and more obviously, where the fitting of RCD protection in the CU is impossible.


    Oh, and by the way, there is no statutory requirement to work to the current regulations.


    Back to the OP: if this was a new circuit, it cannot be compliant without a 61008/9 device at the origin of the circuit.
  • 0
    In amongst all of this the SRCD and FCURCD are UK only products and the British Standard is a UK only standard without a EN equivalent and the equipment is not CE marked.


    Is this the sort of mess everything is going to get into if the UK tries to go it alone on writing product standards?


    Andy B
  • 0
    “Supplementary protection is not defined in either BS 7288 or BS7671, but may have been in previous editions.”


    Wasn’t just that the name was changed?


     Andy B.

  • 0
    Double pole RCDFCU devices are readily available.

    http://www.greenbrook.co.uk/eshop/files/files/H92WPAPN-%26-H92MPAPN-Technical-Product-sheet(1).pdf


    Doesn’t that resolve any potential issues in TT installations?


    The simple truth seems to be that solidly engineered units have been redesigned removing the features that made them usable.


    Andy B.
  • 0

    ASMTECH:


     

    The customer did at one point say that an RCD could 'colour the sound' of the hi-fi. That is clearly subjective but who am I to argue?



     

    Many HiFi 'buffs' read Russ Andrews' Connected magazines, who may have been dissuaded from using an RCD for the reason you mentioned above.  Read page 12 of this archived issue:

    https://issuu.com/russandrewsaccessories/docs/connected39issuu


    With the 18th edition which mandates use of an RCD, Russ Andrews' now suggest complying with this and they supply a specially treated Hager consumer unit for a dedicated HiFi ring.  They are not cheap though but then again, it sounds like your customer has plenty of money to spend on their set up:

    https://www.russandrews.com/upgraded-rcd-consumer-unit-metal-case/

    https://www.russandrews.com/images/pdf/MainsWiringGuide191218.pdf
  • 0
    Russ Andrews:

    Hand picked, hand tested components. Our Hi-Fi consumer unit is suitable for a single Hi-Fi mains circuit, the individual components and breakers have been selected by Russ as being the very best performing ones he tested.

    We're offering it here, fitted in a compliant metal case with the correct RCD and MCB circuit breaker for a single Hi-Fi or Home Cinema circuit The contacts are fully treated with DeoxIT contact enhancer for better performance.


    So what percentage are selected, and what happens to the rejected ones?


    The terminals may be treated, but I am at a loss to see how the contacts (in the RCD and MCB) could be.


    Snake oil!
  • 0
    Consumer unit suitable for one Hi-Fi or Home Cinema ring or spur


    Will a ring introduce more interference that can be detected by a HiFi purist than a radial?


    Given the apparent high leakage current is a ring a better option? Assuming distribution circuit being SWA with a core and the armour as a combined CPC already has earthing bigger than the live and neutral, which also screens it.


    Andy B.
  • Former Community Member
    0 Former Community Member

    Sparkingchip:
    Consumer unit suitable for one Hi-Fi or Home Cinema ring or spur


    Will a ring introduce more interference that can be detected by a HiFi purist than a radial?


    Given the apparent high leakage current is a ring a better option? Assuming distribution circuit being SWA with a core and the armour as a combined CPC already has earthing bigger than the live and neutral, which also screens it.


    Andy B.




     

    A ring will often act like a bloody big radiating antennae  - and SWA offers almost zero EMC screening at frequencies above mains frequency


    Regards


    OMS

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