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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
    The problem is that people have assumed BS 7288 SRCDs were suitable to comply with BS 7671, whereas BS 7671 has never stated they were ...


    Yet, it is clear from the SCOPE of BS 7288, that the


     

    I am sitting on my settee at home with a printed copy of the July/August 2019 Professional Electrician magazine that I picked up off the counter of the WED the electrical wholesaler I use.


    In the magazine is This article explaining how under floor electric heating can be installed.

    There is this quite clear and unambiguous statement by the NICEIC 





    Hang on, that advice is Industry Guidance, and should always have considered changes to the product standard.


    BS 7288:1990 did not have the same statements about limitations in terms of isolation, and requirements for the installation to already have Fault Protection and Additional Protection. Hence, before 2016, the advice was correct, but now, it's not the case.



    So we are in a position that some organisations' guidance needs to change - and BS 7671:2018 is wholly correct as things stand.
  • 0
    By the way, I've checked the following IET 18th Edition guidance:
    • On-Site Guide

    • Guidance Note 1 Selection & erection

    • Guidance Note 3 Inspection & testing

    • Guidance Note 6 Protection against electric shock


    None of these say that BS 7288 SRCDs can be used for fault protection or additional protection.


    They do, however, discuss testing and RCD tripping characteristics of such devices to prevent nuisance-tripping. (Although agree it's debatable whether testing of the devices is actually necessary if they aren't providing fault protection or additional protection is open to discussion, but the device may of course be a pre-2016 device, and installed to an earlier version of BS 7671, so such inclusion is, I believe, quite pertinent.)
  • 0
    SCRDs are not intended to provide isolation.


    What if you have a SCRD fused connection unit supplying a 20-amp double pole switch with a flex outlet which the appliance is actually connected to, does that fulfill requirements for isolation?


     Andy Betteridge
  • 0
    Does beg the question, though, what the actual purpose of a BS 7288 SRCD is now, following publication of BS 7288:2016 in which the functionality seems to be dumbed down in preference of protection already being in the electrical installation?


    The standard talks about "supplementary protection downstream of the RCD" - but if additional protection is already provided for the socket-outlet in accordance with Regulation 411.3.3, then ???
  • 0

    Sparkingchip:

    SCRDs are not intended to provide isolation.


    What if you have a SCRD fused connection unit supplying a 20-amp double pole switch with a flex outlet which the appliance is actually connected to, does that fulfill requirements for isolation?


     Andy Betteridge 




    Definitely not, it's the device providing ADS that has to be suitable for isolation... Regulation 531.1.1



    In any case, it's no good skirting around BS 7671:2018 for a way through the myre ... BS 7288:2016 clearly states in the scope that SRCDs to that standard must have fault protection and additional protection upstream of the SRCD.



    SRCDs are intended for use in circuits where the fault protection and additional protection are already assured upstream of the SRCD.



    So, if you were to install an SRCD to BS 7288:2016, you should ensure that it is protected by a suitable overcurrent protective device, and additional protection is provided in accordance with BS 7671, upstream of the SRCD or FCR RCD



    In fact, since BS 7288 requires additional protection is in place, does that mean even an FCU-RCD will always require an upstream 30 mA RCD, even though it may not come under 411.3.3 of BS 7671 ???

     

  • 0
    If you install a replacement consumer unit or 30 mA RCD upfront of an SRCD then you usually have to take the the the SRCD out as every time someone presses the button on it to test it or disconnect the load it trips the upfront RCD.


    Fitting a SRCD in a 30 mA RCD protected circuit is only sensible if you install an 10 mA SRCD device, but never actually having done that I cannot tell you if pressing the button on the 10 mA device would take the upstream 30 mA device out or not.


    I can foresee a few discussions about this at the Coventry Elex Show this year, there’s a few manufacturers to question and questions to be raised on the IET stand as well as at the Q an A sessions, is JP on the forum panel this year?


    Andy Betteridge
  • 0
    I agree that what I've said about BS 7288 itself isn't necessarily good news.


    But it seems that the product standard has effectively scoped itself out of its most common "use case" in existing installations in the UK?



    I think this is definitely a question for product manufacturers, rather than the Wiring Regs committee, at this stage. Basically, it would be nonsense for BS 7671 to include BS 7288 specifically, if BS 7288 states it's not intended for the purpose for which BS 7671 would permit it?


  • 0
    Okay.

    Here is the latest version of the Timeguard BS8277:2016 RCD connection unit

     

    RCD Fused Connection Unit for additional protection against electric shocks and fire risk. The unit will remain latched if the power supply is tripped.



    • Provides additional protection against the dangers of electrocution.

    • Eliminates the need for plug-in residual current circuit breakers.

    • Protects appliances from possible fire hazard.

    • Extra fast action – trips out within 40 milliseconds (BS7288).

    • Latching for continuous operation after supply interruption.

    • Disconnects both Live and Neutral connections leaving Earth connection intact.



    I am sure you can see what I am about to point out, that it states twice that the device provides additional protection.


    I think it is acceptable to say that anyone other than an electrical engineer that would be additional protection as referred to be BS7671:2018.

    Is it additional protection, but not as we know it?






  • 0

    ASMTECH:

    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?


     




    A compliant option - not cheap though, but since the hi-fi kit costs £££ and you've already got the SWA installed ...


    Install a local dedicated CU for the hi-fi equipment, close to it, with a dedicated RCBO for each piece of equipment, each supplying its own socket-outlet.


    In domestic, I don't really see an option, and I'm not sure a "disclaimer" from the customer would necessarily provide a "get out of jail card" if the worst happens.

  • 0

    Sparkingchip:

    Okay.

    Here is the latest version of the Timeguard BS8277:2016 RCD connection unit

     



    RCD Fused Connection Unit for additional protection against electric shocks and fire risk. The unit will remain latched if the power supply is tripped.



    • Provides additional protection against the dangers of electrocution.

    • Eliminates the need for plug-in residual current circuit breakers.

    • Protects appliances from possible fire hazard.

    • Extra fast action – trips out within 40 milliseconds (BS7288).

    • Latching for continuous operation after supply interruption.

    • Disconnects both Live and Neutral connections leaving Earth connection intact.



    I am sure you can see what I am about to point out, that it states twice that the device provides additional protection.


    I think it is acceptable to say that anyone other than an electrical engineer that would be additional protection as referred to be BS7671:2018.

    Is it additional protection, but not as we know it?

     


    Agreed that it's confusing information, but disconnection is not isolation, and still the Scope of BS 7288:2016 says what it says ...


    I guess a question to the product manufacturer regarding what they are saying?

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