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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
    Going back to the HiFi equipment, does the customer have daisy chained multi gang trailing socket outlets with surge suppressors in them?


    One very large trailing socket with one surge suppressor would be less problematic than multiple daisy chained trailing sockets, though hardwired sockets could be better still.


    Andy Betteridge
  • 0
    Last year I arrived at the Elex show and being a man of a certain age decided to nip to the gents at the back of the hall before going anywhere else.


    As I walk across the hall that only had those involved with the show in it I met John Peckham and Tony Cable talking to a couple of IET engineers, John greeted me and I passed the time of day with them, I said to John you’re okay this year as I don’t have an axe to grind, so I won’t be on a mission when you’re doing the live forum.


    Assuming I make it to the show this this year it looks like the mission is to sort out if SCRDs have any purpose in life and seek some clarification as to their uses.


    I should add that at last year’s Elex show a manufacturer representative assured me they were fit for purpose as generally installed, but that was last year.


    Andy Betteridge
  • 0

    gkenyon:




    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?


     




    I was about to leave this alone for a bit, but a thought went through my mind.


    The protective measure we provide is ADS, Automatic Disconnection of Supply.


    We don’  provide automatic isolation, that is manual isn’t? 

     

  • 0
    Okay.  I went and had a shower and stood there thinking about this (I know that is not a pretty image, but remember Archimedes had his eureka moment soaking in the bath.).


    I can knock the idea that you cannot use a SCRD such as those made by BG as an isolator on the head.


    Here is one I have fetched in from my van lying on top of table J1 in the IET Onsite Guide giving guidance on the selection of protective, isolation and switching devices,  reproduced from BS7671.


    The BG SCRD connection unit is complies with both BS7288:1990 and BS1363-4:1995


    Although BS8277 does not approve it as an isolator BS1363-4 does.


    If push comes to shove you can take the fuse out of it.

    6e306ce1f17ab5a41dc714dd032afc5c-huge-20190811_155356.jpg


    Andy Betteridge 

  • 0
    Hmm. Now BS7671 is not a legal requirement, and there are plenty of situations were it is not applied, and installations can be perfectly safe.  The DNO side of the wiring comes to mind as the most obvious that most folk will have come accross, but as one who has worked in both university and commercial R and D establishments,  I can say that there are several situations where the RCD and for that matter the MCB/fusing requirements of the regs are simply inappropriate, and a properly engineered approach specific to the situation is needed. It is not sensible to demand that the regulations are modified to cover all possible cases.

    If this was an industrial setting with a difficult load, I'd be well on the way to  risk assessing the RCD out of circuit,  or fitting an earth fault relay set to a high level, and using  cables with earthed armour or braids, and perhaps  additional bonding to cases of class 1 equipment, to give an equivalent protection. By this I mean that more than 2 credible and independent faults must occur to reach a dangerous state. In such a case, the paperwork is not quite the BS7671, but incudes the design authority calculations and description documents and risk assessment, that shows how the credible faults are safely mitigated.


    This however is more the case of an enthusiastic amateur, and I agree this is trickier. First eliminate the possibility there is a real fault. Then confirm it is earth leakage that is causing the problem.  Could the conduit take another set of cables, and split the load over two circuits ? This adds a slight risk if there is ever a  very odd fault occurs between the two circuits, but should halve the stress on any one RCD.

    You may decide to remove the RCD (or change it for one that is not providing protection at <30mA) but this needs a savvy customer who understands what the increase in risk actually means, and if you belong to a part P notification scheme that insists you must follow BS7671, then you may not be able to.

    It is of course no more dangerous than any number of installations in current use, problems arise if there is a faulty load connected, and a person is connected between L and E, and the RCD should trip and it does not.


  • 0

    Sparkingchip:

    Okay.  I went and had a shower and stood there thinking about this (I know that is not a pretty image, but remember Archimedes had his eureka moment soaking in the bath.).


    I can knock the idea that you cannot use a SCRD such as those made by BG as an isolator on the head.


    Here is one I have fetched in from my van lying on top of table J1 in the IET Onsite Guide giving guidance on the selection of protective, isolation and switching devices,  reproduced from BS7671.


    The BG SCRD connection unit is complies with both BS7288:1990 and BS1363-4:1995


    Although BS8277 does not approve it as an isolator BS1363-4 does.


    If push comes to shove you can take the fuse out of it.

    6e306ce1f17ab5a41dc714dd032afc5c-huge-20190811_155356.jpg


    Andy Betteridge 

     




    I fear a little misunderstanding.


    The device providing automatic disconnection has to provide isolation.


    Hence, whilst the socket-outlet of an SRCD (or in non-TT systems, a fuse in an FCU RCD) may provide isolation ... but does the RCD?

     


    • If the RCD in the SRCD complies with BS 7288:1990, then PERHAPS YES

    • If the RCD in the SRCD complies with BS 7288:2016, the DEFINITELY NO (as the scope of the 2016 standard says so).



    Here is Note 4 to the Scope (Section 1) in BS 7288:2016:

    NOTE 4 For SRCDs intended to provide isolation or fault protection, or to be used in IT systems, BS EN 61008-1 or BS EN 61009-1 should be used, as applicable, in conjunction with the requirements of BS 1363-2 for socket-outlets.




    It's also worth noting that the 1990 version of BS 7288 had requirements for 3 mm isolation, the 2016 version of the standard does not.


    Hence, as I said, guidance will need to change, and BS 7671:2018 is correct to not include BS 7288 in the list of acceptable RCDs.


  • 0

    mapj1:

    Hmm. Now BS7671 is not a legal requirement, and there are plenty of situations were it is not applied, and installations can be perfectly safe.  The DNO side of the wiring comes to mind as the most obvious that most folk will have come accross, but as one who has worked in both university and commercial R and D establishments,  I can say that there are several situations where the RCD and for that matter the MCB/fusing requirements of the regs are simply inappropriate, and a properly engineered approach specific to the situation is needed. It is not sensible to demand that the regulations are modified to cover all possible cases.

    If this was an industrial setting with a difficult load, I'd be well on the way to  risk assessing the RCD out of circuit,  or fitting an earth fault relay set to a high level, and using  cables with earthed armour or braids, and perhaps  additional bonding to cases of class 1 equipment, to give an equivalent protection. By this I mean that more than 2 credible and independent faults must occur to reach a dangerous state. In such a case, the paperwork is not quite the BS7671, but incudes the design authority calculations and description documents and risk assessment, that shows how the credible faults are safely mitigated.


    This however is more the case of an enthusiastic amateur, and I agree this is trickier. First eliminate the possibility there is a real fault. Then confirm it is earth leakage that is causing the problem.  Could the conduit take another set of cables, and split the load over two circuits ? This adds a slight risk if there is ever a  very odd fault occurs between the two circuits, but should halve the stress on any one RCD.

    You may decide to remove the RCD (or change it for one that is not providing protection at <30mA) but this needs a savvy customer who understands what the increase in risk actually means, and if you belong to a part P notification scheme that insists you must follow BS7671, then you may not be able to.

    It is of course no more dangerous than any number of installations in current use, problems arise if there is a faulty load connected, and a person is connected between L and E, and the RCD should trip and it does not.


     




    It's not simply the case of BS 7671:2018 here.


    The product standard BS 7288 has changed in 2016, so the product we used to fit for the job is no longer suitable for that job. The product standard itself says so.

  • 0
    " 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?"


    It would be unwise to reduce the safety aspect provided by the use of  30mA RCD. So I wonder how long this HiFi set up been in use before the new supply circuit had been installed, whether there has been any changes to the electrical earthing and/or audio grounding. Changes in the way the audio grounding has been set up might have caused changes in resistance in the ground with the possibility of an active ground loop producing spurious voltages in the audio earthing system. This may be responsible for tripping the RCD.


    Legh
  • 0
    "

    The product standard BS 7288 has changed in 2016, so the product we used to fit for the job is no longer suitable for that job. The product standard itself says so."


    So further to that comment can you suggest what they can be used for?


    Andy B.
  • 0

    Sparkingchip:

    "

    The product standard BS 7288 has changed in 2016, so the product we used to fit for the job is no longer suitable for that job. The product standard itself says so."


    So further to that comment can you suggest what they can be used for?


    Andy B. 




    Good question ... to be honest I was very surprised when I read that in the 2016 standard.


    Based on the 2016 standard, I would suggest that a socket-outlet (or FCU) complying with both BS EN 61008 or BS EN 61009, plus the relevant part of BS 1363. This is stated specifically in Note 4 to the Scope of BS 7288:2016 - see quote in my earlier post.


    It seems that BS 7288 outlets are now intended to be an "over and above what is in BS 7671 (including BS 7671 requirements for RCDs)" type product.

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