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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

    Timeserved:



    On my last Inspection which was March 2019 my assessor said it was perfectly fine. I'm with the NIC EIC if it helps. 


    Regards TS


     




    Not sure what the rationale is, to be honest. BS 7671 clearly lists the standards for RCDs recognised for providing Additional Protection. If you haven't used an RCD complying with one of those standards, that's a departure (but may be no less safe, therefore OK, but still need to state the departure).


    Of course, whatever the situation, additional protection by 30 mA RCD may be required for other reasons, e.g. cables concealed in walls < 50 mm from the surface.

  • 0
    I am still puzzled by the requirement for one socket.


    One socket, one circuit, one CU. All a bit odd, but HiFi buffs have some funny ideas. What really amuses me is that by the time one can afford to spend £tens of thousands, most of us cannot hear the high frequencies which have cost so much. ?
  • 0
    No one yet has said if it really  is earth leakage is it not likely to be just the collective high inrush currents of all those PSUs causing a trip and if it is then wouldn't switching to a type D RCBO be the answer. Also if the customer is listening to let's say vinal then why would he need the CD player or Tuner switched on? That really makes no sense. I know of some hi fi buffs who by oxygen free gold plated leads for everything to try to improve the sound waste of money if you ask me. Oh watch out grumpy Kellys out again!
  • Former Community Member
    0 Former Community Member

    Chris Pearson:

    I am still puzzled by the requirement for one socket.


    One socket, one circuit, one CU. All a bit odd, but HiFi buffs have some funny ideas. What really amuses me is that by the time one can afford to spend £tens of thousands, most of us cannot hear the high frequencies which have cost so much. ?




    Most certainly! Although there are advantages to power conditioning where amplifiers are concerned, the perceivable difference by human ears approaches zero when we are talking about this particular installation. Now, an independent ground connection, isolation transformer with voltage stabilisation/regulation and a system of capacitors to smooth the waveform MIGHT make a difference but unlikely to hear it.


    It's what makes people happy I suppose. You could buy a Ferrari which does 0-60 in 2 seconds and a top speed of 200mph but you still have to drive it on the terrible British road network and get stuck behind people like me driving a van. Pointless, but makes people happy.



    Kelly Marie:

    No one yet has said if it really  is earth leakage is it not likely to be just the collective high inrush currents of all those PSUs causing a trip and if it is then wouldn't switching to a type D RCBO be the answer. Also if the customer is listening to let's say vinal then why would he need the CD player or Tuner switched on? That really makes no sense. I know of some hi fi buffs who by oxygen free gold plated leads for everything to try to improve the sound waste of money if you ask me. Oh watch out grumpy Kellys out again!


    Customer reports it's the RCD (61008) tripping, not the MCB but I haven't been to verify it myself and you could well be right and the answer would be replace the MCB with a B20 or C16 maybe.


    I agree that the trip is linked to inrush current but if it is tripping the RCD it could be the overload which is being leaked to earth. For example, if the unit has a startup current of 6-10x usual use and typically leaks ~1ma to earth then we are talking ~6-10ma earth leakage at startup. So 3-5 power supplies of similar construction would leak ~30ma to earth on startup causing a trip.


    Of course you are right, the simple solution is to only turn on what you need. Or just leave everything on all the time. Which is probably what I will tell the customer on Monday (although worth testing the equipment to check it's within specification).
  • 0
    An article dated 17th June 2019 RCD protection is it fit for purpose?


    It is really isn’t good that the manufacturers did not know how BS7671 had been written up until they had a phone call to tell them. Similarly it really isn’t good that they are apparently promoting and selling equipment that is not listed in the regulations.


    It really appears that the left hand does not know what the right hand is doing and some people really aren’t on the ball.


    Andy Betteridge
  • Former Community Member
    0 Former Community Member

    gkenyon:




    Timeserved:



    On my last Inspection which was March 2019 my assessor said it was perfectly fine. I'm with the NIC EIC if it helps. 


    Regards TS


     




    Not sure what the rationale is, to be honest. BS 7671 clearly lists the standards for RCDs recognised for providing Additional Protection. If you haven't used an RCD complying with one of those standards, that's a departure (but may be no less safe, therefore OK, but still need to state the departure).


    Of course, whatever the situation, additional protection by 30 mA RCD may be required for other reasons, e.g. cables concealed in walls < 50 mm from the surface.


     




    Hi Graham,


    Sorry forgot to mention that the cables were surface ran in galv conduit, so in this instance the additional protection was only for the user and not required for the fixed electrical installation. 

    Regards TS

  • 0

    Hi Graham,


    Sorry forgot to mention that the cables were surface ran in galv conduit, so in this instance the additional protection was only for the user and not required for the fixed electrical installation. 

    Regards TS





    Sadly, still the case that BS 7671:2018 doesn't recognise SRCDs for Additional Protection.


    The "additional protection" mentioned in BS 7671 is for the socket-outlet (411.3.3 says that, and doesn't mention the "user").


    Therefore, I'm still unsure as to the rationale for the assessor stating it was OK ... having said that, the assessor isn't signing the cert ...


    So, yes, departure in this case, and probably "equivalent safety", and I guess that is where the assessor is coming from (so long as the departure is documented).
  • 0

    Therefore, I'm still unsure as to the rationale for the assessor stating it was OK



    Possibly the assessor wasn't aware of the rather subtle change in the 18th, that wasn't at all advertised and might not have been explicitly covered in an 18th update course?

       - Andy.
  • 0
    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.


    Perhaps it's worth considering the following
    • RCDs to other standards than those listed in BS 7671:2018 were never explicitly referenced by BS 7671 prior to the 18th Edition.

    • Further, SRCDs and FCURCDs were never included in Table 53.4 in BS 7671:2008+A3:2015.



    In fact, in terms of BS 7671:2018 compliance, it's a little more subtle than Regulations 531.3.4.1 and 531.3.6. For example, neither BS 7288 nor BS 7071 are included in Table 53.4, so the installer must ensure that such devices can provide isolation in accordance with Regulation 531.1.1.


    I quote from BS 7288:2016 (first part of last para in section 0)

    The residual current device at socket-outlet level is normally intended to be installed by skilled or instructed persons. It can be operated several times per day. The isolation function is not necessary since pulling out the plug from the socket-outlet is recognized as providing effective isolation.



    And in the Scope:


    SRCDs are neither intended to provide an isolation function nor intended to be used in IT systems.



    and also in the Scope


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






    Therefore, it is clear that SRCDs to BS 7288 are not suitable for either fault protection or additional protection in accordance with BS 7671:2018.
  • 0
    Okay, we are seriously in a situation where the left hand doesn’t know what the right hand is doing and it really is time that the IET should have issued clarification on the intentions of the Wiring Regulations committee regarding the use of BS8277 RCD protected socket outlets and connection units.


    THERE IS ABSOLUTELY NO DENYING that the NICEIC officially promotes the use of BS8277 RCD connection units as an acceptable means of providing RCD protection.


    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 


    Typically, in older installations fuses to BS 3036 or circuit breakers to BS 3871 maybe found. Where this is the case and it is not practical to include RCD protection at the consumer unit a 13 A RCD spur maybe installed locally from an existing circuit as shown in Fig 2. 


    Someone at the IET needs to get their act together and resolve the potential issues of using BS8277 equipment.


    Andy Betteridge 

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