The IET is carrying out some important updates between 17-30 April and all of our websites will be view only. For more information, read this Announcement

Rules and Guidelines
Want to participate in the discussions? Please read our Community Rules and Guidelines. Need some help? Visit Support and Answers.

  • State Not Answered
  • Locked Locked
  • Replies 91 replies
  • Subscribers 31 subscribers
  • Views 10478 views
  • Users 0 members are here
Options
You may also be interested in
This discussion is locked.
You cannot post a reply to this discussion. If you have a question start a new discussion

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
    Interesting situation. Can't use the HiFi system with the RCD in situ due to high start up currents causing high earth leakage. What sort of HiFi system requires 10 power supplies one wonders? unless its a test bench for assembling and testing disco type players with large PA ampifiers ie some form of commercial set up.

    I might start by running each power supply separately, with an oppropriate load, to see how much leakage current flowed. then? ..............

    Now " The customer has now decided he doesn't want RCD anyway for 'reasons' " must be a qualified electrical engineer!


    Legh
  • 0
    I don't see how you can omit the 30mA RCD protection and claim either BS 7671 compliance or 'no less degree of safety' - without the RCD it lacks additional protection even if ADS is fine. A 100mA RCD isn't likely to be much better on that score either (no point in an 100mA RCD if 50mA can kill).


    Some 30mA RCDs can be better than others at handing inrush currents. A couple of different effects can cause some to trip - a high initial protective conductor current obviously, but some can also trip just due to a high L-N current - if the inrush current is well beyond the RCD's current-carrying rating (e.g. 40A or 80A, not 30mA) then the L & N coils can saturate the torroid and combined with manufacturing tolerances can mean the L and N coils don't quite balance as they should and the sense coil sees an imbalance larger than it really is.


    So things to try might include a 30mA RCD with a higher current carrying rating, (or just a different make/model), or a type with better transient resistance (e.g. Dopeke suggest that AKV, B, F and EV types do better than traditional AC and A types) - more expensive of course, but that's perhaps the market you're in?


    Alternatively, design out the problem by using a sequential start unit between the socket and various items of equipment (or just let the user switch things on one at a time).


      - Andy.
  • 0
    Maybe the RCD is serving its intended purpose.


    A debatable solution is to remove the upfront RCD and replace it with as many BS8277 30mA RCD protected socket outlets as are required to spread the leakage or identify the actual problem.


    The customer won’t like that, so the other solution is to walk away if you cannot identify a specific problem with the connected equipment.


     Andy Betteridge
  • 0
    Agreed, I would fit a number of socket outlets each of which incorporates its own RCD. No need for RCD protection to the circuit if in SWA, provided that each socket includes an RCD.

    With the starting inrush and earth leakage spread over several RCDs tripping seems unlikely unless there is a significant and potentially dangerous fault on the hi fi equipment.
  • Former Community Member
    0 Former Community Member

    Legh Richardson:


    What sort of HiFi system requires 10 power supplies one wonders? 




    Pre-amp, power-amp, record player, cd player, tuner, both electrostatic speakers... I've guessed as far as 7 but yeah, no idea beyond that. Hi-fi is an industry full of magic and snake oil and you'll be surprised what audiophiles spend their cash on! EDIT: Some of it could be quite old too - 'vintage mojo' I believe it's called!




    "The customer has now decided he doesn't want RCD anyway for 'reasons' " must be a qualified electrical engineer!


    Legh




    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?

     



    AJJewsbury:





    I don't see how you can omit the 30mA RCD protection and claim either BS 7671 compliance or 'no less degree of safety' - without the RCD it lacks additional protection even if ADS is fine. A 100mA RCD isn't likely to be much better on that score either (no point in an 100mA RCD if 50mA can kill).






    I agree. It would be deviation from the regulations once a 30ma RCD is omitted. I suppose it's shades of compliance...

     




    If the inrush current is well beyond the RCD's current-carrying rating (e.g. 40A or 80A, not 30mA) then the L & N coils can saturate the torroid and combined with manufacturing tolerances can mean the L and N coils don't quite balance as they should and the sense coil sees an imbalance larger than it really is.





    That's really interesting, I'll look into that for future reference. However, this circuit has a 63A 61008 and the overload protective device is a B16 60898. It's difficult to imagine this scenario. There will also be a 13A BS1362 in the multi strip he has plugged all the equipment into (hopefully!).

    (or just let the user switch things on one at a time).


      - Andy.



     





    This is my preferred option.
     



    Maybe the RCD is serving its intended purpose.


    A debatable solution is to remove the upfront RCD and replace it with as many BS8277 30mA RCD protected socket outlets as are required to spread the leakage or identify the actual problem.


    The customer won’t like that, so the other solution is to walk away if you cannot identify a specific problem with the connected equipment.


     Andy Betteridge 




    The customer definitely won't like that. We had many discussions on aesthetics during installation and a single, un-switched socket outlet was the only agreeable option.


    I think I need to look at this hi-fi system for myself and work out why >15ma of current is leaking to earth.


    I wonder if 411.3.3 is unnecessarily strict with regards to dwellings. I liked the 'specially labelled socket for a specific piece of equipment' in the 17th AM3 because it makes sense in scenarios like this, particularly for those who work from home with a home workshop or multiple pieces of IT equipment. Oh well, I don't make the rules! 


    Thanks everyone.
  • Former Community Member
    0 Former Community Member

    broadgage:


    unless there is a significant and potentially dangerous fault on the hi fi equipment.




    That's definitely a concern which I will investigate first. He won't be happy if one of his £3000 separates fails a PAT test though! ?

  • 0

    I liked the 'specially labelled socket for a specific piece of equipment' in the 17th AM3 because it makes sense in scenarios like this, particularly for those who work from home with a home workshop or multiple pieces of IT equipment.



    But if the cluster of IT equipment was in danger of tripping a 30mA RCD in normal service then the leakage would have to be 15mA or above and then you're well into high protective conductor current territory and a BS 1363 socket is then verbotten.

     

    A debatable solution is to remove the upfront RCD and replace it with as many BS8277 30mA RCD protected socket outlets as are required to spread the leakage or identify the actual problem.



    Indeed debatable - as the 18th lists RCD types that are acceptable for additional protection - and BS 8277 isn't one of them. (Although there is a theory that that's just an oversight - I presume no-one's had any reassurance from the IET on that score yet?)


      - Andy.

  • Former Community Member
    0 Former Community Member

    AJJewsbury:




    I liked the 'specially labelled socket for a specific piece of equipment' in the 17th AM3 because it makes sense in scenarios like this, particularly for those who work from home with a home workshop or multiple pieces of IT equipment.



    But if the cluster of IT equipment was in danger of tripping a 30mA RCD in normal service then the leakage would have to be 15mA or above and then you're well into high protective conductor current territory and a BS 1363 socket is then verbotten.

     

    A debatable solution is to remove the upfront RCD and replace it with as many BS8277 30mA RCD protected socket outlets as are required to spread the leakage or identify the actual problem.



    Indeed debatable - as the 18th lists RCD types that are acceptable for additional protection - and BS 8277 isn't one of them. (Although there is a theory that that's just an oversight - I presume no-one's had any reassurance from the IET on that score yet?)


      - Andy.


    Good point on the high protective conductor currents - I'm sure a 60309 is out of the question in a living room though! Haha. ?


    I'm going to offer a test on his equipment to check how much earth leakage each device is producing. >15ma is excessive for properly operating domestic electronics, never mind being in the realm of high protective conductor currents.


    I agree, the lack of BS8277 is an oversight.

  • 0
    Each piece of Hi-Fi equipment could plug into its own socket-outlet, each outlet could be protected by 30 mA RCD individually (although slight deviation from BS 7671:2018 as currently SRCDs are not "recognised" by the 18th Ed for additional protection) - and the requirements for high protective conductor currents applied for the final circuit.


    The only other option (but high protective conductor currents still apply) would be to fit BS EN 60320 couplers onto the hi-fi equipment, and provide a corresponding PDU connected to a connection unit (e.g. SFCU). Since BS EN 60320 outlets are classed as "cable couplers", no RCD is required.


    The only other issue with RCDs in dwellings, though, is the "at a depth of less than 50 mm" requirement which also often invokes 30 mA RCD for cabling in walls - but of course there are ways around that too.


  • Former Community Member
    0 Former Community Member

    AJJewsbury:




    I liked the 'specially labelled socket for a specific piece of equipment' in the 17th AM3 because it makes sense in scenarios like this, particularly for those who work from home with a home workshop or multiple pieces of IT equipment.



    But if the cluster of IT equipment was in danger of tripping a 30mA RCD in normal service then the leakage would have to be 15mA or above and then you're well into high protective conductor current territory and a BS 1363 socket is then verbotten.

     

    A debatable solution is to remove the upfront RCD and replace it with as many BS8277 30mA RCD protected socket outlets as are required to spread the leakage or identify the actual problem.



    Indeed debatable - as the 18th lists RCD types that are acceptable for additional protection - and BS 8277 isn't one of them. (Although there is a theory that that's just an oversight - I presume no-one's had any reassurance from the IET on that score yet?)


      - Andy.

     

     




    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

Join us to get the best from IET EngX.

Joining EngX lets you personalise your experience so you stay up to date on the topics that interest you, plus you’ll be able to make connections who are looking to collaborate, exchange ideas and more.

Join us Not now

Community Rules & Guidelines