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Rcd discrimination

NGUACE
Any Information greatly appreciated.


Each outdoor socket has to be RCD protected, so if you have a junction box supplying more than one outdoor 30ma socket, how do you achieve RCD discrimination all the way back. I have an older C/U and the rcd on it is also 30ma?


A spur from internal plug to fused switch outside, which then feeds multiple outdoor sockets?


this is just a scenario I am trying to figure out.


thanks for anyone’s time.


Andy
  • Former Community Member
    0 Former Community Member
    It's not possible to achieve discrimination (or selectivity) in that set up


    It would be achievable with a 100mA S type on the front end, plus the 30mA devices acting on both poles - and you may then need further 30mA devices to protect existing circuits


    Is it really a problem  - my house has a single front end 30mA device - I have no further RCD's anywhere - a single fault knocks us out - I can live with it


    Regards


    OMS
  • 0
    Hi,


    thank you for the response.


    It’s not so much a problem but standards dictate the out side plugs are to be rcd protected, I am trying to figure out how you achieve this with multiple sockets?


    thanks again


    Andy
  • 0
    you cannot say with any confidence which RCD will operate, or indeed if both will - they may have both started moving before the contacts in either one remove the power.

    However, what does happen is that the chance of a dangerous no-trip due to an RCD failure and then a fault (about 5-10% chance according to some historical data) is now reduced to the chance of 2 independent failures, so  more like 1%, assuming as part of the reset process you then test both after clearing the fault, and replace any that do not trip.

    As per OMS, if you need discrimination, then you need to delay and to increase the threshold - typically by about a factor of 3 to be reliable.

    so for example, 30mA instant on a 32A final circuit,  fed by 100a supply and 100mA 1/10 second, fed by 300A genset at 300ma 300ms etc. by the time you get to a few seconds you are looking at seriously expensive earth fault relays 

  • 0
    NGUACE:

    It’s not so much a problem but standards dictate the out side plugs are to be rcd protected, I am trying to figure out how you achieve this with multiple sockets?


    I think that there may be a misunderstanding here. You can protect a whole circuit or even installation with one RCD. You do not need to have sockets with built-in RCDs: in fact arguably that is no longer an acceptable way of achieving RCD protection.


  • 0
    Hi Chris,


    My understanding was installing an outside socket required RCD protection?


    installing an outside socket without rcd would then trip the C/U if there was any issue?


    probably my misunderstanding of the regulations?


    thank you for your response.
  • 0
    If you have an RCD in the supply to the socket, either at the socket or back at the dis board, then the socket meets the regs, as it has RCD protection. The regs do not care about where.
  • 0
    Hi Mike,


    I guess that makes perfect sense.


    Thank you for your time and assistance, very much appreciated.


    stay safe.


    Andy
  • 0
    mapj1:

    If you have an RCD in the supply to the socket, either at the socket or back at the dis board, then the socket meets the regs, as it has RCD protection. The regs do not care about where.


    They do, however, care about the standard that the RCD is manufactured to. The RCDs in the socket-outlets, if none are provided upstream, should comply with the standards stated in either Regulations 531.3.4.1 or 531.3.4.2 for fault protection, or Regulation 531.3.6 for additional protection.


    BS 7288 is currently not accepted in BS 7671 for RCDs - but if the installation design was conducted, and any additional sock-outlets installed, under BS 7671:2008+A3:2015 there is no such constraint.


  • 0
    Like OMS says. I am also a "Frontender" there`s a few of us here and quite a lot in the UK methinks. It was quite "normal" a while back and for quite a long time to have one mainswitch RCD feeding all MCBs for an installation. In an otherwise decent set up nuisance trips do happen but not regularly. The main thing about RCDs is "are they working correctly?" regular testing to check and regular tripping to keep the mechanism mechanically able to act are important to me. Some statistics suggest 7% failure rate (Note - failure includes those that trip but outside the stated standard), math theory would suggest therefore 0.49% failure might be acheived by two cascading RCDs. I`m sure that this figure would seldom be case though. Probably somewhere in between. Some of us come from an age before RCDs and relied soley on a rewireable fuse . I do not remember the council having to clear away piles of corpses each evening.
  • 0
    Thank you for your response.


    Feel old now, I remember wire fuses, lol. 


    Andy


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