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100mA RCDs TT system

Nikkid
Afternoon all,

What is the stance on using 100mA RCDs for TT systems now?

I was under the impression that they are no longer required as long as all circuits were protected by a 30mA RCD within the consumer unit (which in this case they will be).

Just looking for some clarity. 


Thanks! 

  • 0
    As our poster knows, I am talking about an all steel box, but let's not let the facts get in the way of a good topic.

    Regards, UKPN
  • 0
    Sparkingchip:

    Why did councils specify 100 mA main switches in consumer units with TNCS earthing back in the late 1970’s, rather than a 30 mA?


    Not a trick question, I do not know why. 


    I have not come across that one Andy, could it be a regional thing


  • 0
    UKPN:

    As our poster knows, I am talking about an all steel box, but let's not let the facts get in the way of a good topic.

    Regards, UKPN


    But you still haven't told us which law it is.


  • 0
    Ian Hildrew:

    Nowadays to reduce the susceptibility to nuisance tripping, good electricians fit every circuit with its own RCBO. Not the cheapest option but it saves call outs from customers. In these cases there is no need for another upstream RCD unless the setup was with the S curve 100mA RCD situated remotely from the CU.


    Yes but the RCBOs would need to be double pole to avoid the S type coming out on a neutral earth fault after the final circuit RCBO had tripped.


  • 0
    ebee:
    Sparkingchip:

    Why did councils specify 100 mA main switches in consumer units with TNCS earthing back in the late 1970’s, rather than a 30 mA?


    Not a trick question, I do not know why. 


    I have not come across that one Andy, could it be a regional thing






    I have found them all over the country, one sticks in my mind, I drove from Worcester to Morecambe to do a job knowing that the survey photos showed a RCD main switch and when I got there it was a 100 mA.


    Mind you quite often I get sent photos of the CU with the cover closed, so as to retain life’s little mysteries.


  • 0
    UKPN:

    As our poster knows, I am talking about an all steel box, but let's not let the facts get in the way of a good topic.

    Regards, UKPN


    The point I'm trying to make is that a steel box does not necessarily mean that something is entirely class I - in the case of steel consumer units some manufacturers provide 'insulation kits' that provide double/reinforced insulation to the incoming tails - thus removing the need for an external upstream RCD where there is an RCD incomer or the internal construction is double double/reinforced insulation up to the RCBOs etc.


    Personally I'm not entirely convinced by some of those kits, and so would prefer an upstream RCD in its own insulating enclosure - but I can only claim that as a personal preference, not a legal requirement.


        - Andy.


  • 0
    I am afraid none of these "kits" claim to change the class 1 enclosure [as it is manufactured and stated in the data sheet] into a class 2 enclosure or a particular part of the enclosure. These kits do not also claim to provide "double or re-inforced insulation". They are provided as a a glorified grommet.
  • 0
    a glorified grommet

    Pretty much, yes. That's what prevents contact between the tails and steel enclosure at the point of entry. Leaving the outer sheath on the tails inside the enclosure is deemed to provide the equivalent of double/reinforced insulation inside (as per 412.2.4.1) - then some manufacturers even add a clamp to ensure that the bare ends of the conductors can't pop out of the incomer's terminals. So hopefully meeting the requirements of 531.3.5.3.2.201.


    You're quite right that manufacturer's don't directly claim Class II compliance - usually using words like "similar to Class II". I gather part of the issue is that true Class II equipment never connects exposed metalwork to a c.p.c. (and any PE conductors have to be insulated as if they were live parts) - that way Class II equipment is immune to importing shock hazards from elsewhere in the system and so can be specified where that would be a problem - e.g. outside of an equipotential zone or some special locations (BS 7671 doesn't really define the requirement for Class II equipment, but 412.2.2.4 hints at that). Since the CU case still has to be Earthed to provide ADS for faults on the outgoing wiring (and possibly internal wiring), we can't claim Class II for the enclosure itself - but that's not what is required - we just need to provide protection according to section 412 for the incoming tails.


    (As I mentioned before, personally I don't like that approach. The consequences of a L-PE fault before the first RCD is particularly nasty - imposing a hazardous voltage on the entire installation for a long duration (almost certainly until someone suffers a shock), so I think exceptional care is needed for this particular case. Also if we've now decided that CUs have to remain safe even if they catch fire internally, for the safety (from shock) of the entire installation to rely on plastics that'll melt at the first sign of a fire doesn't seem entirely consistent to me. But what I think is logical and what BS 7671 (or the law) requires/permits aren't the same thing.)


       - Andy.

  • 0
    AJJewsbury:


    (As I mentioned before, personally I don't like that approach. The consequences of a L-PE fault before the first RCD is particularly nasty - imposing a hazardous voltage on the entire installation for a long duration........


       - Andy.

     


    And perhaps someone else’s installation!


  • 0
    And in many designs, the tails from the meter coming detachd is not the only possible area of concern - a final circuit CPC tail (which in a worst case may be badly sleeved bare copper from T and E ) could end up poked against the back of the busbar, so connecting to live after the incoming switch and before the outbound RCBOs.

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