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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
    It depends on the circumstances and consumer units that are installed, so they may still be required
  • 0
    I think many people would recommend still having an S-type RCD as a backup to all the RCDs on a system where the RCDs are being used as fault protection rather than just as additional protection. That's due to the propensity of RCDs to fail over time.
  • 0
    You'd still need them for any distribution circuits in TT systems. Likewise upstream of Metal cased CUs if you can't ensure double/reinforced insulation up to the 30mA RCD (e.g. for most split dual RCD CUs).  Some like to have a backup for the 30mA RCD too - as the failure rate isn't insignificant and unlike TN systems there's no relying on the MCB for ADS.

       - Andy.
  • 0
    I have prepared several landlords EICRS so far this year that state the lack of 100 or 300 mA RCD protection to a 16 mm twin and earth distribution circuit is a C2.
  • 0
    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.
  • 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.




    Again, it depends.


    If it is TT earthing it may need an upfront RCD, whereas if it is TN earthing it may not.


     


  • 0
    If its a plastic box, no problem. If it's a steel box a plastic rcd upfront is always reqd. It's the law.

    Regards, UKPN. 
  • 0
    If it's a steel box a plastic rcd upfront is always reqd. It's the law.

    Oh no it isn't. It's perfectly possible to have Class II (double insulated) equipment with exposed steel parts. If you think otherwise please quote the appropriate legislation,


       - Andy.
  • 0
    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.
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    6ccd1785ed52286c37277f990e321f6d-original-20201013_233913.jpg

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