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Selectivity of RCD types

Former Community Member
Former Community Member
In this Voltimum article https://www.voltimum.co.uk/articles/could-rcd-type-s-be-used-upstream-rcd, it discusses some aspects of selectivity of RCD's with regard to different types as follows:

1. A type ‘A’ RCD should not be fitted upstream of a type ‘F’ or ‘B’ RCD.



2. A type ‘AC’ RCD should not be fitted upstream of a type ‘A’, ‘F’ or ‘B’ RCD.



3. A type ‘F’ RCD should not be fitted upstream of a type ‘B’ RCD.




What is the danger of fitting a type A or AC RCD upstream of a type B RCD?

It talks about the smooth DC current blinding of the Type AC, but surely thats the point of the Type B RCD? If a smooth DC current appears from down stream of the Type B, then the Type B will open, leaving he rest of the installation safe? What am i missing?


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  • 0
    Well of course it can be done, the question really relates to which RCD you expect to trip first, and if one or other will be blinded  or might false trip by a condition the other can see.

    If you are happy that the consequences if the 'wrong ' RCD trips are not serious, it is just fine.

    However if you have a load that you know needs for example, a B type - perhaps because it introduces some DC into the LE loop, then that load should not be connected via for example an AC type, that is also shared with other circuits  and is the sole means of protection for those other circuits. It is likely that the known 'bad' load will either cause the other circuits to 'nuisance trip' or worse, to compromise their fault coverage s it never trips , leaving the other circuits exposed.

    It is not the cascade of RCDs as such that is the problem, but the possible effect on the protection of other unrelated circuits on the same RCD that is the concern here.

    Hope this helps.
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  • 0
    Well of course it can be done, the question really relates to which RCD you expect to trip first, and if one or other will be blinded  or might false trip by a condition the other can see.

    If you are happy that the consequences if the 'wrong ' RCD trips are not serious, it is just fine.

    However if you have a load that you know needs for example, a B type - perhaps because it introduces some DC into the LE loop, then that load should not be connected via for example an AC type, that is also shared with other circuits  and is the sole means of protection for those other circuits. It is likely that the known 'bad' load will either cause the other circuits to 'nuisance trip' or worse, to compromise their fault coverage s it never trips , leaving the other circuits exposed.

    It is not the cascade of RCDs as such that is the problem, but the possible effect on the protection of other unrelated circuits on the same RCD that is the concern here.

    Hope this helps.
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