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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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  • Former Community Member
    0 Former Community Member
    Great, thanks to both replies. It's a lot more simple than i thought. 


    I guess my biggest concern really is in a TT installation scenario, where i'm installing the EV chargepoint. A lot of the points I install already have the correct Type A RCD and 6ma Protection built in, but then the whole installation is protected by a 100ma Type AC RCD. I've seen these where people have just come off the RCD to and MCD and the charge point, which is "wrong" in terms of regs and risks really, and i wondered what i will do in this scenario. At the moment, i've only installed a unit without the 6ma protection on a TT system, so i went straight to a Henley block, then to a the MCB and Type B RCD (at significant cost!), before the run to the chargepoint.
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  • Former Community Member
    0 Former Community Member
    Great, thanks to both replies. It's a lot more simple than i thought. 


    I guess my biggest concern really is in a TT installation scenario, where i'm installing the EV chargepoint. A lot of the points I install already have the correct Type A RCD and 6ma Protection built in, but then the whole installation is protected by a 100ma Type AC RCD. I've seen these where people have just come off the RCD to and MCD and the charge point, which is "wrong" in terms of regs and risks really, and i wondered what i will do in this scenario. At the moment, i've only installed a unit without the 6ma protection on a TT system, so i went straight to a Henley block, then to a the MCB and Type B RCD (at significant cost!), before the run to the chargepoint.
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