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Selectivity of RCD types

Former Community Member
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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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    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.



    All this talk of which types RCD can be upstream of what other times of RCD seems a bit like muddled thinking to me. It's the nature of the load(s) that's the important thing when it comes to RCDs being blinded, rather than the downstream RCD type.


    For instance if you had a charge point that tripped out if it saw smooth d.c. leakage current over 6mA (either in-built or from an EV-type RCD) then you could use an "ordinary" RCDs upstream of it.


    If you had a charge point that could produce over 6mA of smooth d.c. leakage then you'd need at least type Bs all the way upstream.


    But if you had two or more of the first types in the same installation each on their own (A-type) RCD and each leaking below 6mA, the cumulative smooth d.c. leakage could still be well over 6mA so you'd be back to needing type-Bs or better all the way upstream - or some means of tripping them both out should the cumulative leakage exceed 6mA smooth d.c.



    BTW - I understand that the maximum tolerable smooth d.c. component for 'ordinary' 30mA RCDs is 6mA - does anyone know if the figure is the same for higher rated ones (e.g. 100mA or 300mA)?


      - Andy.
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  • 0

    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.



    All this talk of which types RCD can be upstream of what other times of RCD seems a bit like muddled thinking to me. It's the nature of the load(s) that's the important thing when it comes to RCDs being blinded, rather than the downstream RCD type.


    For instance if you had a charge point that tripped out if it saw smooth d.c. leakage current over 6mA (either in-built or from an EV-type RCD) then you could use an "ordinary" RCDs upstream of it.


    If you had a charge point that could produce over 6mA of smooth d.c. leakage then you'd need at least type Bs all the way upstream.


    But if you had two or more of the first types in the same installation each on their own (A-type) RCD and each leaking below 6mA, the cumulative smooth d.c. leakage could still be well over 6mA so you'd be back to needing type-Bs or better all the way upstream - or some means of tripping them both out should the cumulative leakage exceed 6mA smooth d.c.



    BTW - I understand that the maximum tolerable smooth d.c. component for 'ordinary' 30mA RCDs is 6mA - does anyone know if the figure is the same for higher rated ones (e.g. 100mA or 300mA)?


      - Andy.
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