You can have a DC leakage current which is too small to trip the B or whatever, but is still large enough to blind the AC or whatever.

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.

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.

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.