I'm not sure what you are saying/suggesting here. If they are both 30mA RCDs of the same type, then I don't see what benefit (other than redundancy) could be gained by having two in series.
If a RCD type AC is being swamped by say a washing machine inverter drive so it will not trip a second RCD even is also type AC feeding from that circuit to a hedge cutter is not likely to be swamped as well as only supply to hedge cutter going through that RCD, so that one would still trip.
What of course is required is a risk assessment, what are the chances of a RCD failing to operate because of the DC component? In the main where a RCD has tripped it is either water or a component failure, like a capacitor used in interference suppression. In other words the RCD trips before a human touches a live part, not as he touches a live part, although the latter is of course possible.
It seems the inverter drive, USB socket, switch mode power supply or other item will not stop the RCD working under normal conditions, only when the item goes faulty, I assume a DC path to earth, but not clear on what fault causes DC to run through the RCD iron core, so the question is how likely are these faults likely to happen at the same time as some one touches a live wire? i.e. what is the risk.
We have identified bathrooms and gardens are the high risk areas, the bathroom shower has its own RCBO so that it seems will still protect, so only item in the bathroom is the lights, garden at rear of my house I would use power from the flat, which is really only a store room, so that RCBO will still work, so the only one for power is front of house so if I fit an outside socket with its own RCD for front of house that is covered.
So only remaining problem area is the lights, but there are no earths to most bulbs, so how would there be a DC through the RCD? I don't know which is why I am asking. Now the lights is the odd one out, I can buy B6 type A RCBO's so it would seem three RCBO's type A for lights and a outside socket with RCD built in for front of house. However what is the risk of getting a shock from lights? Is it really worth it, and would it just be simpler not to use electric outside at same time as running washing machine?
So what is the chance of a DC swamping at the same time as also getting a shock?
) that a Type AC can tolerate whilst still working within spec and/or the amount of DC component and/or the degree of 'non-sinusoidality' that a non-Type-AC can tolerate and still work 'as intended' (whatever that may be).
