You can, and the up-front RCD will meet every specification for it.1. If my understanding is correct, it would hypothetically fine to use Type A time delayed protection upfront of Type AC devices or Type B time delayed (in a future where they exist) upfront of Type A or AC devices.
But usually the bigger concern is the additional shock protection of the down-stream 30mA RCD/RCBO, you would be better to have it the other way round, with the up-front RCD being type AC/A and the downstream being the type A/B so they provide better in-specification tripping on faults with a DC component, is it might be a person that is on the receiving end of the stray current!.
No, I would say for a typical TT install with a good earth rod it is not such a concern.2. The Type AC time delayed RCD I place upstream of my parent's shiny new board, populated with Type A 1P+N RCBOs is far from ideal. This occurred to me after asking questions in the other thread, so I might as well admit this oversight. Not an immediate issue as those RCBOs are functioning as the manufacturer intended (at least they were prior to this lockdown), and there are very few devices in their house capable of causing DC current issues, but it needs to be corrected
Yes, a type A up-front is better in the sense you have the manufacturer's guarantee that it meets the trip current/times with some DC component present, but my experimental results backs up my expectation that having some small DC component (few tens of mA or so) is not serious, and beyond that an AC/A RCD will always trip if sufficient AC component is present so the current transformer gets enough of a fault signal.
So to back up the RCBO in the case of electronics failing and not wanting the earth system to go to 230V as the rod Ra won't disconnect based on over-current, then the AC up-front RCD will do its job short of a massive DC fault.
Such as DC fault is of course possible in many electronic systems with stored charge, but really only a big risk in my mind from PV generation or EV charging where you have lots of fairly high DC volts outside. For those cases a downstream (final circuit) type B RCD would allow safe disconnection and stop the large DC component, so the up-front RCD would still be able to deal with any AC faults above the specified 100mA/300mA/etc that might be needed for high safety (no single point of failure) TT installations.
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