What’s a better board? And what are the limitations of a dual RCD board?
The RCBO board is better in the sense that an earth leak fault on one circuit only trips that circuit, and not a group of circuits off one of the 2 RCD in a dual RCD board.
Historically the first system to protect against a fault making conductive stuff (metal boxes, etc) "live" and so presenting an electric shock risk from touching it was to have an earth connection to any metal work, and so in the event of a fault a lot of current flows, the fuse would blow, and power was disconnected. This was known as EEBADS from "Earthed Equipotential Bonding and Automatic Disconnection of Supply" and today is still the starting point under the more generic term of ADS (the Automatic Disconnection of Supply aspect).
To make ADS work you need a low impedance earth so enough current flows to make the supply disconnect on the fuse or breaker that is normally providing fault/overload protection. Where such an earth was provided by the power supply company it would originally been in the form of the 3rd conductor (after L & N) back to the supply transformer (the TN-S supply).
But if you were in the middle of nowhere then often the supply company would not provide that as it makes the supply 50% more expensive which adds up on km of a route. In this case you would end up with a 'TT' supply where your earth was a rod (or similar) buried in the ground by your property. The problem with earth rods is it is very difficult to get a low resistance in to soil, and it can vary due to frost or drought, so in most cases not enough current would flow to blow even a small fuse and so ADS fails to protect.
So for old TT installations they have the VOELCB (voltage operated earth leakage circuit breaker) which was quite a simple device that sensed the voltage between the property's "earth" and the rod and tripped if anything significant happened. While better than not disconnecting they were not very reliable in practice as various different faults could escape detection.
The replacement was the RCCB (residual current circuit breaker) also known by the more generic term RCD (residual current device). This senses the difference between current going out and returning (the "residual") and if it sees enough going astray disconnects. While they don't have to be electronic, in practice they all are, and when they first came out were very expensive and so only one was fitted in the TT supply case to make ADS work at all.
But folk kept dying of electric shock, especially from garden power tools, so folk started fitting a sensitive RCD to outdoor sockets or extension leads. These were sensitive enough to disconnect on the small leakage current that a human could (usually) survive and so protecting against touching L (from a cut cable, etc) while standing in the garden, etc.
Eventually it became a requirement for sockets in general which in a property might be on several circuits. However, if you have one RCD feeding them then a fault on
any of the circuits disconnects power to
all of them. Also this makes fault finding difficult as it is not always the obvious circuit that is actually at fault, it might just be provoking a trip on a neighbouring one due to a N-E fault.
As prices dropped you started seeing dual RCD boards, advertised as "high integrity" since you had less trouble in the event of a fault, typically half you house's lights & sockets would still be working.
But these days with the ever falling cost of electronics and options to miniaturise them, you now can have the RCBO which combines the RCD and the overload MCB in one device. This is ideal as then a fault is only impacting on the faulty circuit, any not the other healthy ones.
