My old house it was common for RCD to trip with power cut or even resetting the other RCD. When fitted the RCBO was not really an option with a single modular width, the only way was for one RCD covering many circuits.
Today we should measure the back ground leakage, but we rarely do, the recommended 9 mA maximum back ground leakage is all well and good in theory, in practice I don't have a meter to measure it. And the instructions given to electricians when going to collage seem flawed, we are told to test the RCD with no load at ½ its rating so with a 30 mA test at 15 mA but with circuits disconnected, if we tested with circuits connected it would be far better, we would then know the RCD would not trip with an extra 15 mA and also it is not being frozen by any DC when tested it does trip at 30 mA, however traditionally we test with all circuits off.
AC current will always leak to a small extent, capacitive and inductive linking means there is always a little, years ago when we used 100 mA this was not enough to worry about, however it is with 30 mA, although the regulations have said the supply should be split into circuits to "reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced by equipment in normal operation" electricians have decided not to think of the RCD as forming a circuit, defined as "An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)." it clearly is an overcurrent, but to earth rather than to power a device, and we have as a trade hidden behind the definition.
It is all down to price, I have just looked on internet and a 10 way consumer unit with 2 RCD's advertised at £50, but a 10 way consumer unit with all RCBO and SPD without the RCBO's is £60 populated £231.60 with each RCBO costing £17.16, and the £50 one is on offer, likely because electricians are now realising they should not really be fitting a 10 way with just two RCD's and as you move to smaller units the RCBO works out cheaper anyway.
I did not measure the leakage with my own house, little point, fitting all RCBO so if over 9 mA not much I could do about it without a rewire, but on that 10 way example the home could leak 10 x 30 mA so 300 mA collectivity before it trips, but on a twin RCD only 2 x 30 mA so 60 mA, we know from past experience the 100 mA trips rarely had a nuisance trip, they only tended to trip with a true fault, and the RCBO with less wiring on each unit also rarely has a nuisance trip, but a 30 mA with multi-circuits it does vary from home to home, some people seem to get away with them, but others it is common for them to trip.
Personally last house fitted around 1992, and I would get a bout of tripping, maybe every week for 6 weeks, then 2 years before next trip, I would test the RCD with a RCD tester not just the button, no fault found, and test the circuits with an insulation tester again no fault found, just before moving lost a pair of freezers full of food, so with this house I fitted all RCBO's. The extra £200 for all RCBO's is less than what I lost when freezers defrosted.
But be it a single or multi RCD (RCBO is an MCB and RCD combined) or fitting a surge protection device, it does not cause a danger to personal, only to loss of equipment or goods, so the electrician can't force the home owner to fit RCBO's or SPD it is the owners call as to if they want them or not. However as electricians we maybe don't explain before doing the work what it means when only two RCD's fitted or no SPD are fitted.
