I made an error and got type AC RCBO's, it does not say what is fitted and picture not really good enough, even the adverts for just RCD in tool station don't say what type, but pictures show type AC. The problem with type AC is DC can stop them working, and switch mode power supplies if they go faulty can result in DC, so we are basic doing a risk assessment, and at the cost to replace all my RCBO's I have decided since a TN supply and any device which causes DC will only effect one circuit, I think the risk is low, but as you group circuits together the chance of DC stopping a RCD working increases, and also the small amount of leakage you always get will AC supplies means the likely hood of a RCD tripping is increased.
In theory 9 mA is the limit for leakage with a 30 mA trip, never measured it, as my clamp on ammeter will not measure below 100 mA. Also when fitting an RCD any strain caused by the tails can cause them not to work, so when fitting them they need testing, and the test set is not cheap, and you can't measure 40 mS with a stop watch.
So even if not worried about the law, fitting a CU is not really a DIY job as you don't have the test gear to either select what is required or to test it all works after fitting, trusting to the lord is not really good enough.
So cost to fit £250 to £1000 depending on type and make of CU used, last thing you want to do is repeat the job because either it is tripping all the time, or not tripping when it should. Not a clue why the name, but it seems if three neutral bars it is called "high integrity" and if using just 2 RCD's you will want a few ways which can still take either a RCBO or MCB with no RCD protection to feed things like electric car charging.
Also in the main lights are split upper and lower floor, but if sockets are split the same, that means if a RCD fails one would need to run extension leads up/down stairs which is clearly dangerous, so sockets are better split side to side, however this means impossible to arrange so if some thing causes a RCD to trip, one is not also plunged into darkness.
As said one assesses the risk, be it adding a SPD, RCBO, RCD it is a case of assessing if acceptable, light from street lights, position of stairs, emergency lights, size of home, all come into the assessment. And why 80 amp RCD's instead of 100 amp, OK I know my house only has a 60 amp supply, but likely some time in the future I will want an electric car, and as a result a larger supply, to be limited to 80 amp seems silly when a 100 amp RCD is so little difference in price. Just penny pinching.
In theory when the new CU is fitted you want the person fitting it to sign for the design, installation and inspecting and testing, normally with one signature and he registers the work with scheme provider, if you do the design, i.e. select the type of CU, then he would need to use a three signature form, and would not be able to use the scheme membership to register the work, so would cost an extra £100 plus vat in Wales not sure on English charges, to register with the LABC. OK he is unlikely to be that pedantic, however if your employing some one to do the work, why tie their hands, let them do their job, and decide what is the best compromise. There is always some compromise, to fit a CU with all type B RCD protection would cost a fortune, even type F hard to find as a RCBO, so likely type A, so not a Rolls Royce install, but let him tell you what you need, then if there is a problem you can get him back to fix it, if you tell him, then it's down to you if the RCD is tripping all the time.
