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When we install a ring final we are told to consider 20 amp load in centre and 12 amp even spread, this is also unlikely to be true most of the time, but it is a valve used to decide ring final length.I think the testing is a bit of a red herring, since there really is no way in which an electrician can undertaking any meaningful tests...
So
this means items plugged in to a ring final have a limit of 3.5 mA. If we like when using 26 amp as the load for working out ring final volt drop have a figure which we consider as reasonable leakage we could come up with a acceptable formula to decide how many RCD's are used, we clearly should allow at least 3.5 mA per socket circuit and we should not exceed 9 mA with a 30 mA RCD so we can say 3 ring finals are the limit.Equipment having a protective conductor current exceeding 3.5 mA but not exceeding 10 mA, shall be either permanently connected to the fixed wiring of the installation without the use of a plug and socket-outlet or connected by means of a plug and socket-outlet complying with BS EN 60309-2.
With items like cookers which are hard wired 10 mA seems to be limit, so cooker on it's own RCD.
So if we have a high integrity consumer unit with three RCBO's two for lights and one for cooker, and supply the rest from the two RCD's it seems unlikely we would have a problem. In fact if the lights are using RCBO's then all the safety arguments go out of the window.
Although the cooker is likely to have some earth leakage, what we have to consider is if to remove from general RCD loads likely to cause a trip or loads where there is a safety or financial implication should it fail, so lights and freezers on RCBO's and rest RCD.
The problem is often size, my old RCD's were 4 module wide, now down to 2 module wide but having a consumer unit with three RCD's a main switch and SPD means 10 ways are taken up before we include any MCB's. But with RCBO only 4 ways taken up. I have 14 RCBO's in an 18 way board, to go to RCD need a 24 way board. I have the room, but when you see some pictures submitted clearly room is a problem. Also although a high integrity board has three neutral bars, never tried fitting three RCD's so not sure you could?
And three RCD's is not two and the question was about using only two. But I look at my aunty's two up, two down house, and with a battery backed light at top of stairs, two RCD's would likely be good enough. She never had a fridge never mind a freezer, and still had gas lights on the wall. One bed room converted to bathroom, originally toilets outside at bottom of yard with door to back ally so muck man could collect the waste.
Just because I live in a 5 bedroom house does not mean everyone lives in a house that big.