Before the RCD it was common to split lights upper and lower, but sockets side to side, as side to side used less cable and so better loop impedance, but when RCD's came in, we were told to not supply power and lighting in the same room from same RCD, so if some one did trip the RCD due to some thing plugged in, it did not also plunge them into darkness. Clearly a general power cut would still mean no lights, but if you get a shock so trip RCD better if lights don't go out.
However with 2 RCD's and lights up/down and sockets side/side it is not possible, some rooms must have lights and sockets on the same circuit, OK today with RCBO's we can get it so not on same RCD, I have argued many times as to if a RCD forms a circuit, we have been told we should divide the installation so as to "(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit." and I can't see how two RCD's complies with that? "Circuit. An assembly of electrical equipment supplied from the same origin and protected against over current by the same protective device(s)." RCD = "Residual current device (RCD). A mechanical switching device or association of devices intended to cause the opening of the contacts when the residual current attains a given value under specified conditions." can't see how any one can say not an over current device? OK may be over permitted current to earth but still over current.
However back in the real world people want things cheap, having lived in a house with 2 RCD's since 1992 when I fitted RCD's to this house I was not going to make the same mistake, so 14 RCBO's in this house, I feel the extra £200 well worth it, if it saves one freezer full of food, it has paid for its self. But why should I force some one to copy me, if they want to take the risk and save £200 that is up to them.
