If the power is cut due to some thing out side the house, then clearly street lights and all house lights other than battery backed fail, I have a battery backed light on the stairs so I will not trip on the stuff wife leaves on the stairs.
However where the power cut is due to some thing in side the house, then some one may be injured or startled at the same time, this is the least opportune time to lose lights.
My battery backed light for stairs is also PIR controlled so if just a lighting circuit is lost, it will still light with movement, as plugged into a socket outlet.
I consider the danger other than stairs is not that high when loosing power, so else where in the house there is no battery backed lights, and should I be plunged into darkness I can likely shuffle my way to some where safe to wait for lights to return. However if for example getting stuck toast out of toaster the person gets a shock, then they are already disorientated, so to be plunged into darkness as well there is a very real danger of them touching some thing hot, or tripping over some thing.
So we have
BS7671:2008 said:
314.1 Every installation shall be divided into circuits, as necessary, to:
BS7671:2008 said:
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
I know a circuit is defined as
BS7671:2008 said:
An assembly of electrical equipment supplied from the same origin and protected against over current by the same protective device(s).
but a RCD does trip when the current leakage goes over a pre-set limit, so it is debate-able if a RCD forms a circuit, and it is clear that not including RCD as forming a circuit when considering 314.1 is being rather blinkered considering the explanation as to why it should be divided.
I can see with a caravan that only 1 RCD on the whole supply is acceptable, one as lights are normally battery backed, and two it is such a small area with windows all around, and with a two up, two down where the lights and sockets are split so lights and sockets in every room are always on separate RCD's considering for years the 4 fuse fuse box was used so never any more than 4 circuits, then two RCD's are likely ample.
However as we look are larger properties, then very quickly we reach a stage when capacitive and inductive linking can result in the differential between in coming and outgoing power increasing, add a few interference reduction networks, and we are sailing close to the wind with a 30 mA RCD, so having just two means both more likely to trip, and also that some areas will end up with the same RCD controlling power and lights.
The easy cure would be one RCD for lights, and one RCD for power, however should a fault happen, you want to keep items like freezers running, clearly without an extension lead running up or down stairs, so splitting sockets front/back or side to side is far better than up/down and also means the earth loop impedance is reduced as cable needed is less in most homes.
However for lighting with use of ceiling roses as junction boxes, up/down is a better split for keeping the ELI low. So it is common to find the home split up/down for lights and side to side for sockets. Pre-RCD that made sense.
So this house 13 rooms or areas, when I bought it, 4 rooms had RCD protection for sockets, rest of house no RCD protection. Needed to do something as fuse box access was through a hole in false ceiling, 13 circuits however I split them, impossible to use 2 RCD's and have lights not on same RCD as sockets is some rooms, fact I need to leave house and walk down a set of steps and re-enter house to reset any trip, and even a 16 slot wide CU is big, add just two RCD's and 20 slot required so it seemed clear RCBO was the way forward. But I know the risk, and so I am doing my own risk assessment.
However with neighbours houses, they are guided by an electrician, and I question if using 2 RCD's for whole house complies. To my mind there is a danger forcing old people to leave the house and walk down steps at night in the dark to reset power which they have to do to get heating, adding two RCD's makes the home more not less dangerous when a failure of one RCD means the home becomes uninhabitable. I can if I wish plug in a oil filled radiator and wait until morning, there is a way around any RCBO tripping, and should I need to reset the one running central heating, at least there are outside lights so I can see my way down to where the CU is located.
Adding a two RCD only CU would make this house, and most houses in the street into a death trap.