Her house has the stair well on outside of house so even with a power cut no problem seeing how to descend the stairs.
So she never experiences a power cut where the street lighting is also knocked out?
We always lose the lot.
Of course there can be a general outage however if that is the case it does not matter how many circuits you split the house into or if you count a RCD as forming a circuit or not.
Personally I like the idea of emergency lighting and I have such a unit at top of my stairs.
With emergency lights fitted the "(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit" is taken care of so one could fit a single RCD covering all circuits where it's unlikely to trip in normal circumstances which is exactly what is done with supplies to caravans and boats.
A small house or flat is not different to a caravan.
However forgetting the regulations what I am saying to fit a new RCD protected consumer unit first without taking the rest of wiring into consideration is wrong. Even with emergency lights before changing one wants to test the existing wiring to see if there are any existing earth leakage problems.
Also in spite what I have said one really does need to have two circuits each with their own RCD protection feeding at least a socket each so when there are problems one has an option to try each appliance in a different RCD supplied socket to be able to identify which appliance is faulty.
My house when built had a ring final wired through the cavity wall and to separate the sockets to form two circuits would mean converting the ring to two radials and with the lights it would be just impossible to split. I wired a second ring which serves the kitchen extension and a few odd sockets many years ago so I do have two independent RCD protected circuits both made RCD protected well before the regulations required it.
But as standard with my house the best one could have done was use a RCBO to feed the 4 original circuits with a house just 34 years old. Since the immersion was removed and the kitchen remote switch and immersion heater outlet was replaced with two sockets I would with original wiring have two sockets on a different RCBO so in my case without splitting into two radials I would have two sockets I could use if something caused the ring to fail and it could not be reset.
As to splitting a ring final into two radials the question has never been answered as to what the design current is for a radial? If we used the same formula as with a ring final then first 20 amp at centre or end means the radial design current = MCB size so the amount of cable permitted with two radials to comply with volt drop is less than half that permitted for a single ring. So before splitting careful measurements need to be made.
I have discussed this at length when I tried to make a program to work out volt drop from the prospective short circuit current and after tests I realised the meters are just not accurate enough to get a useful measurement so again it would be up to the judgement of any electrician doing the job as to if the existing design could be changed.
Clearly where changing the design then an installation certificate is required and the electrician has to sign as being responsible for the design. Unless there is an existing installation certificate then I see it as being a problem to issue and installation certificate for work which one has not been responsible for so we then come to the big questions.
One could you find an electrician willing to do the work?
Two if you can with the issues talked about can you trust his judgement?
Much depends on work load all electricians must find enough work and when work coming in is low feeding the family may come before ensuring all the boxes are ticked but when there is plenty of work those jobs which may in the future come back and bite you are back healed. And if the electrician is short of work the question is why. Maybe he's not that good.
It may be the original design will allow easy splitting of circuits and the 4 way box can be replaced with a 10 way box leaving room for expansion without a problem. However you will not find out that on here it needs some one actually there in the house to give you the options.
