433.2.2 and 434.2.1 are the two regulations talked about with this in mind. I have been told I am miss reading the regulation but as far as I can see there is a 3 meter limit to a un-fused spur.
The 20A grid switches will not take two cables larger than 2.5 mm sq so it is common rather then feed the switch bank with a single 4 mm sq or 6 mm sq to use two 2.5 mm sq. However they are really two conductors in parallel regulation 523.8 among many others refers to this with the comment
This regulation does not preclude the use of ring final circuits with or without spur connections.
so I will admit it is a grey area in some ways.
Where the grid switches feed single sockets then feeding from a 32A MCB is not too bad but where one want to feed multi-sockets then using a 20A MCB would ensure both the switches and the 2.5 mm sq cable is not overloaded.
Of course you can get fuse carriers that fit the grid so one could fit fuses if you wanted. 434.2.1 does talk about point of reduction in cross sectional area and I would say one needs to use some common sense. Where the grid switch is in a central position and cables radiate out to for example 4 points fridge/freezer, washer, dish washer, and drier for example with some cables up to 10 meters long one starts to ask why use a single 32A MCB when two 20A MCB's could have been used giving less chance of overload and also less likely that any two appliances would trip the MCB.
I think it is a case of lack of thought although in some cases there may be a loop impedance consideration.
By using Ali-tube rather than twin and earth the MCB could be up-graded to 25A which would mean two 13A appliances are very unlikely to ever trip the MCB.
However there is still a cost cutting system being used by many builders of using semi-skilled people to wire new houses with a skilled guy testing it is considered having two people work on the system reduces the chance of short cuts and bad workmanship specially with bonus systems in place. As a result there is a tendency to adopt the same system for all houses this also means an as built plan can be provided without it needing to be changed house to house.
So there is a general "This is the way it is done" without really much thought going into if it really suits the house in question.
