The main regulations are given below. 522.6.5 has also a similar one 522.6.6 which covers walls. Together it means nearly every circuit needs RCD protection in some way. Also circuits small enough to ensure 314.1(iv) is complied with. With a TN supply the RCBO is best method but expensive using a number of RCD's covering a group of MCB's is cheaper in the main. But one must consider what happens if they trip. Using emergency lamps to light stair wells does avoid hazards and can reduce the number of RCD's required.
A risk assessment is only method. One can't really just say that's the way I wired the last house.
Sockets are a problem as many items have EMC filters which allow a small amount of earth leakage and this builds up. The norm is to use a separate RCD or RCBO for every floor. But were computers are used sometimes radial circuits with far less sockets are used and one could have a radial for every room.
Consumer units can be stacked so giving two rows of MCB's / RCBO's and using that method one can fit 4 RCD's in one box but in the main the boxes are designed to take just 2 RCD's and maybe also some RCBO's using RCBO's for ring mains does reduce risk of faulty item or many items tripping the lights. However down to risk assessment and there are no fixed rules.
Forgetting about the RCD there is the loading on each circuit. Most lights use the ceiling rose as a junction box and are rated at 5/6 A so a 6 A MCB/RCBO is largest that can be used. Spot lights can take 0.2 A each plus inrush so one needs to limit how many on each switch and how many on each circuit.
Again normally at least a separate circuit for each floor. Where there is a bathroom then lights need to be on a RCD/RCBO.
Earth loop impedance, volt drop, and prospective fault current will also limit the lengths of circuits using 2.5mm twin and earth 106 meters to top limit for ring main. i.e. no more than one role for each circuit. Depending on beams one can easy exceed this limit and some times side to side rather than up/down splits are required to comply.
Using heavier cable or smaller than the standard 32A MCB/RCBO can increase the cable used in a ring so you do need to calculate or estimate what will be required. All the formula is in the big red book. BS7671:2008
If you have a TT supply then using the single RCBO with no switched neutral is a problem and special boards may be required. There is also a problem with large houses with ELI and Volt drop and using sub-mains can help. I used a three phase fused isolator on one very large house and three separate consumer units. The idea was that if the supply turned out to be not enough then the supply could easy be split to two or three phase or a larger than 100 A single phase supply. Remember most consumer units rated at 100 A only so using a 160 A fused switch meant I could use a 160 A supply if offered. However in most cases 100 A is enough.
In the main since limited to 100 A the consumer unit only has 16 ways and they can soon be filled. 4 ring mains kitchen and one for each floor. At least 3 lighting circuits. Oven, Hob, Immersion heater, shower, garage, heating, Out door lighting and very quickly you have used 16 ways up. Add to that door bell transformers and you can easy fill 32 ways of double consumer unit.
Basic a house system needs designing not just added to as you go along. Regulations on lighting types means that today much has to be energy saving type but still one has to allow for as soon as the building inspector turns his back for these to be replaced with non energy saving so still one allows for 60W per lamp.
314.1 Every installation shall be divided into circuits, as necessary, to:
(i) avoid hazards and minimize inconvenience in the event of a fault
(ii) facilitate safe inspection, testing and maintenance (see also Section 537)
(iii) take account of danger that may arise from the failure of a single circuit such as a lighting circuit
(iv) reduce the possibility of unwanted tripping of RCDs due to excessive protective conductor currents produced
by equipment in normal operation
(v) mitigate the effects of electromagnetic interferences (EMI)
(vi) prevent the indirect energizing of a circuit intended to be isolated.
314.2 Separate circuits shall be provided for parts of the installation which need to be separately controlled,
in such a way that those circuits are not affected by the failure of other circuits, and due account shall be taken of
the consequences of the operation of any single protective device.
411.3.3 Additional protection
In a.c. systems, additional protection by means of an RCD in accordance with Regulation 415.1 shall be provided for:
(i) socket-outlets with a rated current not exceeding 20 A that are for use by ordinary persons and are intended for general use, and .......
522.6.5 A cable installed under a floor or above a ceiling shall be run in such a position that it is not liable to be damaged by contact with the floor or the ceiling or their fixings. A cable passing through a joist within a floor or ceiling construction or through a ceiling support (e.g. under floorboards), shall:
(i) be at least 50 mm measured vertically from the top, or bottom as appropriate, of the joist or batten, or
(ii) incorporate an earthed metallic covering which complies with the requirements of these Regulations for a protective conductor of the circuit concerned. the cable complying with BS 5467, BS 6346. BS 6724, BS 7846, BS EN 60702-1 or BS 8436, or
(iii) be enclosed in earthed conduit complying with BS EN 61386 and satisfying the requirements of these Regulations for a protective conductor, or
(iv) be enclosed in earthed trunking or ducting complying with BS EN 50085 and satisfying the requirements of these Regulations for a protective conductor, or
(v) be mechanically protected against damage sufficient to prevent penetration of the cable by nails, screws and the like.
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