The regulations until the 16th Edition were refereed to as the edition number, shortly after the 16th Edition came out the IEE as it was then joined with BSi to produce BS7671. These have a date behind the number so BS 7671:1992 and BS 7671:2001 both called 16th Edition but major changes, then BS 7671:2008 and BS 7671:2018 the 17th and 18th Edition. BS 7671:2001 brought in the split consumer unit with half RCD protected and BS 2008 brought in the twin RCD consumer unit, seem to remember the old BS 7671:2001 brought in the RCD for supplies that could be used in the garden, normally a RCD socket. Each edition also had updates.
The RCD when it first came out was around 100 mA and was used on all circuits when using an earth rod (TT), it was not used with installations where earth came from the supplier (TN) it replaced an earlier version called the ELCB-v which tripped it voltage went over 50 volt and was unreliable. It has had many names ELCB-c, RCCD, and RCD in the UK. The type now fitted are 30 mA and have caused problems with mineral insulated stuff, the mineral insulations is hydrophilic it attracts water, and has a seal at the ends, if this seal fails then they often trip the RCD, there is a method to temporary fix, when it is an element, but it will not last, so only real fix is a new element.
The second problem is many devices only switch the line, neutral is left connected, when no current is used neutral and earth are same voltage, but as one uses current the voltage difference between earth and neutral increases, so current flow increases, so a bit of damp toast in the toaster can trip the RCD when the kettle is used due to voltage between neutral and earth increasing.
With BS 7671:2008 there was a rethink on earthing in bathrooms, mainly due to use of plastic pipes, and the need for earthing everything was relaxed, on condition that all in the bathroom was RCD protected at 30 mA, this is the main reason why lights are now RCD protected. There was also a consideration about wires hidden in walls and floors under 50 mm deep, but bathroom was main thing.
You can get the MCB and RCD combined, but they cost more, called a RCBO, so the RCD feeding the row of MCB's was a money saving idea, the problem is loss of supply to essential equipment like freezer or intruder alarm or lights, so the was a high integrity board which mixed the two so less likely to loss essential services, however the cost of RCBO's has dropped so now all RCBO's boards are becoming more common.
So 10 way consumer unit with surge protection and main switch £82.20 and the RCBO's type A curve B £18.60 the type AC down to £12.12 and SPD is £31.20 so one can save money, but for 7 RCBO's type A and the box looking at £212.40 including vat. But can be reduced to £135.84 by using type AC RCBO's and no SPD. But using twin RCD's feeding the MCB's again reduced to £77 pounds all prices same supplier (consumer unit world) and for FuseBox make which is around the cheapest, so one can see why penny pinching we still use the twin RCB option. However the loss of one freezer full of food as a result would cost far more than the extra paid for the better design. There is some debate as to if SPD and type A RCD is required, I don't really want to get into that debate on this thread. I have looked a 7 devices is that is what you have, there are offers from likes of
Screwfix for 6 way at £60 but your not allowed to select MCB sizes, and so often it works out more expensive.
I found my new house had no RCD protection, and so I fitted a new consumer unit, I made an error in my mind, got type AC RCBO's not type A, written type B on packet, but was actually curve B. I say this as many ask for a quote and get different prices due to different products being fitted. Most electricians allow a day to fit a consumer unit, maybe they are lucky and take ½ a day, but they can't really plan on that, and can't really say sorry will finish tomorrow and leave without power over night, so most allow for a full day. Cost will depend on where you live.
