I have posted a piece here which is also on the TT topic, but is more general and I think a new thread would be better. Your voice is heard. See below.
I have looked at the information provided by the references from Chris above, and they do not make much sense. They come up with the "magic" diversity equation, in some way supposed to describe real diversity decisions. The problem which it does not consider is this:
Real diversity is not about how many circuits are installed, it is actual loads' characteristics that matter. It seems that Hager (and therefore possibly BEAMA as a whole) do not understand this. If I fit 2 ring circuits to a house, does the actual maximum demand double? Of course not! It is true that I could then use 7 or 8 electric fires of 3kW at the same time and for long periods, but the house would have to be very large or become far too hot. I know that I have been commenting on diversity quite a lot recently, but it is badly understood by many, and depends on load, period of use, and its AVERAGE power consumption over a long period. Because all these things need Engineering Judgement which is often in short supply, many "rules of thumb" have been invented, and over time, seem to have received unwarranted status.
The instruction from Hager seem to be designed to sell either RCBOs or higher rated RCDs, or perhaps to limit the number of final circuits provided which surely is counterproductive to their sales. The references used from BS7671 are taken out of the full context, and used as reasons to change our use of the Consumer unit. I would say that it is trying to solve something which is not and never has been a problem, but the important bit is that is a MISUSE of the manufacturers' instruction (MI) clause in BS7671. I think that it is trying to change BS7671 via the back door for manufacturer benefit, and at best is highly confusing to Electricians and particularly Inspectors, who have to decide if the MI now need many installations to be given a C2 because the MI say that the installed CU ratings are inadequate. This is the worst possible outcome because they are generally perfectly satisfactory, and have been for a long time, but the MI have to be taken as fully retrospective, or if not at the Inspectors risk! Repairing an existing installation may be quite difficult as it may require further splits to the CU to 3 RCDs, or its replacement. This means that all the latest regulations on SPDs AFDDs etc may come into force as a result of the change, which will be very expensive indeed, and at the stroke of a pen in the MI.
Overall this is highly undesirable to both the industry and the consumer, and there is no significant benefit to anyone is there (except BEAMA members perhaps)? Remember that the average power use in each house is less than 2kW, and most of the time very low. This is the real diversified figure for around 20 million installations. CU fires are not and never have been caused by overheated RCDs, just loose connections caused by poor screw quality and inadequately designed terminals.
I have looked at the information provided by the references from Chris above, and they do not make much sense. They come up with the "magic" diversity equation, in some way supposed to describe real diversity decisions. The problem which it does not consider is this:
Real diversity is not about how many circuits are installed, it is actual loads' characteristics that matter. It seems that Hager (and therefore possibly BEAMA as a whole) do not understand this. If I fit 2 ring circuits to a house, does the actual maximum demand double? Of course not! It is true that I could then use 7 or 8 electric fires of 3kW at the same time and for long periods, but the house would have to be very large or become far too hot. I know that I have been commenting on diversity quite a lot recently, but it is badly understood by many, and depends on load, period of use, and its AVERAGE power consumption over a long period. Because all these things need Engineering Judgement which is often in short supply, many "rules of thumb" have been invented, and over time, seem to have received unwarranted status.
The instruction from Hager seem to be designed to sell either RCBOs or higher rated RCDs, or perhaps to limit the number of final circuits provided which surely is counterproductive to their sales. The references used from BS7671 are taken out of the full context, and used as reasons to change our use of the Consumer unit. I would say that it is trying to solve something which is not and never has been a problem, but the important bit is that is a MISUSE of the manufacturers' instruction (MI) clause in BS7671. I think that it is trying to change BS7671 via the back door for manufacturer benefit, and at best is highly confusing to Electricians and particularly Inspectors, who have to decide if the MI now need many installations to be given a C2 because the MI say that the installed CU ratings are inadequate. This is the worst possible outcome because they are generally perfectly satisfactory, and have been for a long time, but the MI have to be taken as fully retrospective, or if not at the Inspectors risk! Repairing an existing installation may be quite difficult as it may require further splits to the CU to 3 RCDs, or its replacement. This means that all the latest regulations on SPDs AFDDs etc may come into force as a result of the change, which will be very expensive indeed, and at the stroke of a pen in the MI.
Overall this is highly undesirable to both the industry and the consumer, and there is no significant benefit to anyone is there (except BEAMA members perhaps)? Remember that the average power use in each house is less than 2kW, and most of the time very low. This is the real diversified figure for around 20 million installations. CU fires are not and never have been caused by overheated RCDs, just loose connections caused by poor screw quality and inadequately designed terminals.
