80A PME Fuse

[plugwash]

The total of the 'ratings' of MCBs/RCBOs in a CU will nearly always be considerably greater than the 'rating' of the main switch - but 'so what?' ! Much the same is true of the situation when the MCBs protected by an RCD 'add up' to al lot more than the 'rating' of the RCD - but, again, "so what?", given that the final circuits are never going to all be loaded to anything like the total ratings of their OPDs.

We should not design installations we anticipate will be overloaded, but we have to consider the possibility that people will use an installation in ways the designers did not anticipate. One of the functions of overcurrent protective devices is to protect the installation against overloads caused by unanticipated usage,

While it's not normally listed as such on the circuit schedule, there is effectively a "circuit" consisting of the tails, isolators, busbars RCDs etc between the main fuse and the OCPDs. I see no reason why the components in this circuit would be any less deserving of protection against overload than any other components in the installation.

Earlier editions of BS7671 required overcurrent protection for cables, but had a blind spot around switches and RCDs, the 18th edition has apparently tried to fix that, though was somewhat ham-fisted reffering to manufacturer guidance. The manufacturer guidance seems to be that the RCD rating should be based on the lower of either the upstream protective device, or total downstream proective devices.

https://www.beama.org.uk/static/uploaded/6861a2e1-d8d4-4d81-b5fa710ba60ca4a7.pdf

 

[JohnW2]

Seem to remember this was one of the things clarified by BS7671? Which means not a change, just clarified, so if you read it wrong, what one has done is non-compliant even with older editions.

What "this" are you referring to?

 

[JohnW2]

.... We do not need to add up the ratings of our fuses/breakers etc but can apply diverity on circuits dependant an maximum possible loads of what is actually connected or maximum expected loads per circuit and then sometimes apply some diversity to the whole system.

Indeed, and it's not only 'diversity'. Many circuits in domestic installations have 'design currents' (hence OPD ratings) well in excess of likely loads. As you go on to imply, now that we are in 'LED era', this is perhaps most dramatic in relation to lighting circuits - since even a 1A circuit would be more than adequate for the average domestic property. Indeed, given that there are usually at least two lighting circuits, a lot less than 1A would probably be appropriate! Less dramatic, but similar, can probably be said of sockets circuits, since it would be incredibly rare that such a circuit would be asked to supply anything like 32A in a domestic property!

.... Or we might think it more realistic to apply a sensible Watts per square Metre of floorspace approach. .... This applies in similar fashion to our maximum cut out rating and maximum consumer unit Mainswitch rating and RCD switch rating. .... Working it all out realistically then adding a reasonable "safety factor" can be a bit of a Dark Art but given that the bog standard Cooker Circuit Diversity calculation usually works very well then all methods may have their merits.

Quite so - and although I don't want to 'put words into your mouth', I think what you are really talking about is 'common sense' - which I definitely think is needed here!

Kind Regards, John