It seems that BAS (hence the 'oft-quoted story') and your elderly engineer were probably both right. If you look at this article about the hisrtory of ring finals and the BS1363 plug/socket concept, both of those considerations appear to have been part of the thinking.
Yes, it seems that economic use of copper was considered but was only one aspect of the design of the then new installation system.It seems that BAS (hence the 'oft-quoted story') and your elderly engineer were probably both right. If you look at this article about the hisrtory of ring finals and the BS1363 plug/socket concept, both of those considerations appear to have been part of the thinking.
Kind Regards, John.
Edit: I must learn to type faster than Paul
It's not quite as obvious that there will be a significant copper saving as you seem to be suggesting - it depends entirely upon the layout of the circuits. If you simply break a 2.5mm² ring to make two 2.5mm² radials, you'll be using exactly the same amount of copper. Similarly, the layout may be such that one can wire a set of sockets on a 4mm² radial using not much more than half the length of 2.5mm² that would be needed for a ring - yet 4mm² /2 = 2mm², not 2.5mm². The copper savings, if/when they exist, can therefore be very small.
In the absence of faults, it's actually quite a clever concept. It arose at a time when ('clever' to some) statistical concepts (like diversity and statistical quality/ process control) were coming very much into fashion, and that's probably got something to do with the beliefs that it was 'revolutionary'. As for the risk under fault conditions you describe, I suspect that those who introduced the idea had in the back of their minds the belief (quite probably correct) that the degree of overloading which could occur would not actually result in any serious harm; as we often discuss here, there appear to be some very large 'safety margins' built into what we regard as maximum current-carrying-capacities.
Erm...fused plugs are necessary.
Faults of any sort can obviously occur, but one has to decide how far to go in trying to design with all manner of possible faults in mind. In any event, it can work both ways - "broken CPCs" occur (not that uncommonly) - and in the face of such a fault, a ring is safer than a radial. Also bear in mind what I said before - that, although no-one would suggest that one should design/install a circuit involving 2.5mm² cable (or the Imperial equivalent) carrying 30A, 'those in the know' probably knew, or at least believed, that no actual harm would come from such an occurrence.
I can't say that I've ever seen or heard of anyone installing a 1.5mm² radial sockets circuit, and you certainly won't find one mentioned in BS7671 or the OSG.
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