Overload protection

The most extreme thought experiment example would be a wire that feeds the light switch in the substation - the lights probably draw single figure amps, and yet the at the origin PSSC is as high as it is ever going to be. 

So does the wire to that  HRC fuse get done in 240mm2 or something - even if it is only 5A ?

Generally, no  - we may prefer to accept that if disaster struck in the hop before the fuse, that whole (short ) length would be heat damaged (maybe even blown open circuit)  and have to be replaced.  It won't be as thin as 1mm2, though from a current rating consideration perhaps it could be, but that is more about sensible mechanical robustness, and lug sizes available. It does need to be routed and restrained so that it cannot fail in a way that causes any damage though.

Mike

The most extreme thought experiment example would be a wire that feeds the light switch in the substation - the lights probably draw single figure amps, and yet the at the origin PSSC is as high as it is ever going to be. 

So does the wire to that  HRC fuse get done in 240mm2 or something - even if it is only 5A ?

Generally, no  - we may prefer to accept that if disaster struck in the hop before the fuse, that whole (short ) length would be heat damaged (maybe even blown open circuit)  and have to be replaced.  It won't be as thin as 1mm2, though from a current rating consideration perhaps it could be, but that is more about sensible mechanical robustness, and lug sizes available. It does need to be routed and restrained so that it cannot fail in a way that causes any damage though.

Mike