When I started a radial was a supply radiating out from an over current device so a FCU would supply radials, but the wording seemed to change and they are now called fused spurs, a supply from a FCU is a circuit, however since a ring final is the final circuit, you can't have any more circuits from a final circuit so supplies from a FCU are not considered as circuits, at least if supplied from a ring final, since a radial is simply called a radial it could I suppose have a further circuit, but we don't consider it as such.
But at the end of the day it is all about stopping over load, so a 4 mm² or 2 x 2.5 mm² can take 32 amp, but a single 2.5 mm² can only take 20 amp depending on type of cable and the way it is installed to allow cooling.
A 13 amp socket can only supply 13 amp (except in a hospital) as there is a 13 amp fuse in the plug, so if supplying a single BS 1363 device it should be limited to 13 amp, as to why a double socket is permitted and two single sockets are not I have never worked out, I was told is was because you can tell if socket is part of a ring final or not when supplied with 2.5 mm² cable by how many cables, but that is clearly not true as could be a 20 amp radial.
To my mind we check if a ring by testing continuity between the pair of cables, and size of overload supplying it. So have not idea why a double socket is allowed on an unfused spur with 2.5 mm² cable. Or how it is permitted to market a double socket with a combined rating of 13 amp, it seems wrong, one would expect a double socket to take 26 amp. However there are double sockets, in the main specials like filtered sockets, where the combined rating is only 13 amp.
The system was developed during WW2 to assist with re-build after the war, but at that time the cable was around 3 mm² as an imperial size, 7/0.029 also the heat distribution was better as the line pin was solid brass and could conduct heat into the socket better than today with a necked pin with insulation over the necked part. Also brown radiators heat better than white old plugs/sockets were bakelite so black or brown.
Mineral insulated cable can be 1.5 mm² for a ring final but ali-tube cable has to be 2.5 mm² yet both seem to have same current carrying spec and temperature range. OK in real terms mineral insulated can run hotter, but not sure the terminations would take the heat?
So I can see the problems, and in a concrete factory i.e. all electrical work done by an electrician no DIY permitted, I found a 4 mm² radial which had been extended half way down the run with 2.5 mm² cable, the person doing it had clearly thought it was a ring final, and had not checked for continuity.
This is why filling in the minor works certificate is good, as it forces one to test so details can be entered on the certificate.
To extend a ring or radial one needs to test to show the loop impedance had not been exceeded to comply with both volt drop and ability to open or rupture the MCB/RCBO or fuse, but I have met electricians who did not have a loop impedance meter, or low ohmmeter so can't see how they tested?
