"Why a ring?" is a question to which I've never been able to find a definitive answer myself.
As I said in the other thread, I've seen various claims and counter-claims about whether the ring circuit involved the use of less material. (It wasn't actually introduced until about 1949, but shortages and rationing in the the U.K. continued for quite a few years after WWII -- Some items were rationed until about 1953.)
The undersized cable for a 30A OCPD is one of the things I don't like about the system. The theory that so long as the ring stays intact then the wiring is protected sounds fine. But as my recent example shows, in practice a break can go undetected for a long time because no outlets stop working. In the worst case, the break would show itself only when the cables start a fire.
There's also the point that unknowledgable DIYers can easily break the ring during alterations without realizing it.
Another point: Our "code" allows a single outlet to be wired as a radial spur off the ring. According to the IEE, there's no chance of overloading the spur cable because of the fuse (13A max.) in the appliance plug. The one problem there is that a "twin socket" (duplex receptacle) can be counted as a single outlet for this purpose, so it is theoretically possible to overload the spur by connecting two heavy appliances to that duplex outlet. Unlikely maybe, but possible.
If somebody extends a spur cable to another receptacle elsewhere, then the chance of overloading that spur cable becomes much greater. In fact, in the original specifications, a spur was allowed to feed two separate single receptacles.
I may be in a minority here, but I'd like to see rings abandoned from our IEE Regs. altogether.
