Originally posted by sparky:
i guess someone's gotta ask sometime here, why a ring???
I've never been able to find a definitive explanation, but here are a few relevant points.
I guess the first thing to bear in mind is that the typical British home in the late 1940s was quite different to today. Central heating was rare, and most people had open coal/wood fires, gas fires, and portable electric heaters.
Up to then, there were outlets of three different ratings used: 2, 5, and 15A, with non-interchangable plugs. A 15A socket to run a large electric heater was often fitted next to a fireplace and wired back to its own 15A fuse. It was rare to have more than one such outlet in each main room, and sometimes none upstairs in the bedrooms.
Other "general purpose" outlets were 5A, and wired on various 5 or 15A branches. (The IEE Regs. still recognize these circuits with up to three 5A outlets permitted on a 15A branch.) Wiring was what the IEE now calls "radial" circuits - like American wiring.
The 2A outlets were intended mainly for plug-in table lamps etc. and were often wired on 5A branches with fixed lights.
To add to the confusion, both 2-pin ungrounded and 3-pin grounded versions of the 5A outlet were very common. Unlike American receptacles, however, the spacing of the hot & neutral is slightly different, so a 2-pin plug won't fit a 3-pin socket. 2A sockets also came in 2 and 3-pin types, although lighting circuits were rarely run with a ground wire at that time. 15A only came in 3-pin grounded versions by then, but a lot of 2-pin 15A sockets installed in the 1920s/1930s were still around (the old house I moved to as a kid in 1970 still had some!).
The mix of sockets meant that a plethora of adapters were often used. A typical type plugged into a 3-pin 5A outlet and provided a 3-pin 5A socket on the front plus a 2-pin 5A outlet on each side. Another type allowed 5A plugs to be connected to a 15A socket, but many different combinations were available.
It was this "electrical lottery" of plugs apparently, which led to the search for a universal connector and the design of the 13-amp plug with its built-in fuse.
That's the background. OK,back to those big 2 and 3kW portable heaters.....
I've heard it said that the IEE wanted to work out a simple circuit arrangement which would allow two 3kW heaters to be run simultaneously and still leave capacity for smaller appliances. The design was to be such that the heaters weren't restricted to just one or two outlets as before, but could use any sockets in the house.
Whether that's true or not, I'm not sure, but as a 30A circuit provides 7200 watts, it's certainly a possibility. And a massive 30A fuse at the panel, of course, necessitated the adoption of fuses in the plugs.
OK, that still doesn't answer "Why a ring?" From the practical standpoint, it allows the use of smaller cable; e.g. the IEE Regs. permit a 30A radial circuit, but this means the use of (in modern sizing) a 4 sq. mm cable - about 20% larger than #12 AWG.
I've also seen various claims (and counter-claims!) that in the design of homes being built at the time, the ring resulted in the use of the least material - important in post-WWII Britain with shortages and rationing.
I've never actually sat down & tried to work this out. It might prove interesting.
Other than those arguments, I've never been able to find out why for sure. I guess it seemed like a good idea at the time, and we've had rings ever since.
Oh boy, 2 a.m. already! Must've been talking too long!