UK Electrical Systems
Way back in the dim and distant past our Electrical system was based around 2 amp, 5 amp, and 15 amp 3 pin plugs, these were round pins with the earth being the largest.
The plug and socket shown above is 5 amp, the 2 amp and 15 amp are similar, just a different size.
The system was wired as a radial circuit, so one circuit starts at the fuse box, runs out to the first socket, then on to the next, the maximum for a circuit was 15 amps, so you could only have one 15 amp, or three 5 amp, the 2 amp plugs were mainly for standard lamps controlled by the lighting circuit.
The fuse board often consisted of several boxes, one would have sockets and lighting, another the cooker, another for the immersion heater, and all were usually cast iron, they were a bit unsightly.
A new system was introduced, the sockets were wired as a ring circuit, and it was generally considered to have a 'downstairs' ring and an 'upstairs' ring, so instead of multiple fuses, you only needed 2 for the sockets, 2 for lighting, plus oven and immersion heater, all contained in what was known as an '8 way board' that looked something like this
This is a diagram of a ring circuit
And this is a typical modern 'consumer unit' (distribution board)
Our plugs and sockets changed too, as can be seen in the ring circuit drawing, we now have square pins, originally the pins had no insulation, many people think the insulation was to prevent electric shock, but we've had bare pins for years without much problem, but a serious incident meant as a prank forced us to include insulation, you can read about it in 'Pinsulation Story' in the menu.
A 13 amp plug
showing the inside view, notice the fuse for the live, the neutral is on the left, with the earth at top