Dorman Smith 13A Ring Main plug
Alternative to the BS1363 plug and socket, this plug has the fuse as the live pin.
Ring Main because that is what the circuit was called at the time - ring final circuit today.
Plugs
The plug has three round pins, the top earth pin being longer than the others and having a groove along one side.
As with many other three pin plugs of this age, there is a hole in the top above the earth terminal for inspection purposes.
These examples are model number M692.
Live pin as the fuse
The most notable feature of these plugs is the fuse. The fuse serves as the live pin, and can be unscrewed to allow replacement.
The fuse shown here is a 13A type, coloured white. Other colours and ratings were available.
The fuse ends are metal, with the main body being a very hard ceramic material. This fuse also has '13A' stamped onto the end.
A supposed problem with these was the live pin unscrewing and remaining in the socket. Whether this actually happened is unclear - the plug contains a spring insert which should prevent the pin accidentally unscrewing. If the pin was left in the socket, it would be a serious hazard, as it would be live.
Internals
The cover is secured with two brass screws. Both the top and base are made from a hard moulded plastic material.
The terminal blocks inside are brass, as are the neutral and earth pins.
A small threaded insert with spring allows the fuse pin to be screwed in from the outside of the plug.
Flex is secured with a flat strap and two small screws.
Socket
This socket is circular, with two fixing holes 2 inches apart. This is designed to fit onto a standard round conduit box.
The front has the DS logo, and '13 AMP MAX.
Line and neutral holes are covered with shutters, which are opened by the insertion of the longer earth pin on the plug.
The back has three terminals, L N and E. The earth terminal also connects to a small ring around one of the fixing holes, as the conduit box this was fixed to would most likely be made of metal and therefore would be earthed.
This example has signs of corrosion on the screws, which are steel even though the terminals themselves are brass.
Plug and socket
The shutter mechanism is very simple - a single pivot at the top, with a spring to return it to the closed position when the plug is withdrawn.
The plug has the DS logo and the word FUSED - this was necessary as the majority of other plugs in use at the time did not contain fuses. One of the key concepts of this design is that the fuse in the plug should be the one to fail, rather than one in the fusebox or distribution board.
For those not familiar with this new (at the time) system, not indicating the plug was fused would have resulted in people checking their fusebox, only to find nothing was wrong.
