Why does 13A plug and socket system have flat pins when most others are round?

[marconi]

(A sad admission: as a lad I collected plugs by make and colour and displayed them on a shelf in my bedroom - I liked for example the style of the 'Rock' make. I had therapy and am now recovered but some interest remains. Who remembers makes such as Rock, WG, Nettle, Wylex, Empire, Walsall(?)?)

Does anyone know why the designers opted for square pins rather than continue with round pins, as other plug/socket systems eg. MK commando have done.

And why did the old plugs have an aperture above the earth pin?

 

[Midwest]

Finally an answer, from here, "Socketman";
13 amp plug question - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum - https://www.vintage-radio.net/forum/showthread.php?p=563154&posted=1

Getting back to the original point about the hole, clause 15 of BS 1363:1947 is quite clear, it states "The connection of the flexible conductor to the earthing plug pin shall be visible when the cover of the plug is in position". I can confirm that I inspected a copy of BS 1363:1947 held in the National Library of Scotland earlier this year, and took notes on that very point.

Do'er, I was about to post that link; see Lucien was posting in that.

 

[Midwest]

Our History - http://www.mkelectric.com/en-gb/AboutUs/Our_history/Pages/default.aspx

If you do read that potted history, it has no date, it boast that MK products are manufactured in the UK?

 

[telectrix]

If you do read that potted history, it has no date, it boast that MK products are manufactured in the UK?

must be quite old then.

 

[Marvo]

Maybe the square/rectangular X-section was simple about ease of tooling for manufacture or, as is often the case, quantity of raw materials/price.

 

[marconi]

#13 From second reference:

The company was based on the Multy Kontact socket, patented by Charles Arnold. Before the Multy Kontact pretty much all that was available was split pin-style sockets. They were mostly made from thick gauge slotted brass tubes, offering practically no flexibility. Plug pins were split to allow compression; but poorly made small pins would often produce a loose fit and poor contact. Large pins, on the other hand, needed too much force to insert and remove them.

Multy Kontact was better and safer, though no-one knows why it was decided to spell Multy with a y rather than an i or Kontact with a k rather than a C. Just as well they did though, since the whole company eventually got named MK Electric after that one product. MC Electric doesn't sound right somehow.

At the heart of the MK socket there are "numerous flexible spring tongues which actually grip the pin in much the same manner as the legs of two caterpillars on opposite sides of a flower stem", according to the original patent filed in April 1919. You don't get flowery language like that in many modern patent applications.

 

[marconi]

I have no reference but I seem to remember that the inspection hole was to make it easy to see that the CPC had indeed been connected to the top pin because the idea of earthing appliances supplied by flexible cords and plugs was new and some folk simply did no bother to connect the green wire ( and cut it back) because they failed to understand its importance to safety.

 

[marconi]

 

[Lucien Nunes]

Square pins = 'Modern wiring system in use here.'

Here is my reasoning:

In 1947 we had a bewildering selection of round pin sizes in the UK: 3-pin plugs to BS546 in three current ratings: 2A, 5A & 15A.
2-pin versions to BS372 in 2A, 5A and 15A that weren't interchangeable - the spacing was different, you couldn't put a 5A 2-pin plug in a 5A 3-pin socket.
A few 10A still survived, having been part of the Lundberg range that spawned BS73 / BS372.
Manufacturer-specific types included Wandsworth and original MK gauges in various current ratings (MK offered both their own gauge and BS gauge plugs at one time. Theirs were more compact for the same rating).
European plugs based on the 19mm pin spacing would turn up in the UK and loosely fit the 5A 2-pin.

About the only plug on sale that did not have round or modified round pins was the original Wylex type. All in all it was difficult for the householder to be sure of which was which.

With the coming of BS1363 there was a paradigm shift. The size of plug no longer defined the rating of appliance it was to be used with. A fuse was essential due to the 30A circuit rating. The presence of an earth was pretty much guaranteed. The visually distinctive square pins carried the message that this was a new plug or socket using the new system, not just another unknown size. It also permitted a new design of contact and I believe MK patented one form of the socket construction but I would need to look this up carefully to see how the dates indicate the cause and effect in the development path.

The designers of the other competing 13A plugs didn't go this route. The DS plug had modified round pins (the earth was grooved, the line pin was a screw-in cartridge fuse) and closely resembled BS546 5A (it was the most compact of the three). The Wylex 13A was an alteration of their existing system with an ingenious twist. You could plug a new 13A fused plug into an existing 15A Wylex socket (on a 15A radial), but not an unfused 15A plug into a 13A on a 30A ring. The idea presumably was to encourage existing Wylex users to stick with them. Wylex 13A plugs and sockets were engraved with 'Ring Main System' in a circle or similar to distinguish them, without which the 13A and 15A were confusingly similar.

Finally, it could be called a 'square pin plug' and everyone would know what you were talking about, even if they didn't know their amps from their volts. In contrast look at the situation with BS EN 60309, the plug formerly known as BS4343. Some people call it by a brand name (Ceeform, Marechal, Commando) and others by its colour (blue plugs, red plugs). No-one can agree what to call it because it doesn't have an 'official' name (discounting the long-winded title of the standard) nor one outstanding distinguishing feature.

 

[pirate]

PA Testing may be boring, but it does provide some amusing plugs and extension leads for the museum, especially those ones from the big river which have a fuse which is not connected to anything, and cables smaller than bell-wire...

 

[Midwest]

must be quite old then.

Possibly written in 1969

 

[Midwest]

Square pins = 'Modern wiring system in use here.'

Here is my reasoning:

In 1947 we had a bewildering selection of round pin sizes in the UK: 3-pin plugs to BS546 in three current ratings: 2A, 5A & 15A.
2-pin versions to BS372 in 2A, 5A and 15A that weren't interchangeable - the spacing was different, you couldn't put a 5A 2-pin plug in a 5A 3-pin socket.
A few 10A still survived, having been part of the Lundberg range that spawned BS73 / BS372.
Manufacturer-specific types included Wandsworth and original MK gauges in various current ratings (MK offered both their own gauge and BS gauge plugs at one time. Theirs were more compact for the same rating).
European plugs based on the 19mm pin spacing would turn up in the UK and loosely fit the 5A 2-pin.

About the only plug on sale that did not have round or modified round pins was the original Wylex type. All in all it was difficult for the householder to be sure of which was which.

With the coming of BS1363 there was a paradigm shift. The size of plug no longer defined the rating of appliance it was to be used with. A fuse was essential due to the 30A circuit rating. The presence of an earth was pretty much guaranteed. The visually distinctive square pins carried the message that this was a new plug or socket using the new system, not just another unknown size. It also permitted a new design of contact and I believe MK patented one form of the socket construction but I would need to look this up carefully to see how the dates indicate the cause and effect in the development path.

The designers of the other competing 13A plugs didn't go this route. The DS plug had modified round pins (the earth was grooved, the line pin was a screw-in cartridge fuse) and closely resembled BS546 5A (it was the most compact of the three). The Wylex 13A was an alteration of their existing system with an ingenious twist. You could plug a new 13A fused plug into an existing 15A Wylex socket (on a 15A radial), but not an unfused 15A plug into a 13A on a 30A ring. The idea presumably was to encourage existing Wylex users to stick with them. Wylex 13A plugs and sockets were engraved with 'Ring Main System' in a circle or similar to distinguish them, without which the 13A and 15A were confusingly similar.

Finally, it could be called a 'square pin plug' and everyone would know what you were talking about, even if they didn't know their amps from their volts. In contrast look at the situation with BS EN 60309, the plug formerly known as BS4343. Some people call it by a brand name (Ceeform, Marechal, Commando) and others by its colour (blue plugs, red plugs). No-one can agree what to call it because it doesn't have an 'official' name (discounting the long-winded title of the standard) nor one outstanding distinguishing feature.

I said all that in #3

 

[Octopus]

The I.E.E had 22 meetings (no surprise there then).

Before they could on which biscuits to have with their tea?

 

[static zap]

And why did the old plugs have an aperture above the earth pin?

As a kid I've seen it , attached to bell wire ... and stuffed in back of 2 core valve radio Earth ... (Ant ..more of same going to bed frame)

Who wants bell wire -passing live terminals ! Woolworth days ...arrr

(Vaguely remember up market brown 5A round pin, round bodied plug + pull knob)

 

[sparkdog]

I remember when I was an apprentice being told that the aperture was so you could see the earth wire was connected, I was suitably impressed.

 

[GeorgeCooke]

I wouldn't agree most other socket systems have round pins. The whole of North America and most of South America have flat pins. As do Australia, New Zealand, Japan, China, and several SE Asian countries.