Another unusual plug and some other questions

[AndiiT]

Hi,
A recent purchase of mains plugs to add to my collection brought with it a strange round pinned 3 pin MK branded plug which at first sight appears to be a "standard" 5 amp one. Closer investigation reveals that the pin spacing lies somewhere in between that of a 5 amp 3 pin and the Dorman and Smith Britmac "Wandsworth" 13 Amp plug. I have enclosed pictures of all three for comparison purposes.

It has been suggested that the plug in question may have been used in industry and, as the earth pin is of an odd design, maybe with a latching socket.

My other questions relate to the progression of plugs and sockets over the years - from a logical point of view it would seem that the two pin 5 amp was first followed by the 2 pin 15 amp, the three pin variants being produced later. That said where does the 2 and 3 pin 2 amp plug/socket combination fit in?
I know that Wylex plugs had an earth pin so were they produced after the 2 pin plugs and sockets?

These questions arise out of pure curiosity.

Thanks in advance

Andrew

[Herald1360]

ISTR a 10A 2-pin plug and socket existed- maybe some evolution of that?

[MikeyPP]

Does this help > http://en.wikipedia.org/wiki/BS_546

[Leon Crampin]

I have a dim recollection of a 3 pin 5A plug being made which was interlocked with the switch on the socket. When the plug was inserted, the switch would move to "OFF" and once switched "ON" the plug was locked into position.

This would explain the ramp and the notch on the earth pin.

Leon.

[ThePillenwerfer]

I've got some old text-books from the '30s which say that on DC mains any socket above 2A must be switched and interlocked so the plug can't be pulled out with the juice on. I would suggest that these plugs were to work with such a socket.

- Joe

[AndiiT]

Originally Posted by Leon Crampin

I have a dim recollection of a 3 pin 5A plug being made which was interlocked with the switch on the socket. When the plug was inserted, the switch would move to "OFF" and once switched "ON" the plug was locked into position.

Hi,
I once owned a plug and socket combination like this some years ago, this was of the standard BS546 5 amp pin spacing, unlike the one shown in my pictures, the difference being that the socket I owned had a switch which would not move to the on position until the plug was inserted and the plug could not be removed once the switch was in the on position.

Regards

Andrew

[dseymo1]

That would make a lot of sense for DC, as it would of course prevent an arc being drawn on withdrawal of the plug.
Wouldn't be a bad safety idea for all mains sockets though.

[MikeyPP]

Maybe as a sort of precursor to sleeved pins to avoid partially removed plugs

[russell_w_b]

Wouldn't be to automatically operate the (interlocked) switch mechanism as the plug was inserted and withdrawn, would it?

[Peter.N.]

Regarding progression I can give my experience of this, which is not a lot. My grandparents house which was built in the early 1900s and presumably had electricity installed later only had two sockets in the whole house, a 2 pin 5a in the kitchen which looked like a tumbler switch with the socket on the top and a two pin 15a upstairs.

The house I was brought up in was built about 1937 and had 2 pin 5a and 2 pin 15a plugs, the houses the other side of town had Wylex plugs.

Just after the war, probably about 48/49 a new large council estate was built near us and was universally fitted with the 13a fused pin plugs centre picture.

Some houses of the period did have 3 pin plugs but I think it depended on the price of the house more than anything whether you got 2 or 3 pin plugs.

My grandparents house had ceramic fuse carriers house in a nice wooden box with a glass front. Ours had metal clad boxes with the exception of the lighting box which if I remember correctly was a MEM whith a knob on the front that pulled the fuse carrier out to switch the supply off.

All these houses were in the Bromley area.

Peter

[Peter.N.]

Having thought about my last post there were a couple of errors. Our house had 5 amp and 10 amp plugs not 15a as I said, also we did have 3 pin plugs in the kitchen, one for the kettle and one for the cooker but all others were 2 pin.

Peter

[ThePillenwerfer]

I think that in the old days, ie pre-war, a lot was down to the preference of whoever did the installation.

My house only had 15A 3-pin sockets, and not many of them, whereas further up the road they had a mixture of 5A and 15A in both 2 and 3-pin versions.

Also around here, and I've heard of them in other places 'Up North,' were some with a round central Earth pin with flat live and neutral at either side.

- Joe

[Peter.N.]

"Also around here, and I've heard of them in other places 'Up North,' were some with a round central Earth pin with flat live and neutral at either side."

I remember those too, mostly on the 'posh' side of town. What you say is probably quite right, the two halves were by different builders.

Peter

[russell_w_b]

'Posh' side of town, eh?

A glance through the 1929 'Sunco' catalogue finds component reference to 'competitive' installations, which I take to mean tendering for contracts, and components suitable for 'high-grade installation work' - which I take to mean the posh side of town: country houses, more salubrious residences, etc...

I was looking at lampholders, but the above applies to plugs and sockets. I particularly like the description which says: '...Designed to meet the modern requirements where lampholders are employed to carry the comparatively heavy load of irons, kettles, toasters, bowl fires, etc...'

[AndiiT]

Hi again,
Thanks for the information about "chronological progression" It's interesting that the choice of 2 or 3 pin sockets may have been down to nothing more than cost.

In the Village I spent my childhood in, which was a small mining village close to the North East coast of what used to be North Yorkshire, there were a multitude of different sockets fitted in houses all over the place.
One house had nothing but 2 pin 5 amp sockets, I seem to recall that this house had a gas oven and whistling kettle which may have been due to the the absence of any "high current" sockets in the kitchen. A house in the same street as this, a few doors away, had 2 pin 15 Amp sockets in two of the rooms and a 5 amp 3 pin in the Kitchen which was used for the Kettle!

Some houses had a combination of 3 pin 5 amp, 3 pin 15 Amp and the odd 13 Amp BS1363 socket which must have been added later.

The council estate was split in to two roads, with Dorman and Smith Britmac 13 Amp types (as shown in my picture) in most of the houses in one of the roads although some of the larger houses had 15 Amp 3 pin sockets for some reason. Whilst the other road had BS1363 13 Amp sockets.

One of my grandmothers friends owned a drapery and the living quarters at the back of the shop had Wylex sockets.

Has anyone been able to confirm yet whether my "unusual" 5 amp(?) plug is an industry special yet?

Andrew

[ThePillenwerfer]

Of course something else which should be borne in mind was that the supply voltage wasn't standardised so there may have been a convention to install different sockets to stop people from borrowing other peoples appliances and blowing them up.

- Joe

[Herald1360]

Originally Posted by AndiiT

and a 5 amp 3 pin in the Kitchen which was used for the Kettle!

Not unreasonable at a time when a typical kettle was 1kW rather than the 2.4 or 3 common today. My parents had a SWAN 1kW kettle in the late 50s which I don't think was more than about 7-8 years old.

[dominicbeesley]

Going back to the original plugs, does anyone have more information on the plugs and sockets that were used for DC and any associated standards? There's plenty about AC on the net and in textbooks but not a lot on DC.

Dom

[Lucien Nunes]

At first, no plugs were standardised - every design was proprietary. In these days of standardisation and global trade we wonder at such an idea, but as the original concept of a socket outlet was only to allow a householder to move his appliance from one position to another, there was no great need. It didn't matter that Mr. Smith's standard lamp would not plug directly into Mr. Jones' living room, as they both bought the appliances without plugs and probably never moved them anyway.

The BS round pin designs developed out of one of the competing systems offered by Lundberg. There was a 10A gauge but this fell out of use in the 1920s. I don't think the type of socket chosen was much influenced by the voltage in use, again because it was unusual to move appliances around. It would have been decided by budget, habit, availability and the requirements of the architect, electrician or householder. For example, if 2-pin 5A sockets had been installed for lighting, and the electrician was called upon to add some points for electric heaters, he might choose 3-pin 15A for safety, without considering this to be a conflict. I.e. the points remained allocated for specific functions; lighting, heating, wireless etc and could be of different sizes on differently rated circuits. Thus 2-pin and 3-pin sockets co-existed for a very long time. In the early days, current for heating was sometimes charged on a different tarriff than that for lighting and heater points would often have had different plugs.

Take care not to confuse the many non-standard types. Britmac, Dorman & Smith, Niphan, Wandsworth (round pin), 'Wandsworth gauge' 13A, Lewden, Santon, Wylex, etc are all unrelated, some based on BS gauges, others not. Then there were the proprietary interlocking adaptations of BS gauges. MK, Crabtree and others made interlocked versions in addition to standard ones, that would usually interchange without interlocking, nor would they interlock with each other.

The MK plug in the picture above is an early interlocking type not made to BS gauge. This system captivated the plug when switched on and prevented the switch being operated without a plug present. The alternative method mentioned by Russell, in which the action of inserting and withdrawing the plug operated the switch, was adopted on the Wandworth type, another non-BS gauge. This had a slotted earth pin with an actuator pawl across, that engaged the lever of a tumbler switch as it went past. The other plug in your original picture, a DS fused plug, belongs to a different era. It was made as an alternative to BS1363 for ring-main circuits.

Most early sockets were intended for either AC or DC. There were no separate standards for the two, although some special features did develop that were unneccessary for AC. MK introduced shutters as an arc-quenching device, Wylex included a detent arrangement with sprung-loaded balls that ejected the plug suddenly as it passed the disconnection point. When electrical accessories with micro-break switches were introduced, they were marked 'AC only' to indicate their unsuitability for DC.

Lucien

[dominicbeesley]

Thanks Lucien for a good overview!

Dom