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Old 1st Jul 2011, 10:40 am   #19
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
Posts: 2,508
Default Re: Another unusual plug and some other questions

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
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