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Old 3rd Jul 2011, 7:57 pm   #28
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

Quote:
I seem to recall one which, as well as allowing two plugs to be connected, also had connections for a fixed appliance too
Yes, this adaptor is of that type, see pic. Whilst it is generally well engineered, the cable entry layout is crude, which suggests it was an afterthought. There is no positive clamping arrangement and very little room for the conductors.

Quote:
commercial products are legislation driven
I don't think this was as generally true then as it is now, perhaps because it was not necessarily an offence to supply non-compliant goods provided that no claim of compliance was made. Nonetheless, the IEE regulations did require the use of plugs and sockets meeting applicable British Standards, although the regulations themselves were non-statutory. It is probably safe to say the tide of opinion on which the regulations were based was reflected in the best quality products of the time.

The critical period for the emergence of 3-pin was during the 20's and 30's. I don't have a copy of the 9th edition of the IEE wiring regs (1927) but in the 10th edition (1934) we find in regulation 1001(E) a requirement to earth all exposed conductive parts of appliances, and a note specifically deprecating the use of reversible 2-pin plugs and lampholder plugs while recommending appliances of all-insulated construction. The implication here is that non-reversible 2-pin plugs would be suitable if such appliances were chosen, and we find reference to the proper connection of non-reversible 2-pin plugs and sockets in 607 (H) and (K). Plugs of this type were available in the era but never caught on, probably due to the lack of all-insulated appliances. Thus, if these regulations were in part responsible for the obsolescence of 2-pin, the lack of polarisation of existing types seems to have been of primary concern.

Incidentally, when visualising the wiring of a 3-pin plug in 1934, don't forget that brown was the colour for earth in a flexible, green at that time being one phase of 3-phase AC.

Lucien
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