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Old 5th Apr 2012, 12:28 am   #131
Lucien Nunes
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Join Date: Apr 2008
Location: London, UK.
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Default Re: Another unusual plug and some other questions

Thanks for those pics Pete - that's what I was trying to recall when I mentioned plugs with open wire fuses in post 112. Late 20's design, obsolete by mid 30's, see attached pic from Sunco 1929 listing three versions, 2 & 3 pin of MK's 15A gauge and 2-pin 15A BS gauge which MK rated 25A on account of their improved contact system. The 'new IEE rules' referred to would presumably have been the 9th edition of 1927

I have a 3-pin 15A BS gauge version not mentioned in that catalogue, pics below. What I find most interesting about it is that the polarity of the socket is reversed relative to that of the plug. Admittedly in its day the use of a 3-pin socket did not imply reliable polarisation but to deliberately reverse it I think is a bit cheeky. DP fusing was the norm in distribution boards and the 3-pin plug was somewhat novel, so perhaps they thought it trivial as DP fusing would be provided in the plug anyway.

The fuses in yours seem to have been subjected to regular incidents. Mine has had one fuse fail, probably just once, however the arc flashed over to the opposite pole (echoing the trouble with MEMDIX fuses I mentioned in the recent thread about Wylex CU's). Metal has been evaporated from both fused and unfused sides of live and neutral fuse terminals and earth, suggesting that rupture of the 5A fuse might have then triggered failure of the 15A in the board.

Prior to the 1920s it had been generally accepted that switches and fuses of opposite poles should be physically separate, the main objection having been the possibility of leakage. The relaxation of that rule seems to have led to a few products like this with insufficiently confined fuses that were prone to inter-pole flashover.

Lucien
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