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Old 5th Mar 2012, 1:09 pm   #97
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Join Date: Mar 2008
Location: St. Frajou, l'Isle en Dodon, Haute Garonne, France.(Previously: Ellesmere Port, Cheshire, UK.)
Posts: 2,813
Smile Re: Another unusual plug and some other questions

Somewhere here I've got a couple of those 5amp Crabtree plugs with a groove around the earth pin. It is indeed an interlock and works well.
The earth pin, you'll notice, is also hollow because the mating socket has a rod in the earth hole with a sliding spring loaded disc that blocks the latch & the switch from moving when no plug is present. Insert a plug and the disc is pushed down by the pin. The groove then coincides with the latch and the switch can thus be operated and the plug trapped.
There were two snags with this design:
1. Only Crabtree plugs could be used as only they had hollow earth pins.
2. W&G used to make sockets with shutters that twisted sideways when a plug was inserted. OK with ungrooved earth pins, but grooved Crabtree plugs would get stuck and could not be removed. I know, 'cos I've done it!
Cheers, Pete
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