On the Russian 220V thread, Paul said ...
The little hole on British plugs for the earth pin is a curious feature though (or I should say "was" I suppose, as you don't find it on current plugs.
It seems more likely to me that it was provided simply as a convenient means for grounding to the house electrical system, e.g. to provide a signal earth for a radio.
I only vaguely recall this feature Paul. Do you have any pictures of that you could post? Was it new to the older 13A type? - I don't recall seeing anything like described on the older 15A standard.
