The long earth pin on a UK plug
Of the three prongs on a British plug, the top one is longer, and in a sense it is the odd one out.
Category: Home & kitchen
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Feature. It unlocks the shutters on the live and neutral holes.
The earth pin on a BS 1363 plug is longer than the others. It goes into its socket first and pushes open the shutters guarding the live and neutral holes, so a poked finger or key cannot reach them.
Source: BS 1363, Wikipedia.