Why are there holes through the male part of a US electrical plug?

That’s a great question.

A mold retainer is another possible use. The majority of plugs are injection-molded assemblies. The prongs must stick out of the mold. The holes will provide a positive retention for them, so they don’t slip and cause a bad molding.

Another possible use that I’ve seen is as a lockout, e.g. To prevent the device from being plugged in or activated by someone else, thread a rod, wire or bolt through the holes.

A quick internet search will reveal all.

They match bumps on the contacts of the outlet receptacle to help retain the plug and improve the contact. It acts as a detent because the contacts are springy.

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