Alec Watson of Technology Connections takes many electrical plugs which are used widely in the United States to find out why every plug has a hole on each prong. He explains why one of the prongs is wider than the other one because of the polarization but has a bit more difficulty finding out why the holes are there.
Today we’re answering the age-old question “why do US plugs have holes in them?” That’s a great question! And many people have asked that. Especially people who have noticed that plugs have holes in them. …Well, the toads over at NEMA actually do tell us what the deal is with the holes. The holes are, and I quote, ‘Optional, and it is intended for manufacturing purposes only. However, if used, it must be located as per dimensions shown above.’ Interesting.
Watson posits that the holes are there in order to align the plugs as they are being forged.
The worker simply has to thread an alignment rod through this holder, place the blades and wires in there while pushing the rod through as they go, give a tug to ensure its aligned, and boom. Plugs made to spec.