Several months ago, I saw a video on YouTube about why plugs have holes. I’d always wondered and was under the impression that it was to help hold the plug in place. That turns out to be a myth, at least with modern plugs.
The original 1913 patent for the wall plug (aka the Separable Attachment Plug) did, in fact, have a mechanism inside the socket to engage notches in the plug. Those notches eventually turned into holes. Over the decades, it was apparently decided that this plug security feature was not necessary. Sockets stopped including a means of utilizing these holes. So, why are they still there on so many plugs? Is it just that they’re expected now?
This video runs through the history of the plug and the notches/holes, the patent, and the eventual loss of their utility.
What the video doesn’t mention (somebody in the comments does) is that the holes are apparently now re-purposed, used in an alignment process for molding the plastic plug jacket. You can see that in action here.
The holes have also been adopted as a lockout mechanism where a small padlock is attached through the holes to prevent someone using a machine without proper access (i.e. key key).
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