This is the type of plug I’m referring to: [https://www.dhcsupplies.com/store/p/6483-9-x-16/3-Power-Supply-Cord-with-U-Ground-Plug.html](https://www.dhcsupplies.com/store/p/6483-9-x-16/3-Power-Supply-Cord-with-U-Ground-Plug.html)
In: Engineering
Polarity!
While for many applications (especially an incandescent light bulb) it doesnt matter which prong power goes in / comes out, sometimes it does. Especially electric motors, and having them all push/ pull in the same “direction” is more efficient for the electrical grid as a whole.
The holes have a couple of uses:
1st they allow for locking out/tamper proofing, along the line of a plastic tag with instructions like “you must do x,y,and z before plugging in.
2nd, they align with bumps in the socket that help keep the plug snugly secured and prevent accidental removal.
The smaller prong is the “hot” one (i.e. the one that has voltage applied to it). The larger prong is the “neutral” one.
The holes are to let the outlet grip the plug a little bit better.
It’s an application of the original phrasing of Murphy’s Law: “If there are multiple ways of doing something and one will result in failure, people will do things the way that results in failure” which was a statement of design principles: if you have a symmetrical plug that blows out fuses when plugged in one way but not the other, you need an asymmetrical plug.
As someone else stated, it’s easier on the grid when electric motors draw current in the same direction, and plugs like that along with sockets like that ensure that this happens as uniformly as possible.
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