Why are some US electrical outlets keyed to go in only one way?
It used to be that you could plug in something and it didn't matter which pole went into which outlet hole in the wall. Now many plugs have one side slightly bigger than the other so it will only fit one way. Why is this if the current is AC and so it doesn't matter which plug goes into which hole?
Answer:
There are, of course, two styles of polarized plugs in the US.
One style which some posters refer to has three plugs, one of which is round -- those three plugs are "hot", "neutral" and "ground."
However, as the asker noticed, SOME US plugs are two-prong polarized. In these plugs, the right-hand-plug is larger than the left. The right hand plug then is "hot" and the left is "neutral." (These are called "type B" plugs.) The older style had two narrow blades and are refered to as "Type A" plugs.
Why polarize plugs? Because in an effort to hold down costs, the makers of some electronic equipment used what is called a "hot chassis." This was common in low end radios and televisions, and extended to other appliances. In those cases, while the appliance would WORK properly with the plug inserted either way, if the plug was in one way, it was safe to touch the bare metal parts of the equipment and the other way, it was most assuredly NOT safe to do so. So, the National Electrical Code people, and the equipment manufacturers came up with the polarized plugs.
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