eli5: why do some plugs only have 2 flat prongs and others have a third cylindrical one?
In: 3
The bottom one is ground.
The ground is to prevent a short from electrifying the case. Imagine a metal toaster. It would be quite dangerous if there was a short, so you will always find a ground plug on a toaster, to increase safety.
If you build a different device, with no metal exposed parts, then you don’t need a ground.
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