Prongs might not be the right word for it.
In: Engineering
There are three connections. Hot, Neutral, and ground.
Under normal operation, hot and neutral carry the current, and neutral is at the same voltage as ground.
If you have a double insulated device(for example, with a plastic case), a single failure won’t cause part of the device you can touch to become electrified(so you don’t need a ground). Though if part of it can be accessed more easily, that will be connected to neutral. This is why a desk lamp will have a polarized plug(two prongs with one side bigger), it’s easier to touch the screw part of the bulb, so it’s supposed to be connected to neutral rather than hot.
As for ground, it’s supposed to be connected to stuff like metal housings, so that if a hot wire comes loose and touches the case, it will short circuit and trip the breaker or fuse instead of sitting there at high voltage waiting for someone to touch it.
The reason neutral isn’t used for this is that if the neutral line comes loose somewhere between the device and the breaker box, turning something on will cause neutral to become hot.
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