Why can some (US) outlets fit a plug from either way you put it in, but some plugs have a fatter and skinnier prong?
In: 8026
I have an old house now. The old wires are all black and the few original sockets have two equal blades. The old style filament light bulbs do not care which way they are wires, as did many other appliances.
The new light fixtures I installed were LED’s which use a LOT less electricity, BUT…they care very much which prong is hot and which is neutral. I bought a $20 no touch pen-style sensor, and when testing wires, I would color-code them before installing the fixture. White is neutral, black is hot, and bare copper is ground, (or green wire is ground).
The wide-narrow blades on modern appliances make it easy to make sure to plug it in correctly. A double-insulated vacuum cleaner with a plastic body may only have two prongs instead of three, and it may not care which way its plugged in, but some devices still need correct polarity.
Simple answer: polarity. In the US, the 120V outlets have two (parallel) flat holes but only one of them could potentially shock you (in practice we call this live or hot). Now say you place a switch between a plug and an appliance. Most switches sold in the US are single-poled. A single pole switch only breaks an electrical connection in one wire. It is important that you break the electrical connection of the wire that is connected to the flat hole which is live/hot. If you didn’t do that, then the device could still have potential to shock you even if the switch is turned off.
That’s only half of the story though. In Europe, they also have a potentially dangerous (live/hot) round hole and a less dangerous (neutral) one. Regions in Europe are divided whether it’s important to differentiate between the two or not. For example, the German plug (Schuko), and Italian plug show that it doesn’t matter much as long as the switch you’ll use with it is always double-pole (breaks the electrical connection in both wires). On the other hand, the French, Swiss, and British plugs say otherwise as you could only insert their plugs one way. Especially for the British, the orientation of their plug is very important since their plugs are also fused and that fuse is directly connected to the live/hot wire.
Edit: added more example
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