If alternating current goes both ways, why do some plugs only allow you to put them in one way?
In: Engineering
It’s all to do with safety devices on the device itself.
The fuse, if fitted is generally fitted on the live/hot side of the circuit, as is the switch. This is to prevent the device sitting there, with most of its internals sitting at mains potential in the event of the fuse being popped, or the switch being off.
This is where the old advice of never stick a fork in a toaster comes from.
In the old days, a lot of mains power connections were unpolarized, so if you plugged it in such that the switch was on the neutral side of the circuit, just the act of plugging it into the wall would make fork-accessible parts inside the slot live, even if the toast switch wasn’t pressed down. This is an example where a polarized plug would increase safety, because it would ensure that the hot side was the switch side of the circuit, so everything inside the slot would remain dead until you pressed the switch down.
Nowadays, things that are double insulated, it generally doesn’t matter, which is where figure-eight mains power connections come from.
Devices where situations like the above could occur, the mains connection is still polarized.
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