If alternating current goes both ways, why do some plugs only allow you to put them in one way?


If alternating current goes both ways, why do some plugs only allow you to put them in one way?

In: Engineering

The current flows both ways but only one of the prongs has *voltage* on it. In other words one side is neutral while the other side alternates between “pushing” and “pulling” the current in different directions.

So only one of them is dangerous if you touch it. The neutral would be connected to any outer shell or other part of the device you could touch, while the “hot” is not connected to anything you might touch.

This is not totally safe because it relies on the outlet having the hot and neutral switched, which is why some devices have 3 prongs. The third one is a neutral/ground and that type of outlet is less likely to be miswired so this is the best standard of safety.

Safety. AC (hot wire) goes both ways, but it needs a center point to push-pull against. We call that center the “neutral” wire, which goes back to the power station, but also to earth-ground.

The metal cases of appliances also have a grounding/bonding wire, so that if a wire ever came loose inside the power would go to ground through that wire, instead of through you when you grab the handle. That’s different from the neutral though.

The neutral and ground are connected, so that the center point “neutral” can’t float to some undefined value, with the hot going along for the ride. Why it might float is beyond what I can explain here.

Reversing hot and neutral would connect the case of the appliance to power, through the neutral-ground link in your power panel. Risk of death.

Some appliances don’t have a ground and the hot and neural can be flipped because there’s no metal casing or polarity sensitive devices inside (some kinds of motors care about polarity, etc.).

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.

AC is a wave. Picture a skipping rope – wiggle it so it goes up and down in the same place – a standing wave. Which way the wave is moving is meaningless – but you are wiggling it from one end, not both. Similarly, one wire is providing the power, but the wave is wiggling in both directions.