Why can some (US) outlets fit a plug from either way you put it in, but some plugs have a fatter and skinnier prong?


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

Electricity flows from the positive side of the plug, through the device and out the negative. Some devices don’t care which side goes in the positive side or the negative so both prongs are the same. When it does matter they make them different sizes like that so you can only plug them in one way.

From my non-expert understanding is that the more universal style are unpolarized, where as the design that has the mismatch prongs are polarized

In regular 110V US household electrical systems the two wires is the live wire with high voltage and the neutral wire which have a very low voltage difference to ground. The concept is that if anything goes wrong in an appliance and you end up with a short circuit through the user to ground the fault should be in the neutral wire and not the live wire. So the appliances which have a higher chance of a fault in one wire then the other have a wider prong on the neutral wire so that it can only be plugged into the neutral side of the plug. Appliances where this does not matter have two small prongs which can go inn either way.

There is also some outlets which cheat and have two wide holes instead of one wide and one small. But these do not follow the standard and if something goes wrong in the appliance it is more likely to shock you.

In a US home the electricity travel on three conductors (wires). Line 1, Line 2 and the Identified Conductor.

Connecting between line 1 and line 2 will produce 240 volts while connecting between either line 1 or 2 and the identified conductor produces 120 volts.

Wall plugs you speak of are 120V and the difference between the conductors is that the Identified conductor is grounded at the panel and the other is not.

Some devices are built in a way that requires polarity.

Polarity (same) wiring requires a specific wire be connected to the Identified Conductor. So to solve this, manufacturers make plugs that only fit one way.

This is to ensure that the plug can go into the outlet in only one orientation.

The wider blade is for the “Neutral” wire which is actually equal to ground. The narrower blade is for the “Hot” wire, or the source of electricity. Connecting a device between Hot and Neutral allows current to flow from Hot to Neutral, powering the device.

Some devices don’t have a user-operable switch, so it doesn’t matter which way the plug goes into the outlet, and both blades on the plug will be narrow.

Devices that do have a user-operable switch generally will have blades of different width in the device’s plug. Such a plug is “polarized.” The purpose of a polarized plug is to ensure the device’s user-operable switch is always connected to the Hot wire when the device is plugged in. This makes the device a little safer to use, and less likely to harm someone if they open the device without unplugging it first (but at least have it turned off). Most of the device won’t be hot, only a small portion of the wiring inside will be Hot—that part before the switch.

Imagine a lamp with the bulb removed. You could theoretically stick your fingers into the empty socket. When plugged in, it’s live up to the switch, but assuming it’s turned off you won’t get a shock. If plugged in backwards, it’s live up to the socket… From the other direction! Stick your fingers in and you can complete the circuit no matter what the lamp switch is doing.

Your electric device doesn’t care what direction power flows. It only cares that it flows. This is important in Any device that the user could possibly encounter a condition where they touch a live electric component.

So many comments that are so close to the whole story.

Plugs are polarized or not depending on the device being plugged in.

If there is a chance of exposed electrical parts, like a lamp, the plug will need to polarized so that the screw shell is not “hot” when bulb gets removed. The neutral side is connected to the ground at the panel so you shouldn’t get shocked if you touch it, as long as the rest of the house wiring is done right.

If the device is double insulated, marked by a symbol looking like square inside a square, there is no chance to get zapped, so no need to polarize the plug. Think of your cellphone charger.

Last is old stuff that was built before polarizing was a thing.

If it has a ground/bond prong on the plug, you can’t put it in backwards, so no need for polarizing.

Extra safety.

As long as your house is properly wired, the dangerous “hot” wire will be connected to the narrow side. The wide side will be the safer neutral wire that won’t shock you.

Designers of small appliances and other plug-in devices consider this when designing their devices to be safe. They may take measures such as putting extra insulation on the hot wire, putting the on/off switch on the hot wire, or routing wires so the hot wire is less likely to make contact with the case if it happens to break.

This isn’t relevant for the ELI5, but in Britain, even though there are only 2 wires (live and neutral), there is an amazingly complicated system. Sets of plugs are connected in a ‘ring main’ configuration, all in parallel one to the next and back to the beginning, so neutral, live, and ground are all three in parallel. Then, neutral and live are brought separately *outside the house* and connected together *outside the house* and also ground is connected to something in contact with the ground. Also the ground (but NOT neutral) is connected to all pipes inside the house.


This is so safe that people have electric showers and no-one ever gets hurt, even though at twice the voltage, there is four times the shocking energy.


One reason this works is, if any one wire breaks behind the wall, connecting one plug to another, it will not matter (that is the point of the ‘ring main’ configuration). But also since the ‘ring mains’ are in small groups, there is not too much power dependence on any one wire even after the break.


Another feature of this is, with all these wires securely intact and with redundancy, if there is ever a short between live and ground in the house, electricity will have to go outdoors and go through the place outdoors where neutral and live are connected, hence through the fusebox and exceed the current capacity and blow a fuse.


Nowadays there is RCD circiutry which also detecgts this, but the point is, this system works independently of RCD.


So, going back to your question about switching the orientation of a plug, if it ever happens that in say an electric drill, the neutral and live wires were switched internally, and neutral connected to the case of the drill such that now live and ground are both connected to the case, that connection will create a circuit whereby electricity has to leave the house to cmplete the short circuit, blowing a fuse, even if the RCD system fails.


Neutral and ground are functionally equivalent, but there is this system that detects that they are kept distinct everywhere inside the house.


This makes sure that it can never happen that there can be like a pipe next to an electric shower which is live, while the water in the shower is neutral, etc.

Polarity sensitive devices.

Like a lightbulb you dont want the whole ring around the bulb to be the part with the zappy zappy you want the little pin in the back so that when you unscrew the bulb you dont chamce getting the zappy zappy in your fingers.

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