Why are the prongs on electrical cord asymmetric?

Forget about the third “prong.” I know that is for grounding purposes.

On modern appliances, the two main prongs are not symmetric and, of course, neither are the holes in the outlet. There is only way to plug it in.


I have a degree in physics and can confidently say that the appliance would not care which way you plugged it in. You could cut the cord, reconnect the wires in the opposite way, plug it in and your DVD player would work just fine. This is because it is alternating current. At one moment the electricty flows in one direction, the next moment the other. And it doesn’t care what the absolute voltages are (since their strictly is no such thing). One side could be at +50 V (relative to ground) and other -50 V, or one side at +500,100 V and other at +500,000 V and the appliance would not know the difference.

So what’s up with the prongs?

Polarity. Many devices require the posiive and negative poles to be stansardized.


It is an attempt at keeping the hapless consumer from executing himself in cases where the chassis is used as a return path for electrical current (even when it’s not, the chassis may internally come into contact with a live wire). Lean on the edge of the (earth grounded) kitchen sink and reach for the (hot chassis) toaster, and this realization may strike you all too powerfully.

True, but it may be unsafe.

The “wide” prong is neutral; the “narrow” prong is “hot.” There is a difference. If an appliance has a SPST power switch, then it should switch the hot, not the neutral. Furthermore, the threaded conductor of a lamp socket should be connected to the neutral, not the hot.

So what they’re saying is that people who don’t supply polarized plugs (i.e., symmetrical) are trying to kill us all? Bastards.

I’ve always wondered why the prongs on electrical plugs have holes in them. Is it so that you can secure a plug in the socket by putting a screw or bolt through it?

Also, if, as indicated in the question linked to above, if “boondoggle” and “boon” are words, is “doggle” really a word, and does it mean what the writer seemed to think it meant?

But a chassis is never connected to neutral. At least than I’m aware of.


If the appliance doesn’t have a power switch or light bulb socket, then there’s usually[sup]1[/sup] not a reason to have a polarized plug.

[sup]1[/sup] [sub]There are (obviously) exceptions to this “rule.”[/sub]

Not intentionally. Drop that toaster enough times, though…

Interesting. If one prong is indeed neutral, while the other fluctuates between + and -, the safety idea makes sense.

But then why the need for a ground wire? Why not just use the neutral wire as ground?

Also, I once upon a time asked this question of an electrical engineer and have since forgotten the answer. I didn’t really understand it at the time since we only talked briefly. He said it had to do with a large-scale effect on the system as whole, not with regard to individual outlets.

Because the NEC says you’re not allowed.

According to NEC, you’re not allowed to come in contact with a current-carrying conductor (under normal circumstances), even if it’s a neutral line. Since the neutral carries current, you’re not allowed to connect the chassis to neutral. And the neutral can be dangerous if there’s a break in it.

Because of the above reasons, it was decided to run a separate ground wire to certain appliances. But unlike the neutral, this wire does not carry current under normal circumstances, and can thus be connected to the chassis.

I think you are confusing a couple of things.

The ground and nuetral are tied together at the service entrance to your house. The reason we use two seperate wires is that the nuetral carries current, which means that since copper isn’t a superconductor, there is a voltage difference between the nuetral and earth ground. It’s a very slight voltage difference, but its not zero. Since the “ground” wire carries no current, it is always at the same voltage potential as earth ground.

The large scale thing relates to why do we bother to ground the system in the first place? Hospitals use isolated systems, and the benefit of an isolated system is that you can touch ground and either wire and not get shocked. The only way you can get shocked in an isolated system is to touch both conductors. The problem with doing this in residential wiring is that hospitals have a hard enough time keeping their systems isolated, and they are contained within a single building. If you don’t have a ground, mother nature likes to randomly insert a few into your system, and there goes your isolation. The larger the system, the greater the chance that somewhere it is going to make an accidental connection to ground. For residential wiring, the system is just too large and there are too many opportunities for mother nature to sneak in. So instead of having a randomly grounded system, we intentionally ground one conductor to make the system much more predictable and safe.

Thanks. All of the responses were appreciated.