US electricity = AC 60hz 110-to-120v correct? So why are the plugs polarized?

[CZroe]

If the polarity of the current changes 60 times a second and spends and equal amount of time with each polarity, then why the hell are the plugs "polarized?" AC means that there is no positive and negative pole except for 1/60th of a second! What is the danger of circumventing this polarization with a non-grounded plug?

Not sure if it's the right forum, but I doubt it would be received well in HT because it's probably basic electronics.

 

[PrincessGuard]

One prong is hot, the other is connected to neutral/ground.

For normal operation it doesn't really matter if you reverse the polarity. If you have a metal chassis or other large exposed metal pieces (e.g. the threads of a light bulb socket), however, it's nice to connect them to neutral to minimize the chances of touching a hot contact.

 

[ai42]

Well the reason the plugs go in one way is that if you have 2 devices connection inversely then it is extremely likely that you will create a loop. So if you touch say a toaster and a fridge that are inversely connected and create a loop you can electrocute yourself. It's unlikely it will kill you but will hurt. They just make the plugs only fit one way so you can't do this.

 

[CZroe]

Originally posted by: PrincessGuard
One prong is hot, the other is connected to neutral/ground.

For normal operation it doesn't really matter if you reverse the polarity. If you have a metal chassis or other large exposed metal pieces (e.g. the threads of a light bulb socket), however, it's nice to connect them to neutral to minimize the chances of touching a hot contact.

I've heard it described this way. But what makes it "Hot?" After all, "hot" can not mean positive or negative because both contacts switch between both states 60 times a second.

Originally posted by: ai42
Well the reason the plugs go in one way is that if you have 2 devices connection inversely then it is extremely likely that you will create a loop. So if you touch say a toaster and a fridge that are inversely connected and create a loop you can electrocute yourself. It's unlikely it will kill you but will hurt. They just make the plugs only fit one way so you can't do this.

That makes sense. Thanks!

Now I'm still wondering why they say one is "hot" and the other isn't when they are both live.

 

[Dustswirl]

Am really bad in explaining so i hope this link will be useful!

Cheers :beer:

 

[TheWarden]

I'm not a guru on electrical code, but as I understand it...

First, the difference between hot and neutral. In household wiring, you only have one wire incoming from the power company, which is called the hot. The other half of the circuit is formed by the ground, and the neutral wire is tied directly to the ground at each house. The way you get a shock is by grabbing a wire so that you complete the connection to ground. The electricity flows down the wire, through you and into the ground. Or it flows from the ground, through you and into the wire. Back and forth, either way, it doesn't matter; you have completed the circuit. But if you grab a neutral wire, you get no shock, as the electricity can't flow from the ground through you and back into the ground. The only way you complete the circuit is by grabbing the wire coming in from the power company, thus why it's called hot. So hot and neutral really refer to the potential they have compared to you.

As for polarized plugs, as I understand it, the appliance couldn't care less which way the plug goes in, as, like you pointed out, the current is reversing directions 60 times a second anyway. Polarized plugs were implemented for safety reasons. Normally, the hot wire comes into an appliance and goes through the power switch first thing, then continues on through the rest of the appliance and out the neutral. This means that when the switch is off, none of the appliance is "hot." But if the plug is reversed, the inside of the appliance is still hot, so that if you touched it, you could complete the circuit to ground and get an unexpected shock. A lamp is a good example of this. If you turn off a lamp and change the bulb, but the lamp is plugged in backwards, then you could get a shock from the metal ring on the bulb as you screw it in, even though the lamp is off.

Cheers,
Warden

 

[mechBgon]

Think of the ground as the middle of the sine wave, with the AC on the hot blade oscillating above & below that. You can touch the ground all day long... in fact, you do, because it's got the same electrical potential as the earth, the faucet handle, etc.

Touch the "hot" blade while you're somehow NOT touching ground, and nothing will happen. Touch the "hot" blade while you ARE touching ground... you is a circuit!

By polarizing the plug, the manufacturer of the appliance controls where the "hot" parts will be, and where the harmless grounded parts will be. You've seen power tools that say they're "double-insulated," I imagine... these have both the hot AND ground parts isolated from anything you can touch, so you aren't going to come into contact with a hot part no matter which way they're plugged in.

Bigger picture: "Regulations." 'nuf said, eh?

 

[CZroe]

OK, now it makes more sense. I didn't realize that an outlet had two ground prongs

Have I been mistaken to believe that ground is the path to the opposite charge? ie, on a schematic showing a 1.5v AA battery as the powersource for 1.5v, and place a ground arrow is shown simply means "connect to negative terminal" correct? I've always assumed this and therefore I assumed that the power company sends two lines to every house and they alternate between positive and negative together (Effectively swapping which prong is ground and which one is the power source 60 times a second).

The thing that finaly prompted me to ask was a Sony PSone AC adapter. I has DC7.5v output no matter which way you insert it, and there is no on/off switch yet the manual specifically states that you "Do not defeat the polarization of the AC plug." It makes no sense when you are talking about an AC to DC adapter plug, which is ALWAYS live as long as it's plugged in. So I guess it's safe to defeat the polarization on any device which doesn't have an on/off switch huh?

 

[oldfart]

There isn't 2 ground prongs. There is a phase, a neutral, and a ground. Phase is the prong with the AC, neutral is the return or reference, and ground is electrical ground.

 

[CZroe]

Originally posted by: oldfart
There isn't 2 ground prongs. There is a phase, a neutral, and a ground. Phase is the prong with the AC, neutral is the return or reference, and ground is electrical ground.

So, does "return" go back to the electrical company or what? If it's grounded, then it's the same as ground. If it's not, then that means that the flow of electrons coming from the company truely does reverse making both slots identical. Getting confused again

 

[Mark R]

The difference between neutral and ground is relatively subtle, but important.

Ground is a safety conductor. It is connected to a ground point at your premises (or close to it). It ensures that any metal appliance that is plugged into the mains has its chassis connected to the ground point - i.e. the metal chassis is brought to the same potential as metal pipes and fixtures in your house. Under normal conditions no current flows through the ground wire.

In the event of a fault - e.g. the live wire inside an appliance coming loose - the ground wire prevents the chassis of the appliance becoming live. If the live wire touches the chassis it causes a short circuit and blows a fuse cutting off the power.

The neutral wire carrys current back to the transformer on the power grid - it completes the circuit. For safety reasons it is connected to ground at the transformer, so under normal circumstances the neutral wire cannot deliver a shock (its voltage is always very close to that of the safety ground - because it is connected to ground). Under fault conditions, you can get a shock from the neutral (e.g. if the neutral wire breaks on the way back to the transformer - it loses its connection to ground, and is instead connected to live via all the appliances connected to it). If you are ever working on a circuit, you should assume that both the live and neutral wires are dangerous, unless both have been disconnected from the mains supply.

 

[kursplat]

If you are ever working on a circuit, you should assume that both the live and neutral wires are dangerous, unless both have been disconnected from the mains supply.

for sure. if you open the neutral , then get in series with it you might as well just grab the hot leg.

 

[Varun]

AC is truly a beast of an idea, and I don't think you will ever understand it from just reading on this board. I would suggest that you a)Assume that the hot is like DC+ and the neutral is like DC- and be done with it, or b)Take a course in electrical waveform theory or something to that effect because there is just so much theory that you need to know to understand how AC works.

Just wait till you realise there is such a thing as imaginary power that you are paying for on your monthly power bills...

 

[kursplat]

if you really want to learn about AC visit the experts

 

[justly]

It might help if you realize that household AC current is single phase. This means that only one line carries AC that fluctuates back and forth between 120V+ to 120V-. When done in this way the other line is allways neutral, and provides 120VAC.

If both lines where "hot" (duel phase) each line would only carry 60V, and each line would always have opposing polarities therefore giving a total voltage (difference between the two lines) of 120V. The only standard 120VAC outlets that uses duel phase (that I am aware of) exist onboard Naval ships.

I think the rest of your question has already been sufficiently explained.

 

[kursplat]

When done in this way the other line is always neutral, and provides 120VAC

actually most homes in this country have 2 hots and a neutral , 120v to ground \240v phase to phase , with the electric panel keeping it divided in 2 busses. that way if you need 240v for A\C or an air compressor you use a 240v breaker that connects to both busses.

 

[justly]

Originally posted by: kursplat

When done in this way the other line is always neutral, and provides 120VAC

actually most homes in this country have 2 hots and a neutral , 120v to ground \240v phase to phase , with the electric panel keeping it divided in 2 busses. that way if you need 240v for A\C or an air compressor you use a 240v breaker that connects to both busses.

Correct, I was only commenting on a standard 120VAC household outlet, but when 240VAC is needed it is done just as you say.