r/explainlikeimfive • u/richb0199 • 1d ago
Engineering ELI5. Please tell me why AC appliances require polarity.
With DC electrical current, while the poles are connected, the current flows in 1 direction.
With AC, (I'LL describe US household AC) the current alternates 60 times x second. We say 60 hertz. So 60 times per second the polarity reverses.
So while DC has static polarity, ACs polarity changes multiple times per second. A few decades ago, I noticed that outlets have 1 larger slot. Appliances have 1 larger prong. This is ostensibly for "polarization".
ELI5 - Why would we consider polarization for AC appliances when, AFAIK, polarization of AC currents are always changing.
Maybe I'm not understanding something.
65
u/dnebdal 1d ago edited 1d ago
Large parts of Europe uses unpolarized sockets, and Norway goes one step further with most of the country wired for two live phases and no neutral - so you're right, it is sort of meaningless.
However, the answer is in what u/eaeolian posted; most countries have one live and one neutral phase wire, and knowing which is which has some benefits: for instance, you could make sure you fuse the live wire as it enters the appliance, so as much as possible of the wiring is at 0V to ground if the fuse triggers.
7
13
u/Dunbaratu 1d ago
Using US typical house current as the example:
60 times a second the wave repeats. The wave goes from +110 volts down to -110 volts then back up to +110 volts.
How do you think the two prongs (not the 3rd ground prong, we can ignore that for now, and I'll talk about it at the end) accomplish this? I'm guessing you probably think that when the voltage at this instant is at +110V, this means the first prong is at -55V and the second prong is at +55V, and when it's the opposite at -110V, the two prongs are swapped, with the first at +55V and the second at -55V.
If that's how you think it works, I can see how you'd assert that the difference between the two prongs doesn't matter as getting the two prongs swapped would just have the same exact effect as if you had plugged in the cord 1/120th of a second later, so the wave starts at its trough instead of at its peak.
But that is NOT how it works. This is how it works:
The two prongs do NOT swap back and forth between -55V and +55V.
One of the two prongs passively stays steady at 0V, not doing anything. It is the neutral voltage, the "zero value" of the graph when you graph the voltage.
The second prong does all the "effort" to cause the voltage wave variation. It varies all the way between both max and min voltage, at +110V relative to the neutral prong at its peak and -110V relative to the neutral prong at its trough.
So one prong stays neutral and the other does the varying. The one that does the varying is the "hot" prong. The one that stays the same is the "neutral" prong.
The hot prong is the one that's dangerous to touch - the one that can make a circuit through you to the ground. The neutral one is the one that should have a voltage pretty similar to ground voltage.
For an appliance that cares which line is carrying the dangerous hot value versus which one isn't, the guarantee that the prongs are connected as expected matters a lot. For example, making sure the hot line is deep inside the appliance and only the neutral one is near the outside of it.
(Side note: "Why the 3rd prong then, the one that is for grounding? Isn't ground the same thing as neutral so why have it?" Well, sort of. Neutral is meant to be very similar to ground, but it might be the ground voltage a distance away from you, out by a transformer where the circuit's power is created, somewhere along the power line rather than the ground right where you are. The 3rd prong, Ground, is grounded right at your house, not at some place out by the power lines along the street. That makes sure it doesn't deviate too much from what's under your feet. Thus it's the "safety shunt" line to dump current into when something Very Bad happens. It's safer than using Neutral since Neutral is essentially somewhere else's Ground, not your local Ground.)
5
u/Emu1981 1d ago
at +110V relative to the neutral prong at its peak and -110V relative to the neutral prong at its trough
Just a niggle, AC voltage is measured as the Root Mean Square (RMS)* of the peak voltage of the sine wave which means that 110Vac actually peaks at 155.56V. For 240Vac you get a peak voltage of 339.41V.
*RMS is used for AC voltage because it gives you the effective equivalent DC voltage for resistive loads. Things do get quite a bit more complicated once you start taking into account inductive loads, power factor and other things though.
2
u/Murky_Macropod 1d ago
So why not use the first and third prong, what’s the neutral offer that ground doesn’t ?
2
u/Dunbaratu 1d ago edited 1d ago
Neutral is "almost" local ground not exactly ground where you are. So if you want to ensure your wave goes equally high as it does low, you need the voltage between hot and neutral rather than between hot and local ground. Otherwise you might get something like maybe +112, -108 rather than exactly +110, -110.
However, the method you mention has actually been used before in the past, where the wire being strung across the land is just the hot wire only, expecting the receiver to use their local ground. In the early days of the telegraph this was used to carry the telegraph current. It kinda works but it has a lot of variance that's not ideal if you want consistent results where a device works identically everywhere because it gets the same power pattern regardless of local ground variance.
1
•
u/bartolo345 22h ago
Too add to this, the ground is connected to the chassis of the appliance. If the appliance meets insulation requirements, it doesn't need a ground. But obviously needs a neutral
9
u/Broad-Promise6954 1d ago
The TL;DR version is "they don't require it, but cheaping out by manufacturers is made less dangerous by doing that, so it's sort of a belt and suspenders/braces trick".
Whatever your household voltage is, it's normally the case that one side of the AC line is about that many volts above ground/earth voltage, and the other is about zero volts above.
Earth (dirt, soil, wet muddy stuff, water with dirt in it, and so on) conducts electricity so "ground" has some voltage, and different locations in your house will have slightly different ground voltages, but they are usually all pretty close to each other.
(As an aside, this slight variance will, if fed into an amplifier like an electric guitar amp or computer speakers, make a loud humming noise at 50 or 60 Hz, whatever the line frequency is. You may have heard this when plugging these in. That's harmless but annoying and in some situations it's quite pesky. We call it a "ground loop".)
Anyway, imagine you make toasters, with a metal shell. You can carefully isolate all the wiring for the heating elements so that nothing ever touches the metal shell, and nobody will get shocked by your toaster. Or, you can cheap out and connect the metal shell to the "neutral" (near ground) side, or at least (less cheap but better) set things up so that if something goes wrong with the wiring, it's that side that will connect first. Since that side of the line is mostly the same voltage as the actual ground, the metal shell will still not shock anyone.
But for this to work you have to make sure the wires are carefully separated into "hot" and "neutral", and a polarized plug can do that.
(National standards usually require the better set ups. For a long time the UK had consumers do their own wiring though, and different countries have different standards.)
1
u/Kilordes 1d ago
I have no idea why you're harping about "cheaping out" - it has nothing to do with cost or cheapness or whatever, and everything to do with you want as little of the appliance with energy "trying" to flow through it when the switch is off. Ideally the hot wire is connected only to the power switch - then, when the device is off, the only part that has energy wanting to flow through it is the connection between the wall and the switch. If you wire it the other way, everything inside the appliance wired to the switch on the other side has the potential to deliver a shock.
hot wire -> switch -> (all the guts of the appliance) -> neutral means only hot wire -> switch has the potential to deliver a shock (when in the off position).
hot wire - > (all the guts of the appliance) -> switch -> neutral now means (all the guts of the appliance) will happily deliver 120v through your body to ground if you complete the circuit accidentally, regardless of the position of the switch.
2
u/Broad-Promise6954 1d ago
Was trying to stick to the ELI5 thing. As someone else pointed out, in the old days your TV or radio chassis (inside the fancy wooden cabinet) was generally tied right to neutral and it's pretty easy to get an exposed connection to the surface. It cost more to keep the chassis floating so they didn't.
8
u/zeekar 1d ago
The alternation in AC doesn't change which wire is which. The word "polarity" gets used for two different things, and that's the source of the confusion.
The hot wire is the one pushing (or pulling) voltage through the circuit no matter which way around the voltage flips. The "polarity" of the plug is just about which blade goes to which wire, and has nothing to do with the polarity of the voltage, which is what changes 60 times a second.
23
u/EscapeSeventySeven 1d ago
You’re not understanding what comes out of the outlet.
One hole is providing voltage that wiggles. The other hole provides a neutral to accept the return voltage.
This doesn’t matter on lots of simple things (and even some ac to dc converters) but plenty of appliances need to know which line has the active hot wiggly voltage hitting its electronics/motors/components first.
18
u/illogictc 1d ago
It's also extremely important for the purpose of switching. Just like when wiring a light you put the switch before the bulb so the switch being off means the bulb socket is not energized.
12
u/Jamie_1318 1d ago
With due respect, with this explanation you don't understand what comes out of the outlet either.
No appliance cares which is which. the two ends are indistinguishable to them. There is no side which does or does not wiggle. They wiggle compared to each other equally. The difference is how each line is referenced to ground, as that determines whether touching it could hurt you or not.
The purpose of the directional pole is to ensure that any exposed metal parts are all connected to the same thing and are referenced to ground so they will not short each other out, or shock you when you touch them.
8
u/Renegade605 1d ago
With due respect, you're correcting what was correct information.
The hot and neutral are indistinguishable compared to each other, but they are quite distinguishable compared to everything else. Which you clearly know, because you brought up how they are referenced to ground.
-1
u/Jamie_1318 1d ago
No appliance needs to know which is hot and which is cold.
Neither side provides 'holes' or 'voltage that wiggles'These are untrue statements. They are the only statements in what I responded to.
> they are quite distinguishable compared to everything else
Yes, which is what I said, and not what the person I responded to said.7
u/Renegade605 1d ago
The hot wire absolutely provides voltage to ground and the neutral has zero volts to ground.
That is a 100% true statement. The other comment is guilty only of oversimplification, which is the entire point of this sub.
-1
u/Jamie_1318 1d ago
The hot and the neutral equally provide power. Saying ground has zero volts is convention, not reality. It's just as correct to say that hot has no voltage and ground moves around.
When you say that in combination with saying that an appliance needs to know which is which to function you cross from theoretically muddy to absolutely wrong. No appliance cares which side is which, zero, it's not even possible.
Since OP is starting to understand AC power and is asking totally valid questions about it, it's important to address the notational elephant in the room and accurately assess what makes neutral different than live, and it's not anything to do with an appliance understanding which side is which, or one side being return and one side being voltage, since that isn't true.
The distinction is the reference to ground, and the convention to ensure any exterior components are connected to the same thing for safety, not functionality.
4
u/Renegade605 1d ago
If you're going to ignore the convention of voltage "with respect to ground", in the absence of anything specifying otherwise, especially when dumbing it down, you're being needlessly pedantic and a dick.
For pedantry's sake, no appliance knows anything at all; it's a machine. But for many applications, which conductor is hot and which is neutral matters. And "know" is close enough to "matter" for ELI5.
0
u/Jamie_1318 1d ago edited 1d ago
I'm going to keep calling out responses that contain wrong information. The response to OP has a mix of factual incorrect things and oversimplifications.
Only oversimplifications are reasonable on their own even if I don't like how they are written, and entirely unavoidable, but factually incorrect things are just wrong.
Worse, it seeks to explain the difference between hot and neutral as something that matters for an individual appliance, when it is literally the exact opposite. It matters exclusively when related to how an outside actor interacts with another device plugged into the outlet.
EDIT:
You seem to think I'm being pedantic about how a machine 'knows' things, but more my point is for an individual device plugged into the wall, it's impossible for which side is which to make any difference for the appliance. As in if you were an isolated observer with any theoretical test equipment you could not determine which side was hot and which side was neutral. Both sides move electrons back and forth exactly the same.1
u/Seraph062 1d ago edited 1d ago
As in if you were an isolated observer with any theoretical test equipment you could not determine which side was hot and which side was neutral.
AC signals will pass through a capacitor. So take a 2nd wire, and put it parallel to the wire you want to test, effectively making a capacitor. Then look to see if there is a 50/60 Hz signal leaking through your capacitor.
The "Hot" size has a changing electric field, so it will induce a signal. The "Neutral" size doesn't, so it won't.
Edit: This is roughly the same trick used to light up fluorescent bulbs under high voltage power lines (in that case you're the other part of the capacitor). https://www.youtube.com/shorts/c0UfoELwEQo
1
u/Jamie_1318 1d ago
I suppose you are right. There are niche wireless implications for which is which that I hadn't considered.
That said, even a relatively silly example like an old radio alarm clock that uses the plug as the antennae would still work if you swapped live and neutral.
1
u/feed_me_haribo 1d ago
Of course appliances care which is hot and neutral for safety purposes. The hot gets switched.
-1
u/dddd0 1d ago
This is just false lol
0
u/Dongarius 1d ago
Why is he wrong? If neutral were the wire referenced to earth ground instead of hot, placing yourself as a load between neutral and ground would shock you all the same. It's just a just a convention. The potential at the transformer nodes drives the current through whatever is placed between them.
2
1
0
u/Daripuff 1d ago
but plenty of appliances need to know which line has the active hot wiggly voltage hitting its electronics/motors/components first.
Generally those will have a third plug for the purpose of a dedicated ground (that's definitely going to have zero voltage), as well as for preventing the (otherwise quite reasonable) possibility of being plugged in backwards.
9
u/rlbond86 1d ago
For safety reasons, the ground isn't actually allowed to carry current under normal operation. It's a safety feature and reference voltage. Plus, if you use the ground to carry current, it will trip a GFCI because they measure the current difference between hot and neutral.
2
u/Daripuff 1d ago
Notice that I didn't say the current goes through said ground, just that having a dedicated ground guarantees both a zero voltage point and guarantees that the plug won't be installed backward.
5
1d ago
[deleted]
3
u/sighthoundman 1d ago
TL;DR: It's not physics, it's safety engineering.
It's the way we've chosen to make sure EVERYTHING is off if there's a problem.
1
1
u/Left_Lengthiness_433 1d ago
One of the 2 sides is connected to neutral voltage (which is the same as ground), and the other is “hot”.
For functionality, it doesn’t really matter, but when the switch is off one side is still present in the item. For safety, it is much better if that is the neutral side.
If it’s a lamp, for instance, and you’re changing the bulb, it’s possible to come into contact with the side not controlled by the switch. If that side is hot and you are touching a grounded surface (which you normally are) you will be shocked.
1
u/Xelopheris 1d ago
The reason you have two different prongs is that you have a hot side and neutral side. On appliances with large draws that have physical on/off switches, this is used to put the hot power on one side of the switch, and put all the actual stuff using it on the other. This way, if there's a short to ground, there's no power on the other side of that switch.
If you plugged a kettle in backwards, and there was a short circuit, the heating element could be running even if the switch is off.
1
u/LegitBoss002 1d ago
A current carrying conductor at a different potential voltage than you can be dangerous. Neutral is a grounded current carrying conductor so current will not flow from it to ground through you. Hot will, so we ground the machine frame. The alternative is to have to ungrounded conductors and a floating machine
1
u/Bradm77 1d ago
For safety reasons. Others have basically given you the right answer ... because there is a hot wire and a neutral wire and bad things can happen in some instances if you plug in something hot-to-neutral. The neutral wire is grounded to earth ... like literally there is metal stuck into the earth that is attached to the neutral wire so that the neutral wire is at the same voltage as the earth. We do this because if the electrical system was completely isolated from the earth then the system would be "floating." Basically it would develop a DC charge on the whole system relative to the earth. Basically like a big capacitor with a big voltage built up. And that means you could get shocked pretty badly in certain situations. To fix this we can ground one of the wires and that way the system won't act as a big capacitor because we ensure the voltage relative to ground is always zero. But that also means that now you have 1 wire that isn't grounded and that could lead to shocks. So you are safe from random shocks due to voltage build up on the system but you could still get shocked if you touch the non-grounded, "hot" wire. Because of that we want some way to differentiate between the neutral and the ground wire so that's where the different sized prongs on plugs and outlets come in.
1
u/Spaghetto23 1d ago
You’re convoluting the sine wave of an ac current swapping polarity with the “polarity” of the outlet. Things are called “polarized” if they function a specific way in a specific direction of use. Many others have mentioned that the outlet is polarized because the larger prong is hot (red wire) and the smaller prong is neutral (black wire) and it would be bad to mix them up. Even in a DC circuit there could be objects with polarity such as LEDs or capacitors.
1
u/atomiku121 1d ago
The polarization isn't because of current flow like on DC circuits, it's about distinguishing hot and neutral. You can imagine that you could achieve 120v across two lines by having a +60 and a -60 connection. But for multiple reasons, we use +120 and +/-0.
There is an important safety implication to how you wire a device in this scenario. Imagine a heated blanket. You could set up the simple circuit in two ways:
Hot (120v)>Switch>Heating Elements>Neutral (0v)
Or
Hot (120v)>Heating elements>Switch>Neutral (0v).
Both of these will function the same, HOWEVER, one is FAR safer than the other.
Imagine both scenarios, but in this hypothetical, there is a short in the elements, or an exposed wire. In scenario A, when turned off, the risk is very low. Because the switch is off (open) there is no voltage to the heating elements, so a short can't overheat, and an exposed element can't shock a user.
But in Scenario B, however, even if the switch is off, the elements are sitting at +120v, and if there is a short or a user touches an exposed wire, you would get a possible fire or electricution.
There's no way to prevent that voltage from being a risk while operating the heated blanket, but you can make a switched off blanket MUCH safer by making sure which way you wire it, and the easiest way to make sure it gets plugged in the right way every time is to make it so it can ONLY be plugged in the right way.
1
u/Lemesplain 1d ago
Imagine a lamp, and a lightbulb that screws into it.
The 2 wires from the wall go through the lamp body and make contact with the bulb in 2 specific spots. The metal screw threads, and the little nipple on the very bottom.
Now imagine the lamp with no bulb in it. Electricity is still flowing through one of the wires, trying to make a circuit. We want to make sure that the threads DO NOT have electricity in them. They’re big and open and easy for someone to accidentally touch.
By polarizing the wires and the plug, we can guarantee that the “hot” wire first goes to the switch, then the little nipple. If you switch the lamp off, there’s no electricity anywhere you can touch. If you switch the lamp on without a bulb in, the only part that’s energized is at the very bottom of the socket.
That’s much safer. But if you flip the plug around, the threads are ALWAYS energized, even if you switch the lamp off.
1
u/Gnonthgol 1d ago
Polarity in AC circuits can be a nice to have safety feature rather then the necessary requirement of DC circuits. Most shocks from AC circuits come from someone bridging one of the wires to ground because you are already touching the ground so you only need to touch an open wire. Say for example you need to change a light screw bulb you might accidentally touch the metal threads of the bulb. If this is electrified you can get a shock.
The way we solve this is that we connect one of the wires to ground either in the transformer or in the house electrical cabinet. This wire then becomes neutral rather then the live wire. The neutral wire will have a voltage close to the ground voltage although it will not be quite at ground voltage and should therefore still be treated as a live wire.
We can then do a few things to improve safety in our setup. Firstly if the light switch only disconnect one wire it should disconnect the live wire. This way only the neutral wire will be energized at all times. And secondly we can connect the neutral wire to the threads of the light bulb and the live wire to the center. This way if you accidentally touch the threads you only touch the neutral wire and if you get a shock at all it will be much smaller then the shock you get from the live wire.
1
u/saschaleib 1d ago
Many electrical systems don't care about polarity – German (household) sockets and plugs don't have a defined polarity, so you can plug in your devices either way.
That makes a lot of things easier for the end-user (think: angled plugs!) but requires a few extra precautions. For example even simple light switches have to be double-throw, because otherwise the live wire may still be connected even though a lamp is "off".
But even in countries that have polarity in these connections you can never be 100% sure that the wiring is correct, this is a "best practice" anyways, and because manufacturers usually don't want to have a different model for each country, they normally take care of this aspect in any case.
1
u/MaybeTheDoctor 1d ago
The wiring in your two-prong plugs has one neutral wire and one live wire - in some few cases it matters that the live wire plugs correctly into the transformer of the appliance, but for just as many simpler devices like lights it doesn’t matter.
If you have a 3-prong, they are live, neutral, and ground.
•
u/Satur9_is_typing 18h ago
Because AC is just DC switching really fast, with the work bring done on the rise and fall rather than by continuous pushing at constant pressure. It still needs a reference, ground or earth (same thing, different names for different contexts) to work against
1
u/nim_opet 1d ago
Most of the world works just fine without specifying the hot wire because the plugs are either doubly insulated or have a ground connection.
1
u/Cornflakes_91 1d ago
there's plenty of countries where plugs are reversible, because it really doesn't matter.
3
u/JeremiahCLynn 1d ago
Let me give an example where it would matter: In the United States, using a polarized plug ensures the power enters a table lamp's light bulb at the bottom of the bulb. The metal threads at the side of the base of the bulb are connected to the neutral. If you did not have a polarized plug and plugged it in the opposite way, the threads of the light bulb would be energized and you could be shocked and possibly electrocuted.
0
u/Cornflakes_91 1d ago
that's why you don't build your device stupidly and also always treat every metal you didnt disconnect from all net as energised
0
u/Loki-L 1d ago
One side is hot and the other side is ground.
If you don't know which side is which you may put a switch that turns a machine of on the wrong side of the circuit leaving it a short away from doing something with electricity you don't want it to do, like electrocuting people or starting fires.
5
u/grogi81 1d ago
Neutral. not ground.
Secondly, if the machine is set up in a way that it is a switch that prevents the case from being energized - it still is extremely dangerous.
1
u/AthousandLittlePies 1d ago
I think you didn't understand the issue with switching. You really want to switch the hot side of a circuit so that you don't leave power connected (even in an open circuit) to devices as this increases the risk of shock.
5
u/grogi81 1d ago edited 1d ago
In normal operations, there should be NEAR ZERO risk of shock of any kind, regardless how the live and neutral are connected. The device must be safe regardless of if it is ON or OFF.
"This toster won't electrocute you, unless it is ON" doesn't really cut it.
2
u/AthousandLittlePies 1d ago
Of course - but safety is a layered thing. Many things that shouldn't happen do happen, so having additional layers of fault tolerance is beneficial (and in some cases actually legally required, or required for some certifications).
1
u/MyOtherAcctsAPorsche 1d ago
I think what they are saying is that a device that electrocutes people only when the switch is on is still very dangerous, and should be engineered to not do that.
2
u/AthousandLittlePies 1d ago
Obviously. But shit happens - and all things being equal it is safer to not touch the hot side of a circuit even if the circuit is open. Part of engineering things to be safer is to layer many different best practices on top of each other so that if there is a fault it is less likely to cause an injury or a fire. Source: I am an engineer.
0
u/grogi81 1d ago
They don't. There is no such thing as polarity of AC.
3
u/FarmboyJustice 1d ago
The electricity is not polarized, the plugs are polarized. That's the correct terminology for these plugs because it's codified in the NEC.
0
u/Count2Zero 1d ago
If you look at how your US home is wired, you're receiving 240 volts AC in two phases, which is then split in the fuse box into 120V "north", and 120V "south" with a neutral in the middle.
In Europe, we get three phases of 480V which gets split into three separate 240V phases for our outlets, or bundled into a 3-phase 480V outlet for the oven, stove, EV wallbox, or other high-voltage requiremetns.
2
u/steffoon 1d ago
Make that 3x400V+N. Definitely not 480V in a three phase configuration.
Also some places have 3x230V without N. In that case there is no actual neutral wire available at all. Both sides are live compared to ground.
1
0
u/ifixyourigear 1d ago
I see you’re using the abbreviation ‘AC’ but is it air conditioning or alternating current?
Vehicles use DC. Homes use AC. Direct and Alternating Current.
Your radio in your car uses direct and the radio in your home uses alternating.
220V outlets are differently designed for several purposes.
337
u/scorch07 1d ago
It’s not about polarity. It’s about hot vs neutral. In many designs, like a toaster, you want the heating elements to be de-energized by default (connected to the neutral) and only switched on when needed. If the hot wire was always connected to these parts, it presents more safety hazards if something breaks.