Why is the correct polarity important with AC?

[HomeExperiement]

TL;DR Summary

(Why) does the correct polarity of AC matter?

Hi!

I was watching some random videos about electricity on you youtube and then got this question: if AC changes direction constantly (50 times per second) then why is it still important to connect live to live and neutral to neutral in wall outlets? For example some wall outlets have different size prongs for live and neutral so that you wouldn't be able to connect it with reverse polarity. For example: here is image of what I mean:

But in EU we have mostly non-polarized plugs. Does european electrical appliances work differently that they don't need correct polarity? For example here I found youtube video about old radio that electrocutes you when you plug it in wrong polarity but doesn't when you plug it in using correct polarity so the previous owner marked the correct polarity:
But I don't understand that if AC changes direction 50 times per second, then how does the polarity of plugging it in determine whether you get shocked or not? At least from this video I understood that with AC the direction of current changes periodically:


But doesn't that mean that live and neutral also change 50 times per second like live becomes neutral and neutral becomes live? When why outlet connecting the the radio with correct polarity shock?

An other question is that if for example I look at some AC to DC converter circuits on internet - not that I want to build one as I don't feel that to be my call - I notice that sometimes they kinda assume that one side of AC is live and other side is neutral. For example when input filter is added to such circuits it looks like it is always marked on hot side. Is there any reason for this other than conventions? And what would happen to such circuits in europe where you can plug stuff in with any polarity?Offtopic: does this forum allow me to post youtube videos as links and not as videos? Previews take up much space in post.

 

[sysprog]

Summary: (Why) does the correct polarity of AC matter?

Hi!

I was watching some random videos about electricity on you youtube and then got this question: if AC changes direction constantly (50 times per second) then why is it still important to connect live to live and neutral to neutral in wall outlets? For example some wall outlets have different size prongs for live and neutral so that you wouldn't be able to connect it with reverse polarity. For example: here is image of what I mean:

View attachment 251623

But in EU we have mostly non-polarized plugs. Does european electrical appliances work differently that they don't need correct polarity? For example here I found youtube video about old radio that electrocutes you when you plug it in wrong polarity but doesn't when you plug it in using correct polarity so the previous owner marked the correct polarity:
But I don't understand that if AC changes direction 50 times per second, then how does the polarity of plugging it in determine whether you get shocked or not? At least from this video I understood that with AC the direction of current changes periodically:


But doesn't that mean that live and neutral also change 50 times per second like live becomes neutral and neutral becomes live? When why outlet connecting the the radio with correct polarity shock?

An other question is that if for example I look at some AC to DC converter circuits on internet - not that I want to build one as I don't feel that to be my call - I notice that sometimes they kinda assume that one side of AC is live and other side is neutral. For example when input filter is added to such circuits it looks like it is always marked on hot side. Is there any reason for this other than conventions? And what would happen to such circuits in europe where you can plug stuff in with any polarity?Offtopic: does this forum allow me to post youtube videos as links and not as videos? Previews take up much space in post.

Although AC itself, as you have correctly observed, has no polarity, the direction of current flow, especially in a non-isolated circuit (e.g. one in which neutral is connected to ground), can be important -- e.g., an appliance that has an internal fuse should have the hot side go directly to the fuse first, rather than letting an excess current run through the rest of the circuit first and then letting that excess current blow the fuse -- I think it's a good practice to put 4 diodes in a square formation at the start of the power connection to ensure that the current flow will be in the desired direction even if the electrician wired the outlet with the wrong side hot.

 

[anorlunda]

I agree with @sysprog 's answer.

But on another level: When everything works OK, there is no preferred polarity. However house wiring schemes, including neutral, hot, grounded and not are there for protection when things are not normal. Open circuits, short circuits, insulation failures, and wiring errors are all things that happen in real life, either singularly one at a time, or in combination. That is why we have polarized plugs and systems for wiring conventions and naming conventions.

This is a very frequent question on PF. It sometimes triggers unending debate because our PF members come from many countries and practices vary around the world. It would be most welcome if one of those members would write a PF Insights article on that topic that is truly international in scope. Hint. Hint.

 

[berkeman]

Summary: (Why) does the correct polarity of AC matter?

why is it still important to connect live to live and neutral to neutral in wall outlets?

Here is a hint. In the US, by Code the Neutral wire is connected to Earth ground at the breaker panel (at least in home installations). Why do you think it might be important for this lamp's 2-prong power cord to be polarized?

https://li1.rightinthebox.com/images/190x250/201609/dyfw1473216613785.jpg

 

[Averagesupernova]

Here is a hint. In the US, by Code the Neutral wire is connected to Earth ground at the breaker panel (at least in home installations). Why do you think it might be important for this lamp's 2-prong power cord to be polarized?

https://li1.rightinthebox.com/images/190x250/201609/dyfw1473216613785.jpg
View attachment 251634

That's the explanation I would give.

 

[HomeExperiement]

fuse should have the hot side go directly to the fuse first, rather than letting an excess current run through the rest of the circuit first and then letting that excess current blow the fuse

But how can you guarantee that to happen? According to my understanding, it's only possible with DC because with AC the same fuse would be 50% of times on hot side and 50% of times on neutral side, wouldn't it? Or how would you guarantee that in europe for example? Even if house wiring is correct, end user can still plug their stuff in with any polarity - in other words as a manufacturer you have no guarantee
that what you designed to be hot side is actually hot side. I also haven't heard of any AC circuit tricks to ensure that the side with fuse is always hot.

I think it's a good practice to put 4 diodes in a square formation at the start of the power connection

That goes for designing AC to DC converter? At least I can't remember seeing this in AC.

Why do you think it might be important for this lamp's 2-prong power cord to be polarized?

Not sure. Maybe to reduce the risk of touching the live wire? I mean it would be easier to touch this big outer terminal (not sure what's the correct word for this in english) than this one at the bottom.

 

[anorlunda]

According to my understanding, it's only possible with DC because with AC the same fuse would be 50% of times on hot side and 50% of times on neutral side, wouldn't it?

Because the heat generated in the fuse is proportional to I²R. I is the AC current. So even though I changes sign every AC cycle, I² is always positive. Note that voltage V does not enter into it, only current matters for a fuse.

 

[berkeman]

Not sure. Maybe to reduce the risk of touching the live wire? I mean it would be easier to touch this big outer terminal (not sure what's the correct word for this in english) than this one at the bottom.

Exactly. The big outer threaded metal piece must be connected to Neutral, and the small button at the bottom of the socket must be connected to Line/Hot. That is to reduce the risk of electrical shock.

The type of shock that this helps to prevent is a "Ground Fault" shock, where you touch Line/Hot accidentally somehow, and also have some sort of a ground reference touching you (like you are leaning on a metal appliance that is grounded). Since the Neutral wire is Earth Grounded at the distribution panel, there typically will only be a very small AC voltage between Neutral and Earth Ground everywhere in the house (or commercial building, etc.). So it is not usually a risk that you accidentally contact a Neutral wire while you are exposed to Earth Ground in some way. (It's still not a good idea to risk it, since there can be wiring faults or miswires in electrical installations).

 

[Averagesupernova]

But how can you guarantee that to happen? According to my understanding, it's only possible with DC because with AC the same fuse would be 50% of times on hot side and 50% of times on neutral side, wouldn't it?

You are confusing direction of current flow with what we name the conductors that are connected to the earth. Neutral connects to Earth no matter which direction the current flows. The conductor we refer to as 'hot' shocks you equally bad regardless of the direction of current.
-
Think of it this way: Take 10 automobile batteries (12 volt) and hook them in series. The group wired in this manner should have 120 volts between the ends. Now ground the negative side. Touch the positive side (hypothetical, don't do this) and you would expect a shock as long as your are standing on the ground. Now turn the batteries around and ground the positive side. Touching the negative side (again, hypothetical, don't do it) will result in the same shock. Now imagine if you were to reverse the polarity many times a second while continuing to touch the ungrounded terminal. This is what our AC system is doing. The fuse ALWAYS belongs in what we call hot, or the ungrounded conductor.

 

[sophiecentaur]

Summary: (Why) does the correct polarity of AC matter?

if AC changes direction constantly (50 times per second) then why is it still important to connect live to live and neutral to neutral in wall outlets?

This AC stuff takes quite a bit of getting used to and a lot of it is not 'obvious'.
The term "polarity' is really a misnomer for AC. Yes the Polarity changes many times per second but the RMS value of the Volts on the Live conductor are high (say 240V and lethal) whilst the RMS value of the Volts on the Neutral will be low (well less than 10V and safe). The two conductors are not just fluctuating at +/-120V about Earth with 240V between them but (mentioned above) the Neutral is deliberately kept near Earth (0V)

So you can touch the neutral wire (don't do it though) and the fuse in the Live leg can blow when the current is too high and an appliance will be isolated from the 240V, with no more than the neutral volts anywhere on it.

Summary: (Why) does the correct polarity of AC matter?

I found youtube video about old radio that electrocutes you when you plug it in wrong polarity but doesn't when you plug it in using correct polarity so the previous owner marked the correct polarity:

Haha. That was in the old days when sets took a lot of power and they were supplied without an expensive Mains Transformer to isolate the electrics. The neutral had to be connected to the 'low voltage' side of the circuit and the rectified high voltage DC was supplied directly from the Live side to the valves. If you connected the mains, the wrong way round you were probably quite safe because all the electrics was (should have been) inside the box and all the knobs and buttons and fixing screws were insulated. If you tried to connect an external loudspeaker, though, that could kill you! Sets that used an external loudspeaker used an isolating transformer to take the signal outside safely.

 

[sysprog]

Here is a hint. In the US, by Code the Neutral wire is connected to Earth ground at the breaker panel (at least in home installations). Why do you think it might be important for this lamp's 2-prong power cord to be polarized?

https://li1.rightinthebox.com/images/190x250/201609/dyfw1473216613785.jpg
View attachment 251634

Maybe it depends on what you're connecting to the socket. Here's a simple blinker that uses a fluorescent lamp starter (bimetallic contacts in argon) connected to an incandescent lamp bulb. The blink cycle goes as follows: when power is turned on, the starter begins to warm up, and when it warms up enough, the contacts connect, and the lamp lights up; then the starter cools while the lamp is on, and when it cools enough, the contacts disconnect, and the lamp goes out; then the contacts heat up again, and soon return to a connected state, and the lamp lights again.

If the current-carrying capacity of the 2 devices is about equal, and you swap the blue and brown wires, that may change which device gets blown first in the event of excessive current being applied.

 

[HomeExperiement]

Thanks for explanations.

That cleared some stuff for me. I saw that when multimeter was connected to live and ground then it showed 240V but when it was connected neutral and ground it showed 0V and that conflicted my current understanding.

Sets that used an external loudspeaker used an isolating transformer to take the signal outside safely.

Since you mentioned isolation transformer I would also like to know how exactly do they make it safer? I have watched many youtube videos on it and people there tend to say that it is important that the ground wire on output side is not connected to ground wire on input side so that you would not shock yourself when you touch other grounded object. But that has left me with this question: why not just use extension cord that has no ground wire at first point? What's the difference between isolation transformer and extension cord with no ground? I've learned that is also blocks some noise from going back from item to house. But according to my understanding if you create short circuit on output side of isolation transformer the input side will still see the short circuit?

 

[sophiecentaur]

Since you mentioned isolation transformer I would also like to know how exactly do they make it safer?

The output from a transformer has no conduction path to the input (the power travels via magnetic fields) so the two wires out are 'floating' and at no particular potential with respect to Earth. If you touch one, a tiny amount of current will flow through you and that side of the output becomes at Earth potential. Safe. If, at the same time, someone touches the other wire then it's not so safe because there is 240V between your hand and the other guy's hand and you have 120V across each of you (you will share the volts approximately). Not good but you can only cater for one idiot at a time.

There's a good trick with a Neon Tube in each leg and that will turn on when there are enough volts across it and it registers that there is a path to Earth via the other end. Time to check out the circuit.

Some transformers use a Centre Tap (half way along the secondary winding) and that keeps each output at 120V which is quite a bit safer than the full 240V. (Just like the 120V used in USA.)

 

[sophiecentaur]

But according to my understanding if you create short circuit on output side of isolation transformer the input side will still see the short circuit?

Yep. Absolutely true. You can just as easily blow a fuse when using a transformer as without - but that's yet another topic of conversation. You can spend a long time talking about transformers and many people just don't understand them at all, so there's a lot of BS talked about them.

 

[Averagesupernova]

I have watched many youtube videos on it and people there tend to say that it is important that the ground wire on output side is not connected to ground wire on input side so that you would not shock yourself when you touch other grounded object.

That is wrong.

 

[berkeman]

I have watched many youtube videos on it and people there tend to say that it is important that the ground wire on output side is not connected to ground wire on input side so that you would not shock yourself when you touch other grounded object.

That is wrong.

I mainly use Isolation Transformers to isolate AC Mains powered equipment that I need to be able to touch and probe without being too worried about getting shocked by a ground fault. I often need to wear an ESD wrist strap to avoid inadvertently latching up the circuits I'm working with, so if I don't use an Isolation Transformer, it's way too easy to get shocked.

I also used to use Isolation Transformers to be able to repair CRT TV sets for my friends, because they often used 2-prong power cords and were not doubly-insulated once you got inside the chassis. Heck, some of them were even "Hot Chassis" designs, which make it pretty hard to use your oscilloscope to probe the circuits unless you use an Isolation Transformer with no Earth ground reference at the output.

 

[essenmein]

AC "polarity" is also important if you want your 3ph machines to spin in the correct direction.

 

[Averagesupernova]

If what you are 'plugging in' to an isolation transformer has a 3rd prong, it needs to be connected to the 3rd prong socket in your wall, AKA conduit ground. The hot and neutral need to be isolated from everything on the output of the isolation transformer.

 

[sophiecentaur]

AC "polarity" is also important if you want your 3ph machines to spin in the correct direction.

I think you'd get worse than the wrong direction of turn if you connected the L and N of one phase the wrong way round! Zaaaaaap!

 

[essenmein]

I think you'd get worse than the wrong direction of turn if you connected the L and N of one phase the wrong way round! Zaaaaaap!

Not saying they don't exist, but no poly phase machine I've worked with at least has a neutral connection...

 

[sophiecentaur]

You could well be right if there is only ever a delta connection but I'm not sure that "polarity" is a term that applies to the vertices of a triangle.
It all comes down to terminology.

 

[essenmein]

You could well be right if there is only ever a delta connection but I'm not sure that "polarity" is a term that applies to the vertices of a triangle.
It all comes down to terminology.

Even with star the star point is not typically brought out or connected (just speaking from what I work with, line operated machines may well connect the neutral/star, I don't know). With PWM inverters the star point jumps about a lot based on the PWM scheme. So star, star/delta hybrid or delta all just get 3 wires for 3 phase machine (as a power conversion guy, star-delta hybrids break my brain a little lol).

I agree that the term "polarity" doesn't make too much sense in the AC context, line-neutral or line-line are better.

 

[sophiecentaur]

(just speaking from what I work with

You guys who use 3ph get very familiar with it whilst I always have to start from scratch each time.
@jim hardy ("Old Jim" - RIP ) used tp have a really good grasp of these things and made some great contributions to this sort of thread. So helpful and never got cross with idiot questions - some of them were mine.

 

[russ_watters]

Here is a hint. In the US, by Code the Neutral wire is connected to Earth ground at the breaker panel (at least in home installations). Why do you think it might be important for this lamp's 2-prong power cord to be polarized?

https://li1.rightinthebox.com/images/190x250/201609/dyfw1473216613785.jpg
View attachment 251634

I learned this while changing a lightbulb my dad wired in my parents' basement...

@HomeExperiement something perhaps less evident is consistency in the circuit itself - not just the devices plugged into it - is part of the safety equation. If you aren't paying attention to polarity, you may wire a circuit that works fine, but due to the switch being on the neutral side, it's always live even when switched off.

 

[JBA]

Please pardon my interjection but:
While all of the above inputs are on point and well presented, this discussion has caused me to recall what launched one of the most heated (and unquotable) responses I ever received from a long past girl friend when she asked "Which is the best direction for the ceiling fan to blow, up or down" and I started my response with: "Well, it depends ..." but never got any further than that.

 

[sophiecentaur]

And on the subject of lightbulbs. The Edison Screw is very dodgy because the coarse threaded outer contact is connected (should be) to neutral. If it is wrongly connected then it can be live and easily touched when being screwed in. You cannot rely on the switch being off. Many light fittings are very badly specified and, despite being just as potentially lethal, they are supplied with bare wires and the regs for house wiring seem to change from needing or not needed an Earth conductor. You just can't buy other domestic appliances without plugs these days (moulded, usually) but home lighting circuitry is just a mess (at least in the UK).

 

[Averagesupernova]

And on the subject of lightbulbs. The Edison Screw is very dodgy because the coarse threaded outer contact is connected (should be) to neutral. If it is wrongly connected then it can be live and easily touched when being screwed in. You cannot rely on the switch being off.

As long as the switch is kept out of the neutral, the shell won't be live if the switch is off.

 

[sophiecentaur]

As long as the switch is kept out of the neutral, the shell won't be live if the switch is off.

That's true but it needs two conditions for safety, The Bayonet connector has two contacts at the tip and you need to stick your finger actually into the socket itself to touch either conductor. Polarity is of much less consequence. ES was designed at a time (stoneage) when safety was the last thing they thought of.
I have seen ES sockets with a second contact at the bottom of the socket and also sockets with a short powered section of thread, leaving the threaded sleeve isolated until there's a bulb in there.

 

[essenmein]

Please pardon my interjection but:
While all of the above inputs are on point and well presented, this discussion has caused me to recall what launched one of the most heated (and unquotable) responses I ever received from a long past girl friend when she asked "Which is the best direction for the ceiling fan to blow, up or down" and I started my response with: "Well, it depends ..." but never got any further than that.

Depends on what polarity you want for your air flow?

 

[Guineafowl]

Forgive me if I’m repeating things already said, but I’m often asked this question and would like to craft a perfect answer.

We need to define polarity:
For DC, it refers to the direction of current, conventionally from pos to neg. We always bear in mind that the real electron flow is the other way.

For AC, the direction reverses at the supply frequency, so AC has a different definition of polarity: Live and neutral.

A raw single phase AC supply doesn’t have live or neutral, just two equivalent terminals. We ground/earth one terminal, and call that neutral. The other is live. Why?

1. So that the live conductor does not rise more than the supply voltage above ground/earth, which might lead to clearance/arcing issues and may degrade wire insulation.

2. So that there is a dedicated path (ground/earth wire) for fault currents to flow, thereby tripping the protection. This applies mainly to metal-cased appliances and armoured cable.

3. If we left the supply floating, inevitably one wire will get a ground/earth reference from a fault, but this will be a silent fault. All other users on that supply will be affected, but will not notice until a second fault occurs.

4. For polarised plugs, it ensures it’s always the live conductor that is broken by a switch or breaker, rendering the load relatively safe.

 

[JBA]

Depends on what polarity you want for your air flow?

Well, you might well state it that way, at that time about 4 factors immediately came to mind:
1. This was a kitchen with the fan directly over the kitchen table so it would depend on whether people sitting at the table are comfortable feeling a direct downdraft from the fan.
2. Whether or not you are looking for a direct local region cooling under the fan or for a more general air circulation for the entire room.
3. Air blowing down the walls and across the kitchen's hard floor might pickup and circulate more dust back upward from the floor than the fan blowing downward on the table.
4.Whether or not there might be loose papers on the table that could be scattered by a direct fan downdraft.

In a different situation, in a discussion with a fellow university engineering student one day we were disagreeing on some technical issue that I don't remember; and, at some point he became frustrated and pointed upward and declared "Well that is up, right!"; and, in all candor, I replied "Relative to what" and that definitely didn't improve the discussion.

I think that type of approach is what tends to separate those of us in our sciences and engineering worlds from a good number of those want a world with simple and quick answers. It is the desire and maybe even compulsive need to seek out and understand, to the maximum extent possible, all the elements that explain the whys and what's of an issue; and is the driving force that makes our forums possible and valuable.

 

[russ_watters]

Please pardon my interjection but:
While all of the above inputs are on point and well presented, this discussion has caused me to recall what launched one of the most heated (and unquotable) responses I ever received from a long past girl friend when she asked "Which is the best direction for the ceiling fan to blow, up or down" and I started my response with: "Well, it depends ..." but never got any further than that.

"Whichever direction you prefer, honey."

 

[sysprog]

I mainly use Isolation Transformers to isolate AC Mains powered equipment that I need to be able to touch and probe without being too worried about getting shocked by a ground fault. I often need to wear an ESD wrist strap to avoid inadvertently latching up the circuits I'm working with, so if I don't use an Isolation Transformer, it's way too easy to get shocked.

I also used to use Isolation Transformers to be able to repair CRT TV sets for my friends, because they often used 2-prong power cords and were not doubly-insulated once you got inside the chassis. Heck, some of them were even "Hot Chassis" designs, which make it pretty hard to use your oscilloscope to probe the circuits unless you use an Isolation Transformer with no Earth ground reference at the output.

(underlining added)

Every ESD wrist strap that I have used has included an inline $1\text{M}\Omega$ resistor to disallow strong current at normal mains voltages.

 

[sysprog]

"Whichever direction you prefer, honey."

 

[Guineafowl]

(underlining added)

Every ESD wrist strap that I have used has included an inline $1\text{M}\Omega$ resistor to disallow strong current at normal mains voltages.

Likewise. Strapping your wrist directly to Earth is asking for a fatal shock.