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.

 

[berkeman]

(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.

Agreed, you pass the test.

Unfortunately, there are lots of grounded things on my workbench that I touch at times, and there is no 1Meg resistor in line with them to ground. The worst shocks I've gotten are arm-to-arm when I'm sweating (sometimes because it's warm in the lab, sometimes because of nervousness...). Luckily I've never been grabbing something (which could result in a prolonged shock from contracting muscles).

 

[Tom.G]

Luckily I've never been grabbing something (which could result in a prolonged shock from contracting muscles).

Lucky you! I did. ONCE.

I was a teenager with a Model-T Ford Spark Coil (ignition coil). The interrupter (ignition points, on the primary side) was tightened down to remain closed and it was connected to a step-down transformer from a toy electric train. Could still draw a spark from the secondary. Somehow I ended up with my hands on opposite ends of the secondary (high voltage winding). Hand and arm muscles stayed contracted so I couldn't let go. Fortunately I was experimenting while sitting on the bed and used the remainder of my body to throw myself away from that alligator.

As I said above, it happened ONCE!

Cheers,
Tom

p.s. Don't try this at home.

 

[sysprog]

Agreed, you pass the test.

Unfortunately, there are lots of grounded things on my workbench that I touch at times, and there is no 1Meg resistor in line with them to ground. The worst shocks I've gotten are arm-to-arm when I'm sweating (sometimes because it's warm in the lab, sometimes because of nervousness...). Luckily I've never been grabbing something (which could result in a prolonged shock from contracting muscles).

In the '80s I worked as a Systems Engineer for Amdahl Corp -- they had a practice of cross-training the SEs, who were mostly system(s) programmers, to be stand-ins for the FEs (field engineers -- the hardware guys -- to some of those guys a 'file' was a disk drive unit -- back then a drive with about 2GB capacity was about half the size of a domestic refrigerator), and we had to be able safely (for ourselves and for the expensive equipment) to swap boards in the back-planes, and if we could handle it and thereby save time and money, do board-level diagnostic and repair work.

The Amdahl boxes that were marketed to compete with high-end IBM mainframes, used emitter-coupled logic, and (for somewhat complicated reasons related to that) used 400-cycle AC, instead of the usual 60-cycle supplied by the utility company. Amdahl offered 2 basic methods for converting the supply frequency -- one was an MG (motor generator -- also about the size of a fridge -- the device would spin a big motor at 60 cycles, and it would drive a generator, from which 400-cycle AC would be tapped), and the other was a 3-or-4-fridge-sized device known as 'frequency converter' -- I asked wasn't the MG functionally also a frequency converter, and they said "yeah, but this works by a completely different method".

The guys opened the cabinet, and asked "what do you think about that". I looked and saw sets of arrays of oatmeal-box-sized cylindrical electrolytic capacitors teamed up in gangs of 8, with maybe 3/8"-thick slabs of steel joining them, and replied "don't touch" -- the guys told me that was the right answer, and laughed, and closed the cabinet.

 

[berkeman]

"what do you think about that". I looked and saw sets of arrays of oatmeal-box-sized cylindrical electrolytic capacitors teamed up in gangs of 8, with maybe 3/8"-thick slabs of steel joining them, and replied "don't touch" -- the guys told me that was the right answer, and laughed, and closed the cabinet.

LOL.

In contrast, when I was a young and stupid engineer at HP (no really, there was a time when I was young and stupid), I was wandering around the cubicles getting to know other EEs there, and asking about what they were working on.

I ended up in the cubicle of an engineer who was testing his new prototype of the power supply for one of our new data terminals. He had the top off the terminal exposing the power supply, and as we were chatting, I reached over and expertly felt the heat sinks to see if they were running too hot (Quiz Question -- how can you tell by feel that the transistors are running too hot?).

Bzzzzzzttt! I got a pretty strong shock, because the heat sinks were not grounded (which is common), and I had enough of a parasitic path back to Earth ground at the time that I completed that circuit. The engineer was very annoyed with me, and said in a loud voice, "Why does everybody keep touching my heat sinks!?"

So I kinda' failed that test, but learned more about what not to touch when you don't know how the circuit is connected...

 

[Svein]

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.

I am not qualified to do a truly "international scope" answer, but since I have designed some control circuitry for dealing with the mains voltage, I know something about the way it works in Scandinavia.

Firstly you have the "old-fashioned" three-wire distribution (I think it is called an "IT" distribution). This is a delta-type distribution where each side of the delta supplies 230VAC. No "neutral" exists (the center of the delta may be connected to Earth somewhere, but do not depend on it). You still need a "protective ground" in your system, but that protection is supplied locally.

Secondly we have the more modern four-wire distribution, which is a "Y"-type distribution. Here all three phases and the center of the "Y" are distributed. The center of the "Y" is designated "neutral" and the phases are at 230VAC from "neutral". This means that the voltage difference between the phases are 230VAC*√3 or about 400VAC. In theory the current in the "neutral" wire should be ≈0 but in practice it is not. The "neutral" may be connected to Earth somewhere, but you cannot depend on it being at "earth" potential. Therefore you still need a "protective ground" in your system, and that protection is supplied locally.

Each distribution has its advantages and disadvantages:

The three-wire system is cheaper to deploy, but in any outlet both wires are "live". This means that wet-room or outside deployment require double-pole switches.

The four-wire system is more expensive to deploy (four wires are more expensive than three wires) and there is the 400V hazard. A "neutral" wire exists in every outlet, but since the plugs are symmetrical, you must treat both wires as "live" anyhow.

 

[HomeExperiement]

Thanks for this great information. I was also wondering if there are any single phase AC devices that would break if L and N on the input was reversed? I have not heard of any but was just curious.

 

[sophiecentaur]

Thanks for this great information. I was also wondering if there are any single phase AC devices that would break if L and N on the input was reversed? I have not heard of any but was just curious.

If there is an internal connection (or a component with a low maximum operating voltage) connected between the marked neutral and Earth then 240V connected to the should-be-neutral would not be good. Ideally, a fuse should blow but a supply fuse has to pass the full operating current plus extra headroom; fuses are very crude devices. Current much less than the fuse rating could do real damage to part of the innards.

 

[zoki85]

I was also wondering if there are any single phase AC devices that would break if L and N on the input was reversed?

I bet 99% of the appliances should work normally. However some may experience problems or not work at all:

 

[artis]

@Guineafowl in your post #30 you said

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?

Now I have a feeling you are talking about the US "split phase" system here aren't you?

I think there has to be a clear distinction made here, because if I would do as you said "We ground/earth one terminal, and call that neutral." and the terminal I grounded ended up being phase trust me there would be fireworks going on and magic smoke (hopefully not from my burning flesh)

In the US you have a transformer with a center tap and two secondary outputs so essentially an "isolation transformer" so you can maybe indeed take one of the legs of that transformer's secondary winding and ground it while the other then becomes the 120v potential.

Here in Europe and especially in my country we have a 20kV to 0.4kV (3 phase 400v) (I believe the same goes on in UK etc)distribution system , so most private houses and especially highrise apartments are fed from the nearest distribution transformer which is not like the small pole pig transformers in US but instead a large house like facility with a big transformer inside, this transformer simply transforms the 20kV city/suburban 3 phase voltage to 400v 3 phase and adds a neutral.
Now if I have (which I do) in my apartment a single phase supply that means that my supply is nothing more than the neutral (taken from the distribution transformers neutral) and a single phase (which is one of the phases from the distribution transformer)
This means that the phase is always live and always phase with 240v in the wire no matter what you do about it, and the neutral is always neutral. There is no mixing of them, and if someone does indeed ground the phase he either blows his fuses or blows himself.!
In my apartment block like in most others we have incoming 3 phase 400v with a neutral so 4 wires and then to spread out the load evenly each apartment is connected to a different phase but the same neutral.
So If I was to measure between live in my socket and live in my neighbors socket I would see 400v which is the voltage between two phases.
Our EU system doesn't allow any messing around and mistakes , people get fried easily with these voltages and currents.

 

[Guineafowl]

Hi Artis - answers in the text:

@Guineafowl in your post #30 you said
Now I have a feeling you are talking about the US "split phase" system here aren't you?

I meant a transformer with two poles in (at distribution voltage) and two out (at supply voltage). Before any connections are made onwards from the output, the terminals are exactly equivalent, neither live nor neutral. Just a plain old stepdown transformer. I’m in the UK, but was trying to generalise.

I think there has to be a clear distinction made here, because if I would do as you said "We ground/earth one terminal, and call that neutral." and the terminal I grounded ended up being phase trust me there would be fireworks going on and magic smoke (hopefully not from my burning flesh)

Back up a little here, and consider the transformer with its unconnected secondary. Remember that neither terminal is yet phase or neutral, just equivalent AC poles. Whichever one you choose to ground will become neutral by definition. If you then decide to connect the other (now phase) to ground, as you say there will be fireworks, but you wouldn’t. Why? Because now the live/neutral safety system has been imposed on the supply, the wire colours show you which is which.

In the US you have a transformer with a center tap and two secondary outputs so essentially an "isolation transformer" so you can maybe indeed take one of the legs of that transformer's secondary winding and ground it while the other then becomes the 120v potential.

As I say, not in the US but I believe they have three secondaries, the centre of which is grounded and becomes the neutral. The country specifics are not important, as the principle of L/N still apply there.

Here in Europe and especially in my country we have a 20kV to 0.4kV (3 phase 400v) (I believe the same goes on in UK etc)distribution system , so most private houses and especially highrise apartments are fed from the nearest distribution transformer which is not like the small pole pig transformers in US but instead a large house like facility with a big transformer inside, this transformer simply transforms the 20kV city/suburban 3 phase voltage to 400v 3 phase and adds a neutral.
Now if I have (which I do) in my apartment a single phase supply that means that my supply is nothing more than the neutral (taken from the distribution transformers neutral) and a single phase (which is one of the phases from the distribution transformer)
This means that the phase is always live and always phase with 240v in the wire no matter what you do about it, and the neutral is always neutral. There is no mixing of them, and if someone does indeed ground the phase he either blows his fuses or blows himself.!

My house is supplied with three phase 400V, with the neutral taken from the star point just as you describe. However, the OP was about a common confusion with polarity of AC. I avoided tackling three phase and country-specific systems in general as this can be confusing - I feel the principle of the L/N system can be more easily understood by starting with a simple two-in, two-out transformer. This way, we can see how it’s a connection system that we impose on the supply, not something inherent in the AC itself.

In my apartment block like in most others we have incoming 3 phase 400v with a neutral so 4 wires and then to spread out the load evenly each apartment is connected to a different phase but the same neutral.
So If I was to measure between live in my socket and live in my neighbors socket I would see 400v which is the voltage between two phases.
Our EU system doesn't allow any messing around and mistakes , people get fried easily with these voltages and currents.

It IS a good system!

 

[artis]

yes I agree @Guineafowl , over the history people made different choices in different places so now whenever we talk about distribution we can only be specific about one place at a time, this messes it up a bit.
True I agree the basic idea is still the same. And it is easier to understand using a simple single phase example.
The system seems good just that in the US you have a bigger chance of staying alive when things go wrong.PS.
So you yourself are from UK? I wonder then you have the same system as we here do or you also have these small transformers next to private houses which are effectively single phase transformers just with a higher voltage then in the US, or do you have the 3 phase star/delta just distributed as single phase to houses that don't have 3 phase connection?

 

[Guineafowl]

The system seems good just that in the US you have a bigger chance of staying alive when things go wrong.

Voltage-wise, yes. But then what about unsleeved plug pins, unshuttered outlets and lower take-up of RCDs/GFCIs in the US? Then again, we’ve been slow to implement AFDDs... The debate rages on.

PS.
So you yourself are from UK? I wonder then you have the same system as we here do or you also have these small transformers next to private houses which are effectively single phase transformers just with a higher voltage then in the US, or do you have the 3 phase star/delta just distributed as single phase to houses that don't have 3 phase connection?

Same as EU, except we never bothered to harmonise the voltage to 230/400 (more like 240/415). Rural areas often have a single phase transformer near the house. Otherwise it’s as you say, and some lucky houses (like mine) have three phase.

 

[jack action]

Maybe I don't understand all the answers correctly (or don't understand the problem), but it sounds simpler to me. I wanted to give my own answer, but someone already done it on the web (with nice drawings too!), even if it's mostly about three-prong plugs.

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

The concept is mostly to isolate the conductive casing of an appliance. You want the casing to be connected to ground, just like the neutral wire. This way, if the live wire touches the casing, the current will select the path with the least resistance, i.e. NOT the operator holding the appliance by its casing (the operator have some resistance). With a two-prong plug, the neutral may be connected to the casing (or some circuitry that need a similar protection), thus why having a correctly identified neutral plug is important.

Does european electrical appliances work differently that they don't need correct polarity?

From the previous source:

Why are some appliances still sold with two-prong plugs? These have nonconducting cases, such as power tools with impact resistant plastic cases, and are called doubly insulated.

In other words, without a conductive casing, there are basically no safety issues possible.

 

[sophiecentaur]

the current will select the path with the least resistance,

Please please do not say that. What happens is that a low resistance path to Earth reduces the voltage when the fault path resistance is high (it's a potential divider). If the fault path is low resistance then enough current will pass to blow the fuse. The current is always shared by both paths; most current flows through the lowest resistance path.

 

[cosmik debris]

LOL.

In contrast, when I was a young and stupid engineer at HP (no really, there was a time when I was young and stupid), I was wandering around the cubicles getting to know other EEs there, and asking about what they were working on.

I ended up in the cubicle of an engineer who was testing his new prototype of the power supply for one of our new data terminals. He had the top off the terminal exposing the power supply, and as we were chatting, I reached over and expertly felt the heat sinks to see if they were running too hot (Quiz Question -- how can you tell by feel that the transistors are running too hot?).

Bzzzzzzttt! I got a pretty strong shock, because the heat sinks were not grounded (which is common), and I had enough of a parasitic path back to Earth ground at the time that I completed that circuit. The engineer was very annoyed with me, and said in a loud voice, "Why does everybody keep touching my heat sinks!?"

So I kinda' failed that test, but learned more about what not to touch when you don't know how the circuit is connected...

I had a similar experience when I was at a similar age and experience. I was fixing a 749 series Textronix scope, it had a mixture of valves (tubes) and transistors. I looked at the circuit diagram and saw that one particular transistor was a 12V variety, suspecting failure I did the finger test on the transistor's metal can. I got a huge shock, the transistor was in the anode circuit of one of the tubes, the can was at 345V.

Cheers