Why is the correct polarity important with AC?

sophiecentaur

Science Advisor
Gold Member
23,720
4,023
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

Science Advisor
Gold Member
3,543
564
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

Science Advisor
Gold Member
23,720
4,023
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.
 
434
190
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?
 
520
225
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

Science Advisor
Gold Member
1,507
427
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 whats of an issue; and is the driving force that makes our forums possible and valuable.
 

russ_watters

Mentor
18,967
5,130
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."
 
767
351
berkeman said:
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.
 
520
225
(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

Mentor
55,886
5,947
(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. :wink:

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

Science Advisor
2,821
1,593
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. :oldwink:
 
767
351
Agreed, you pass the test. :wink:

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

Mentor
55,886
5,947
"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. :smile:

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... o0)
 

Svein

Science Advisor
Insights Author
1,970
611
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.
 
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

Science Advisor
Gold Member
23,720
4,023
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.
 
1,003
128
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:
 
381
62
@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.
 
520
225
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!
 
381
62
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?
 
520
225
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

Science Advisor
Insights Author
Gold Member
1,814
1,246
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 dont need correct polarity?
From the previous source:

https://philschatz.com/physics-book/contents/m42416.html said:
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

Science Advisor
Gold Member
23,720
4,023
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.
 
706
152
LOL. :smile:

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... o0)
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
 

Want to reply to this thread?

"Why is the correct polarity important with AC?" You must log in or register to reply here.

Related Threads for: Why is the correct polarity important with AC?

Replies
1
Views
2K
  • Last Post
Replies
4
Views
3K
  • Last Post
Replies
4
Views
6K
Replies
2
Views
1K
Replies
2
Views
3K
  • Last Post
Replies
3
Views
3K
Replies
18
Views
3K
Replies
11
Views
986

Hot Threads

Top