Why is the correct polarity important with AC?

[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

 

[Merlin3189]

I wonder if anyone can put up a diagram of how this is done. It's a new one on me and I haven't been able to figure out how it would work.

Although AC itself ... 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.

 

[jack action]

I wonder if anyone can put up a diagram of how this is done. It's a new one on me and I haven't been able to figure out how it would work.

It is call a diode bridge:

 

[berkeman]

Since this thread has been revived, I wanted to post a follow-up about polarized 2-prong AC Mains plugs in the US. I had previously used the 2-prong power cord connected to a light bulb socket to illustrate why polarized plugs are important:

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

But a couple days ago I noticed that my toaster also has a polarized 2-prong power plug, and realized why. Thoughts?

https://images.homedepot-static.com...ilton-beach-pop-up-toasters-22790-64_1000.jpg

 

[sophiecentaur]

But a couple days ago I noticed that my toaster also has a polarized 2-prong power plug, and realized why. Thoughts?

The outwardly friendly electric toaster is really a deadly accident waiting to happen. The elements are so easy to touch with a metal table knife when the toast gets stuck. It's amazing that type approval still applies. The only 'safety' feature is to make sure that the element wires at the top end, near the slots, are at the Neutral End. That helps to ensure that a knife will only have a relatively low voltage on it. A tingle and some sparks should be the limit of the 'accident'. But it does require that L and N ('polarity') connections are carried through from mains socket to the elements.

I believe I have seen a toaster with IR elements in glass but never bought one. That should be a required feature in all new toasters.

 

[berkeman]

Yep, but the polarized plug also ensures what about the on/off switch?

 

[sophiecentaur]

Yep, but the polarized plug also ensures what about the on/off switch?

Double pole switch IIRC.

Edit: They are very popular for throwing into a victim's bath in crime movies.

 

[Merlin3189]

Well yes, it is.
But I thought they were talking about AC and ensuring that one of the outputs remained at ground potential whichever way the input was connected.

Here it looks to me that whether this cct is connected to AC or DC, neither output is safe unless you know which input is grounded.
For AC, of course, neither output is safe whatever you do! You might prefer to use a transformer, or keep well insulated from both outputs.

BTW I use 100 V just for ease of typing. It might be 250 V or whatever.

I still wouldn't fancy 100 V, if I happened to have my other hand on a good ground (as is made mandatory here, IMHO, by equipotential bonding of random bits of metalwork.)

 

[berkeman]

But I thought they were talking about AC and ensuring that one of the outputs remained at ground potential whichever way the input was connected.

It is, so your diagrams should show the negative output of the rectifying bridge connected to Earth Ground.

 

[sophiecentaur]

But I thought they were talking about AC and ensuring that one of the outputs remained at ground potential whichever way the input was connected.

Possibly, in certain posts but the thread has lost its way, I think. What did the OP want from it?

 

[anorlunda]

All EE threads that use the words ground or neutral seem to become inevitably confused. Confusion is no quite as certain as the 2nd Law, but close.

1. Earthing practices and wiring codes are as much art as science.
2. Practices vary widely from country to country, and threads are read by PF members from several countries, so they talk past each other.
3. The reasons behind wiring practices have everything to do with abnormal circumstances and possible wiring errors. Yet OPs seek clarification using normal circumstances and correct wiring in their local reference frame, and the only abnormality they ask about is touching one terminal versus another.

 

[darth boozer]

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.

I found an even worse wiring situation in an old pub in South East London, UK, where my friend, the landlady, had accidentally hit a light with a brush and broken the lamp socket. A fuse blew and I at first assumed the power was off. Being an electronics technician, I had learned to test things rather than assume and this may have saved my life, or at least a severe shock. Finding the lamp socket still live, I rechecked the fuses in the pub's cellar and found a blown fuse in a neutral line on a separate switchboard to the live fuses, which of course left the circuit live and unsafe to work on without extra safety measures. Removing both fuses, I replaced the lamp socket and then rewired and replaced the fuses.

The ancient switchgear in the pub cellar reminded me that electrical practices in the the pre-WW2 era were different to the more modern (early 1980s) times and reinforced the concept of testing before attempting any sort of electrical work.

In the same pub, the landlord had replaced a broken plug on a pie warmer, but it wasn't warming so he asked me to check it. A slight tingling shock (I was insulated from ground) suggested that I should check the plug wiring. Sure enough, the landlord was red/green colour blind and had interchanged the live and Earth wires (the old colour scheme, red-live, black-neutral, green-earth).

 

[Guineafowl]

I found an even worse wiring situation in an old pub in South East London, UK, where my friend, the landlady, had accidentally hit a light with a brush and broken the lamp socket. A fuse blew and I at first assumed the power was off. Being an electronics technician, I had learned to test things rather than assume and this may have saved my life, or at least a severe shock. Finding the lamp socket still live, I rechecked the fuses in the pub's cellar and found a blown fuse in a neutral line on a separate switchboard to the live fuses, which of course left the circuit live and unsafe to work on without extra safety measures. Removing both fuses, I replaced the lamp socket and then rewired and replaced the fuses.

The ancient switchgear in the pub cellar reminded me that electrical practices in the the pre-WW2 era were different to the more modern (early 1980s) times and reinforced the concept of testing before attempting any sort of electrical work.

In the same pub, the landlord had replaced a broken plug on a pie warmer, but it wasn't warming so he asked me to check it. A slight tingling shock (I was insulated from ground) suggested that I should check the plug wiring. Sure enough, the landlord was red/green colour blind and had interchanged the live and Earth wires (the old colour scheme, red-live, black-neutral, green-earth).

Both of these stories illustrate why, at least in the UK, electricians are taught a standard ‘proving dead’ procedure. This involves a dedicated voltage indicator, which has no off button or requirement for batteries, and a test unit for the proving unit. They also must test all combinations of L, N and E as there is no guarantee that polarity is correct.

I’m surprised at your landlord/pie warmer story - even without the colours, L and E should be very different lengths, unless he chopped off and started again.

 

[berkeman]

Sure enough, the landlord was red/green colour blind and had interchanged the live and Earth wires (the old colour scheme, red-live, black-neutral, green-earth).

Oh yikes!

 

[Svein]

In Norway, both wires are "live". They may or may not have any close relation to protective ground.

 

[Averagesupernova]

In Norway, both wires are "live". They may or may not have any close relation to protective ground.

I find that very hard to believe. In the USA it is common knowledge amongst those who have an interest in electricity that the neutral it grounded. However, those with an ounce of sense still respect the neutral wire as much as the hot. Sometimes it seems the neutral wire gets more people into trouble than the hot.

 

[Guineafowl]

I find that very hard to believe. In the USA it is common knowledge amongst those who have an interest in electricity that the neutral it grounded. However, those with an ounce of sense still respect the neutral wire as much as the hot. Sometimes it seems the neutral wire gets more people into trouble than the hot.

It’s an IT system in Norway, I believe. Isole-Terre.

It’s pointless attempting an Earth return path through the rocky terrain, so the supply transformer is not earthed at all, or only through high impedance. The installation has a local earth.

 

[artis]

I am not sure why would the Norway system differ from others here in Europe? Maybe @Svein can elaborate , as far as I know , in the link below
https://energifaktanorge.no/en/norsk-energiforsyning/kraftnett/

it is said that they have 400v and 230v supply, which is the same as I have here below Scandinavia, 400v would be for the 3 phase system and 230v single phase. Here also neutral exists only in the lowest distribution part from the local step down 20kv/0.4kV traffo to the user, before that there are only 3 wires for each phase.
As for the US split phase system I understand that they ground the center tap because that is normally always used as one of the two pins for a 120v outlet, what confuses me a bit is why in most of the diagrams they show the US split phase 240v as two sines 180 degrees out of phase because as I understand the 240v is the real full secondary of the transformer and being single phase I would expect to see a normal typical sine with a 240v amplitude if I probed it with a scope should I not?

 

[Guineafowl]

I am not sure why would the Norway system differ from others here in Europe?

The rocky terrain, as I said.

As for the US split phase system I understand that they ground the center tap because that is normally always used as one of the two pins for a 120v outlet, what confuses me a bit is why in most of the diagrams they show the US split phase 240v as two sines 180 degrees out of phase because as I understand the 240v is the real full secondary of the transformer and being single phase I would expect to see a normal typical sine with a 240v amplitude if I probed it with a scope should I not?

If you probed the outer taps, you would see a 240V rms sine. The diagrams you have seen are two simultaneous waveforms, commoned to the centre tap and connected to each outer.

 

[artis]

Sure but they only look like that on drawings because in real life when one uses the 240v outlet the center tap is not used by that device/outlet.

Well here also we don't use ground for neutral (only for lighting rods and ground fault current protection) the neutral runs as a separate (4th ) wire from the nearest step down to load, this is so for every transformer.
I think that almost nowhere neutral runs back through ground as in most cases that would be a very inefficient/dangerous solution.

 

[Averagesupernova]

Sure but they only look like that on drawings because in real life when one uses the 240v outlet the center tap is not used by that device/outlet.

Phase is relative. Relative to the center tap, the outside ends of the secondary are 180 degrees out of phase. Been there, done that.