Why is the correct polarity important with AC?

[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.

 

[Guineafowl]

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

I’m not sure how that relates to my answer, so I guess I’ll try again.

1. Scope between outer taps - one 240v rms sine. Centre not involved.

2. Using a two-channel scope, connect one probe common to centre, and its tip to one outer tap. Connect the other probe tip to the other outer tap. Displaying both channels simultaneously, you will see two sines, 120V rms 180deg apart.

^^^ This is a thought experiment. There are safety issues to consider when probing mains voltages.

 

[artis]

@Guineafowl thanks, I already understood that by the fact that @Averagesupernova said about the measurement being relative to center tap but you beat me to it

 

[Svein]

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.

Spot on!

 

[chipresistor122]

Hello just stumbled upon this.. trying to understand how diode bridge works. Is it an assortment of 4 different diodes , are they configured a specific way ? I am a bit confused on the construction. Any help would be great. Also why would this be necessary in a situation like this ?

 

[sysprog]

Hello just stumbled upon this.. trying to understand how diode bridge works. Is it an assortment of 4 different diodes , are they configured a specific way ? I am a bit confused on the construction. Any help would be great. Also why would this be necessary in a situation like this ?

This wikipedia article is pretty good : https://en.wikipedia.org/wiki/Diode_bridge

There is a non-planar crossover (the little half circle) in this bridge circuit $-$ that crosser can be replaced with 3 XOR gates to make it planar, arranged as shown here:

. . . and each those can be relaced by 4 NAND gates arranged as follows:

That can matter in micro-circuits, but generally doesn't matter on a breadboard circuit.

 

[darth boozer]

This wikipedia article is pretty good : https://en.wikipedia.org/wiki/Diode_bridge

View attachment 273546

There is a non-planar crossover (the little half circle) in this bridge circuit $-$ that crosser can be replaced with 4 XOR gates to make it planar, arranged as shown here:

View attachment 273544
. . . and each those can be relaced by 4 NAND gates arranged as follows:

View attachment 273543

That can matter in micro-circuits, but generally doesn't matter on a breadboard circuit.

"... that crosser can be replaced with 4 XOR gates to make it planar, arranged as shown here:"
There are only three XOR gates shown.

I don't see why a diode bridge was suggested in the context of the OP, anyway. It certainly wouldn't work for an appliance requiring an AC supply, and neither would using logic gates as part of a diode bridge in a power circuit.

 

[sysprog]

"... that crosser can be replaced with 4 XOR gates to make it planar, arranged as shown here:"
There are only three XOR gates shown.

I don't see why a diode bridge was suggested in the context of the OP, anyway. It certainly wouldn't work for an appliance requiring an AC supply, and neither would using logic gates as part of a diode bridge in a power circuit.

You right $-$ it's 3 XORs, 4 NANDs each to replace them, and 4 diodes in the bridge $-$ thanks for the correction $-$ I'll edit the post $-$ as for appliances that use bridge rectifiers to convert AC to full-wave-rectified DC, there are many such devices.

Regarding using logic gates as part of the bridge circuit, they could be used in the DC part of the circuit to allow a planar crossover, like this:

The 3 XOR gates could go where the left-tilted X with the adjacent - + and + - labels is in the circuit illustration.

 

[chipresistor122]

Sysprog , thank you for the thoughtful response. It is much appreciated I needed a refresher on how Bridge Rectifiers work.

 

[jacobgoona]

Yup that is a good explanation. Overall a bridge rectifier is just an assortment of 4 or more diodes. It comes in a pre-assembled module. They convert AC current to DC current and are generally used in power supplies or battery chargers. The assortment of diodes are used to allow current to flow in one direction only and not flow backwards. It is a really common component in our daily lives. I found another article for you to reference that should help refresh you. Talks about how it works, different applications and also has a video to go along with it Bridge Rectifier Explanation . Hope that helps ... let me know if you have any other questions. I am happy to help.

best,