Why is the correct polarity important with AC?

[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,