# AC polarity

Discussion in 'General Electronics Chat' started by stoopkid, Jun 17, 2012.

1. ### stoopkid Thread Starter Member

Mar 3, 2011
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I was wondering what the difference is between positive and negative when using AC power? Why do some electronics require you to plug them in with the correct polarity? How can you have a hot and neutral wire when they alternating? Seems like if you reversed them you would simply be inverting the phase, so why does it matter?

2. ### WBahn Moderator

Mar 31, 2012
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In theory, what you say is true. However, for safety reasons, the neutral wire is electrically connected to the ground (as in physical earth ground) and the other wire is alternately positive and negative with respect to that wire. So grabbing the neutral, even on a circuit in heavy use, will only result in your body seeing a few volts relative to the ground that you are standing on while grabbing the hot will result in hundreds of volts relative to that same ground.

Now, you might say, if the neutral weren't grounded then you could grab either side and simply be floating relative to ground and thus have very little danger of any significant current flowing through your body. Under ideal conditions, this is true. But now if someone somewhere else has something plugged in one way and has it sitting on a decent electrical ground, they are defining the grounding point and the case of your radio may now be several hundred volts above ground just waiting to zap you real good. Worse, if the transformer that steps down from 13kV (or whatever in your area) to your house voltages shorts, it could pass the full 13kV to your house.

By intentionally defining a solid earth ground on one of the wires (the neutral), we make is to that all of these other unintential circumstances blow fuses to disconnect the power until the problem is identified and fixed.

3. ### vpoko Member

Jan 5, 2012
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The neutral prong is always common with the earth while the hot prong isn't.

4. ### paulktreg Distinguished Member

Jun 2, 2008
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I don't know how it's done in the USA but over here in the UK the neutral is connected to earth at the nearest substation that drops the voltage to 230VAC. It will be local and that's why you always measure a couple of AC volts between neutral and earth.

5. ### WBahn Moderator

Mar 31, 2012
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Here the transformers that drops it to 230VAC (center tapped) generally aren't at a substation but rather are distributed throughout the area. For instance, where we are is pretty sparsely populated and the pole that serves are house has a transformer on it and only serves our house. Where I lived before there were several houses served by each transformer. I'm pretty sure that the neutral is earth-grounded only at the each service drop and not at the transformer. The goal here, I believe, is protect the customer in case a high voltage line falls on a low voltage line. It may improve the safety against certain failure modes, but the multiple grounding points (one at each service drop) also permit ground currents that can lead to hard to identify and correct problems. I know that the code requires the water feed pipe to be grounded on both sides of the meter. Of course, that assumes that there IS a water feed pipe and that it is copper coming up from the ground.

All metal systems in a house (HVAC ducts, hot water pipes, metal chassis on appliances, etc.) must be bonded to the earth grounding system.

If I've screwed any of this up, hopefully someone will correct me.

6. ### #12 Expert

Nov 30, 2010
16,704
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No objection WBahn. I see 4 to 6 houses supplied by each pole mounted transformer in a city block with lots about 60 feet wide. Arranged in 2x2 or 3x2 pattern. I do not see an earthing cable running down each wooden power pole. I do see a grounded wire as the top conductor on the pole, perhaps to carry the current resulting from atmospheric activity like lightning and corona (St. Elmo's Fire). I also see local transformers connected to (1) one high voltage wire, so I must believe that a current carrying ground is up there, too.

I am not looking at a power pole with binoculars. I am looking at all the pictures in my head that resulted from examining several power poles over the years. (I was surprised when I learned that not everyone can do that.)

In rural areas, it is normal to supply one transformer per house because the IR losses would be too high to go more than 100 to 200 feet with a reasonable size of wire.

7. ### wmodavis Well-Known Member

Oct 23, 2010
737
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A safety issue not a polarity issue. And AC does have polarity. It just alternates over time at usually 50 or 60 Hz.

Last edited: Jun 18, 2012
8. ### takao21203 Distinguished Member

Apr 28, 2012
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Electric polarity and phase polarity are two different things.

If for instance you want to use 2x secondary windings in series/parallel, phase polarity must be observed.

AC does not have electric polarity as such but also can have bus polarity, depends on circuit. In the end bus polarity is the same as phase polarity.

9. ### PackratKing Well-Known Member

Jul 13, 2008
850
216
I likely missed it in a quick scan, The hot and neutral leads are polarized at the plug, to ensure that the hot wire always goes to a given equipments main power switch...........switching the neutral would leave the entire appliance "hot" in the event of a snafu.

10. ### Audioguru New Member

Dec 20, 2007
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There are no power poles in my neighborhood. Electricity, telephone, cable TV and others are all underground.

Do you also have natural gas, water and sewer pipes on poles in your neighborhood??

strantor likes this.
11. ### strantor AAC Fanatic!

Oct 3, 2010
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LOL. It's pretty common to have electricity on poles AG. Gotta laugh though when I picture turds in transit overhead. Clear pipes anyone?

12. ### Audioguru New Member

Dec 20, 2007
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I saw a photo of a street in India with millions of wires all over the place up high on poles blocking light so the street was dark during daytime.
Is it like that in Texas??

13. ### strantor AAC Fanatic!

Oct 3, 2010
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haha yeah just like that. Actually i took this picture when the guy came to troubleshoot why my multi KW "sine wave inverter" UPS wasn't powering my house.

14. ### #12 Expert

Nov 30, 2010
16,704
7,354
There is a water tower about 2 miles from my home and I have signed a petition to have it moved underground so it isn't so ugly

15. ### WBahn Moderator

Mar 31, 2012
18,088
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A much sounder idea is to circulate a petition to have all of that dihydrogen monoxide banned within city limits altogether.

16. ### strantor AAC Fanatic!

Oct 3, 2010
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You would think that with all the masses suffering from aqueous rhytides, something would have been done already

17. ### Audioguru New Member

Dec 20, 2007
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Water tower??
The water reservoir near my neighbourhood is also underground. It is fed high voltage high power from overhead high voltage lines.

18. ### strantor AAC Fanatic!

Oct 3, 2010
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So, you do have power poles? or are your overhead lines suspended by maglev?

19. ### dataman19 Member

Dec 26, 2009
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OK - ya;ll got me wound up...
...
When a person grabs a hot wire in the dark - how can you tell whether he has a hold of an AC power line or a DC Power line?...
...