Polarity relating to AC

[Sparky Joe]

As the title says; Is there such a thing?

In another forum there was a question raised of a handyman being found at fault for burning up a TV because the 'hot' and 'neutral' were reversed in the receptacle he wired.

I say there is no such thing as reverse polarity when dealing with alternating current, But I wanted to know what the experts had to say on the subject?

-Joe

 

[Bob NH]

As the title says; Is there such a thing?

In another forum there was a question raised of a handyman being found at fault for burning up a TV because the 'hot' and 'neutral' were reversed in the receptacle he wired.

I say there is no such thing as reverse polarity when dealing with alternating current, But I wanted to know what the experts had to say on the subject?

-Joe

Hot and neutral are not interchangable. In some appliances, the neutral was/may still be connected to the chassis.

The voltage on the hot wire varies between positive and negative when measured relative to the grounded conductor. The neutral is "grounded" and the only voltage on it should be the neutral current times the resistance.

 

[Sparky Joe]

Yes I understand all this, as I understand AC theory to upmost.

The question was not whether the "lamp shell" is gounded or not. I hate to sound arrogant, but I thought my question was well stated. Though I guess my understanding of the subject was not very clear.
I am a union electrician and know more than a construction electrician needs to. My question is;

is there such a thing as polarity when dealing with AC?

I know there is not, but of course no one likes to hear me say so.

 

[LarryFine]

Yes, in the DC sense, AC has no polarity. The term is often used to refer to the line and grounded conductor (neutral), and yes, we know it's not technically correct.

That being said, some equipment is "polarity" sensitive. For example, some cooking equipment with electronic ignition will not stop sparking with hot and neutral reversed.


"You sent all the way to Chicago, and no slaw?"

"Forgive us; we're draftees." ~ M*A*S*H

 

[winnie]

I believe that Bob's answer was right on the money. Note however that he did not use the term 'polarity'.

Having the ungrounded and the grounded conductor swapped can cause damage to anything that uses the grounded conductor as the 'zero voltage reference'. This is IMHO tremendously bad design, but bonding the grounded conductor to chassis or tieing the grounded conductor to signal ground is not unheard of.

The arrangement of grounded and ungrounded conductors at the receptacle is commonly called 'polarity'. We can argue that this use of the term is not correct, but we cannot argue that it is uncommon.

If the AC supply is properly isolated from signal and chassis, then _as far as the load is concerned_ there is no difference between the grounded and ungrounded conductor, and no polarity issues.

-Jon

 

[Minuteman]

For example, some cooking equipment with electronic ignition will not stop sparking with hot and neutral reversed.

That EXACT scenario happened to me a few years back. My green cub reverse wired a receptacle. Spark igniter sparked all night and damaged the circuit board. (Home owner thought it was "normal".) I had to buy a new board.

 

[plate]

Sparky Joe,

When considering if something is sensitive to "reverse polarity" the type of load connected to the supply is important. Lets remove from consideration any tie betweeen a grounded conductor and the EGC for this discussion.

If you have a simple resistor, reversing of the hot and grounded (neutral) conductor won't cause any problems. However, if you do the same to equipment with some power electronics you may end up back feeding some of the electronics.

As Bob has said,

The neutral is "grounded" and the only voltage on it should be the neutral current times the resistance.

Keep in mind that most TV's consume power even when they are "off." (I have a panasonic 32" tube type that pulses about 0.25 amps rms) I would think that this constant back feed can eventually damage some of the electronics, although I have not done a internet searches to back this up. But this would make sense to me.

 

[jtester]

In transformer theory, polarity is the drection of the secondary current relative to the primary. It shows up as higher or lower voltage between a high and low voltage terminal when you apply voltage to two high voltage terminals.

Jim T

 

[LarryFine]

In transformer theory, polarity is the drection of the secondary current relative to the primary. It shows up as higher or lower voltage between a high and low voltage terminal when you apply voltage to two high voltage terminals.

And thus the buck-boost transformer was born.

 

[steve66]

Very old TV's had a metal chassis (inside the TV) that was connected to one prong on the plug. The old style plug could be inserted into the outlet either way. So depending on how you plugged it in, the chassis could be grounded, or it could be hot. That made TV repair somewhat dangerous if you didn't use an isolation transformer.

The next step in the TV evolution included polarized plugs so the chassis was always grounded.

Modern TV's usually use a polarized plug without any kind of metal frame or chassis. The only way I see that reversing the receptacle polarity could hurt the TV is if a coax connector was connected to neutral inside the TV and inside another piece of equipment (like a DVD player or cable box). But I don't think any TV's (or other equipment) would connect the coax to the neutral - that would be a shock hazzard.

I think the TV most likely died for some other reason.

 

[charlie b]

One more possible consideration, though I do not know electronics well enough to know whether it matters. When the cord enters an appliance, the first thing it should encounter is the on/off switch. That will be connected to the ?hot? conductor. Thus, with the switch ?off,? there will be no power internal to the appliance. But if you switch the ?hot? and ?neutral? connections at the receptacle, then it is the neutral leg that will connect to the on/off switch. As a result, the entire internals of the appliance will be energized at all times, even with the switch ?off.?

Can that have had an influence in this case? I doubt it. I had thought that the polarized plug concept was a safety measure for troubleshooting and repair, and not related to the device?s operation.

 

[steve66]

I think you are right about the polarized plug, Charlie. In normal operation, it shouldn't make any difference if you reverse the hot and neutral.

Regarding the "off switch", they usually aren't the first thing anymore. Too many electronics suck power 24x7 to keep their programming intact.

I can't see any UL listed appliance connecting a neutral wire to anything someone can touch. That's a job that should be reserved for the ground wire. And I can't see any UL listed appliance bonding the neutral and ground inside the equipment. Electricians can't do that in the wiring, so I bet its not allowed inside appliances.

With those two rules, the TV shouldn't care what wire is hot and what wire is neutral.

Steve

 

[growler]

One more possible consideration, though I do not know electronics well enough to know whether it matters. When the cord enters an appliance, the first thing it should encounter is the on/off switch. That will be connected to the ?hot? conductor. Thus, with the switch ?off,? there will be no power internal to the appliance. But if you switch the ?hot? and ?neutral? connections at the receptacle, then it is the neutral leg that will connect to the on/off switch. As a result, the entire internals of the appliance will be energized at all times, even with the switch ?off.?

Can that have had an influence in this case? I doubt it. I had thought that the polarized plug concept was a safety measure for troubleshooting and repair, and not related to the device?s operation.

Charlie this is the very same argument that I used 25 years ago at a computer company. I noticed that many of the receptacles at the work stations had reversed polarity. The techs. didn't unplug when servicing they would only turn off the switch to change out a board. I could see an accident waiting to happen.

They would not let me make the correction and then a few days later someone burns up a very expensive proto-type board ( hard to replace ).
Suddenly polarity is important and changes are made.

 

[rattus]

Pedantics:

Pedantics:

As the title says; Is there such a thing?

In another forum there was a question raised of a handyman being found at fault for burning up a TV because the 'hot' and 'neutral' were reversed in the receptacle he wired.

I say there is no such thing as reverse polarity when dealing with alternating current, But I wanted to know what the experts had to say on the subject?

-Joe

Strictly speaking, an AC signal reverses polarity twice per cycle, or 120 times per second @60Hz. However, in common usage, "reverse polarity" indicates that the hot and common are swapped--as others have said.

 

[Sparky Joe]

Thanks to everyone for all the posts.
I definetely found what I was looking for........

Although the ignitor for a stove or even perhaps a furnace; why would it keep trying to ignite with the switch off?
Because it's bolted to the steel frame of the appliance and when neutral(according to the appliance) becomes opposite polarity of the frame it will spark except backwards?
Interesting stuff, thanks guys, I enjoy the 'sparks' in my brain when reading the posts around here.

 

[johnny watt]

Polarity can describe charge (voltage) or current direction or the timing of the waveform.

Some video circuits use the AC voltage as a timing signal for the vertical synchronization. Reversing the leads could throw this timing off by 180 degrees regardless of the other repercussions, such as a hot chassis.

 

[Sparky Joe]

That EXACT scenario happened to me a few years back. My green cub reverse wired a receptacle. Spark igniter sparked all night and damaged the circuit board. (Home owner thought it was "normal".) I had to buy a new board.

This scenario has been bugging me, as this is the second time I've heard these 'ignitors' being sensitive to hot and neutral reversed.

I've racked my brain and drawn a few drawings, but still can't come up with a feasible answer for my self. Perhaps someone else could show me a drawing or even explain how this is possible?

Most my drawings showed a direct short if polarity were reversed, though I did one relating to the iron core of the coil being bonded with ground, and though it doesn't entirely make sense, it's the best my meager mind could do.

Here's what I drew; Any comments?

Oh and the actual spark comes into play somewhere after the coil(forgot to include that)

 

[Sparky Joe]

Nevermind; now that I think of it, if there were an 'open' in the circuit before the coil(as if polarity were reversed) there wouldn't be any spark.

Which leads me to the conclusion of; I really need help on how this constant sparking could happen?

At least so I can sleep easy at night, haha