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#129946 - 11/28/05 11:32 PM the neutral wire issue?
tbtkdz Offline
Member

Registered: 07/29/05
Posts: 13
Loc: London, UK
Hello every one!
I was taught that in alternating current, this latter change its direction in the circuit 50 time per second (50 hertz), or 60 in some countries. Does this not mean that there is actually no fixed phase and no fixed neutral, since the live wire is sometimes the current provider and the neutral is the return current and vice versa!!!

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#129947 - 11/29/05 01:34 AM Re: the neutral wire issue?
Alan Belson Offline
Member

Registered: 03/23/05
Posts: 1801
Loc: Mayenne N. France
'Neutral' is grounded, (earthed), somewhere between the generator and the consumer. The relative voltage on neutral [to Ground/Earth/User!] is thus always nominally zero, irrespective of the polarity. At certain points in the cycle, the hot/live voltage on single phase is also zero, which is the nature of the beast. It's alternated in order to get better transmission efficiencies by the use of static transformers- dc voltage transformations in the past requiring complicated, expensive and unreliable rotating machinery. Today, of course, dc can be transformed efficiently using solid-state switches, as in modern railway-locos and High Voltage DC transmission systems.

Alan



[This message has been edited by Alan Belson (edited 11-29-2005).]
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#129948 - 11/29/05 04:43 AM Re: the neutral wire issue?
pauluk Offline
Member

Registered: 08/11/01
Posts: 7693
Loc: Norfolk, England
The ground/earth on the neutral simply provides a reference point by making sure that the neutral is always at zero potential with respect to the earth.

During one half-cycle of the generator the live is the positive and the neutral negative. As the neutral is grounded, that means that the live will show a voltage which is positive with respect to ground.

During the next half-cycle, the polarity reverses, with the live being negative and the neutral positive. Because the neutral is bonded to ground, however, it still maintains zero potential. Thus the live wire is driven negative with respect to ground during that half cycle.

So yes, the current is actually flowing from live to neutral half the time, and neutral to live the other half, but the neutral is always at earth potential and thus the live goes positive and negative with respect to earth.




[This message has been edited by pauluk (edited 11-29-2005).]

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#129949 - 11/29/05 07:50 AM Re: the neutral wire issue?
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
In other words, while the instantaneous DC polarity between hot (phase) and neutral can be either + or -, the phase wire is always hot relative to earth.

Whether using either the conventional (+ to -) or electron (- to +) theory of current flow, we don't assign a current direction when discussing AC circuits.

Some people say that current is supplied by the hot wire and returned by the neutral, but this obviously wouldn't apply to a line-to-line circuit.

In a multi-wire circuit, current could be said to flow in both directions in the shared neutral at any moment, which is mathematically true. Neutral current is the difference current.
_________________________
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Fine Electric Co.
fineelectricco.com

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#129950 - 11/29/05 10:15 AM Re: the neutral wire issue?
Dnkldorf Offline
Member

Registered: 12/12/04
Posts: 1091
Loc: nowhere usa
I may sound like a dead horse here, but, this is a really good site for the basic understanding, and for "brushing up" as Trumpy says...
http://www.allaboutcircuits.com/


Dnk.....

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#129951 - 11/29/05 12:50 PM Re: the neutral wire issue?
tbtkdz Offline
Member

Registered: 07/29/05
Posts: 13
Loc: London, UK
Hi, the neutral is then earthed! does this not mean that since the potential of the earth is the lowest, that during the negative half of the alternating wave (when the neutral is the positif), all the current goes to the earth rather than going to the circuit?
regards

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#129952 - 11/29/05 01:10 PM Re: the neutral wire issue?
tbtkdz Offline
Member

Registered: 07/29/05
Posts: 13
Loc: London, UK
Hi, I don't agree with you Larry when you said that in a multi-wire circuit, current could be said to flow in both directions in the shared neutral at any moment.
If you are talking about the 3-phase currents system ,if the system is balanced, then at any specific time one of the 3 phases is a return current wire.
"Neutral current is the difference current" means that if the system is unbalanced then the neutral would be flown by the difference current.
regards

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#129953 - 11/29/05 04:51 PM Re: the neutral wire issue?
maintenanceguy Offline
Member

Registered: 12/02/01
Posts: 303
Loc: Southern NJ, USA
tbtkdz:

You are correct: nothing about alternating current is fixed. It's alternating all the time.

Imagine the electrons sloshing back and forth in the wire 50 or 60 times each second. The movement of the electrons is the "current". Current is how much electricicty is moving.

The thing that is fixed is the voltage. Voltage is how much electricity is available, moving or not. On the neutral wire, voltage will always be the same. The current sloshes back and forth but voltage will always be the same as ground. We call that zero just so we have something to compare other voltages to.

On the hot wires, voltage goes up and down from positive to negative. The average voltage is what's given as the voltage but its actually going up and down all the time.

This is a little simplified, and lots of people are now going to tell you that the neutral voltage isn't always zero and that the voltage given isn't the average but the average of the square root of the square of the voltage and other stuff. Some are even going to get all fired up and type in capital letters or tell me that I'm going to get you killed but they're just trying to make it more complicated than it needs to be.

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#129954 - 11/29/05 05:19 PM Re: the neutral wire issue?
tbtkdz Offline
Member

Registered: 07/29/05
Posts: 13
Loc: London, UK
Hi, once again, another one who I don't agree with:
Voltage is not the amount of electricity available, it's the power to push or pull the electrons in the circuit; if you want to compare this with water, then the current is the water and the voltage is the pump.
And the voltage is not fixed in an alternating current, it's constantly alternating in value as the current does.

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#129955 - 11/29/05 10:06 PM Re: the neutral wire issue?
Larry Fine Offline
Member

Registered: 02/18/05
Posts: 684
Loc: Richmond, VA
tbtkdz, so you want to pose questions for the sake of arguing with the answers? Okay, I'm cool with that. Remember, the title of this forum is "Electrical Theory and Applications."

Of course, when the phases are balanced, there is no neutral current (ignoring harmonics). Is not the difference between two or three of the same number zero? That the difference is zero does not negate the validity of the math.

And also of course, at the moment of zero-crossing of the sine wave, no current (ignoring reactances) is flowing in either direction, nor into, or out of, the earth. We're talking two steady-state moments here; near the positive and negative peaks.

When one models a pair of current loops that share a conductor, the currents are diagramed and studied individually, even if that means showing two opposing currents in the shared conductor, and the resulting current is the mathematical difference.

However, your posts suggest that you believe that the instantaneous polarity of the alternating current affects whether any current is diverted to earth instead of directly back to the supplying transformer's terminals.

That one conductor is grounded in no way alters the current flow from its intended path. (Remember, we're talking theory here, and ignoring impedances and leakage currents.) In practice, we are never to intentionally use the earth as a conductor.

The reason a solid neutral is required from the source to the main disconnect, even if there are no line-to-loads, is to assure a low-impedance path to the source for the proper operation of over-current/ground-fault protective devices.

Neither half of a sine wave is "higher" or "lower" than the earth as far as current flow is concerned. A negative earth (relative to the hot wire) absorbs no more current than a positive earth does. Only a rectifier can cause the halves of an AC wave to flow differently.

In theory, no circuit current should ever flow through the earth, like an EGC, but in reality, the whole planet is like a giant equipotential grid, and some current is always in the earth, but that's not what we're discussing here.

The main difference between a system EGC and the earth is that an EGC is intended to carry fault current during a ground fault, but the earth shouldn't even then, because it is shunted by a low-impedance EGC and grounding electrode system.

As you obviously know, voltage is the difference between two points, and a circuit between those points is required for current to flow. No current flows into the earth because there is no potential difference between the neutral and the earth; the neutral to the source sees to that.

So yes, alternating current flows back and forth, like a see-saw, but the two halves of the waveform should be the same, and take the same conductive pathway. Again, only a rectifier can cause a difference between the positive and negative wave halves.

Therefore, the polarity at any given moment has no affect on the current flow's path. Whether the electrons are moving from earth to hot, or hot to earth, they take the route through the transformer's secondary, through the load, and back to the secondary.

Whether that circuit involves a grounded conductor or not is of no relevance. Only a ground fault, whether bolted or high-impedance, involves an outside path. Hopefully, that path does not involve a person. That's why we have GFCI's.


Whew! My fingers are tired!
_________________________
Larry Fine
Fine Electric Co.
fineelectricco.com

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