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!!!

'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).]

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).]

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.

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.....

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

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

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.

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.

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!

t,
The /water/pressure/pump' analogy doesn't work very well with ac. I prefer a very simplified rail-wagons in a circular shunting-yard analogy. The loco ('volts') shoves the wagons (electrons) forward and back in a continuous cycle, and their being no 'links', only buffers, the first wagon must touch the back of the loco for the circuit to work. The mass of the wagons and their contents are accelerated / decellerated continuously, and their net inertia is analageous to 'current'.
Rail and other friction is analageous to resistance, ohms.
Now, how far does the 'current' go? Answer- practically nowhere! 'It' cycles on the spot, like someone doing pointless step-ups at the gym!

Alan

Hi, Larry I don't want to pose questions for the sake of arguing with the answers, if I wanted to do so I would have chosen the journalist job and my topic is in the right place: "Electrical Theory and Applications", and no one should force anyone to accept anything. In fact all what you have just said I've actually studied it when I was in the university in 1987, when I was preparing my diploma in teaching electrotechnics to teenagers. I understand exactly every thing in theory. But since I went for Electrical installation job I've noticed that there are many differences.and among those differences is the famous neutral issue which I'm exposing here, and apparently haven't found the right answer yet. Any way thanks a lot, I'll keep searching to get the right answer and "I'll be back!!!"
Regards

Ahh, now that you've described where you are coming from, I think that I can take a stab at this.

The first thing to remember is that 'neutral' is a misnomer. Neutral has the implication of 'dead' or 'unpowered'. This is a false implication. The 'neutral' conductor is a full current carrying conductor, and is as important as the phase conductor. The 'neutral' conductor is perhaps better termed the 'center tap' conductor. It is connected to the source of electrical power (battery, transformer, generator, inverter) just like any of the 'phase' conductors.

Rather than thinking of this connection to the transformer (I'll stick just to transformers for the rest of the discussion) as being in any way special, simply think of it as just another tap on the winding. You will be able to measure voltage between any pair taps. You could (as you wish), measure instantaneous voltage, RMS voltage, peak voltage, phase angle, etc. If you connect a load between any two taps, you will see current flow that depends upon the load characteristics (resistance, reactance, etc.) and the supply characteristics. Select a different pair of taps (and thus a different potential difference), and you will see different current flow. But the key issue is that you will have a voltage and the potential for current flow between any of the transformer taps.

The current flow through the load between any of these pairs of taps will be alternating, as the supply frequency. The flow of current will cycle at the supply frequency, and all of the terminals connected to the load will in turn act as current source or current sink. The center tap is in no way special in this regard. Sometimes it is a current source, and sometimes it is a current sink.

Once you have the concept of voltage between _any_ pair of taps, you can think of the _set_ of voltages associated with a single tap. Think about all of the pairs of output terminals on the transformer. You have a voltage between any pair. Pick _one_ terminal, and measure the voltage between that terminal and each of the others. The thing that makes the center tap unique is that the 'overall' voltage between the center tap and all of the other terminals is lower than the 'overall' voltage of any of the other terminals.

For example, consider a standard 415Y/240 transformer set. You have 4 terminals: 3 phase terminals and the 'neutral' terminal. From any of the phase terminals, you have 415V, 415V, and 240V to the three other terminals. But from the neutral terminal you have 240V, 240V, 240V. This is the only thing that makes the neutral special. It is the transformer tap from which the set of all other voltages are minimized.

If you connect a load to the neutral, sometimes electrons will flow toward the neutral, and sometimes they will flow from the neutral, and as far as current carrying is concerned, the neutral is just a transformer tap.

The function of the loads and the transformer is essentially independent of the grounding of the transformer. The circuits would function the same if ungrounded, if _one_ of the phases were grounded, if the neutral was grounded, or if the entire transformer and load were loaded into a spaceship and entirely separated from the earth. Current would simply flow between the transformer terminals just as if nothing was different.

The fact that the neutral is at 'center potential' also means that current flow from one phase conductor to the neutral might be balanced by current flow from another phase conductor, through another load, to the same neutral. This is the basis for reducing the size of the neutral. Because of the principal of superposition, it is mathematically reasonable to claim that current for one load is flowing in the neutral, and current from the other load is also flowing in the neutral, in the opposite direction. But at the level of the electrons moving around, current is flowing in the single aggregate direction.

For safety, electrical systems _intentionally_ connect the transformer neutral to the local 'ground'. For the most part, only very slight current will flow through this connection, because it is not part of an intentional circuit. It is part of a capacitive circuit between all of the phase conductors, their insulation, and all of the surrounding conductive materials, but this is a _very_ low curent circuit. What this intentional connection does is minimize the voltage between any of the phase conductors and local earth, in doing so minimizing the voltage that would be present in the case of any unintentional contact between phase and earth. Remember that the neutral is the tap for which the voltage to the other taps is minimized. This connection minimized the voltage between the phase conductors and all the other conductive things that simply happen to be around, but which are _not_ supposed to be carrying current.

The intentional ground connection is also their to provide a safe discharge path for electrostatic charge, for example lightning, or much lower power zaps from walking across the carpet. When you don't have a ground connection, the voltage between the electrical system and ground can build up in a number of different ways, reaching the point of punching through insulation and damaging hardware. But the ground connection is _not_ necessary for operation, and does _not_ need to be made at the neutral point.

In addition to the common balanced supply systems used in the US, you will see two systems which ground a conductor that _isn't_ the system neutral. One is a 'high leg' system, where you have a center tapped single phase transformer in a three phase delta bank. You ground the center tap, which _is_ neutral for that single phase, but is not the system neutral. The other system is corner grounded delta, where you make the ground connection to one of the supply phases of a delta secondary. In the corner grounded delta, the grounded conductor is full size and delivers as much power as the other two 'hot' phases.

-Jon

Jolly good show, Jon.
Great explanation there!.

It seems that earth connections always seem to cause a lot of confusion. You'll often hear things like "The current will always try to get back to earth," which is simply not true.

The current will try to flow in a complete circuit back to its source. If one pole (any pole) of the supply is grounded, then the earth just happens to provide a convenient path.

Connecting the neutral (or one phase of a corner-grounded delta, mid-point of a high-leg delta etc.) to ground does not suddenly mean that current flows to ground all the time, or that negative portions of an AC waveform are absorbed by the earth and do nothing useful.

I've found that the concept of voltages which are negative with respect to ground is one which many people find hard to visualize. You need to look at the relative polarities between two points to see which way the current will flow. Whether one side or the other is grounded is immaterial, until such time as a second connection to ground is made.

The ground reference simply sets the zero reference point from which other voltages can be measured. The batteries providing power in telephone exchanges, for example, always have the positive side grounded, so the "live" supply is at -50V or so with respect to earth.

As the original poster is in London, I think we also need to mention that the term "neutral" is very often used in Britain to mean a grounded conductor in general, not just a true neutral in the strictest sense such as the center of a star or the middle wire on a 3-wire circuit.

Doesn't electrical power distribution as used everywhere in the world use the earth itself as a giant white wire? I mean, we don't exactly have neutral wires going back to the power stations.

No.

There are some very rural areas that use SWER (single wire earth return ) systems.

Here is a link to a page with more info.
http://www.ruralpower.org/


3 phase power can be distributed with just 3 'hots' when a neural is required you install a delta / Wye transformer and derive a neutral from there.

Also the utilities use an MGN (muiltigrounded neutral) that does indeed run from the end user back to the power station.

If you think about it you always see a bare conductor run between poles just under the phase conductor(s), that is the MGN.

[This message has been edited by iwire (edited 12-04-2005).]

I thought the TOP bare wire was the multi grounded neutral, and that it's earth grounded at every pole & not a current carrier, and more of a lightning arrester than anything else?

At any rate, I didn't mean to imply ALL power generation systems use the ground as a white wire, just that it's very common and used all over the US. Three-phase delta might not require a current carrying neutral, but once you go wye or connect two of those to a center-tap transformer to get 120V 1-phase, the actual earth is carrying considerable current, is it not?

Edit: ah, answered my own question: http://www.mikeholt.com/mojonewsarchive/SV-HTML/HTML/GroundCurrents~20020918.htm
EG, even when there is a neutral wire (MGN) the earth is also acting as a neutral wire.

[This message has been edited by SteveFehr (edited 12-04-2005).]

The original HVDC transmission line between Gotland and Sweden, (Vastervic-Ygne), used the sea as the return leg. I wonder if it stunned any fish? The old telegraph systems that spanned the American continent used the Earth as return. It's just a question of trading the cost of power-losses against the cost of the copper line and the labour.
The problem in cities with dc returning in the ground was electrolyic action disolving the water and gas mains. I'm not sure if ac has that effect.

Alan

Alan Belson

The problem in cities with dc returning in the ground was electrolyic action disolving the water and gas mains. I'm not sure if ac has that effect.

Alan

Yes it does. No it doesn't. Yes it does. No it doesn't.

Joe

"Hey, is my turn signal working?"

"Yes, no, yes, no, yes, no..."

Hello everyone!

I know the question has been somehow answered, but I will do it anyway.

“tbtkdz” asked:
“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”

And my short answer is: NO; it does not mean that!

A better one could be something like this (putting together some pieces already answered by some other members and with a little addition):
The “Neutral” is neutral because it is grounded. The name Neutral does not refer to an absence of electrical currents, but to an absence of electrical tension between it and the ground electrode. It does not depend on phase configuration (monophase or polyphase) or load distribution (balanced or unbalanced).
In a typical commercial building’s three phase wye 120/208, the Neutral is the transformer’s secondary central point (!); but in a residential 120/240 the Neutral is a middle point between two phases. I both cases the Neutral is explicitly bonded to ground.
I know, someone could ask what happen if suddenly the Neutral-to-ground bounding gets broken? Well, we have a safety issue on hands, the system still works (electrically [at least most of it]); but the Neutral is not neutral anymore (it is not acting as a Neutral; is now a floating phase measured against ground [looking at it from the supply-side]).

Hope this helps,

Joe (another Joe).-