ECN Forum Forums Technical Discussion Electrical Theory and Applications the neutral wire issue?

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