explaining the purpose of neutral wire?

[ban-all-sheds]

I take it you are talking about floating systems (electrically separate)?
Neutral needs to be at about 0v with respect to earth, otherwise it isn't a neutral. In floating systems you effectively have two phases.

Wouldn't the centre point of a 3-phase Wye system be neutral, even if it were not at 0V wo earth?

 

[matt1e]

I take it you are talking about floating systems (electrically separate)?
Neutral needs to be at about 0v with respect to earth, otherwise it isn't a neutral. In floating systems you effectively have two phases.

yes and no mate It's just we are looking at it from a different perspective what I mean is that the 0v in this statement

The peak voltage between 0v (Neutral) and the top of the sine wave will be +340v,

is not the solely the neutral connection, it is as much the phase connection as it is the neutral
just found a cool clicky that shows what I mean its got buttons to play with too

but yes talking in reference to earth you are of course correct

same here

Nope, 0v and neutral are essentially the same. If you put a scope on a bog standard AC supply the sine wave will oscillate around 0v.
Don't try connecting a scope to 240v unless it is rated for it tho - it'll go up in smoke!

the 0v on the neutral is not the same 0v you see on the scope

if that makes any sense
but you are still right too,if that makes any sense

I take it you are talking about floating systems (electrically separate)?
Neutral needs to be at about 0v with respect to earth, otherwise it isn't a neutral. In floating systems you effectively have two phases.

Wouldn't the centre point of a 3-phase Wye system be neutral, even if it were not at 0V wo earth?

yes that's why its called neutral (not just because it's most the time neutral to earth)

matt

 

[Spark123]

I take it you are talking about floating systems (electrically separate)?
Neutral needs to be at about 0v with respect to earth, otherwise it isn't a neutral. In floating systems you effectively have two phases.

Wouldn't the centre point of a 3-phase Wye system be neutral, even if it were not at 0V wo earth?

I don't think it will be tbh - never come across it yet.

 

[davelx]

It all relative, innit ?

The regs define the Neutral Conductor as a conductor connected to the Neutral Point of the supply system. However they don't define the Neutral Point AFAICS.

So we can arbitrarily select any point on the supply side and define that as the Neutral Point. If there is no point in the system connected to Earth, i.e., the system is floating, then the Neutral Point, together with all other parts of the circuit, has no voltage WRT Earth anyway.

We can arbitrarily decide to connect any point of the system to Earth. If, as is usual, we elect to connect our defined Neutral Point to Earth, then that Neutral (Point) is, by definition, at 0v wrt Earth. But we are not compelled to do that. We could connect some other point in the supply to Earth, in which case Neutral would not be at Ov wrt Earth.

In a three phase supply, we generally define Neutral as the star point of the 3-phase transformer feeding the distribution network, and we generally connect that star point to Earth. That fixes the Neutral Point at 0v wrt. Earth.

The voltage at any point on the Neutral Conductor will, of course, vary from 0v where it connects to the Neutral Point, up to some value determined by the resistivity of the Neutral Conductor, its size, distance to the Neutral Point and the current flowing through it.

So it seems to me to depend on your frame of reference. (If that is on a moving vehicle, does your ammeter read a lower current? )

At least, that's how it looks from where I am standing. Your frame of reference may vary.

 

[ban-all-sheds]

It is all relative - my Q really was about a theoretical possibility, not something that ever has, or ever would, exist, but I can imagine that with a bunch of multi-tapped transformers with you could create a 3-phase wye with the neutral at >0V wo earth, and each phase therefore offset from earth, even if within the frame of reference of the 3-phase supply the voltages between phases and neutral were normal.

 

[matt1e]

It all relative, innit ?

The regs define the Neutral Conductor as a conductor connected to the Neutral Point of the supply system. However they don't define the Neutral Point AFAICS.

yes it is an strange one

the star point of wye configured transformer is classed as a neutral point
a cable connected to this point becomes a neutral conductor

the centre tap on the secondary on a transformer can also classed as a neutral point as in the american domestic system they also earth (or ground) it for safety purposes

yet would we ever call the centre tap on the secondary of a 110 site transformer neutral even though its technically a neutral point? no
as Bas and yourself say its all relative its also depending on your perspective
and a think hellmouths head is probably done in by now

 

[Spark123]

The thing that is still playing on my mind with it is you would still need to have overcurrent protection on all poles with a floating system. Overcurrent protection isn't required to be placed in a neutral conductor however on a floating system derived from a wye connection I can see danger arising if overcurrent protection wasn't there on all poles.
Or am I confusing myself with ADS vs Separate??

 

[Goldberg]

and a think hellmouths head is probably done in by now

 

[bernardgreen]

There is another less obvious function of the neutral wire. That is to be able to have the returning current in the same cable as the outgoing current ( or currents if more than one phase ) so that the sum of all currents in the cable is zero resulting in no magnetic field or induced currents occuring in adjacent cables or equipment. ( No mains hum on audio equipment etc ).

 

[Stoday]

I'm rather curious to know where the "return currents" go in a cable without a neutral. I thought that a phases' return current went back up the other two phases.

 

[matt1e]

and a think hellmouths head is probably done in by now

 

[ericmark]

It all relative, innit

So it seems to me to depend on your frame of reference. (If that is on a moving vehicle, does your ammeter read a lower current? )

At least, that's how it looks from where I am standing. Your frame of reference may vary.

The star point on a motor vehicle is NOT neutral in fact no part of the AC output is neutral as it in normally the negative which is connected to earth so non of the three phases or star point will be neutral.

Thank you Davelx for pointing this out. Yes some single phase twin wound alternators do have one end of the winding within 0.6 volts of chassis voltage before pedantic BAS picks me up.

But to be a neutral it must be bonded to earth at some point. In parts of USA they use delta secondary and centre tap one winding so they have a neutral not connected to any star point.

I'm rather curious to know where the "return currents" go in a cable without a neutral. I thought that a phases' return current went back up the other two phases.

It was found back in the 1980's that by rectifying the star point as well as the three phases it reduced eddy currents and increased the alternator output and reduced heat. So even with what should have been perfectly balanced system on a vehicle it was better to use the star point.

I am sure if I hunt out my books I can find out more but I think that should be separate post it would only confuse.

 

[holmslaw]

..

 

[bernardgreen]

I'm rather curious to know where the "return currents" go in a cable without a neutral. I thought that a phases' return current went back up the other two phases.

If there is no neutral conductor then the return currents for each phase do return via the other phases. If the loading is evenly balanced between phases then this situation is fine and the voltage on each phase is close to the nominal supply voltage. But if the loads on the individual phases are is not evenly balanced then the voltages across phases are not equal and can be far from nominal supply voltage.

 

[mikhailfaradayski]

To the OP,

Are you keeping up with this?

I say its the blue one

No earths on an aircraft

lol, good job too, could you imagine the length of your earthing conductor for your average heathrow/newyork flight? Does copper float? Wouldn't like to calculate Zs for that install. def wont get satisfactory disconnection times.
Argh, the skies are filled with green and yellow streamers!!!!