Ac basics, Live and Neutral

[Burrah Boy]

Hi, i would like an explanation on the function of the neutral wire on a simple single phase supply. From what I understand is that your live (hot) wire comes in from the power companies transformer, into your house at 230v. It then say is connected to a light bulb, which causes current resistance, the bulb glows and the neutral wire (which has a 0 - 5v reading between earth) then acts as a return path to the power companies transformer. This neutral is connected to ground at the transformer. In my ignorance this is what is confusing me. How come the light will still glow if I reverse the wire around? if neutral is connected to ground at the transformer, its as if it is returning the 230v into the ground and bang?? I know that there is no electron flow in AC, like in DC, because of the reversing effect of the sinusoidal waveform. The electrons start to move way and then are pulled back in the other direction, and in DC there is electron flow through the conductor from negative through the load and back to positive, returning the electrons to the battery. On a simple transformer magnetic induction cause the volatge, there are two wires that come off either side of the transformer, one pair being supply, and the other pair, either stepped up or stepped down voltage. In our case the 230v. As you can see I am confused. Is there anyone who knows how to explain it in simple terms, or an informative website?
Thanks
Marc!

[Brian_UK]

I think that you have answered your own question ---------


because of the reversing effect of the sinusoidal waveform

[nike123]

When you have power transformer which is not earthed, both wires are line and both have some potential. Because it is not earthed, current cannot close circuit thru earth and you cannot measure any potential to earth. That is principle of isolation transformer. Only potential is between two wires of transformer. When you earth one end of coil, you than have only one wire at 220V (it goes from -220V to 0V to +220V in one period) and that become neutral since her potential is always at 0V.


I know that there is no electron flow in AC, like in DC, because of the reversing effect of the sinusoidal waveform.Electron flow is always present in both AC and DC current. Current is flow of electrons and without that flow you don't have any current.
Reversing polarity doesn't mean that current flow is void.

[Electrocoolman]

Burrah Boy,
AC Electrical power is generated and distributed as 3 phases.
A 3 phase system can be connected in DELTA or STAR and is normally 'balanced'.
A DELTA system does not have a 'Neutral' point (by definition), whereas a STAR system does, however when balanced, the potential of the Star / neutral point remains steady (ie Neutral) and does not need to be connected.
When a STAR supply is connected to a balanced STAR load using 4 wires, the sum of the currents flowing in the wire connecting the star points is ZERO, and thus it does not require a 'Star / Neutral' wire.

Most electrical supplies are derived from STAR connected transformer secondary windings for voltage and final distribution practical considerations. In this case the STAR point is used and is commonly called the Neutral. The phase to phase voltage in UK normally 415v and the phase to star/neutral voltage is 230v.

As Nike says, a transformer provides isolation, and potential only exists (and can only be measured) between the circuit conductors. Only when one of these conductors is referenced (i.e. connected) to earth, will you measure a potential to earth.
Thus the Neutral / STAR point of the supply transformer secondary is connected to EARTH in order to give the supply a reference to Earth. This is the main basis of the supply protection system and gives us a safe supply.

In your house, depending upon the 'type of supply' the neutral is close to earth potential. This can be effected by how out of balance the system is (different loads on the 3 phases, will result in a current flowing in the neutral), and how far you are from the point where the neutral and earth are tied together. This is the reason for your measured 0 - 5 v on the neutral (unless you have a fault on your neutral).
(Certain types of supply have the house earth connected to the neutral at the supply point, in which case you would not measure a voltage.) The 'earth' in your house then becomes the equipotential reference for circuit protection
As you correctly say the neutral is the RETURN path for the current flowing from the PHASE.

The Phase (Live or Hot) supply is fed through a circuit protection device (fuse or circuit breaker) before it feeds a load. If one side of the supply was not tied to earth, then in the event of a fault (e.g. short to metalwork) the fuse would not operate. It would only operate if a short circuit or overload were to occur between the circuit conductors.

The load (your lamp) does not care wich wire is connected to phase or neutral, the current (and electons) will flow between the two supply wires, however we normally require a switch in the circuit feeding the load, and convention is that this is in the phase conductor for safety. We also (in the UK), need to sub-fuse the phase leg in the spur circuit or plug tops. Thus we need to identify the phase conductor which is done through the colour of the cable insulation.

You could also have added protection from a RCCD (Residual Current Circuit Device). This checks that the current flowing in the phase and neutral are equal.
Your lamp will still light if the return circuit is via the earth, but this would be detected by the RCCD which would trip.

[Burrah Boy]

Thanks guys, it makes much more sense now!

[tbirdtbird]

In the USA, the power company transformer is actually a center-tap transformer, which you can google what that is for a diagram. The center tap is what is referred to as the 'neutral'. So you can draw 115v between the neutral (the center of the winding) and either end of the winding (the so-called 'hot leg'). If you connect to both ends of the winding at the same time (to both 'hot legs') you get 230 V. The center tap just happens to be grounded.

You should NOT be reading a voltage between neutral and ground. If so you have a 'floating' neutral somewhere...a loose connection *somewhere* even possibly in the main electrical panel or I have even seen it at the transformer itself on the pole.

What is more difficult to explain is that if your system is *perfectly* balanced then the neutral is carrying no load (amperage) at all. Perfect balance is hard to do esp. depending on the varying loads. But if a user has a lot of motor and heat loads, you should try to balance them out. I have seen panels where 80% of the load was on one hot leg. Get your amprobe out and compare the amps in the 2 hot legs. Any difference is being carried in the neutral.

Not sure about the UK but must be similar. Draw it out on paper it will make more sense.

[Toosh]

Hi, i would like an explanation on the function of the neutral wire on a simple single phase supply. From what I understand is that your live (hot) wire comes in from the power companies transformer, into your house at 230v. It then say is connected to a light bulb, which causes current resistance, the bulb glows and the neutral wire (which has a 0 - 5v reading between earth) then acts as a return path to the power companies transformer. This neutral is connected to ground at the transformer. In my ignorance this is what is confusing me. How come the light will still glow if I reverse the wire around? if neutral is connected to ground at the transformer, its as if it is returning the 230v into the ground and bang?? I know that there is no electron flow in AC, like in DC, because of the reversing effect of the sinusoidal waveform. The electrons start to move way and then are pulled back in the other direction, and in DC there is electron flow through the conductor from negative through the load and back to positive, returning the electrons to the battery. On a simple transformer magnetic induction cause the volatge, there are two wires that come off either side of the transformer, one pair being supply, and the other pair, either stepped up or stepped down voltage. In our case the 230v. As you can see I am confused. Is there anyone who knows how to explain it in simple terms, or an informative website?
Thanks
Marc!

Hi Burrah Have a look at this

http://www.behlman.com/applications/AC%20basics.pdf

Norm