What is the neutral used for!?!

[kris jones]

Hi, Im in my first year of training 2357 and often find that college or work don't actually tell you the most basic of things. I dont understand neutral very well.

What I can tell you is:

Is the return path. That suggests to me current flows via the live, to the load and then back through neutral to its origin? Why then do people refer to it as having a value of '0'? surely neutral is as live as live!

Also and perhaps easier to grasp is that I understand that when phase loads are imbalanced then the neutral takes a proportion of current to equal that imbalance (I think ive worded that right) however Ive seen many people on you tube comments etc argue this.

Im sorry if these seem like fundamental q's however don't know the answers and when it clicks in my head Ill be away! Im fine with star/delta, ohms law etc it just keeps playing in my head all the time so i had to ask!

please be as basic as possible in your explanation!

thanks

 

[SparkyChick]

For electrical devices to operate, you need a potential difference (the voltage) across which current can flow.

So in your home, you have two live conductors. Line and neutral.

Line will be carrying a sinusoidal waveform that provides 240v AC RMS. That is -330v to +330v peak to peak. For devices connected to it to work, they need the required potential difference which is provided by neutral.

On the supply side of the DNOs transformer, it's a set of three windings connected in a star configuration. One side provides the lines for the three phases, the other sides are connected together at the star point which is where the neutral conductor originates. In the UK, this is also earthed, which references it to the general mass of the earth and means it will be 0v.

It's worth pointing out that under certain fault conditions, the neutral can rise to line voltage. A break in the neutral line for example. In this case, because the circuit is broken, no current will flow and the devices connected across the supply won't be dropping the voltage (by consuming power) and so the neutral conductor will rise to line voltage.

 

[kris jones]

Thank you for your reply. I'll try and get my head around it. I understand the star point etc. Still I do t fee this answers my q. Many thanks

 

[SparkyChick]

The one thing I missed out, was this....

When you measure a voltage, you measure it 'with respect to' something. So a typical single cell battery has a voltage of 1.5v on the positive terminal 'with respect to' the negative terminal.

In the mains supply, line is at 240v RMS with respect to neutral and earth, which are considered to be 0v (by virtue of neutral being connected to earth at the origin). Whilst phases have a voltage of 400v RMS with respect to the other two phases.

 

[Ian1981]

For exam purposes state the single phase voltage as 230v and not 240v unless they've changed the nominal voltage again

 

[SparkyChick]

For exam purposes state the single phase voltage as 230v and not 240v unless they've changed the nominal voltage again

Quite right.... U0 should be 230v, despite it being more like 240v nearly everywhere I've ever measured

 

[D00DY_T00DY]

For exam purposes state the single phase voltage as 230v and not 240v unless they've changed the nominal voltage again

The value can be 230V +/- 5%?(off the top of my head) voltage on site tends to be on the higher side of the spectrum some closer to 250V.
So most electrical accessories(if not all) are rated as 250V X amps.

 

[telectrix]

bloody obvious what the neutral is for...... somewhere to put your black wires into. otherwise they'd be flopping about all over the place.

 

[D00DY_T00DY]

bloody obvious what the neutral is for...... somewhere to put your black wires into. otherwise they'd be flopping about all over the place.

Black wires?!
Times have moved on now tel.. you still using twisty porcelin connectors? Lol

 

[Ian1981]

The value can be 230V +/- 5%?(off the top of my head) voltage on site tends to be on the higher side of the spectrum some closer to 250V.
So most electrical accessories(if not all) are rated as 250V X amps.

230 +10%-6%

 

[Ian1981]

Just stating that for exams/ calculations test certification Uo is the nominal voltage of 230v and 3 phase between lines 400v even tho your test instruments may measure a true RMS value

 

[D00DY_T00DY]

Just stating that for exams/ calculations test certification Uo is the nominal voltage of 230v and 3 phase between lines 400v even tho your test instruments may measure a true RMS value

Just remember OP exams may try catch you by stating the voltage as something different so do remember to read and re-read the question!

 

[telectrix]

Black wires?!
Times have moved on now tel.. you still using twisty porcelin connectors? Lol

why, is there something better now?

 

[D00DY_T00DY]

why, is there something better now?

Try blue! It's the new black!

Twist and tape my friend. Best thing since sliced bread, save your money from those connectors. For good results use two passes of tape. Don't be stingy.

 

[davesparks]

230/400 is the nominal voltage, and the voltage you will be using in your exams unless stated otherwise.
Don't confuse this with the real world however, when you are are calculating current from a power rating use the voltage stated on the data plate for that power rating. So if you have a shower which is 8kW at 240V then calculate it at 240V not 230V otherwise you will get an incorrect answer and in some cases end up installing a cable which is oversized for a job.
Most rating plates these days will also have the 230V power rating which you will see is a lower value.

It is also worth noting that, regardless of what the books say, all regular substations in the U.K. output 250V/433V and that won't change anytime soon.