Basic circuit theory - neutral

[RMAK]

Hello to all maybe a silly question but here goes, in a single phase domestic situation
where does the current flow to in the return leg of its journey through a load for example a lamp, is it sent back to the point of origin the generator?

 

[Softus]

Terra Firma.

 

[JohnD]

is it sent back to the point of origin the generator?

yes*

but no but yes but no but yes.

(via substations, transformers, distribution network. However the Neutral from one phase will mostly be balanced by currents from other phases into the same neutral, which cancel each other out once they get into a 3-phase supply, which is where your single phase comes from. *So maybe "not exactly" would have been a better reply. A 3-phase supply if fully balanced may not need, or have, an Neutral conductor, which would only end up carrying net imbalances between the 3 phases. Some 3-phase loads, like large motors and industrial heating devices, do not have a Neutral connection.)

there is a method of return via earth, but this is not used in UK.

 

[RMAK]

Hello Softus and JohnD thankyou for the quick response
I am currently on the 2330 course and wanted to understand the basics of what happens in a circuit but this has got me even more confused

 

[JohnD]

we need someone better at explaining

Have you studied 3-phase yet? I think you will find it easier to grasp what happens to a SP neutral once you have looked at 3P supplies.

 

[RMAK]

The tutor has briefly touched on 3 phase via a diagram on the board, but not in any great detail. In my opinion they seem to gloss over a lot of information, the first week or so we looked at atoms and the flow of electrons in a circuit and then suddenly he is showing us how to construct ring and radial circuits

 

[JohnD]

have a rummage through your text book. there should be a diagram of a generator.

It is easier to understand with a diagram

 

[333rocky333]

Do you have 17th regs book
look at page 81

 

[Adam_151]

Perhaps, in that case then, it would be best to startt right from the beginning, and an analogy might help clinch the picture.

You say you've looked at atoms, and electrons, etc, if we take that idea and simpify it down to a wire being somewhat similar to a piece of garden hose filled with large ball bearings (these are electrons), a battery is a pump for pushing these bearings around, and a heating load is a much thinner bit of pipe, which heats up as the bearings rub aginst the sides of it.

Clearly with this system the 'circuit' must be complete for heat to be generateed, if you disconnect a pipe the ball bearings will go all over the floor! (this is where it breaks down, if you want a better picture, imagine this was a computer similation and we'd set the rules such that a bearing could only move into a place previously occupied by another bearing - then if a pipe were disconnected, they'd just stop moving, which is what happens in reality)

In AC power, the pump changes direction many times a second, the bearings down't move far (just sit there and shuffle back and forth), but energy is still transfered to the load, the bearings going back and forth still generate heat in the load.

 

[RMAK]

JohnD will do I will see what I can find

333rocky333 I dont have the new regs book yet

Adam151 sorry the picture is still unclear I have spent many hours today searching the net in the hope of finding the answer to the above, or am I being thick
its strange because I can build the lighting circuits including 2 way lights , intermediate switching, spurs , fcus aswell as the power circuits and various others in the practical sessions we have in the workshop from memory ,so its frustrating I can do all that without knowing how it ACTUALLY works if that makes sense

 

[333rocky333]

look on the net for
TNS
TNC
TT
TNCS
systems may help
There is a few drawings in the preface of the 15th, near the definititions, but not as detailed as the 17th.
http://www.marcspages.co.uk/pq/3110.htm

 

[JohnD]

for single-phase domestic work, you can work on the principle that you have two wires coming in, like from a battery, and the electrons whiz from one to the other through the loads, changing direction 50 times a second.

The 3-phase distribution and supply systems, and what happens to the neutral outside the house, you will come to later, and you will learn things that will probably surprise you. However you will need to understand them well if you hope to be more than a Domestic Installer.

There are other surprising things (they surprised me!) you will learn, like why caravans shouldn't be on PME, and what large conductors can do under overload, but hopefully these will be covered later in your course. there are also any number of books available.

 

[holmslaw]

..

 

[ban-all-sheds]

for single-phase domestic work, you can work on the principle that you have two wires coming in, like from a battery, and the electrons whiz from one to the other through the loads, changing direction 50 times a second.

Not sure I'd describe something travelling at a few mm/sec as "whizzing".....

 

[jujubean]

JohnD will do I will see what I can find

333rocky333 I dont have the new regs book yet

Adam151 sorry the picture is still unclear I have spent many hours today searching the net in the hope of finding the answer to the above, or am I being thick
its strange because I can build the lighting circuits including 2 way lights , intermediate switching, spurs , fcus aswell as the power circuits and various others in the practical sessions we have in the workshop from memory ,so its frustrating I can do all that without knowing how it ACTUALLY works if that makes sense

Here ya go RMAK....you have asked a simple question and this is a nice easy simple answer...assuming the supply is AC and to make it easier to understand pretend the frequency is 1HZ or 1 cycle per second instead of the usual 50
The size of the voltage will vary from 0 to its maximum (325v) for half a second and 0 to the second maximum (-325v) for the second half of the second
As you know with AC the voltage reverses every cycle. Naturally the current will also reverse. This makes no difference to most loads as for example multiplying + currents to +voltages will give + watts as will - volts and - amps (two minuses multiplied will give a plus)
The current in both situations will end up flowing through the transformer star point and to earth while the voltage will be "lost" or "dropped" across whatever resistor. Try take a voltage reading from the load side or the resistor (neutral )to earth and it "should " read 0 volts
But wait I hear you ask ...if the voltage reverses then surely for half a second the neutral will be on the "supply" side of the resistor and should measure a reading. Technically it is, but for the second half of the second the supply voltage is minus volts and current will always flow from the highest value to the lovest. In this case 0 is higher that - 230 or -325 max
Takes a bit of getting your head around I know ..using arithmetic the lowest number is zero but in algebra we use negative numbers and this is what you would see on a simple sine wave
Hope this was helpfull
Cheers