The way our homes are connected to the grid?

[Dellathane]

Hello all,

Perhaps this is a little far off topic even for the Frontroom, but I hope someone can help.

Below is a diagram I made about how I think our houses are connected to the electricity grid. It is of course a horrible over simplification, but I made it because I was unsure of the relationship between the "neutral" line in plugs, which carry the same current as the "live" wire, and the neutral cable in 3 phase power distribution, which in a perfectly balanced system should carry no current. This is the only way I can imagine that these two can be connected and for the currents to balance out. I was wondering if anyone can confirm it.

For those who have no idea what is going on but are curious, see below the image for an explanation.
(I'm from the UK so I'll be using our names for things, hope it won't prove too confusing)



Basics of Generators
This is a decent and above all quick explanation of what is going on in the box labelled generator

Three Phase Power
Starting from where the above link leaves off, the generator is producing three sinusoidal voltages, each 120 degrees behind the last. Each line is it's own circuit, separate from the others. The thing to realise about these three sine waves however, is that if you add all three voltages together at any instant (i.e the vertical black line on the graph above) they will equal 0, and they will ALWAYS equal 0, all the time. That's what's so clever about three phase power. Instead of needing 2 wires per circuit to carry the current of each phase there and back again (a total of 6), you only need 3, and you just connect the three circuits together at the end to complete them. In the real world though things aren't quite so perfect and the three voltages may not equal exactly 0, so a fourth much thinner wire is used to carry any little currents caused by a non-0 voltage back to the generator. This is called the neutral line, because in theory it shouldn't be carrying anything.

Connecting to homes
Having three phase power in our homes would give more power than we need and adds to wiring complexity (all though some electric ovens and hot water tanks use all three phases), so instead (in the UK at least) only one phase is connected to your house, and the other two go to the neighbours. This phase is hooked up, in parallel, to all the sockets in your house (remember that parallel circuits have the same voltage at each socket) on what's called the "live" ("hot" if you must) wire. Check out the picture above. Since we are only dealing with one phase the circuits are normal again, all the current going into a gadget must come out again. So you plug in your iHoover and current flows in from the "live" pin, the voltage is drops to zero by doing work, and the current flows out through what is unhelpfully also called the "neutral" pin/wire. Its called neutral because it has the same voltage as ground (i.e 0). This cannot be the same thing as the generator neutral I described earlier though, as there is a significant current going through it. So now you understand what I'm asking. Where does all the current flowing through this "neutral" wire go?

What I think
Look at the diagram, specifically the arrows on lines 1 and 3 near the generator, which represent the flow of current, and the currents flowing out of houses 1 and 3. I'm assuming the answer to my question lies in the magic of three phase power. If line 1 has a positive (+ve) voltage of Y, and line 3 has a -ve voltage also of Y, then current will flow from the +Y to 0, and then from 0 to -Y (since 0 V is more than -Y V). In a perfectly balanced system, the current coming out of house one will be the same as the current going into house 3. If they are not the same (and the difference will only ever be small) then current either comes out of or flows into the ground (that is the actual earth by the way, i.e it's a long metal stick hammered into the mud somewhere near your house. There are in fact only three wires coming out of power stations after all, the fourth "wire" is just the earth, which is electrically conductive. The power stations will have also have a stick in the mud somewhere (there's a joke in there somewhere)).

So to summarise I am pretty sure that is what's happening but I was wondering if anyone has the expertise to confirm it, as I've been trying to figure it out all day.

Hope that wasn't too tedious a read and that some of you may even have found it interesting.

[Tellos Athenaios]

In short, this is not how your house is wired up. You are misunderstanding two key definitions: that of voltage and ground.

Voltage between two points is simply a potential difference (delta) between two points. This is fairly arbitrary and therefore it is important to have the proper frame of reference when you want to assign fixed numbers or quantities to the potentials. Potential is measured in Volt, which is why Voltage is also measured in Volt.

Ground or mass is not, necessarily, Earth. It is an arbitrary reference which is considered to be at 0V potential, that is, all voltages are measured relative to this arbitrary fixed reference. Therefore ground is basically your frame of reference which allows you to quantify all the other potentials (relative to ground).

From this definition a few facts become apparent: the ground, or neutral in your house must be connected to the ground (reference) of the transformer in some way, or else the the RMS voltage between "live" and "neutral" is ill defined. It could be 240V, it could be a bit less, it could be rather more.

This does not mean that ground (neutral) in your home is connected to the ground (neutral) of the generator; in fact, one of the major benefits of most transformers is to electrically insulate the two circuits. This is more of an implementation detail of the transformer, albeit a highly desirable one.

Secondly the copper peg is not, in fact, to earth your home. Your home is perfectly earthed already and potential at the top of your roof is roughly equal to the potential of actual Earth (the actual ground you plant the peg in). The peg exists solely to make sure that if something bad were to happen, like, say a lightning strike you have a safe pathway for the current to flow to Earth as opposed to burning down your house or melting the wiring -- big fat copper pegs are far more conductive than drywall and thin wires, for example.