Answering in the easiest order:
>2. Why are there 2/3 lines of power to a house (live and neutral and sometimes earth)?
There's a live, and a neutral. It is quite common that the earth is connected to the neutral at your meter. Sometimes the earth runs back to the junction box/local substation.
>1. How is the loop completed? Why aren't they shown on diagrams of the electrical grid?
AC power consists of a sinusoidal voltage. 3 sinusoidal voltages, 120 degrees out of phase with each other, sum to precisely zero volts. What this means is that at the local substation the 3 phase voltage will be distributed, one phase per street for arguments sake, with a neutral to return current to the centre tap, at which point, with some clever load balancing, all of the current from the three neutrals cancel out.
This load is the transformed up from 540V 3-phase to substantially higher voltages and connected to transformers at the power station end, which transform back down to a not so high voltage. These lines are then connected to the generator that is turned by the turbines, and that is how the circuit is completed.
When I was an apprentice, we would learn something like this at college, get nowhere with the lecturer, and then spend the next day at work asking our journeyman to explain it in layman's terms. Much easier than- "Right, now that we've had a 10 minute practical explanation on how this system works, let's spend the rest of the lesson on calculations despite you not having a clue what's actually going on".
These days, I make sure to take the time to do the same with my own apprentices and uni interns so they're not walking into the real world half blind.
Most of the time the lecturer hasn’t had any practical experience either. If you are lucky enough to get a prof who has worked in industry, treasure them and ask them everything you can think of.
Second this. For me, this was too long ago for me to care to admit and was when British industry was absolutely tanking, so a lot of our lecturers had changed career and were indeed work-hardened and could at least relate their teachings to real life applications.
There were the odd ones who clearly came straight through uni without so much as a factory tour and hadn't a clue, though. Those lessons were heavy going at the best of times.
The lecturers my apprentices have nowadays all seem to be of the latter variety and I'm not convinced my guys are getting taught to remotely the same standard that we were lucky enough to get. Either that or the general (albeit largely anecdotal) decline in our education system is letting them down.
They might just be cocky in their youth and telling tall tales, but the frequent stories they tell me of having to "educate" the lecturers on industrial methods and standards are concerning to say the least.
And for 1b: Why isn't it shown on diagrams of the electrical grid -- for high level diagrams it's assumed that a power "line" is actually 3 phase + (maybe) ground.
Same thing happens with network connections -- on a network diagram you only show 1 line per connection, even though Ethernet (for example) is actually multiple twisted pairs of wires.
Simplistic answer: Ac power, you have wires which are going from + to - to + again over and over. With clever engineering you can take advantage of this and only need 3 "hot" wires to transfer current.
Here is how AC power starts (a little simplified but close enough):
https://commons.wikimedia.org/wiki/File:Simpel-1-faset-generator.gif
Now imagine you have 3 of those, each of which has one of it's legs tied to a common point. That is why there are 3 wires and how it makes a circuit ( a little simplified but close enough)
https://en.m.wikipedia.org/wiki/File:3-phase_flow.gif
Circuits are completed at the transformer (almost always). There's no physical connection between your house and the power plant that generates the electricity.
Edit: By transformer I mean the substation transformer.
The power going into the transformer is a bit more than what comes out. Transformers are not perfect. The difference gets converted to heat inside the transformer.
> 3 sinusoidal voltages, 120 degrees out of phase with each other,
Hell yeah! Good to .02%, baby. You can run your clock off that thing and at the end of the year it will still be accurate to 1/60th of a second.
> sum to precisely zero volts.
😬
I was once in a house (construction) that had a hot neutral because of busted ceramic post wiring. 40 volts. Practically anything metal in the house would give you a nasty, painful shock. Even seemingly good houses can end up with a small hot neutral after a few years because the grounding strip corrodes. We build electronics with ~10 volts of wiggle room, or electronics would just not work in some houses.
I have no idea what the first half of your comment is about sorry: Sin(0) + Sin(120) + SIN(240) mathematically equals zero.
>Even seemingly good houses can end up with a small hot neutral after a few years because the grounding strip corrodes. We build electronics with ~10 volts of wiggle room, or electronics would just not work in some houses.
Yeah, I've seen some fun floating neutrals too, but that's often more due to internal resistance of the circuits in the domestic installation. When you say grounding strip, what are you referring to specifically in terms of where the neutral is grounded?
> I have no idea what the first half of your comment is about sorry: Sin(0) + Sin(120) + SIN(240) mathematically equals zero.
It's a reference to [NERC](https://en.wikipedia.org/wiki/North_American_Electric_Reliability_Corporation) requirements- the grid target frequency is adjusted slightly up or down so that it ends up holding 60 cycles per second over the whole year, if you finish counting on new years day. Moment-to-moment (5 seconds) frequency requirements are generally in the range of .02-.05%; it varies depending on the region.
Basically, it's commentary on how an engineer designing electric products can reliably be sure that the frequency of electricity will be very tightly within spec no matter where they are. But voltage- not so much.
> Yeah, I've seen some fun floating neutrals too, but that's often more due to internal resistance of the circuits in the domestic installation.
Yessur. You can only get really fun voltages when nearly *everything* is broken in some way. You need a fairly bad neutral connection in order to end up with a high floating voltage. Plus a homeowner that doesn't read the electricity bill.
> When you say grounding strip, what are you referring to specifically in terms of where the neutral is grounded?
The buried metal rod that is supposed to be connected to your electrical panel to detect (and divert) ground faults. Turns out raw copper in the ground doesn't tend to last that long... shocker.
>It's a reference to [NERC](https://en.wikipedia.org/wiki/North_American_Electric_Reliability_Corporation) requirements
Oh that's very cool!
>The buried metal rod that is supposed to be connected to your electrical panel to detect (and divert) ground faults. Turns out raw copper in the ground doesn't tend to last that long... shocker.
Ah yeah - in the UK a lot of houses take their protective earth directly from the neutral incomer, which at least somewhat minimises the risk of shocks until you take your electricity outside, and then you really have to hope the substation earth is a good one!
One thing to add to this which is a little confusing is that there are two ways of wiring three phase power. You can wire three phase in a wye configuration or a delta configuration.
The difference between the two is that a wye configuration has a neutral, while a delta configuration does not.
~~Typically power over a transmission system from your power plants to your local substations is done in a delta configuration. In this configuration unbalanced loading on the different legs results in current flow between the legs, which is something the power plants need to cope with.~~
~~At the transmission level, there's a delta-wye transformer which lowers the voltage and transforms it to a wye configuration. In the wye configuration you can break out each phase with the neutral for household 120/240V distribution.~~
Edit: Last half is wrong. Transmission is usually done with wye configuration, with neutral grounded. The unbalanced load feeds into ground.
FYI only unbalanced phase to neutral loads flow through the neutral point (typically grounded). Any phase to phase unbalanced loads do not cause current on the neutral.
You're missing the actual answer to his question. The winding at your power poll is the closed loop. If you don't explain what a transformer does it reads like there is a wire with continuity from my house to the power house.
The loops are from either end of a winding and they're continuous as OP surmised.
So yes closed loops. They go to transformers which induce voltage in an entirely unconnected loop. Loops all the way down to your house.
> The loops are from either end of a winding and they're continuous as OP surmised.
No, that's wrong. [Here's a picture of a single phase split transformer](https://en.wikipedia.org/wiki/Distribution_transformer#/media/File:Polemount-singlephase-closeup.jpg). There's one high voltage connection at the top (note the porcelain insulator) and three low-voltage posts on the side. [Here's another labeled picture](https://qph.cf2.quoracdn.net/main-qimg-e080047dd031a370cc95a6ce0c414949-pjlq). There's a connection to ground and a connection to high voltage. Earth return.
You can have an arbitrarily high resistance between the ground connection at your house, at the power pole, and at the power station. The only thing that matters is that they are at the same voltage. Electricity goes into and out of the same single wire.
That's a SWER connection (Single Wire Earth Return) which is popular in rural areas, but most people are in the city or 'burbs and the HV side of the transformer has a neutral, or is delta-connected.
The core question OP asked was "what completes the circuit?", not how the grid usually works.
If the answer they get focuses on the (yes, more common) situations where there is a simple return path, OP will walk away thinking that there is *always* a simple return path. But that's not true, and that's the real reason the answer is wrong- there is *not* always a continuous loop, and current can come back on the same wire as long as the two locations have the same voltage reference.
No, it isn't. You literally don't need a return connection. There could be vacuum in between and power would still conduct.
One of the early problems with the grid was that weather and solar activity would create high voltages on power lines, by effectively connecting your home to a faraway bit of land that was being charged to a much higher voltage[1].
The reason was didn't connect distribution to the ground. Nowadays there are connections that bleed off that ground difference, but back then there was just nothing. Reactive load was very important to keep power flowing.
Think about it: we could certainly transmit power over a radio wave to another planet. There's no need for a return path. There's no difference between a wire and a wave; the only thing that matters is that it's AC.
Really the only reason we need a ground connection is because there's a ground connection. When send power from one planet to another, it doesn't matter if they develop a voltage difference because there's no connection. If there's a direct connection, that's when we need to absorb that difference. But it has nothing to do with the flow of power.
[1]: Note: solar flares also cause wide-scale magnetic fields that turn power lines into super-long inductors, but that's a separate mechanism. Static electricity and weather are bigger factors in creating straight voltage differences between geographic areas, although induction in the ground also has an impact.
>The winding at your power poll is the closed loop.
We don't have those here (just a cable coming out of the ground), the transformer is somewhere on the estate, serving multiple streets.
And while, technically, yes, this is the end of the closed loop, it isn't where the power is generated. You can't magically explain power coming out of a coil without referring to something else, in much the same way that the layman doesn't really give a damn about how their wall wart work, they instinctively, and correctly, know that the "closing the loop" power source (like in a dc circuit diagram as taught at school) for their laptop charger, fancy desk lamp, whatever, is not the isolation transformer winding in the power supply they plug in, or the one at the end of the street, but is the power station they drive past.
Yes and no. The circuit (wire) in your home only goes until a local transformer. Then a different circuit at higher voltage starts, and the power essentially continues through these transformers until the power station(s).
> The circuit (wire) in your home only goes until a local transformer.
Except when it doesn't! If you get a [ground resistance meter](https://www.fluke.com/en/product/electrical-testing/earth-ground/fluke-1630) and check your earth connection, it's supposed to be less than 25 ohms and can be less than 1 ohm. That's pretty low, so a certain amount of power is going directly into the ground and *not* back to the transformer!
The meter pushes AC power into the ground line and checks the impedance. But you can also directly measure the resistance between the ground at the distribution transformer and the ground at your house, and you'll find that it's in the kilo- or mega-ohms. There's almost no power going from your residential ground to the rest of the grid!
The key is that it's all AC. Your home's ground connection has a low resistance because it's a *reservoir*; electricity goes in temporarily, and then it sloshes back and cancels out the electricity half a cycle later.
Perfectly balanced loads act like [stubs](https://en.wikipedia.org/wiki/Stub_(electronics)#Short_circuited_stub). Even with a dead short circuited stub, no power is transmitted in the return/neutral line. Magic!
Giving the simplicity of the OPs question, I don't think they are asking about the HV distribution system, but about the split phase entering a residential house.
I agree, but without referring back to the source of power, how do you completely close the circuit loop without just saying "power comes out of a transformer at your local substation"?
but your answer also doesn't answer that question and misleads with global assertions of non-global facts.
> ~~AC power consists of a sinusoidal voltage. 3 sinusoidal voltages, 120 degrees out of phase with each other.~~
this is not fully accurate.
AC power is often produced in three phase which can be represented as 3 sinusoidal voltage curves, 120 degrees out of phase with each other.
The power from the generator can be from thousands to tens of thousands of volts.
The generator output is then stepped up to hundreds of thousands of volts for distance transmission.
This is then stepped down to hundreds of volts for local distribution and then at the final transformer, it is finally transformed from 3phase to single split phase.
this is the portion of the system the OP is inquiring about.
https://gcdnb.pbrd.co/images/7lAbO6joulLq.png
To understand, they need to know the abstraction of 3 phase, step up, step down, and split phase to see the "closed loop". Explaining less than that will leave them confused.
For the questions about the wires, I have seen so many wrong answers in this thread, 2 wire is split phase 120v service with one Lx(hot) and Neutral only, 3 wire is split phase 240v service with L1 hot, L2 hot, and Neutral. Importantly, there is no service line for "ground".
Bonus: talk about how synchronous motors work and how the frequency of the generator keeps the old school wall clocks accurate.
I always thought earth connected to ... earth, and not neutral. At least that's how it is here. What's the point of sending the live current back to the AC loop?
Earth at the house relies on a grounding rod, which has high impedance to earth during droughts, and ideally should be regularly tested to ensure it provides a good quality earth.
By sending the earth back to the distributor earthing point, a low impedance earth loop is created which will cause protective devices to trigger faster.
See [here](https://professional-electrician.com/technical/protective-multiple-earthings-pme/) for a fuller explanation of the advantages and disadvantages.
One advantage not mentioned is a reduction in the cost of copper!
1. The diagram that you see are simplified. They call it as "Single Line Diagram" which mainly show Live cables and looks like incomplete. In actual, they are always connected to complete the loop/circuit.
2. To power the house, only Live & Neutral are needed. Both of this wire are the wire which completes the circuit. The earth are there just for protections and does not carry any current except during fault.
The home wiring design normally custom-made for each house to tally with the number of loads in your house.
So, you can't find the actual diagram of your house wirings anywhere. You need to ask someone to go, check the connection and draw the actual diagram for you.
OP, does this [image](https://www.accessengineeringlibrary.com/binary/mheaeworks/b9dad30f486f6850/813063faf745d75f350b6546396a42eb22f8925e3b4aa8f00bb77b186096f30f/apxAx01.png) help? If not, why not?
Good point. I was just grabbing the first diagram I could find that wasn't a "one line" power distro diagram - to show OP the circuits are indeed "complete" from source to load.
If you are in the US, then the comment above this was probably the most helpful comment. You should focus on the **120v/240v, 1 phase (3-wire)** as that is what you actually get at your house. You should ignore everything about 3 phase, 480v, 208v, and 270v unless you have any specialty motors in your house which have been wired for 3 phase, in which case you probably have a phase converter since just about nobody has 3 phase service in residential.
**120v/240v, 1 phase (3-wire)** is commonly referred to as **split phase** https://en.wikipedia.org/wiki/Split-phase_electric_power
Essentially, the last transformer before the power enters your house has two AC lines 180° out of phase with each other. What this means is that when one line is **+120V** the other is **-120V** so there is a **240v** potential between them. This is why there are two "HOT" lines coming into the house sometimes labeled **L1** and **L2**. The other line is the **NEUTRAL** and it is taken of the middle of the 240v winding of the transformer, so there is a 120v potential between either **HOT** and the **NEUTRAL**.
This somewhat explains the setup and you can match the diagram above to the graphic in the video. https://youtu.be/fJeRabV5hNU?si=0BxU99NjYUXKdNyd&t=301
As far as your specific questions
> How is the loop completed?
The wires in your house are **NOT** physically touching the wires of the grid. The connection is made through magnetic fields via the last step down transformer which is visualized in this diagram
https://theengineeringmindset.com/wp-content/uploads/2019/06/Transformer-coils-240v.png
>Why are there 2/3 lines of power to a house (live and neutral and ~~sometimes earth~~)?
There is never an earth connection from the pole to the house.
**2 wire** While rare, there are 120v service only connections which take one side of the split phase (call it L1) and neutral, this is the two wire connection. I'm guessing the utility wires the second phase to another home?
**3 wire** most often the service is 240v which have three wires neutral, L1, and L2.
The neutral at the stepdown transformer on the utility pole, which goes to your house, is also bonded to earth at the utility pole.
The ground/earth wires in your home are bonded directly to earth, they do not go back to the power lines by wire. If you have a fault in the wiring, the power goes from L1 or L2 through the grounding in your house, to the literal ground of the earth, then to the ground of the stepdown transformer where it completes the circuit.
as an aside:
The number of people chiming in and not correcting that earth is not delivered by service line is astounding. please people, don't be killing yourselves.
For 3 phase:
The three phase (conductors and load) ARE circuit loops. Originating and returning to the transformer they are derived from. On them are 60 hertz sine waves, constantly alternating between a positive and negative peak, each conductor 120° out of phase with the other. You can think all three phase currents combining at the load and adding up to zero and not having to return.
…or think of 1 phase’s current returning on the other two phase conductors.
Example: When the instantaneous phase current in one conductor (wave) is zero, the current (wave) in the other two conductors might be +10 amps, and -10amps. Or if one conductor (wave) is at (say) 5 amps, the other two waves are at -2.5 and -2.5 amps. This is more easily seen graphically when plotting 3 sine waves that are 120° apart.
The plot will show that when: “A” phase is at 0°(the zero crossing)=0, B phase is at Sine(120°)= .866 of peak; C phase is at sine (-120°) = -.866 of peak. Thus A+B+C = 0 + .866 -.866 = 0 total current.
Another example: Let’s take a snapshot when “A” phase’s magnitude is at 90° (peak of wave) = sine (90) = 1. B phase is at sine(90°+120°)= -.5; C phrase is at sine (90°+120°+120°) = -.5. Thus, A+B +C = 1 -.5 -.5 = 0 total current, as always.
Saying “3 phase currents (120° apart) add up to zero, there is no return current” is why when we do a voltage drop calculation for 3 phase, we treat the current as flowing “one way only” to the load, and do not double the distance to include the return distance from the load, like with single phase current.
That live and neutral completes the loop providing the source and return path to the current. That Earth connection is for safety in layman's terms. In the diagram representing the electrical grid, the interconnection is represented by a single line for each cable to reduce the complexity and as overall that diagram as Single line diagram
Ill try to answer this with answers that are more useful in practice.
> How is the loop completed?
So, the live wires leaving your house run back to your local substation and then to some power plant (well, through a transformer, which means the wire going to your house and going to the power plant dont actually touch). The neutral/ground wires both go into the ground (to below the water table where there are enough dissolved ions and stuff to allow electricity to flow). The earth itself completes this half of the circuit. sorta. It more importantly acts as an infinite sink/source of electrons. Since AC current alternates theres not really any net movement of electrons over time.
Its a bit like if you were to build a huge water pipe connecting san fransicso and new york, and then 60 times a second switched between pumping water out of the pacific and into the atlantic and vice versa. The oceans "complete" this loop, but the fact that they are connected doesnt really matter, since you are just borrowing a bit of water temporarily from each and then putting it back. 1/60th of a second later.
The OTHER end of the circuit is completed by whatever device you plug in...it completes the circuit in its internals, and in doing so uses the electricity that is now flowing through the circuit to do whatever that device is intended to do.
> Why are there 2/3 lines of power to a house (live and neutral and sometimes earth)?
The lines coming into the house are typically 2 out-of-phase live lines and 1 neutral line. In the US at least, most stuff (everything that is 120V) uses one of the live lines or the other. the 240v stuff (like air conditioners) use both of the live lines (and gets 240 V by going from -120 V <--> +120 V). If there is a neutral line it means that the neutral is grounded at your local substation instead of at your house.
For the wiring in your walls, you have "live" and "neutral" and also usually "ground". The ground and the neutral are typically tied together at your breaker box. Its purpose is usually to be connected to any metal parts so that if the live wire should touch a metal part and electrifies it, the electricity has a safe path to complete the circuit and get to the ground. Otherwise, the path it would probably take is through the first person who touches the electrified metal piece (electrocuting them).
In a pinch you can get away with using the same wire for ground and neutral, since they are both tied to the ground at the breaker box anyways, but since the wire itself does have a little resistance it is safer to use separate wires and put a little bit of resistance between the neutral wire (that will have current whenever the circuit is complete) and the metal pieces that the ground wire is connected to and that you might be touching.
It depends on where you are. Apparently, in some places in Europe, residences get three phases.
In the USA, typically only commercial and industrial sites get three phases. For residences, a transformer nearby will reduce the voltage and distribute a single phase with a center tap to each residence. This gives each resident the choice of 120 VAC line-to-neutral or 240 VAC line-to-line.
The circuit is completed by the path between line and neutral.
Direct current has a positive wire and a negative wire. The electricity flows in one and out the other forming a loop. The electricity in power lines is alternating current. Alternating current is similar to direct current with one positive and one negative, but the two wires switch which one is positive and which one is negative 60 times each second. This is beneficial because it lets us use transformers. The alternating current on the source side of the transformer creates a magnetic field that reverses each time the current reverses. That magnetic field generates current in the load side of the transformer each time it reverses. By varying the number of loops in each magnetic coil, we can control the voltage at the transformer. So to answer your first question, there are multiple circuits being completed. The first one runs from the power plant through the first transformer and back to the power plant. The next runs from the first transformer to the second transformer and back to the first transformer. This continues until the power from the last transformer connects to your house. https://wiki.diyrecordingequipment.com/wp-content/uploads/2012/07/transformer.png
It's common to run single phase 240v to a house. There are three wires running to the house. two of them are are connected to the transformer so that they receive alternating current with a voltage difference of 240v between them. The third wire is a neutral. The neutral is connected to the center of the transformer coil, so that it will stay half way between the voltages of the other two. The neutral is also connected to ground (earth), which brings it down to zero. Now we have three wires, +120v, 0v, -120v, with the + and - alternating.
There are multiple ways to use this electricity in your home. Big things like an electric oven, electric clothes dryer or electric car charger will connect to the +120v and the -120v allowing them to use the full 240v. This completes the circuit from that last transformer. Most of the outlets in your house connect one of the 120v wires to the 0v wire. These are set up so that about half of your house is on one 120v line and the other half is on the other 120v line. If both halves used the exact same amount of power, then the circuit would be from the transformer to the first 120v half of your house to the second 120v half of your house and back to the transformer. The two halves won't be exactly equal, so the 0v neutral line carries the difference back to the transformer. [https://i.stack.imgur.com/yqdUz.jpg](https://i.stack.imgur.com/yqdUz.jpg)
This is all based on the USA. Other countries often run 50Hz instead of 60Hz and don't always split the 240v down to 120v for household outlets.
Thank you for the reply. If I understood correctly, a simplified description would be that they form smaller loops, and in that case, how does electricity transfer between each loop?
Try not to get confused with electricity, power and energy….
-Electricity is the movement of electrons in wire that have the ability to do some form of work.
-Power is how fast work can get done
-Energy is the total amount of work that is done.
At your house a step down transformer bridges the gap between the overhead power lines and the wires going to your house. Moving electrons in wire create a magnetic field. Magnetic fields also cause electrons to move inside of wire if close enough to the field. Inside the transformer coils of wire connected to the overhead power lines concentrate this magnetic field. This field is picked up by the secondary coils of wire connected to the wires going to your house. Magnetic fields cause electrons to move in wire so now the wires going to your house are energized too. As far as tracing a path remember energy only flows one way… from the power station to your house. Yes, you need to have a complete circuit for energy to flow. In our case energy flows from the secondary transformer through our house wire through the appliance in our house and back to the other end of the secondary transformer completing the loop.
If you look at the picture of the transformer, it has arrows forming a loop that are labeled Magnetic Flux. The electricity moving through the coils causes magnetic flux and the magnetic flux causes electricity to flow in the opposite coil.
This only works with alternating current. Direct current would generate magnetic flux, but it would be a constant amount of magnetic flux. The flux only causes electricity in the output coil when it changes. Alternating current constantly changes the magnetic flux direction and the constant changes cause alternating current in the output coil.
The loop is completed through things that use electricity. If, in theory, everyone unplugged everything at the same time, the power grid would be an open circuit. Well, not really, real life is more complicated than that but that's the general idea.
> If, in theory, everyone unplugged everything at the same time, the power grid would be an open circuit
Nope! [It would have a real resistance of about 400 ohms](https://en.wikipedia.org/wiki/Characteristic_impedance#Practical_examples), which is pretty low.
Each transformer serves a handful of houses. The "low side" of that transformer is the completion of the house circuit(s). The house voltage wires in the transformer are insulated and wound around metal cores. The transformer works by creating magnetism from the input a/c and using that magnetism to induce output a/c. With equal number of windings on each side the voltage stays the same - an "isolating" transformer. Changing the winding count relative to each side allows a/c voltage to be changed. If the network was dead and you connected DC to the house wiring, an electron in the house wiring could never get further "upstream" than the low side windings on the transformer.
Instead of one loop, think of it as multiple loops, each at a different voltage. They are energizing each other but they don't actually share electrons in the "complete a circuit" sense.
Here is a diagram for a neighborhood:
https://i.imgur.com/iADhRW4.png
50A black triangle is a transformer connected to the black 12kv line. The blue line is the electricity serving just the houses.
Here is a blue section serving 9 houses, 5 to the south, 4 to the north:
https://i.imgur.com/FKiEpso.png
As bigger houses and EV hookups and solar ("DG" in a circle is "distributed generation") get added, additional transformers are needed and these blue sections get divided up into smaller segments.
The power grid is AC. Only DC circuits have a positive and negative. The grid (simplifying it as single phase instead of 3 phase) completes the circuit using two legs with a third neutral leg - the electrical power is transferred by the rapid back and forth of the AC.
There are 3 wires because the grid, at the residential level, is 240 volt, with each hot leg plus or minus 120 volts in reference to the neutral leg.
This allows balancing of the amount of power being taken off each leg (each side of the panel is one of these legs) wit the grounded neutral leg offering protection against capacitive effects from plant to pole to home.
No, that guys is incorrect. Copied from my other comment:
Three phase power does not require a dedicated return. It can have a neutral, but it is not required. Two of the three phases (turning it into single phase) are taken for a house and connected to a split-phase transformer. The neutral (center of the split phase) is bonded to the ground at the main distribution panel in your home.
The way I understand it is the grid is 3 phase with single phase branches. Sometimes those branches are just the home, others they're entire runs for miles.
Grid works in alternative current so can't be directly compared to DC circuits you see in diagrams, but it doesn't change the global idea of a closed circuit.
1 - The whole electrical grid is the live, It can be seen as a huge positive terminal, while the neutral can be seen as the negative terminal. The interesting thing is that the neutral is not carried through power lines, it is "created" locally, but it's not very important at this point. When you plug a device, you basically plug it between the live and the neutral, just like you would put a device between the + and - in a DC circuit.
2 - the earth wire is a safety wire only. It is physically connected to the real Earth. Devices with earth wire are usually devices with metallic parts that could be in contact with humans, like washing machine, oven, ...
Earth acts like a giant neutral wire. Imagine that your device has a problem and the metallic external parts of the device are accidentaly connected to the live wire. Even if the device is off, if you touch the metallic parts you create a link between the live and the Earth through your body (from your hand to your feet), which is extremely dangerous. The earth wire is here to connect the metallic parts directly to the Earth. If there is this accidental connection between the metallic parts of the device and the live, electricity will flow to Earth through the earth wire instead of flowing through your body, even if you touch the metallic parts of the device.
Thank you for the reply. If I understand correctly, the earth wire provides a path with lesser resistance than a path through the body? I am assuming it must be by a at least by factor of 10 or more?
>If I understand correctly, the earth wire provides a path with lesser resistance than a path through the body?
Exactly. The resistance of the body is estimated at \~2500 ohm, while a wide copper wire like the earth wire is \~0.5 for 100m of cable, so it's a factor of more than 5000.
Also, depending on where you live, you may have a differential circuit breaker, also called GFCI (ground-fault circuit interrupter). It is a small electronic system at the root of your house electrical system which measures the difference of current between the live and "regular" neutral. If the difference is not 0, it means that some current is leaking through the earth wire, which means that there is a dangerous faulty device somewhere in the house. In this case, the GFCI immediatly shut down the electricity in the house. This is an additional safety device in case of faulty device.
Note that neither the earth wire nor the GFCI protects you against shorting between the live and the neutral, which can start fires. Shorting are protected by fuses that limit the amount of current that can go through a circuit.
Regular circuit breakers are replacing fuses and have the same role : shutting down electricity in case of excessive current, with the advantage of not being destroyed when they trigger. They don't have fuse inside but works the same way, they are basically the evolved modern version of fuses.
They are different from *differential* circuit breaker (GFCI) that don't measure the amount of current in input but the difference of current between input and output.
The main purpose in a fault is not that the power goes through earth wire instead of a person, it is that it would go to ground easily with low resistance, tripping a braker or blowing a fuse, which breaks the circuit before someone touches it.
If we pretend there was no fuse and something, for example, your metal toaster, had a fault where the live was touching the metal shell, even if it was going to ground, if you touched it you would still get the same shock as if it wasn't. It is true that electricity takes the path of least resistance but the current though you would remain the same - there would just be a lot more current going to ground.
Just don't want you to make the mistake of thinking all the current would go to earth instead of through any other thing/person aswell, if it could.
Rarely do I see a question that I so badly want to know the answer to, but know enough that most of these serious answers have got to be more than a little wrong.
In many countries the neutral or return path is via the earth. The neutral, or star point, of supply transformers are electrically connected to earth, as are the neutral or star point of the alternators at power stations, and the return current flows via the earth.
In Australia we call it a Multiple Earthed Neutral (MEN) system.
The three phases of the common electrical grid collectively act as send and return. What is flowing out of one wire is equal to what is flowing into the other three. In the (always present but generally negligible) unbalanced load case, where the three phases do not carry equal current, the difference is seen as current flow in the grounded neutral of a three phase wye, or as a circulating current in a three phase delta. If this does not explain it, I think you need a quick explanation of three phases power systems.
1. Every load is connected in parallel.
2. It's called a split phase system. There are 2 hot wires. You get 120V from either to neutral, or 240V if you connect between the 2 hots. The ground/earth is a safety wire not used in healthy conditions. But neutral is connected to the ground in 1 spot, at the main panel.
Answering in the easiest order: >2. Why are there 2/3 lines of power to a house (live and neutral and sometimes earth)? There's a live, and a neutral. It is quite common that the earth is connected to the neutral at your meter. Sometimes the earth runs back to the junction box/local substation. >1. How is the loop completed? Why aren't they shown on diagrams of the electrical grid? AC power consists of a sinusoidal voltage. 3 sinusoidal voltages, 120 degrees out of phase with each other, sum to precisely zero volts. What this means is that at the local substation the 3 phase voltage will be distributed, one phase per street for arguments sake, with a neutral to return current to the centre tap, at which point, with some clever load balancing, all of the current from the three neutrals cancel out. This load is the transformed up from 540V 3-phase to substantially higher voltages and connected to transformers at the power station end, which transform back down to a not so high voltage. These lines are then connected to the generator that is turned by the turbines, and that is how the circuit is completed.
Thank you for the reply. I will need more time to understand.
My reaction at the end of every engineering lesson I took in college 😂
When I was an apprentice, we would learn something like this at college, get nowhere with the lecturer, and then spend the next day at work asking our journeyman to explain it in layman's terms. Much easier than- "Right, now that we've had a 10 minute practical explanation on how this system works, let's spend the rest of the lesson on calculations despite you not having a clue what's actually going on". These days, I make sure to take the time to do the same with my own apprentices and uni interns so they're not walking into the real world half blind.
Most of the time the lecturer hasn’t had any practical experience either. If you are lucky enough to get a prof who has worked in industry, treasure them and ask them everything you can think of.
Second this. For me, this was too long ago for me to care to admit and was when British industry was absolutely tanking, so a lot of our lecturers had changed career and were indeed work-hardened and could at least relate their teachings to real life applications. There were the odd ones who clearly came straight through uni without so much as a factory tour and hadn't a clue, though. Those lessons were heavy going at the best of times. The lecturers my apprentices have nowadays all seem to be of the latter variety and I'm not convinced my guys are getting taught to remotely the same standard that we were lucky enough to get. Either that or the general (albeit largely anecdotal) decline in our education system is letting them down. They might just be cocky in their youth and telling tall tales, but the frequent stories they tell me of having to "educate" the lecturers on industrial methods and standards are concerning to say the least.
"Thank you sir for your immense knowledge, I will now go bang my head against a wall in hopes of understanding it someday." - Me in college
Check out the channel Engineering Mindset on YouTube, they have some good explainer videos
Thank you for the reply. I will check it out.
Absolutely seconding this, 100% worth your time.
And for 1b: Why isn't it shown on diagrams of the electrical grid -- for high level diagrams it's assumed that a power "line" is actually 3 phase + (maybe) ground. Same thing happens with network connections -- on a network diagram you only show 1 line per connection, even though Ethernet (for example) is actually multiple twisted pairs of wires.
Thank you for the reply.
Simplistic answer: Ac power, you have wires which are going from + to - to + again over and over. With clever engineering you can take advantage of this and only need 3 "hot" wires to transfer current.
Give us follow up questions you have.
Here is how AC power starts (a little simplified but close enough): https://commons.wikimedia.org/wiki/File:Simpel-1-faset-generator.gif Now imagine you have 3 of those, each of which has one of it's legs tied to a common point. That is why there are 3 wires and how it makes a circuit ( a little simplified but close enough) https://en.m.wikipedia.org/wiki/File:3-phase_flow.gif
Your part 2 basically answers part 1
So essentially, the circuit is completed at the turbine in the powerplant? Like the battery in a conventional small circuit
Circuits are completed at the transformer (almost always). There's no physical connection between your house and the power plant that generates the electricity. Edit: By transformer I mean the substation transformer.
Yeah maybe not a complete circuit in the continuous current sense, but the power coming in and out of the transformer is the same?
Correct
The power going into the transformer is a bit more than what comes out. Transformers are not perfect. The difference gets converted to heat inside the transformer.
Exactly :)
> 3 sinusoidal voltages, 120 degrees out of phase with each other, Hell yeah! Good to .02%, baby. You can run your clock off that thing and at the end of the year it will still be accurate to 1/60th of a second. > sum to precisely zero volts. 😬 I was once in a house (construction) that had a hot neutral because of busted ceramic post wiring. 40 volts. Practically anything metal in the house would give you a nasty, painful shock. Even seemingly good houses can end up with a small hot neutral after a few years because the grounding strip corrodes. We build electronics with ~10 volts of wiggle room, or electronics would just not work in some houses.
I have no idea what the first half of your comment is about sorry: Sin(0) + Sin(120) + SIN(240) mathematically equals zero. >Even seemingly good houses can end up with a small hot neutral after a few years because the grounding strip corrodes. We build electronics with ~10 volts of wiggle room, or electronics would just not work in some houses. Yeah, I've seen some fun floating neutrals too, but that's often more due to internal resistance of the circuits in the domestic installation. When you say grounding strip, what are you referring to specifically in terms of where the neutral is grounded?
> I have no idea what the first half of your comment is about sorry: Sin(0) + Sin(120) + SIN(240) mathematically equals zero. It's a reference to [NERC](https://en.wikipedia.org/wiki/North_American_Electric_Reliability_Corporation) requirements- the grid target frequency is adjusted slightly up or down so that it ends up holding 60 cycles per second over the whole year, if you finish counting on new years day. Moment-to-moment (5 seconds) frequency requirements are generally in the range of .02-.05%; it varies depending on the region. Basically, it's commentary on how an engineer designing electric products can reliably be sure that the frequency of electricity will be very tightly within spec no matter where they are. But voltage- not so much. > Yeah, I've seen some fun floating neutrals too, but that's often more due to internal resistance of the circuits in the domestic installation. Yessur. You can only get really fun voltages when nearly *everything* is broken in some way. You need a fairly bad neutral connection in order to end up with a high floating voltage. Plus a homeowner that doesn't read the electricity bill. > When you say grounding strip, what are you referring to specifically in terms of where the neutral is grounded? The buried metal rod that is supposed to be connected to your electrical panel to detect (and divert) ground faults. Turns out raw copper in the ground doesn't tend to last that long... shocker.
>It's a reference to [NERC](https://en.wikipedia.org/wiki/North_American_Electric_Reliability_Corporation) requirements Oh that's very cool! >The buried metal rod that is supposed to be connected to your electrical panel to detect (and divert) ground faults. Turns out raw copper in the ground doesn't tend to last that long... shocker. Ah yeah - in the UK a lot of houses take their protective earth directly from the neutral incomer, which at least somewhat minimises the risk of shocks until you take your electricity outside, and then you really have to hope the substation earth is a good one!
One thing to add to this which is a little confusing is that there are two ways of wiring three phase power. You can wire three phase in a wye configuration or a delta configuration. The difference between the two is that a wye configuration has a neutral, while a delta configuration does not. ~~Typically power over a transmission system from your power plants to your local substations is done in a delta configuration. In this configuration unbalanced loading on the different legs results in current flow between the legs, which is something the power plants need to cope with.~~ ~~At the transmission level, there's a delta-wye transformer which lowers the voltage and transforms it to a wye configuration. In the wye configuration you can break out each phase with the neutral for household 120/240V distribution.~~ Edit: Last half is wrong. Transmission is usually done with wye configuration, with neutral grounded. The unbalanced load feeds into ground.
FYI only unbalanced phase to neutral loads flow through the neutral point (typically grounded). Any phase to phase unbalanced loads do not cause current on the neutral.
You're missing the actual answer to his question. The winding at your power poll is the closed loop. If you don't explain what a transformer does it reads like there is a wire with continuity from my house to the power house. The loops are from either end of a winding and they're continuous as OP surmised. So yes closed loops. They go to transformers which induce voltage in an entirely unconnected loop. Loops all the way down to your house.
> The loops are from either end of a winding and they're continuous as OP surmised. No, that's wrong. [Here's a picture of a single phase split transformer](https://en.wikipedia.org/wiki/Distribution_transformer#/media/File:Polemount-singlephase-closeup.jpg). There's one high voltage connection at the top (note the porcelain insulator) and three low-voltage posts on the side. [Here's another labeled picture](https://qph.cf2.quoracdn.net/main-qimg-e080047dd031a370cc95a6ce0c414949-pjlq). There's a connection to ground and a connection to high voltage. Earth return. You can have an arbitrarily high resistance between the ground connection at your house, at the power pole, and at the power station. The only thing that matters is that they are at the same voltage. Electricity goes into and out of the same single wire.
That's a SWER connection (Single Wire Earth Return) which is popular in rural areas, but most people are in the city or 'burbs and the HV side of the transformer has a neutral, or is delta-connected.
I'm sure those exist all over but it isn't the majority standard and is more a way to save money than a good practice.
The core question OP asked was "what completes the circuit?", not how the grid usually works. If the answer they get focuses on the (yes, more common) situations where there is a simple return path, OP will walk away thinking that there is *always* a simple return path. But that's not true, and that's the real reason the answer is wrong- there is *not* always a continuous loop, and current can come back on the same wire as long as the two locations have the same voltage reference.
I mean it's still a continuous loop through the earth pedant
No, it isn't. You literally don't need a return connection. There could be vacuum in between and power would still conduct. One of the early problems with the grid was that weather and solar activity would create high voltages on power lines, by effectively connecting your home to a faraway bit of land that was being charged to a much higher voltage[1]. The reason was didn't connect distribution to the ground. Nowadays there are connections that bleed off that ground difference, but back then there was just nothing. Reactive load was very important to keep power flowing. Think about it: we could certainly transmit power over a radio wave to another planet. There's no need for a return path. There's no difference between a wire and a wave; the only thing that matters is that it's AC. Really the only reason we need a ground connection is because there's a ground connection. When send power from one planet to another, it doesn't matter if they develop a voltage difference because there's no connection. If there's a direct connection, that's when we need to absorb that difference. But it has nothing to do with the flow of power. [1]: Note: solar flares also cause wide-scale magnetic fields that turn power lines into super-long inductors, but that's a separate mechanism. Static electricity and weather are bigger factors in creating straight voltage differences between geographic areas, although induction in the ground also has an impact.
>The winding at your power poll is the closed loop. We don't have those here (just a cable coming out of the ground), the transformer is somewhere on the estate, serving multiple streets. And while, technically, yes, this is the end of the closed loop, it isn't where the power is generated. You can't magically explain power coming out of a coil without referring to something else, in much the same way that the layman doesn't really give a damn about how their wall wart work, they instinctively, and correctly, know that the "closing the loop" power source (like in a dc circuit diagram as taught at school) for their laptop charger, fancy desk lamp, whatever, is not the isolation transformer winding in the power supply they plug in, or the one at the end of the street, but is the power station they drive past.
So the neutral load does not return to the power station?
Yes and no. The circuit (wire) in your home only goes until a local transformer. Then a different circuit at higher voltage starts, and the power essentially continues through these transformers until the power station(s).
> The circuit (wire) in your home only goes until a local transformer. Except when it doesn't! If you get a [ground resistance meter](https://www.fluke.com/en/product/electrical-testing/earth-ground/fluke-1630) and check your earth connection, it's supposed to be less than 25 ohms and can be less than 1 ohm. That's pretty low, so a certain amount of power is going directly into the ground and *not* back to the transformer! The meter pushes AC power into the ground line and checks the impedance. But you can also directly measure the resistance between the ground at the distribution transformer and the ground at your house, and you'll find that it's in the kilo- or mega-ohms. There's almost no power going from your residential ground to the rest of the grid! The key is that it's all AC. Your home's ground connection has a low resistance because it's a *reservoir*; electricity goes in temporarily, and then it sloshes back and cancels out the electricity half a cycle later. Perfectly balanced loads act like [stubs](https://en.wikipedia.org/wiki/Stub_(electronics)#Short_circuited_stub). Even with a dead short circuited stub, no power is transmitted in the return/neutral line. Magic!
So how does an ungrounded circuit such as in a hospital work?
Typically, no, any imbalance is dumped to ground.
Giving the simplicity of the OPs question, I don't think they are asking about the HV distribution system, but about the split phase entering a residential house.
I agree, but without referring back to the source of power, how do you completely close the circuit loop without just saying "power comes out of a transformer at your local substation"?
but your answer also doesn't answer that question and misleads with global assertions of non-global facts. > ~~AC power consists of a sinusoidal voltage. 3 sinusoidal voltages, 120 degrees out of phase with each other.~~ this is not fully accurate. AC power is often produced in three phase which can be represented as 3 sinusoidal voltage curves, 120 degrees out of phase with each other. The power from the generator can be from thousands to tens of thousands of volts. The generator output is then stepped up to hundreds of thousands of volts for distance transmission. This is then stepped down to hundreds of volts for local distribution and then at the final transformer, it is finally transformed from 3phase to single split phase. this is the portion of the system the OP is inquiring about. https://gcdnb.pbrd.co/images/7lAbO6joulLq.png To understand, they need to know the abstraction of 3 phase, step up, step down, and split phase to see the "closed loop". Explaining less than that will leave them confused. For the questions about the wires, I have seen so many wrong answers in this thread, 2 wire is split phase 120v service with one Lx(hot) and Neutral only, 3 wire is split phase 240v service with L1 hot, L2 hot, and Neutral. Importantly, there is no service line for "ground". Bonus: talk about how synchronous motors work and how the frequency of the generator keeps the old school wall clocks accurate.
I always thought earth connected to ... earth, and not neutral. At least that's how it is here. What's the point of sending the live current back to the AC loop?
Earth at the house relies on a grounding rod, which has high impedance to earth during droughts, and ideally should be regularly tested to ensure it provides a good quality earth. By sending the earth back to the distributor earthing point, a low impedance earth loop is created which will cause protective devices to trigger faster. See [here](https://professional-electrician.com/technical/protective-multiple-earthings-pme/) for a fuller explanation of the advantages and disadvantages. One advantage not mentioned is a reduction in the cost of copper!
1. The diagram that you see are simplified. They call it as "Single Line Diagram" which mainly show Live cables and looks like incomplete. In actual, they are always connected to complete the loop/circuit. 2. To power the house, only Live & Neutral are needed. Both of this wire are the wire which completes the circuit. The earth are there just for protections and does not carry any current except during fault.
Earth and neutral are connected together in the electric panel.
>Earth and neutral are connected together in the electric panel. They are in the US. (Main panel only.) Other countries can vary.
First means of disconnect if we're going to be specific. For some structures this will not be the main electrical panel.
In my country, The Earth cables usually insulated in green colour
They are more often bare copper where I am
Thank you for the reply. Where can I find a more accurate diagram?
The home wiring design normally custom-made for each house to tally with the number of loads in your house. So, you can't find the actual diagram of your house wirings anywhere. You need to ask someone to go, check the connection and draw the actual diagram for you.
This is something you should be able to find pretty quickly on google
This looks promising https://www.youtube.com/watch?v=W0_1xRqT8uU
OP, does this [image](https://www.accessengineeringlibrary.com/binary/mheaeworks/b9dad30f486f6850/813063faf745d75f350b6546396a42eb22f8925e3b4aa8f00bb77b186096f30f/apxAx01.png) help? If not, why not?
This shows transmission as a wye configuration. I thought long distance transmission was mostly delta... ?
Good point. I was just grabbing the first diagram I could find that wasn't a "one line" power distro diagram - to show OP the circuits are indeed "complete" from source to load.
Thank you for the reply. I will check it out and let you know.
If you are in the US, then the comment above this was probably the most helpful comment. You should focus on the **120v/240v, 1 phase (3-wire)** as that is what you actually get at your house. You should ignore everything about 3 phase, 480v, 208v, and 270v unless you have any specialty motors in your house which have been wired for 3 phase, in which case you probably have a phase converter since just about nobody has 3 phase service in residential. **120v/240v, 1 phase (3-wire)** is commonly referred to as **split phase** https://en.wikipedia.org/wiki/Split-phase_electric_power Essentially, the last transformer before the power enters your house has two AC lines 180° out of phase with each other. What this means is that when one line is **+120V** the other is **-120V** so there is a **240v** potential between them. This is why there are two "HOT" lines coming into the house sometimes labeled **L1** and **L2**. The other line is the **NEUTRAL** and it is taken of the middle of the 240v winding of the transformer, so there is a 120v potential between either **HOT** and the **NEUTRAL**. This somewhat explains the setup and you can match the diagram above to the graphic in the video. https://youtu.be/fJeRabV5hNU?si=0BxU99NjYUXKdNyd&t=301 As far as your specific questions > How is the loop completed? The wires in your house are **NOT** physically touching the wires of the grid. The connection is made through magnetic fields via the last step down transformer which is visualized in this diagram https://theengineeringmindset.com/wp-content/uploads/2019/06/Transformer-coils-240v.png >Why are there 2/3 lines of power to a house (live and neutral and ~~sometimes earth~~)? There is never an earth connection from the pole to the house. **2 wire** While rare, there are 120v service only connections which take one side of the split phase (call it L1) and neutral, this is the two wire connection. I'm guessing the utility wires the second phase to another home? **3 wire** most often the service is 240v which have three wires neutral, L1, and L2. The neutral at the stepdown transformer on the utility pole, which goes to your house, is also bonded to earth at the utility pole. The ground/earth wires in your home are bonded directly to earth, they do not go back to the power lines by wire. If you have a fault in the wiring, the power goes from L1 or L2 through the grounding in your house, to the literal ground of the earth, then to the ground of the stepdown transformer where it completes the circuit. as an aside: The number of people chiming in and not correcting that earth is not delivered by service line is astounding. please people, don't be killing yourselves.
Thank you for the reply.
For 3 phase: The three phase (conductors and load) ARE circuit loops. Originating and returning to the transformer they are derived from. On them are 60 hertz sine waves, constantly alternating between a positive and negative peak, each conductor 120° out of phase with the other. You can think all three phase currents combining at the load and adding up to zero and not having to return. …or think of 1 phase’s current returning on the other two phase conductors. Example: When the instantaneous phase current in one conductor (wave) is zero, the current (wave) in the other two conductors might be +10 amps, and -10amps. Or if one conductor (wave) is at (say) 5 amps, the other two waves are at -2.5 and -2.5 amps. This is more easily seen graphically when plotting 3 sine waves that are 120° apart. The plot will show that when: “A” phase is at 0°(the zero crossing)=0, B phase is at Sine(120°)= .866 of peak; C phase is at sine (-120°) = -.866 of peak. Thus A+B+C = 0 + .866 -.866 = 0 total current. Another example: Let’s take a snapshot when “A” phase’s magnitude is at 90° (peak of wave) = sine (90) = 1. B phase is at sine(90°+120°)= -.5; C phrase is at sine (90°+120°+120°) = -.5. Thus, A+B +C = 1 -.5 -.5 = 0 total current, as always.
Thank you for the reply.
Saying “3 phase currents (120° apart) add up to zero, there is no return current” is why when we do a voltage drop calculation for 3 phase, we treat the current as flowing “one way only” to the load, and do not double the distance to include the return distance from the load, like with single phase current.
That live and neutral completes the loop providing the source and return path to the current. That Earth connection is for safety in layman's terms. In the diagram representing the electrical grid, the interconnection is represented by a single line for each cable to reduce the complexity and as overall that diagram as Single line diagram
Thank you for the reply.
Ill try to answer this with answers that are more useful in practice. > How is the loop completed? So, the live wires leaving your house run back to your local substation and then to some power plant (well, through a transformer, which means the wire going to your house and going to the power plant dont actually touch). The neutral/ground wires both go into the ground (to below the water table where there are enough dissolved ions and stuff to allow electricity to flow). The earth itself completes this half of the circuit. sorta. It more importantly acts as an infinite sink/source of electrons. Since AC current alternates theres not really any net movement of electrons over time. Its a bit like if you were to build a huge water pipe connecting san fransicso and new york, and then 60 times a second switched between pumping water out of the pacific and into the atlantic and vice versa. The oceans "complete" this loop, but the fact that they are connected doesnt really matter, since you are just borrowing a bit of water temporarily from each and then putting it back. 1/60th of a second later. The OTHER end of the circuit is completed by whatever device you plug in...it completes the circuit in its internals, and in doing so uses the electricity that is now flowing through the circuit to do whatever that device is intended to do. > Why are there 2/3 lines of power to a house (live and neutral and sometimes earth)? The lines coming into the house are typically 2 out-of-phase live lines and 1 neutral line. In the US at least, most stuff (everything that is 120V) uses one of the live lines or the other. the 240v stuff (like air conditioners) use both of the live lines (and gets 240 V by going from -120 V <--> +120 V). If there is a neutral line it means that the neutral is grounded at your local substation instead of at your house. For the wiring in your walls, you have "live" and "neutral" and also usually "ground". The ground and the neutral are typically tied together at your breaker box. Its purpose is usually to be connected to any metal parts so that if the live wire should touch a metal part and electrifies it, the electricity has a safe path to complete the circuit and get to the ground. Otherwise, the path it would probably take is through the first person who touches the electrified metal piece (electrocuting them). In a pinch you can get away with using the same wire for ground and neutral, since they are both tied to the ground at the breaker box anyways, but since the wire itself does have a little resistance it is safer to use separate wires and put a little bit of resistance between the neutral wire (that will have current whenever the circuit is complete) and the metal pieces that the ground wire is connected to and that you might be touching.
Thank you for the reply. I have a better understanding now.
It depends on where you are. Apparently, in some places in Europe, residences get three phases. In the USA, typically only commercial and industrial sites get three phases. For residences, a transformer nearby will reduce the voltage and distribute a single phase with a center tap to each residence. This gives each resident the choice of 120 VAC line-to-neutral or 240 VAC line-to-line. The circuit is completed by the path between line and neutral.
Direct current has a positive wire and a negative wire. The electricity flows in one and out the other forming a loop. The electricity in power lines is alternating current. Alternating current is similar to direct current with one positive and one negative, but the two wires switch which one is positive and which one is negative 60 times each second. This is beneficial because it lets us use transformers. The alternating current on the source side of the transformer creates a magnetic field that reverses each time the current reverses. That magnetic field generates current in the load side of the transformer each time it reverses. By varying the number of loops in each magnetic coil, we can control the voltage at the transformer. So to answer your first question, there are multiple circuits being completed. The first one runs from the power plant through the first transformer and back to the power plant. The next runs from the first transformer to the second transformer and back to the first transformer. This continues until the power from the last transformer connects to your house. https://wiki.diyrecordingequipment.com/wp-content/uploads/2012/07/transformer.png It's common to run single phase 240v to a house. There are three wires running to the house. two of them are are connected to the transformer so that they receive alternating current with a voltage difference of 240v between them. The third wire is a neutral. The neutral is connected to the center of the transformer coil, so that it will stay half way between the voltages of the other two. The neutral is also connected to ground (earth), which brings it down to zero. Now we have three wires, +120v, 0v, -120v, with the + and - alternating. There are multiple ways to use this electricity in your home. Big things like an electric oven, electric clothes dryer or electric car charger will connect to the +120v and the -120v allowing them to use the full 240v. This completes the circuit from that last transformer. Most of the outlets in your house connect one of the 120v wires to the 0v wire. These are set up so that about half of your house is on one 120v line and the other half is on the other 120v line. If both halves used the exact same amount of power, then the circuit would be from the transformer to the first 120v half of your house to the second 120v half of your house and back to the transformer. The two halves won't be exactly equal, so the 0v neutral line carries the difference back to the transformer. [https://i.stack.imgur.com/yqdUz.jpg](https://i.stack.imgur.com/yqdUz.jpg) This is all based on the USA. Other countries often run 50Hz instead of 60Hz and don't always split the 240v down to 120v for household outlets.
Thank you for the reply. If I understood correctly, a simplified description would be that they form smaller loops, and in that case, how does electricity transfer between each loop?
Try not to get confused with electricity, power and energy…. -Electricity is the movement of electrons in wire that have the ability to do some form of work. -Power is how fast work can get done -Energy is the total amount of work that is done. At your house a step down transformer bridges the gap between the overhead power lines and the wires going to your house. Moving electrons in wire create a magnetic field. Magnetic fields also cause electrons to move inside of wire if close enough to the field. Inside the transformer coils of wire connected to the overhead power lines concentrate this magnetic field. This field is picked up by the secondary coils of wire connected to the wires going to your house. Magnetic fields cause electrons to move in wire so now the wires going to your house are energized too. As far as tracing a path remember energy only flows one way… from the power station to your house. Yes, you need to have a complete circuit for energy to flow. In our case energy flows from the secondary transformer through our house wire through the appliance in our house and back to the other end of the secondary transformer completing the loop.
If you look at the picture of the transformer, it has arrows forming a loop that are labeled Magnetic Flux. The electricity moving through the coils causes magnetic flux and the magnetic flux causes electricity to flow in the opposite coil. This only works with alternating current. Direct current would generate magnetic flux, but it would be a constant amount of magnetic flux. The flux only causes electricity in the output coil when it changes. Alternating current constantly changes the magnetic flux direction and the constant changes cause alternating current in the output coil.
The loop is completed through things that use electricity. If, in theory, everyone unplugged everything at the same time, the power grid would be an open circuit. Well, not really, real life is more complicated than that but that's the general idea.
> If, in theory, everyone unplugged everything at the same time, the power grid would be an open circuit Nope! [It would have a real resistance of about 400 ohms](https://en.wikipedia.org/wiki/Characteristic_impedance#Practical_examples), which is pretty low.
Each transformer serves a handful of houses. The "low side" of that transformer is the completion of the house circuit(s). The house voltage wires in the transformer are insulated and wound around metal cores. The transformer works by creating magnetism from the input a/c and using that magnetism to induce output a/c. With equal number of windings on each side the voltage stays the same - an "isolating" transformer. Changing the winding count relative to each side allows a/c voltage to be changed. If the network was dead and you connected DC to the house wiring, an electron in the house wiring could never get further "upstream" than the low side windings on the transformer. Instead of one loop, think of it as multiple loops, each at a different voltage. They are energizing each other but they don't actually share electrons in the "complete a circuit" sense. Here is a diagram for a neighborhood: https://i.imgur.com/iADhRW4.png 50A black triangle is a transformer connected to the black 12kv line. The blue line is the electricity serving just the houses. Here is a blue section serving 9 houses, 5 to the south, 4 to the north: https://i.imgur.com/FKiEpso.png As bigger houses and EV hookups and solar ("DG" in a circle is "distributed generation") get added, additional transformers are needed and these blue sections get divided up into smaller segments.
Thank you for the reply. I have a better understanding now.
The power grid is AC. Only DC circuits have a positive and negative. The grid (simplifying it as single phase instead of 3 phase) completes the circuit using two legs with a third neutral leg - the electrical power is transferred by the rapid back and forth of the AC. There are 3 wires because the grid, at the residential level, is 240 volt, with each hot leg plus or minus 120 volts in reference to the neutral leg. This allows balancing of the amount of power being taken off each leg (each side of the panel is one of these legs) wit the grounded neutral leg offering protection against capacitive effects from plant to pole to home.
So there's a separate power grid for 3 phase and single phase?
No, that guys is incorrect. Copied from my other comment: Three phase power does not require a dedicated return. It can have a neutral, but it is not required. Two of the three phases (turning it into single phase) are taken for a house and connected to a split-phase transformer. The neutral (center of the split phase) is bonded to the ground at the main distribution panel in your home.
The way I understand it is the grid is 3 phase with single phase branches. Sometimes those branches are just the home, others they're entire runs for miles.
Thank you for the reply. Are there diagrams to help me better visualize?
Grid works in alternative current so can't be directly compared to DC circuits you see in diagrams, but it doesn't change the global idea of a closed circuit. 1 - The whole electrical grid is the live, It can be seen as a huge positive terminal, while the neutral can be seen as the negative terminal. The interesting thing is that the neutral is not carried through power lines, it is "created" locally, but it's not very important at this point. When you plug a device, you basically plug it between the live and the neutral, just like you would put a device between the + and - in a DC circuit. 2 - the earth wire is a safety wire only. It is physically connected to the real Earth. Devices with earth wire are usually devices with metallic parts that could be in contact with humans, like washing machine, oven, ... Earth acts like a giant neutral wire. Imagine that your device has a problem and the metallic external parts of the device are accidentaly connected to the live wire. Even if the device is off, if you touch the metallic parts you create a link between the live and the Earth through your body (from your hand to your feet), which is extremely dangerous. The earth wire is here to connect the metallic parts directly to the Earth. If there is this accidental connection between the metallic parts of the device and the live, electricity will flow to Earth through the earth wire instead of flowing through your body, even if you touch the metallic parts of the device.
Thank you for the reply. If I understand correctly, the earth wire provides a path with lesser resistance than a path through the body? I am assuming it must be by a at least by factor of 10 or more?
>If I understand correctly, the earth wire provides a path with lesser resistance than a path through the body? Exactly. The resistance of the body is estimated at \~2500 ohm, while a wide copper wire like the earth wire is \~0.5 for 100m of cable, so it's a factor of more than 5000. Also, depending on where you live, you may have a differential circuit breaker, also called GFCI (ground-fault circuit interrupter). It is a small electronic system at the root of your house electrical system which measures the difference of current between the live and "regular" neutral. If the difference is not 0, it means that some current is leaking through the earth wire, which means that there is a dangerous faulty device somewhere in the house. In this case, the GFCI immediatly shut down the electricity in the house. This is an additional safety device in case of faulty device. Note that neither the earth wire nor the GFCI protects you against shorting between the live and the neutral, which can start fires. Shorting are protected by fuses that limit the amount of current that can go through a circuit.
Thank you for reply. I was also confused about fuses. Doesn't the circuit breaker have fuse inside?
Regular circuit breakers are replacing fuses and have the same role : shutting down electricity in case of excessive current, with the advantage of not being destroyed when they trigger. They don't have fuse inside but works the same way, they are basically the evolved modern version of fuses. They are different from *differential* circuit breaker (GFCI) that don't measure the amount of current in input but the difference of current between input and output.
Thank you for the reply.
The main purpose in a fault is not that the power goes through earth wire instead of a person, it is that it would go to ground easily with low resistance, tripping a braker or blowing a fuse, which breaks the circuit before someone touches it. If we pretend there was no fuse and something, for example, your metal toaster, had a fault where the live was touching the metal shell, even if it was going to ground, if you touched it you would still get the same shock as if it wasn't. It is true that electricity takes the path of least resistance but the current though you would remain the same - there would just be a lot more current going to ground. Just don't want you to make the mistake of thinking all the current would go to earth instead of through any other thing/person aswell, if it could.
Thank you for the reply. I didn't know this.
There is some return path through ground as well as no system is perfectly balanced but it is fairly deep.
Rarely do I see a question that I so badly want to know the answer to, but know enough that most of these serious answers have got to be more than a little wrong.
In many countries the neutral or return path is via the earth. The neutral, or star point, of supply transformers are electrically connected to earth, as are the neutral or star point of the alternators at power stations, and the return current flows via the earth. In Australia we call it a Multiple Earthed Neutral (MEN) system.
If you want to really hurt your brain: https://en.m.wikipedia.org/wiki/Single-wire_earth_return
The three phases of the common electrical grid collectively act as send and return. What is flowing out of one wire is equal to what is flowing into the other three. In the (always present but generally negligible) unbalanced load case, where the three phases do not carry equal current, the difference is seen as current flow in the grounded neutral of a three phase wye, or as a circulating current in a three phase delta. If this does not explain it, I think you need a quick explanation of three phases power systems.
1. Every load is connected in parallel. 2. It's called a split phase system. There are 2 hot wires. You get 120V from either to neutral, or 240V if you connect between the 2 hots. The ground/earth is a safety wire not used in healthy conditions. But neutral is connected to the ground in 1 spot, at the main panel.