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I recently learnt that ground can be used to complete circuits using high voltages.

I'd like to know if this is how overhead powerlines work, and if so where exactly do the cables/circuit terminate - at substations or something?

For that matter, the negative (black) mains wire in my house, where does that go?

I had a google around but I wasn't really sure what to look for, hence asking here. If someone can point me in the right direction I'd appreciate it!

Thanks!

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In North American practice, the black wire is "hot" (120 volts). Red and Blue may also be used for hot wires. The neutral (0 volts, ground) wire is white. Green is a safety ground, and should not normally carry current. The green and white wires will be connected together at the service entrance to the building. –  Peter Bennett Apr 22 at 17:48
    
Consider a battery. It doesn't have ground, it has a positive terminal and a negative terminal. When you connect it to a device, the device may define a ground, and may connect it to one of the terminals, or some other point in the circuit. Similar to a battery, the power company doesn't send a "ground" wire to your house or building. It sends 2 or 3 phase power, and at your building you define a ground - often this is binding one of the power supply wires (neutral) to a ground rod. This is then distributed to your outlets as ground. As such, the overhead lines don't have "ground". –  Adam Davis Apr 22 at 19:48
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Ponder this: why is the electrical ground called the ground? –  Eric Lippert Apr 22 at 20:06
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@PeterBennett: The OP is in the UK, where hot is either red or brown, and neutral is either black or blue, and ground is either green or yellow-green. New work wiring should use the brown/blue/yellow-green convention. –  Eric Lippert Apr 22 at 20:27
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Note that the black/blue wire is not negative but rather neutral. –  Eric Lippert Apr 22 at 20:27

4 Answers 4

This post is mainly about ground/earth/soil as conductor and importance of grounding for safety. Maybe it's not accurrate answer but it may be useful too. For other meanings of ground - see other answers.

I think answer for your question is here:

http://en.wikipedia.org/wiki/Earthing_system

See TN, TN-C and TN-C-S systems.

Cables you are asking about terminate at closest transformer.

More about grounding and why ground/soil is not used as conductor for power lines.

Ground in medium and high voltage power grids is not carrying significant power in normal conditions. High voltage power lines are 3-phase, current flows mainly between phases and ground is just reference "zero" for them. Ground works more as reference and discharges static electricity from high voltage equipment housings and other conductive parts that should be at earth potential. High voltage equipment is well insulated and it may accumulate large electrostatic charges.

Carrying power by ground (soil) would probably end up with very fast electrodes corrosion and maybe some environmental changes in soil, because soil contains water, salts, acids. All of this becomes mix of electrolytes.

Grounding also works as lightning protection. Thats the way to route/control lightning power into ground with low "power losses". When there is no intentional grounding - lightning will find 50 other ways anyway. In that case - grounding can be considered as high impedance/resistance grounding. Very high power can be emitted on high resistance and that may cause uncontrolled fire or explosion anywhere, in many places etc. So it's better to make a "highway" for the lightning by grounding big metal things.

In some networks "functional grounding" is used and in this case earth is indeed used to carry power.

In low voltage networks (110V or 230V in europe) grounding is used as "protective grounding", to allow RCD protection. Older method of protection is connecting conductive devices housings to ground. If device is damaged (burned insulation, mechanical damage etc) and voltage "coming out" to grounded housing - fuses will blow up because there is a short circuit.

Your black wire probably goes to transformer at power station/transformer station. It's grounded there. In some countries ground wire has to be connected to earthing system near house (metal tapes burried under house), but in that case - that wire is not black, but yellow with green stripes. That depends on earthing system used in your country. You can read about diffrent earthing systems on wikipedia (link below).

Grounding/earthing is a thing with "many faces"...

And there is no such thing like "negative" or "positive" line in alternate current. There is phase wire, zero and/or ground wire. Phase wire becomes positive (voltage above zero) or negative (below zero voltage) over time. Zero stays at zero relatively to... earth/ground :)

Can someone correct/check for my language mistakes? My english is bad, I don't want to mislead anyone in such important matter (grounding/earthing).

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It may be worth discussing the difference between ground (i.e. the conductor that is earthed) and neutral (i.e. the conductor that returns the load current in a single-phase power supply - the 'black wire' that John Hunt refers to.) Otherwise your answer is mostly correct. –  Li-aung Yip Apr 22 at 17:52
    
@Li-aungYip Right. That depends on kind of earthing system used to deliver power to his house. –  Kamil Apr 22 at 18:08
    
Sure, it depends on the earthing system. Also, my comment about black wire being the neutral is incorrect - as Peter Bennett pointed out in comments, North America uses black for an active conductor. –  Li-aung Yip Apr 22 at 18:33
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A safety ground normally doesn't carry significant currents. A return ground (a.k.a "common") carries all the current out of a system that goes in. –  Kaz Apr 22 at 21:31
    
@Kaz I meant power grids. I corrected that. –  Kamil Apr 22 at 22:06

Be careful not to confuse 'earth'/'ground' and 'negative'/'neutral'.

  • Earth or ground is the conductor that connects the power system to the mass of the earth, i.e. via buried electrodes. The earth conductor does not normally carry any current. (Exception: SWER lines.)

    All power systems require a connection to earth.

    As Kamil already explained, this is required to provide a voltage reference, dissipate static electricity, dissipate lightning strikes to earth, etc. (Exception: power systems with isolated neutral - not very common.)

  • Neutral is the conductor that provides a return path for load current.

Typical colours for these conductors are given by the Wikipedia article on cable colour codes.


So, which power systems have earths, which have neutrals, and which have both?

Broken down by type of power system (Australian practice; US practice may differ.)

  1. In a high-voltage three-phase system, there are three active conductors. If the load is balanced, as it is for high-voltage overhead lines, all current is carried by the three phase conductors, and no neutral is needed.

    Earth is used for voltage reference and safety grounding purposes amongst other reasons.

    So the conductors that are present are: 3× Active conductors, Earth.

    To provide some intuition, here's a picture of how the current flows in a wye-connected three phase system without a neutral:

    Three phase currents

  2. In a low-voltage three-phase distribution system, the load may not be balanced. Therefore a neutral conductor is required to carry the unbalanced load current.

    A ground is also required in addition to the neutral. Whether this is interconnected between points depends on how the earthing has been designed. For example, two buildings 100 metres distant might be separately earthed, and there would be no earth conductor between them.

    So the conductors that are present are: 3× Active conductors, Neutral, and possibly Earth.

  3. In a low-voltage single-phase circuit, as might be found in your house, you have

    • an active conductor which carries the load current from your local distribution transformer
    • a neutral conductor which returns the load current to your local distribution transformer
    • an earth conductor for voltage reference and safety grounding - this does not carry any current. The only time the earth conductor carries current is when there is a short circuit from active to ground - in which case your residual current device (RCD, Australian term) or ground-fault circuit interrupter (GFCI, US term) will detect this and disconnect your power supply.

    So the conductors that are present are: 1× Active, Neutral, Earth.

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It is late and I am about to go to bed, so do feel free to edit my answer if I have made any egregious mistakes. –  Li-aung Yip Apr 22 at 18:34
    
Im not sure about this, but if you use earth as conductor - electrodes will may react chemically with water. That water is not 100.00% water, it contains some salts, acids etc. Electrodes may degrade very soon, so using earth to carry "everyday currents" between buildings may be bad idea. Good night :) –  Kamil Apr 22 at 19:59
    
"All power systems require a connection to earth." – how do airplanes work then? –  Paŭlo Ebermann Apr 22 at 21:02
    
@PaŭloEbermann "How do airplanes work then?" Airplanes require a connection to earth too and this is extremely important. When plane flies - special "wicks" are used to discharge it partially. While landing - dischargers in landing gear are used. When airplane is fueled in airport it must be permanently grounded and connected to fuel truck that fuels it to prevent fuel explosion. –  Kamil Apr 22 at 21:43
    
I would also like to suggest adding this image to your answer: upload.wikimedia.org/wikipedia/commons/4/48/3-phase_flow.gif. It provides a good intuition about three-phase current flow without a neutral. –  Peter K Apr 22 at 21:52

I'd like to know if this is how overhead powerlines work, and if so where exactly do the cables/circuit terminate - at substations or something?

Go outside and find a power pole with a can transformer, and examine it carefully. Configurations vary considerably, but a typical setup in North America is:

  • There is a high-voltage hot line and high voltage return line making a circuit to the substation.

  • The transformer is attached to the high voltage hot and neutral.

  • The transformer steps the ~7200VAC electricity down to 240VAC, configured as two hots and one neutral, the hots being 240VAC apart from each other, and each 120VAC from the neutral. You being in the UK will likely have a slightly different configuration as I believe you use 240VAC all over the show.

  • A ground wire either goes down the pole to the ground from the transformer, or the pole is guyed in place by a steel cable which is used as the transformer ground.

  • The two hots and one neutral go to your circuit breaker box.

  • The circuit breaker box might itself be grounded by a copper cable attached to a bar buried in the yard.

Ultimately grounds are called grounds because they attach to the ground. Typically we expect dedicated ground lines that are actually earthed to carry current only in emergencies; normally the current on the hot should equal the current on the neutral.

It is helpful for beginners to think of analogies with water. The hot is the pressured supply lines. The neutral is the drain line. The safety ground is an extra drain in the basement used only when the house floods.

There are lots of pictures on the internet showing how this all works. Here's a few:

http://waterheatertimer.org/See-inside-main-breaker-box.html

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How much power typical transformer like this can have? waterheatertimer.org/images/Electricity-from-the-pole-4.jpg There are 7.2kV lines everywhere and every house has separate transformer in USA? –  Kamil Apr 22 at 22:33
    
@Kamil: Voltages from the substation vary, and one transformer typically services a few houses. But basically, yes, this is how it's laid out in residential neighbourhoods that have above-ground electrical service. How it goes for houses that have service run through the ground, I don't know. –  Eric Lippert Apr 22 at 22:53
    
@Kamil: I would expect a small transformer like that to be about 50-100 kVA. Poletop transformers can get as large as 500kVA before they get too heavy to put on poles. –  Li-aung Yip Apr 23 at 0:43

Most industrial power lines use alternating current, so there's no positive and negative wires. Common are 3 phase wires and neutral wire. Electric consumer may use all three phase wires or just one phase and neutral (like most domestic ones).

Common power curcuit consist of these three devices: electric power plant -- electric substation, which transform voltage (or series of them) -- power comsumpting device. Phase wires and neutral wire make closed power circuit between two of them.

For trolley transport (electric buses, trains) direct current is used as well as alternating current. One trolley is often positive and other trolley or rails is negative. In this case power to trolleys is delivered by sub station which transform and also rectify power taken from industrial AC line.

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This is true. In the USA, however, residential customers generally get "Split Phase" power. This means that they get two AC lines and one Neutral. The Lines are 180-degrees out-of-phase, not 120-degrees as in a 3-phase system. This way, you can get 120VAC from line to neutral, or 240VAC from line to line. These connections are made by selecting the appropriate circuit breakers in the main breaker panel of a home. –  bitsmack Apr 22 at 17:04
    
@bitsmack So... these customers have 3-wire system? 2 phases and zero? –  Kamil Apr 22 at 17:31
    
@Kamil Three "current-carrying" wires: Line 1, Line 2, and Neutral. Lines 1 and 2 are "hot", or energized. Neutral provides the return current path. There is also a "grounding" conductor, which either comes from the utility or is connected to a large grounding rod that is driven into the earth at each house. This ground connection is only used for safety reasons: Current will only flow to ground when there is an electrical failure. This current will cause the circuit breakers (and ground-fault breakers) to trip. –  bitsmack Apr 22 at 17:45
    
@bitsmack I know how grounding/earthing works, im an electrican by education. Just was curious about USA systems. Never learned about them. –  Kamil Apr 22 at 18:12
    
@Kamil Very good, I wasn't trying to be condescending. I read your answer, and it's obvious that you know what you're talking about! Wikipedia has a page on this type of system: en.wikipedia.org/wiki/Split-phase_electric_power. Take care :) –  bitsmack Apr 22 at 18:20

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