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When I studied how AC mains power works, I learned that one of the wires is connected to the ground or a body of water so that it can get back to the power station. The concept baffles me. Every source of information I've come across fails to explain how it works or quickly glosses over it as if it is self explanatory.

If power can travel through the water or the earth back to the power station, then why aren't we getting vaporized when we walk on the ground near power lines? Also, how does an isolation transformer prevent you or your equipment from getting fried? If I touch both terminals of the secondary, am I going to get fried?

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    \$\begingroup\$ The wire you remember is of yellow/green color and is called protective earth. And no current should flow thru it. \$\endgroup\$ – ott-- May 6 '16 at 20:50
  • \$\begingroup\$ Your question is touched on in my answer (and a comment on it). There are power systems that use earth return, although its not common in developed regions. The earth is not a very good conductor, but the cross-section area is very large, so it can conduct substantial currents. \$\endgroup\$ – The Photon May 6 '16 at 21:17
  • \$\begingroup\$ I've seen the other questions and the answers are unsatisfactory because they are dealing with a simple circuit and reference voltages as in battery. My question deals with ground as in the real ground you walk on and how it interacts with power from a power plant. \$\endgroup\$ – user148298 May 6 '16 at 21:42
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    \$\begingroup\$ After looking at it more closely I'm voting to re-open because this asks specific questions that weren't addressed in the old question. \$\endgroup\$ – The Photon May 6 '16 at 22:06
  • \$\begingroup\$ @user148298. Read the other answers in detail, especially the last 2. They at least overlap the answers on this question you posted here. Both sets of answers overlap enough to be inclusive of both questions, but not in all aspects \$\endgroup\$ – Sparky256 May 6 '16 at 22:08
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schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A local distribution transformer supplying a house with only one applicance in it - an electric lamp in a metal enclosure, properly earthed.

  • With reference to Figure 1, we can see that the local transformer isolates our supply from the generating station. Whether the generator is earthed or not is of no interest to us. (The transformer symbol indicates that there is no electrical connection between the primary and secondary sides.)
  • One side of the transformer is "neutralised" by connecting it to an earth rod driven into the ground. Neutral, as the name implies, means that this conductor is neither positive or negative. In ideal circumstances one could touch this wire without harm. (We'll see later why this isn't recommended.)
  • Everything in the house is wired between L (live) and N (neutral).
  • Non double-insulated appliances and fittings have their metal cases earthed (grounded).

Now there are a few things to note about this arrangement:

  • In normal circumstances all current is supplied on the live wire and returns on the neutral. (Yes, it's alternating but we can still think of it this way.)
  • In normal circumstances NO CURRENT FLOWS IN THE EARTH WIRING. It just sits there - maybe never being used ever - in case there is a fault.
  • If the L wire in the lamp falls off the switch and touches the metal enclosure a fault current will flow to earth. Why? Because the case is connected to ground which is connected to the transformer neutral.

If we don't earth the appliances we could have a dangerous condition.

schematic

simulate this circuit

Figure 2. An unearthed appliance with an internal fault making the enclosure live.

Now if someone touches the lamp they are in danger of electric shock. This may flow through their body to ground by resistive conduction but since the human body has some capacitance with the Earth at least a small current will flow.

schematic

simulate this circuit

Figure 3. An earth fault.

In Figure 3 an internal fault has occurred.

  • Because the appliance is properly earthed the voltage on the appliance will be low.
  • If the fault is severe (a good contact between the internal live wire and the case) AND the ground return path to the transformer is adequately low, a high current will flow and the fuse will blow. This will render the circuit safe.
  • Note that since one wire has been neutralised we don't need to install fuses in it.

When I studied how AC mains power works, I learned that one of the wires is connected to the ground or a body of water so that it can get back to the power station.

No. Just to the local transformer.

The concept baffles me. Every source of information I've come across fails to explain how it works or quickly glosses over it as if it is self explanatory.

I hope the above helps.

If power can travel through the water or the earth back to the power station, then why aren't we getting vaporized when we walk on the ground near power lines?

  1. We don't normally send current back through the ground. It's only in fault conditions.
  2. To get vaporised you would need a high voltage between your left foot and your right foot. If we had a massive fault in the house and the earth potential at the earth rod rose to 100 V and it was 100 m back to the transformer then the voltage gradient would be 1 V/m. Take the biggest step you can and you'd have < 2 V between your feet.

Also, how does an isolation transformer prevent you or your equipment from getting fried?

It doesn't. It just isolates it from the mains. This is a separate question.

If I touch both terminals of the secondary, am I going to get fried?

Yes, there is a voltage between the terminals. If it's isolated you could touch either terminal and not get a shock (but remember the body's capacitance and that of the transformer - you could be surprised) but if you touch each terminal with separate hands then a current could flow through your heart and kill you.

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  • \$\begingroup\$ Way to end an answer :)...+1 in-depth. Just to clarify, 240V is across the secondary of the transformer, whether neutral has a local reference or not, and the main reason a local reference is set is to provide a return path for fault current from houses, since earth can be reached by all of them? Also you say "the voltage on the appliance will be low" under figure 3; i thought the voltage on the appliance remains the same but all that happens is that most of the current flows through the earth conductor, which has a much lower resistance than the human body; like really tiny parallel resistor? \$\endgroup\$ – TisteAndii May 7 '16 at 0:37
  • \$\begingroup\$ In the second diagram the voltage will be low because the switch is off. \$\endgroup\$ – immibis May 7 '16 at 4:12
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    \$\begingroup\$ @TisteAndii: (1) 240 V always. Correct. (2) Main reason? Yes. (3) I didn't want to overdo the answer but consider a dead-short between live and earth on the appliance. The only resistance in the circuit now is that of the live wiring (from transformer to appliance) and the earth wiring (from appliance back through earth rod to ground). If these resistances were equal the voltage on the appliance would reach half-supply. The lower we can make the earth resistance the lower will be the fault voltage. \$\endgroup\$ – Transistor May 7 '16 at 8:34
  • \$\begingroup\$ @immibis: I think you meant Figure 3. I switched it on now. Thanks. (It was a cut'n'paste error. I remembered to toggle the switch in Figure 2 but forgot in Figure 3.) \$\endgroup\$ – Transistor May 7 '16 at 8:36
  • \$\begingroup\$ @transistor. Thx for the pics. Tough to get such concepts across in just words alone. \$\endgroup\$ – Sparky256 May 7 '16 at 17:30
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You are mistaking the 'Power Station' as being the ground return point. AC power is distributed throughout a city at high voltage to sub-stations, where it is lowered to maybe 7,200 VAC. This in turn is distributed to local neighborhoods where pole mounted or ground pad transformers step the voltage down to 240 VAC with enough current to power 1 to 4 homes.

These substations and transformers isolate us from the power station ground, and in fact the highest voltages go from transformer to transformer and are a 'delta' format which has no ground. "Ground' as you are thinking of it is done by grounding the neutral tap at each transformer secondary that supplies power to industry and residential areas.

The utility electricians insert a copper grounding rod at that location and that sets the local ground reference for that power transformer. This is where your white neutral wire to your house and earth ground first meet. But instead of running a separate ground wire to each house, you have a ground wire (usually bare copper) connected to the neutral block at your breaker panel and it goes to a grounding rod within a few meters of the panel.

The power utility does not need to run a ground wire from the source transformer to your house because copper is expensive, and normally there is NO voltage on the ground wire, or current. If there is then you have an appliance that is leaking current to its chassis, or maybe an old electric drill with frayed wiring. Then the ground wire serves its purpose by shunting the leakage to earth ground through the grounding rod.

The supply of power to a house or business is done normally (I am skipping 'special' power feeds like 3-phase) with 2 black wires, the 'hot' wires, or L1 and L2 as they are sometimes called. Each is 120VAC to neutral but 240VAC between them, as they are 180 degrees out of phase with each other. The white neutral wire is the current return for 120VAC appliances, no matter which black wire is the power source, and connects to earth gnd inside the breaker panel, to keep any voltage on it as low as possible.

You may still get a shock though from very old appliances, which is way many panels now use GFCI breakers that will trip 'OFF' if any current into ground is detected. If you touch a hot and neutral wire or hot and ground at the same time. you will get a shock, which includes isolation transformer outputs if you touch both wires at the same time.

This is a snippet from the Green Grounding Manual.pdf

DESIGNING FOR A LOW RESISTANCE EARTH INTERFACE (GROUNDING)

Roy B. Carpenter, Jr. and Joseph A. Lanzoni Lightning Eliminators and Consultants, Inc. Boulder, Colorado, USA

Introduction

Grounding (or earthing) is the art of making an electrical connection to the earth. The process is a combination of science and “art” as opposed to pure science. This process is required because it is necessary to go through a process of “testing the options,” as opposed to calculations made via some formal process. The options for each site must be determined through visualization and evaluation, individually, using a related analytical process.

The earth must be treated as a semiconductor, while the grounding electrode itself is a pure conductor. These factors make the design of an earthing system complex, not derived from a simple calculation or the random driving of a few rods into the soil.

Knowledge of the local soil conditions is mandatory and is the first step in the design process. This includes its moisture content, temperature, and resistivity under a given set of conditions.

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  • \$\begingroup\$ Ok, but my question is still unanswered, but a fews things have been clarified. Essentially, the power plants power is isolated down the chain, but the question still stands at a micro level. How does electricity flow back to the transformer from you house? \$\endgroup\$ – user148298 May 6 '16 at 22:05
  • \$\begingroup\$ The local ground reference is an interesting clue. How do they "set" the reference? You \$\endgroup\$ – user148298 May 6 '16 at 22:41
  • \$\begingroup\$ Remember there is a grounding rod installed by the power utility at both the transformer supplying your power and one at your house. They use the earths low resistance to 'connect' the two together. Because the rods normally have zero current on them, or a few milliamps at most, this method has proven safe for a century now. That is the local gnd ref for your house, set by the power utility. \$\endgroup\$ – Sparky256 May 6 '16 at 22:47
  • \$\begingroup\$ Electricity travels through the soil made of rocks that are very high in resistance? Yet, the earth is low resistance? I give up. Electricity sucks! \$\endgroup\$ – user148298 May 6 '16 at 22:52
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    \$\begingroup\$ Correct. It's a bad conductor but look at the conductor cross-sectional area! It's huge. It's like a whole pile of high value resistors in parallel. The overall resistance can be very low. \$\endgroup\$ – Transistor May 6 '16 at 23:02
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I learned that one of the wires is connected to the ground or a body of water so that it can get back to the power station. The concept baffles me. Every source of information I've come across fails to explain how it works or quickly glosses over it as if it is self explanatory.

There are power systems that use earth return, but it is very uncommon in developed regions. Such systems work because while the earth is not a very good conductor, its cross-section area is very large, so it can conduct substantial currents.

If power can travel through the water or the earth back to the power station, then why aren't we getting vaporized when we walk on the ground near power lines?

For the same reason birds can stand on a power line -- because the voltage drop across the small distance between your feet is not very large.

Also, how does an isolation transformer prevent you or your equipment from getting fried?

An isolation transformer prevents any circuit being formed from the power source, the earth ground, and your equipment. For example if a person were to touch the chassis of the equipment and the floor at the same time.

It doesn't so much protect the equipment from being damaged as the person who might unwittingly use their body to complete the circuit through ground. I've never used one, but I'd guess you use it in cases where the chassis of the equipment must be energized due to the nature of the equipment. The Wikipedia article gives a couple other examples of use cases for isolation transformers.

If I touch both terminals of the secondary, am I going to get fried?

Yes. If you put one hand on each terminal of the secondary, you are going to be in trouble. Because there's a circuit created between your body and the secondary coil.

Edit

Should the ground get hot if current is flowing through it? Is the resistance too high for it to travel?

Yes, if you forced current through a narrow region of earth, it would heat up. But normally the earth is very large, which reduces its electrical resistance and also spreads the generated heat out so that it doesn't produce a noticeable temperature rise.

If I am on the third floor of a house, how do I get shocked if I am a considerable height above the ground?

Generally there are water pipes and electrical wiring in your house that will electrically connect the structure to the earth.

the wood or concrete frame of the house should provide enough resistance? What if the house is made of thick rubber?

If your house were made out of rubber, you would not likely be shocked, even if you touched the hot wire of an electric outlet.

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  • \$\begingroup\$ Hmmm. Should the ground get hot if current is flowing through it? Is the resistance too high for it to travel? If I am on the third floor of a house, how do I get shocked if I am a comsiderable height above the ground? Still confused. \$\endgroup\$ – user148298 May 6 '16 at 21:34
  • \$\begingroup\$ Furthermore, the wood or concrete frame of the house should provide enough resistance? What if the house is made of thick rubber? \$\endgroup\$ – user148298 May 6 '16 at 21:45
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    \$\begingroup\$ There is a conspiracy to prevent me and others from learning how ground works. The illuminati read your answer and prevented its submission. \$\endgroup\$ – user148298 May 6 '16 at 21:48
  • \$\begingroup\$ How can it be a duplicate? It is fundamentally different. \$\endgroup\$ – user148298 May 6 '16 at 22:07
  • \$\begingroup\$ @user148298, read Phil Frost's answer to the old question. It answers your title question and your first paragraph. \$\endgroup\$ – The Photon May 6 '16 at 22:10
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When I studied how AC mains power works, I learned that one of the wires is connected to the ground or a body of water so that it can get back to the power station.

That is partially true. There are many branches and steps between you and the utility power station. Typically a "ground return" scheme is used only at the "last mile" stage. Many residential and rural areas distribute power (at several thousand volts) via a single wire, and use the earth as the return path. You will see examples where there is a single wire at the very top of a power pole, and a big transformer (called a "pole pig") which steps that high voltage down to the level that comes into your house. In North America, typically 230V.

On the transformer, there is one large insulated terminal that attaches to that high-voltage wire at the top of the pole. In the photo below, it is on the top of the transformer and you can't really see anything but the attached wire. But the other side of that circuit (the transformer "primary" or input) is a ground wire which is attached to the pole and goes all the way down to a ground rod. You can see this ground wire going off from the right side of the transformer, and looping down the pole.

The other ("secondary" or output) side of the transformer is typically 230V, and center-tapped where the center tap is also grounded. That is the three larger wires coming out from the side of the transformer.

typical single-phase, ground-return pole-pig transformer

If power can travel through the water or the earth back to the power station, then why aren't we getting vaporized when we walk on the ground near power lines?

Because the ground (or water) are (by definition) at zero-ground potential.

Also, how does an isolation transformer prevent you or your equipment from getting fried? If I touch both terminals of the secondary, am I going to get fried?

That is a very different question which should be asked separately. Just your first question has already generated a large response.

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  • \$\begingroup\$ Hmmm. I am getting closer to understanding. Still, the concept of current flowing through dirt, wood and whatever else to get to ground still makes no sense given the high resistances involved. If you touch a black live wire to the backyard dirt, it doesn't short, but it shorts if it you touch it? Makes no sense at all. My isolation transformer portion was supposed to be an example, but came off as a pseudo question. An isolation transformer's hot wire won't cook you by itself and neither should a on isolated one, but it does. \$\endgroup\$ – user148298 May 7 '16 at 0:55
  • \$\begingroup\$ If you touch a "live" wire to wet dirt, it will actually flow a little bit of current. But the little piece of wire doesn't form a very good contact with the earth. A typical ground rod is copper-plated (for good connection) and 6-8 feet or even longer. But if you use your own body with your bare feet on wet dirt, and touch a live wire, your body is MUCH MUCH more sensitive to the small amount of current, and that could actually kill you by stopping your heart. \$\endgroup\$ – Richard Crowley May 7 '16 at 1:16
  • \$\begingroup\$ Some high-voltage long-distance links use ground return - because the voltage is very high, the current can be relatively low ("only" a few hundred amps). According to Wikipedia the NZ Inter-Island HVDC link was transferring a few hundred megawatts in monopolar/earth-return mode for a few years. \$\endgroup\$ – immibis May 7 '16 at 4:17
  • \$\begingroup\$ Yes, I have lived near BOTH ends of the "Pacific DC Intertie" which transmits 3.1GW of power BI-DIRECTOINALLY between the Columbia River Gorge and metropolitan Los Angeles. It uses ground return. "Ground" is 1,167 cast-iron anodes buried under a rice field in Oregon at the north end, and 24 silicon-iron anodes suspended under the Pacific ocean off the Will Rogers State Beach. en.wikipedia.org/wiki/Pacific_DC_Intertie \$\endgroup\$ – Richard Crowley May 7 '16 at 5:26

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