How does ground mains work?

Question

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?

Answer 1

^{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.

^{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.

^{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.

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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.

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

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.