Without it life can get somewhat cumbersome. Power travels from the power plant to your house through an amazing system called the power distribution grid. The grid is quite public - if you live in a suburban or rural area, chances are it is right out in the open for all to see. It is so public, in fact, that you probably don't even notice it anymore. Your brain completely ignores all the power lines because it has seen them so often.
Electrical power starts at the power plant. In almost all cases the power plant consists of a spinning electrical generator. Something has to spin that generator - it might be a water wheel in a hydroelectric dam, a large diesel engine or a gas turbine. But in most cases the thing spinning the generator is a steam turbine. The steam might be created by burning coal, oil or natural gas. Or the steam may come from a nuclear reactor. No matter what it is that spins the generator, all commercial electrical generators of any size generate what is called 3-phase AC power. To understand 3-phase AC power it is helpful to understand single-phase power first. Single phase power is what you have in your house. You generally talk about household electrical service as single-phase 120 volt AC service. If you use an oscilloscope and look at the power found at a normal wall-plate outlet in your house, what you will find is that the power at the wall plate looks like a sine wave, and that wave oscillates between -170 volts and 170 volts (the peaks are indeed at 170 volts - it is the average (rms) voltage that is 120 volts). The rate of oscillation for the sine wave is 60 cycles per second. Oscillating power like this is generally referred to as AC, or Alternating Current. The alternative to AC is DC, or Direct Current. Batteries produce DC: a steady stream of electrons flow in one direction only from the negative to the positive terminal of the battery. The power plant therefore produces AC. However, it produces three different phases of power simultaneously, and the 3 phases are offset 120 degrees from each other. Out of every power plant comes 4 wires: the 3 phases plus a neutral or ground common to all 3. There is nothing special or magical about 3-phase power. It is simply three single phases synchronized and offset by 120 degrees. Why three phases? Why not one or two or four? One big advantage that 3-phase power has over 1-phase or 2-phase power is the fact that, at any given moment, one of the three phases is nearing a peak. In 1-phase and 2-phase power there are 120 moments per second when the sine wave(s) cross zero volts. High-power 3-phase motors (used in industrial applications) and things like 3-phase welding equipment therefore have even power output. Four phases would not significantly improve things but would add a fourth wire, so 3-phase is the natural settling point.
And what about this word "ground"? The power company essentially uses the earth as one of the wires in the power system. The earth is a pretty good conductor and it is huge, so it makes a good return path for electrons. [Car manufacturers do something similar - they use the metal body of the car as one of the wires in the car's electrical system and attach the negative pole of the battery to the car's body.] "Ground" in the power distribution grid is literally "the ground" all around you when you are walking outside. It is the dirt/rocks/groundwater/etc. of the earth. The 3-phase power leaves the generator and enters a transmission substation at the power plant. This substration uses large transformers to convert the generator's voltage (which is at the thousands of volts level) up to extremely high voltages for long-distance transmission on the transmission grid. Typical voltages for long distance transmission are in the 155,000 to 765,000 volt range in order to reduce line losses. A typical maximum transmission distance is about 300 miles. High-voltage transmission lines are quite obvious when you see them - they are normally made of huge steel towers. All power towers always have three wires for the three phases. Many towers have extra wires running along the tops of the towers. These are ground wires and are there primarily in an attempt to attract Lightning. For power to be useful in a home or business, it comes off the transmission grid and is stepped-down to the distribution grid. This may happen in several phases. The place where the conversion from "transmission" to "distribution" occurs is in a power substation. A power substation typically does two or three things: It has transformers that step transmission voltages (in the tens or hundreds of thousands of volts range) down to distribution voltages (typically less than 10,000 volts). It has a "bus" that can split the distribution power off in multiple directions. It often has circuit breakers and switches so that the substation can be disconnected from the transmission grid or separatedistribution lines can be disconnected from the substation when necessary. In many suburban neighborhoods, the distribution lines are underground and there are green transformer boxes at every house or two. There is a bare wire running down the pole. It is a grounding wire. Every utility pole on the planet has one. If you ever watch the power company install a new pole, you will see that the end of that bare wire is stapled in a coil to the base of the pole and therefore is in direct contact with the earth 6 to 10 feet underground. It is a good, solid ground connection. If you examine a pole carefully you will see that the ground wire running between poles (and often the guy wires) are attached to this direct connection to earth ground.
There are two wires running
out of the transformer and three wires running to the house. The two from
the transformer are insulated and one is bare. The bare wire is the ground
wire. The two insulated wires each carry 120 volts, but they are 180 degrees
out of phase so the difference between them is 240 volts. This arrangement
allows a homeowner to use both 120 and 240 volt appliances.The power then
enters the home through a typical circuit breaker panel.