Few Things about Electricity

Electrical power is a little bit like the air you breathe: You don't really think about it until it is missing. Power is just "there," meeting your every need, constantly.It's only during a power failure, when you walk into a dark room and instinctively hit the useless light switch, that you realize how important power is in your daily life.You use it for heating, cooling, cooking, refrigeration, light, sound, computation, entertainment... Without power, life can get somewhat cumbersome.Power travels from the power plant to your house through an amazing system called the power distribution grid.

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 or wind 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.The three-phase power leaves the generator and enters a transmission substation at the power plant. This substation 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.No matter what it is that spins the generator, commercial electrical generators of any size generate what is called 3-phase AC power.

Why three phases? Why not one or two or four? In 1-phase and 2-phase power, there are 120 moments per second when a sine wave is crossing zero volts. In 3-phase power, at any given moment one of the three phases is nearing a peak. 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 "ground," as mentioned above? 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" that's all around you when you are walking outside. It is the dirt, rocks, groundwater, etc., of the earth.

The three-phase power leaves the generator and enters a transmission substation at the power plant. This substation 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.
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 separate distribution lines can be disconnected from the substation when necessary.

The power goes from the transformer to the distribution bus:
In this case, the bus distributes power to two separate sets of distribution lines at two different voltages. The smaller transformers attached to the bus are stepping the power down to standard line voltage (usually 7,200 volts) for one set of lines, while power leaves in the other direction at the higher voltage of the main transformer. The power leaves this substation in two sets of three wires, each headed down the road in a different direction.You will also find regulator banks located along the line, either underground or in the air. They regulate the voltage on the line to prevent undervoltage and overvoltage conditions.
And finally we are down to the wire that brings power to your house! Past a typical house runs a set of poles with one phase of power (at 7,200 volts) and a ground wire (although sometimes there will be two or three phases on the pole, depending on where the house is located in the distribution grid). At each house, there is a transformer drum attached to the pole.The transformer's job is to reduce the 7,200 volts down to the 240 volts that makes up normal household electrical service.

Fuses and circuit breakers are safety devices. Let's say that you did not have fuses or circuit breakers in your house and something "went wrong." What could possibly go wrong? Here are some examples:

• A fan motor burns out a bearing, seizes, overheats and melts, causing a direct connection between power and ground.
• A wire comes loose in a lamp and directly connects power to ground.
• A mouse chews through the insulation in a wire and directly connects power to ground.
• Someone accidentally vacuums up a lamp wire with the vacuum cleaner, cutting it in the process and directly connecting power to ground.

• A person is hanging a picture in the living room and the nail used for said picture happens to puncture a power line in the wall, directly connecting power to ground.

A fuse is a simple device designed to overheat and burn out extremely rapidly in such a situation. In a fuse, a thin piece of foil or wire quickly vaporizes when an overload of current runs through it. This kills the power to the wire immediately, protecting it from overheating. Fuses must be replaced each time they burn out. A circuit breaker uses the heat from an overload to trip a switch, and circuit breakers are therefore resettable.
The power then enters the home through a typical circuit breaker panel like the one above.
Inside the circuit breaker panel you can see the two primary wires from the transformer entering the main circuit breaker at the top. The main breaker lets you cut power to the entire panel when necessary. Within this overall setup, all of the wires for the different outlets and lights in the house each have a separate circuit breaker or fuse:
If the circuit breaker is on, then power flows through the wire in the wall and makes its way eventually to its final destination, the outlet.