At the House
And finally we are down to the wire that runs past a typical 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, like this:
In many suburban neighborhoods, the distribution lines are underground and there are green
transformer boxes at every house or two.
Here is some detail on what is going on at 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. Let's look at this pole one more
time, from the bottom, to see what is going on:
There are two things to notice in the previous picture:
- 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 transformer is wired in this sort of configuration:
The 240 volts enters your house through a typical watthour meter like this so that
the power company can charge you for putting up all of these wires:
How Fuses and Circuit Breakers Work
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? For example:
When a 120 volt power line connects directly to ground, its goal in
life is to pump as much electricity as possible through the connection. Either
the device or the wire in the wall will burst into flames in such a situation.
(The wire in the wall will get hot like the element in an electric oven gets hot, which is to say
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.
- 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.
- And so on...
The power then enters the home through a typical circuit breaker panel like this one:
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. Then 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:
What an unbelievable story! It took all of that equipment
to get power from the power plant to the light in your bedroom.
The next time you drive down the road and look at the power lines, or the next time you flip on
a light, I hope you think that you now have a much better understanding of what is going on. The
power distribution grid is truly an amazing system.