The
circuit breaker is an absolutely essential device in the
modern world, and one of the most important safety mechanisms
in your home. Whenever electrical wiring in a building has too
much current flowing through it, these simple machines cut the
power until somebody can fix the problem. Without circuit
breakers (or the alternative, fuses),
household electricity would be impractical because of the
potential for fires and other mayhem resulting from simple
wiring problems and equipment failures.
In this edition of HowStuffWorks,
we'll find out how circuit breakers and fuses monitor
electrical current and how they cut off the power when current
levels get too high. As we'll see, the circuit breaker is an
incredibly simple solution to a potentially deadly problem.
Why You Need a Circuit Breaker
To understand
circuit breakers, it helps to know how household electricity
works.
Electricity is defined by three major attributes:
- Voltage
- Current
- Resistance
Voltage is the "pressure" that makes electric charge
move. Current is the charge's "flow" -- the rate at
which the charge moves through the conductor, measured at any
particular point. The conductor offers a certain amount of
resistance to this flow, which varies depending on the
conductor's composition and size.
Voltage,
current and resistance are all interrelated -- you can't
change one without changing another. Current is equal to
voltage divided by resistance (commonly written as I = v /
r). This makes intuitive sense: If you increase the
pressure working on electric charge or decrease the
resistance, more charge will flow. If you decrease pressure or
increase resistance, less charge will flow.
How does all of this come together in your home? The power
distribution grid delivers electricity from a power plant
to your house. Inside your house, the electric charge moves in
a large circuit, which is composed of many smaller circuits.
One end of the circuit, the hot wire, leads to the
power plant. The other end, called the neutral wire,
leads to ground. Because the hot wire connects to a
high energy source, and the neutral wire connects to an
electrically neutral source (the earth), there is a voltage
across the circuit -- charge moves whenever the circuit is
closed. The current is said to be alternating current,
because it rapidly changes direction. (See How Power
Distribution Grids Work for more information.)
The power distribution grid delivers electricity at a
consistent voltage (120 and 240 volts in the United States),
but resistance (and therefore current) varies in a house. All
of the different light
bulbs and electrical appliances offer a certain amount of
resistance, also described as the load. This resistance
is what makes the appliance work. A light bulb, for example,
has a filament inside that is very resistant to flowing
charge. The charge has to work hard to move along, which heats
up the filament, causing it to glow.
In building wiring, the hot wire and the neutral wire never
touch directly. The charge running through the circuit always
passes through an appliance, which acts as a resistor. In this
way, the electrical resistance in appliances limits how much
charge can flow through a circuit (with a constant voltage and
a constant resistance, the current must also be constant).
Appliances are designed to keep current at a relatively low
level for safety purposes. Too much charge flowing through a
circuit at a particular time would heat the appliance's wires
and the building's wiring to unsafe levels, possibly causing a
fire.
This keeps the electrical system running smoothly most of
the time. But occasionally, something will connect the hot
wire directly to the neutral wire or something else leading to
ground. For example, a fan motor might
overheat and melt, fusing the hot and neutral wires together.
Or someone might drive a nail
into the wall, accidentally puncturing one of the power
lines. When the hot wire is connected directly to ground,
there is minimal resistance in the circuit, so the voltage
pushes a huge amount of charge through the wire. If this
continues, the wires can overheat and start a fire.
The circuit breaker's job is to cut off the circuit
whenever the current jumps above a safe level. In the next
section, we'll find out how it does this.
Breaker Design
The simplest circuit
protection device is the fuse. A fuse
is just a thin wire, enclosed in a casing, that plugs into the
circuit. When a circuit is closed, all charge flows through
the fuse wire -- the fuse experiences the same current as any
other point along the circuit. The fuse is designed to
disintegrate when it heats up above a certain level --
if the current climbs too high, it burns up the wire.
Destroying the fuse opens the circuit before the excess
current can damage the building wiring.
The problem with fuses is they only work once. Every time
you blow a fuse, you have to replace it with a new one. A
circuit breaker does the same thing as a fuse -- it opens a
circuit as soon as current climbs to unsafe levels -- but you
can use it over and over again.
The basic circuit breaker consists of a simple
switch, connected to either a bimetallic strip or an electromagnet.
The diagram below shows a typical electromagnet design.
The hot wire in the circuit connects to the two ends of the
switch. When the switch is flipped to the on position,
electricity can flow from the bottom terminal, through the
electromagnet, up to the moving contact, across to the
stationary contact and out to the upper terminal.
The electricity magnetizes the electromagnet (click
here to find out why). Increasing current boosts the
electromagnet's magnetic force, and decreasing current lowers
the magnetism. When the current jumps to unsafe levels, the
electromagnet is strong enough to pull down a metal lever
connected to the switch linkage. The entire linkage shifts,
tilting the moving contact away from the stationary contact to
break the circuit. The electricity shuts off.
For more information about circuit breakers and other
electrical systems, check out the links on the next page.
