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 article, 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.
2. Electricity
Basics
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.
3. Why You
Need a Circuit Breaker
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 following sections, we'll find out how
it does this.
4. Breaker
Design: Basic
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 (See How
Electromagnets Work 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.
Click on the circuit breaker to release the switch.
A bimetallic strip design works on the same principle,
except that instead of energizing an electromagnet, the high current bends a
thin strip to move the linkage. Some circuit breakers use an explosive charge
to throw the switch. When current rises above a certain level, it ignites
explosive material, which drives a piston to open the switch.
Breaker Video
See how the electromagnet throws the switch open and how the breaker
resets.
5. Breaker
Design: Advanced
More advanced circuit breakers use electronic components (semiconductor
devices) to monitor current levels rather than simple electrical devices. These
elements are a lot more precise, and they shut down the circuit more quickly,
but they are also a lot more expensive. For this reason, most houses still use
conventional electric circuit breakers.
One of the newer circuit breaker devices is the ground fault
circuit interrupter, or GFCI. These sophisticated breakers are designed
to protect people from electrical shock, rather than prevent damage to a
building's wiring. The GFCI constantly monitors the current in a circuit's
neutral wire and hot wire. When everything is working correctly, the current in
both wires should be exactly the same. As soon as the hot wire connects
directly to ground (if somebody accidentally touches the hot wire, for
example), the current level surges in the hot wire, but not in the neutral
wire. The GFCI breaks the circuit as soon as this happens, preventing
electrocution. Since it doesn't have to wait for current to climb to unsafe
levels, the GFCI reacts much more quickly than a conventional breaker.
All the wiring in a house runs through a central circuit breaker
panel (or fuse box panel), usually in the basement or a closet. A
typical central panel includes about a dozen circuit breaker switches leading
to various circuits in the house. One circuit might include all of the outlets
in the living room, and another might include all of the downstairs lighting.
Larger appliances, such as a central air conditioning system or a refrigerator,
are typically on their own circuit.
