Introduction to How Circuit Breakers Work

Introduction to How Circuit Breakers Work
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.
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.
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 (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.
Answer the following questions in your lab book.
According to the article, what do fuses and circuit breakers prevent?
What causes the wires in a building or in an appliance to heat up?
What kinds of things can cause the neutral wire in a circuit to come in contact with the hot wire?
What happens to a fuse when it is forced to carry more current than it was designed for?
What is a major disadvantage of a fuse?
What part of a circuit breaker provides the force to “break the circuit” when it is overloaded?
Fuses are designed to regulate current by the size of the wire used. What part of a circuit breaker can
be changed to allow it to carry more or less current?