As you review Figures 4-5 and 4-6,
realize that the powers of 10 make it easier
to work with very large or very small
numbers. These units (milliamperes and
microamperes) make it much easier to list
very, very small units of current. In the
future, we will work with much larger
units. It will be important for you to use the
powers of 10 in electricity and electronics,
so closely review Figures 4-5 and 4-6 as you
examine these mathematics principles.
Overload Protection of
Circuits
It should be quite clear that a certain
kind and size of wire has a specified ability
to conduct an electric current. All conduc-
tors have some resistance. When a current
overcomes this resistance, heat is produced.
If a wire is operated within its limitations,
this heat is dissipated in the surrounding
air and its temperature does not rise exces-
sively. However, if too great a current is
forced through the conductor, the tempera-
ture will rise to a point where the wire will
become hot. If the wire gets hot enough, the
insulation that surrounds the conductor
may melt off. The wire may even get hot
enough to melt itself and be destroyed. If it
is near combustible material, such as in the
wall of your home, a fire might result.
Overloading a circuit can occur from
two causes:
1. An excessive load that draws beyond a
safe amount of current.
2. A direct (sometimes called a dead)
short circuit.
Circuits and appliances are usually
protected by a fuse or circuit breaker. A fuse
is simply a thin strip of metal that melts at
a low temperature. Those used in the home
are usually designated 15 and 20 amperes.
(Carefully examine your fuse box or circuit
breaker panel at home. ) Note the Safety
Suggestions later in this chapter. Typically,
circuit breakers have replaced fuses in most
home and industrial applications. However,
older installations may still contain fuses,
or a combination of fuses and circuit
breakers.
If a current exceeds the fuse rating, it
will melt and open the circuit, preventing
damage of equipment and danger of fire.
The symbol for a fuse in electrical circuit
diagrams is shown in Figure 4-7.
Some fuses, of course, are made to
carry heavier currents. You will generally
find a main power panel in a home rated
between 100 and 200 amperes. In fact, as
homes utilize more and more electricity,
even larger amounts of current are
38
Electricity
Math Manipulation!
1 milliampere (mA) = 1/1,000 (.001) ampere
1 microampere (mA) = 1/1,000,000 (.000001) ampere
Figure 4-5.
Prefixes used in measuring current.
10
0
·
10
-3
·
10
-6
·
Smaller
Basic
unit
(amp)
Milliamp
(mA)
Microamp
(
μ
A)
Figure 4-6.
Conversion chart for ampere prefixes.
Figure 4-7.
The symbol for a fuse.