provided at the main power panel. You will
often find in the electrical service panel in
your home fuses of the cartridge type,
rated for 100 to 200 amperes or more. These
main fuses carry the total current used by
all of the circuits in your home and give
further protection.
In Figure 4-8, a simple load in the form
of a resistor is connected across a voltage
source. If the insulation should become
worn or frayed so that wire A could touch
wire B, the sparks would fly. This is called a
short circuit. This can happen with lamp
and appliance cords.
One improved safety device is called
the circuit breaker. You will study these in
detail in the unit on magnetism. A circuit
breaker is a magnetic or thermal device that
automatically opens the circuit when an
excessive current flows. See Figure 4-9.
Circuit breakers must be manually reset
before the circuit can be used again. Circuit
breakers have three positions, on, off and
tripped. The tripped position has stopped
current because of an overcurrent condition
(a short circuit or a current above its rated
value). Needless to say, the cause of the
overload should be investigated and
removed or repaired before the current is
turned on again. Circuit breakers are replac-
ing fuses in most applications in the home
and industry. One reason is because they are
reusable, the fuse is not. As you examine
your home or school electrical service
panels, are fuses or circuit breakers used?
Circuits and Switches
There are many varieties of switches
used in electrical equipment. The student
should be familiar with the common types,
Figure 4-10.
If only one wire or one side of a line is
to be switched, the single-pole, single-throw
Chapter 4 Ohm’s Law
39
If you do look into the power panel of
your home, be careful! Be sure to touch
only the panel door to open it and close it.
Fuses or circuit breakers may have been
removed. This can expose dangerous and
fatal electrical connections! If there are
loose wires or metal conductors exposed,
do not touch the panel. If you are unsure
of the safety of the panel box, leave it
alone.You may want to have a qualified
electrician examine this possibly
dangerous situation.
Safety Suggestion!
If you were to examine the cords to
lamps and appliances at home, it is
possible that you may find a damaged one.
Needless to say, these electrical cords are
dangerous.You can receive a serious burn
from a short circuited lamp cord, as well as
the danger of an electric shock. It is best to
refer these problems to a qualified repair
person and/or electrician.
Safety Suggestion!
A
B
Power
Source
R
Figure 4-8.
A short circuit, sometimes called a direct or
dead short, across the power supply connec-
tions A and B will make the sparks fly!
Manual Circuit Breaker
Figure 4-9.
Symbol for a circuit breaker.