Chapter 22 Basic Electrical Tests 341
An open circuit can be caused by a:
• Broken wire conductor.
• Blown fuse.
• Burned switch contacts.
• Loose connection.
• Burned or corroded connector terminals.
• Failed electric component.
When current is not reaching a component, fi nd
out which of these potential problems is present in
the circuit. As shown in Figure 22-4, the open can
occur before the load, after the load, or in the load.
If the open occurs in series with a component, the
component will not function. However, if the open
occurs in a parallel branch, the branches that do not
contain the open circuit will still function. Refer to
Figure 22-5.
High Circuit Resistance
High circuit resistance reduces current, but it does
not stop all current like an open circuit. A high resis-
tance might make a bulb glow dimly or a motor turn
too slowly. See Figure 22-6. High circuit resistance
is usually caused by a poor electrical connection. A
corroded or burned connector terminal is the most
common cause. Faulty components can also have
high internal resistance. See Figure 22-7.
Power up
to here
No power
Open before
load
Power up
to here
Power to bulb
terminal
Open in
load itself
Open in
ground
A
B
C
Both branches dead
Open at main
feed wire
Other branch
still functions
Bulb Out
A
B
Open in
parallel
branch
Fuse
Dim bulb
Normal
resistance
Normal
illumination
Connector has
high resistance
Figure 22-4. See the action of a basic light circuit. A—If the
open occurs before the load, voltage cannot travel to the bulb
socket. B—If the open occurs after the load, the socket does
not have a return ground circuit path. C—An open in the bulb
itself is a common example of an open inside the load.
Figure 22-5. Opens in a parallel circuit may cause different
symptoms, depending on where in the circuit they occur. A—
An open in the main wire feeding the parallel branches disrupts
power to both branches. B—If the open is on one of the parallel
branches, the other branches still function normally.
Figure 22-6. A high circuit resistance is similar to an open, but
some current can still exist in the circuit. In this example, high
resistance in the top connector reduces current through that
branch. Therefore, the bulb in that branch burns dimly.