2-15
Q19. Label the fuseholders in figure 2-12.
Q20. Which connector should you use to connect the (a) power source and (b) load to the fuseholder
shown in figure 2-12(A)?
Figure 2-12.—Fuseholder identification.
CHECKING AND REPLACEMENT OF FUSES
A fuse, if properly used, should not open unless something is wrong in the circuit the fuse is
protecting. When a fuse is found to be open, you must determine the reason the fuse is open. Replacing
the fuse is not enough.
Before you look for the cause of an open fuse, you must be able to determine if the fuse is open.
CHECKING FOR AN OPEN FUSE
There are several ways of checking for an open fuse. Some fuses and fuseholders have indicators
built in to help you find an open fuse; also, a multimeter can be used to check fuses. The simplest way to
check glass-bodied fuses, and the method you should use first, is visual inspection.
Visual Inspection
An open glass-bodied fuse can usually be found by visual inspection. Earlier in this chapter, figures
2-4 and 2-5 showed you how an open plug-type and an open glass-bodied cartridge-type fuse would look.
If the fuse element is not complete, or if the element has been melted onto the glass tube, the fuse is open.
It is not always possible to tell if a fuse is open by visual inspection. Fuses with low current ratings
have elements that are so small, it is sometimes not possible to know if the fuse link is complete simply
by looking at it. If the fuse is not glass-bodied, it will not be possible to check the fuse visually. Also,
sometimes a fuse will look good, but will, in fact, be open. Therefore, while it is sometimes possible to
know if a fuse is open by visual inspection, it is not possible to be sure a fuse is good just by looking at it.
Fuse Indicators
Some fuses and fuseholders have built-in indicators to show when a fuse is open. Examples of these
open-fuse indicators are shown in figure 2-13. Figure 2-13, view A, shows a cartridge-type fuse with an
open-fuse indicator. The indicator is spring loaded and held by the fuse link. If the fuse link opens, the
spring forces the indicator out. Some manufacturers color the indicator so it is easier to see in the
open-fuse position.
