Figure 5-63.Ferrule type of fuse.
The other type of fuse you will be dealing with, as a
CE, is the cartridge fuse. There are two types: the
ferrule and the knife blade. Both types are available
with replaceable or nonreplaceable fuse links. Ferrule
fuses are available in ampere ratings from 0 through 60.
Fuse panels that use the ferrule type of fuse have
specially designed fuse clips in which only ferrule types
will fit. Fuse diameter and length increase as amperage
and voltage increase. Ferrule fuses are used in circuits
up to 600 volts. Figure 5-63 shows a typical ferrule type
Fuse panels that provide distribution for high-
capacity circuits use knife-blade fuses for protection.
The fuse clips are especially designed to receive knife-
blade fuses only. Knife-blade fuses are available in
ampere ratings of 61 through 6,000. The maximum
voltage rating for knife-blade fuses is 600 volts. Figure
5-64 shows a typical knife-blade fuse.
Two factors must be considered when fuses for
circuit protection are to be selected. These are the total
current flow and the voltage of the circuit in which the
fuse is to be installed. Since the purpose of the fuse is to
protect the circuit, it must be the weakest point in the
circuit. Thus the fuse used should be rated no higher
than the lowest rated component to be protected. Before
installing a fuse in a panel; check the condition of the
Figure 5-64.Knife-blade fuse.
fuse holder or clips. These must be clean and hold the
One of the newer types of protective devices, used
more often than fuses because of the way it reacts to an
overload, is the circuit breaker. A circuit breaker trips
on an overload but can be reset to complete the circuit
again without having to be removed or replaced. Circuit
breakers are classed according to their operating
principle. They may be thermal, magnetic, or a
combination of thermal and magnetic. Figure 5-65
shows typical circuit breakers with one, two, and three
poles. Multipole breakers are designed to open all
ungrounded conductors in a circuit at the same time.
A thermal type of circuit breaker has a bimetallic
element within the breaker that responds to temperature
change. The bimetallic element is made by fusing
together two strips of dissimilar metal. Each strip has a
different expansion rate when heated. Current flowing
through the breaker generates heat, which increases as
the flow increases. The heat causes the bimetallic
element to bend and act against a latch. The breaker
mechanism is adjusted so that when the current flow
reaches a set level, the element bends enough to trip the
latch. This action opens a set of contacts to break the
circuit. The thermal type of circuit breaker is
commonly called a time lag breaker because the breaker
Figure 5-65.Typical circuit breakers.