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 of fuse.
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 fuse firmly.
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.Continue Reading