2-20
Q26. You have removed an open fuse from a fuseholder and repaired the cause of the fuse opening. The
parts list specifies a fuse coded F02BI25VñA. There are no fuses available with that identification.
In the following list, indicate if the fuse is a direct replacement, a good substitute, or not
acceptable. For the fuses that are good substitutes, number them in order of preference and explain
why they are numbered that way. If the fuse is not acceptable, explain why.
(a) F03BI25V½A
(b) F02BI25V $
(c) F02GR500B
(d) F02B32V½A
(e) F02DR500B
(f) F02A250V $
(g) F02AI25V½A
Q27. What two things should you check before replacing a fuse?
Q28. List the safety precautions to be observed when replacing a fuse.
Q29. What conditions should you check for when conducting preventive maintenance on fuses?
CIRCUIT BREAKERS
A circuit breaker is a circuit protection device that, like a fuse, will stop current in the circuit if there
is a direct short, excessive current, or excessive heat. Unlike a fuse, a circuit breaker is reusable. The
circuit breaker does not have to be replaced after it has opened or broken the circuit. Instead of replacing
the circuit breaker, you reset it.
Circuit breakers can also be used as circuit control devices. By manually opening and closing the
contacts of a circuit breaker, you can switch the power on and off. Circuit control devices will be covered
in more detail in the next chapter.
Circuit breakers are available in a great variety of sizes and types. It would not be possible to
describe every type of circuit breaker in use today, but this chapter will describe the basic types of circuit
breakers and their operational principles.
Circuit breakers have five main components, as shown in figure 2-16. The components are the frame,
the operating mechanism, the arc extinguishers and contacts, the terminal connectors, and the trip
elements.
