2-5
Circuit Breakers
While a fuse protects a circuit, it is destroyed in the process of opening the circuit. Once the problem
that caused the increased current or heat is corrected, a new fuse must be placed in the circuit. A circuit
protection device that can be used more than once solves the problems of replacement fuses. Such a
device is safe, reliable, and tamper proof. It is also resettable, so it can be reused without replacing any
parts. This device is called a CIRCUIT BREAKER because it breaks (opens) the circuit.
The first compact, workable circuit breaker was developed in 1923. It took 4 years to design a device
that would interrupt circuits of 5000 amperes at 120 volts ac or dc. In 1928 the first circuit breaker was
placed on the market. A typical circuit breaker and the appropriate schematic symbols are shown in figure
2-2.
Figure 2-2.—Typical circuit breaker and schematic symbols.
Q6. How are circuit protection devices connected to the circuit they are intended to protect and why
are they connected in this way?
Q7. What are the two types of circuit protection devices?
Q8. Label the schematic symbols shown in figure 2-3 below.
Figure 2-3.—Schematic symbols.
