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.
Figure 2-16. - Circuit breaker components.
The FRAME provides an insulated housing and is used to mount the circuit breaker components (fig. 2-17). The frame determines the physical size of the circuit breaker and the maximum allowable voltage and current.
The OPERATING MECHANISM provides a means of opening and closing the breaker contacts (turning, the circuit ON and OFF). The toggle mechanism shown in figure 2-17 is the quick-make, quick-break type, which means the contacts snap open or closed quickly, regardless of how fast the handle is moved. In addition to indicating whether the breaker is ON or OFF, the operating mechanism handle indicates when the breaker has opened automatically (tripped) by moving to a position between ON and OFF. To reset the circuit breaker, the handle must first be moved to the OFF position, and then to the ON position.
Figure 2-17. - Circuit breaker construction.
The ARC EXTINGUISHER confines, divides, and extinguishes the arc drawn between contacts each time the circuit breaker interrupts current. The arc extinguisher is actually a series of contacts that open gradually, dividing the arc and making it easier to confine and extinguish. This is shown in figure 2-18. Arc extinguishers are generally used in circuit breakers that control a large amount of power, such as those found in power distribution panels. Small power circuit breakers (such as those found in lighting panels) may not have arc extinguishers.
Figure 2-18. - Arc extinguisher action.
TERMINAL CONNECTORS are used to connect the circuit breaker to the power source and the load. They are electrically connected to the contacts of the circuit breaker and provide the means of connecting the circuit breaker to the circuit.
The TRIP ELEMENT is the part of the circuit breaker that senses the overload condition and causes the circuit breaker to trip or break the circuit. This chapter will cover the thermal, magnetic, and thermal-magnetic trip units used by most circuit breakers. (Some circuit breakers make use of solid-state trip units using current transformers and solid-state circuitry.)
THERMAL TRIP ELEMENT
A thermal trip element circuit breaker uses a bimetallic element that is heated by the load current. The bimetallic element is made from strips of two different metals bonded together. The metals expand at different rates as they are heated. This causes the bimetallic element to bend as it is heated by the current going to the load. Figure 2-19 shows how this can be used to trip the circuit breaker.
Figure 2-19. - Thermal trip element action: A. Trip element with normal current; B. Contacts open.
Figure 2-19, view A, shows the trip element with normal current. The bimetallic element is not heated excessively and does not bend. If the current increases (or the temperature around the circuit breaker increases), the bimetallic element bends, pushes against the trip bar, and releases the latch. Then, the contacts open, as shown in figure 2-19, view B.
The amount of time it takes for the bimetallic element to bend and trip the circuit breaker depends on the amount the element is heated.
A large overload will heat the element quickly. A small overload will require a longer time to trip the circuit breaker.