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| 8 Circuit Breakers: Design and Construction |

| Function |

| Switching operations are a necessary part of any system of electrical circuits, primarily those used for the opening and closing of circuits, a function usually called circuit breaking. Circuit-breaking equipment varies from very simple devices for opening low-voltage circuits that carry small currents to large and very elaborate devices for interrupting high-voltage circuits that transport large currents and great amounts of energy. The term circuit breaker, however, is usually applied to heavy-duty devices capable of interrupting comparatively large electric currents safely. |

| Circuit breakers are designed not only to carry and interrupt the normal load currents flowing in circuits, but also, and more important, to interrupt any abnormally high current that might flow under fault conditions such as a short circuit. Although the same device is used both to close and open a circuit, the requirements associated with its opening under abnormal conditions are what determine many of its principal construction features and materially affect its cost. |

| Interrupting Duty |

| When an electric current flows through a conductor, a magnetic field is produced about that conductor, the intensity of which depends upon the intensity of the current (Fig. 8-1A). When two or more such current-carrying conduc- |

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