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| 11 Fuses.. |

| Fuses constitute a very important element in the protection of electric circuits. They are devices designed to melt at a predetermined current value and thereby serve as protection against abnormal current by opening the circuit in which they are installed. Essentially one-shot circuit breakers, they are perhaps the most common protective device now in use. |

| Principle of Operation |

| All fuses work on the same principle: a metallic fusible link that melts when more than a rated current flows through it. The factors affecting the rating of a fuse include not only the composition and dimensions of the fusible link but also the characteristics of its mounting and enclosure. The former determines the magnitude of the current and the minimum time necessary to melt the link. The latter also determines the melting time, but equally important, it determines the arcing or arc-clearing time. The sum of these two times is known as the total clearing time of the fuse (Fig. 11-1). |

| Fuses come in a variety of sizes, shapes and types, from the common, small household type that screws into a socket, to the larger cartridge types found in industrial plants, to the power fuses that are considerably more complex in construction and operation and greater in cost. The basic reasons for these differences are the amperage of the short-circuit or fault currents that need to be interrupted and the voltage at which they operate. Typical cartridge fuses operate at voltages under 600 V and can interrupt currents of |

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