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| Figure 11-1 Oscillogram showing clearing time. |

| several hundred amperes. Power fuses are designed for voltage values in the thousands of volts and short-circuit currents in the thousands of amperes. Only high-voltage or power fuses will be discussed here. |

| Power Fuses |

| Time-Clearing Characteristics |

| Power fuses are rated in terms of voltage, normal current-carrying ability, and interruption characteristics expressed as time-current relationships. It is obvious that a fusible link will take less time to melt with a larger current than with a smaller one, and the relationship between the minimum melting time and the current applied is usually portrayed in a curve. To this curve must be added arcing times for the various current values. The melting time and arcing time values are obtained from tests carried out by the manufacturer and represent the average values of a number of tests. Moreover, the minimum melting-time curve is adjusted by increasing the minimum time values by a predetermined percentage to allow for manufacturing tolerances and to assure a safe and positive functioning of the fuse by providing adequate clearing time. The second curve thus developed will indicate the maximum clearing time for that particular fuse size and type (Fig. 11-2). |

| A similar set of curves is obtained for each of the different ratings and types of fuses (Fig. 11-3). |

| Fuse Coordination. |

| In an extended electric circuit, such as those that may be found in large commercial and industrial installations or in electric utility systems, it is desirable, if not essential, that the outages caused by a fault, short circuit, or |

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