page_203

< previous page | page_203 | next page >

Page 203

| Figure 11-5 Typical fuse-coordinated system. |

| they must have the correct ratings for carrying normal load current and for responding properly to a fault. |

| If a fault occurs in a branch line, as shown in Fig. 11-4, the fault current flows through the main part of the circuit back to the source. The magnitude of current caused by a fault at the different coordinating points depends on the resistance (or impedance for ac circuits) between the source and the point of fault. When a fault is distant from the source, the impedance of this part of the circuit is high and the magnitude of the fault current is low. Conversely, when a fault is close to the source, impedance is low and the magnitude of the fault current is high. |

| Fuses at the coordinating points furthest from the source will thus have the lowest rating consistent with the normal load at this point, whereas fuses at other coordinating points along the main line will increase in rating as they approach the source (Fig. 11-5). The characteristics of these fuses must also coordinate with those of the circuit breaker at the source and with other protective devices in their path. |

| The time-current values of the several fuses are indicated in Table 11-1. It will be noted that for the faults and fault currents (X) indicated in Fig. 11-5, the fuse at point D will clear before that at point C, that at point C will clear before that at point B, that at point B before that at point A, and that a fault close to the source at point A would clear before the circuit breaker opens. Further reference to the time-current characteristics of fuses will be made in Chap. 13. |

| Fuse Construction.. |

| Probably no part of an electrical circuit is subjected to more severe conditions than the fuse. Power fuses are specially constructed so that they will not only operate safely at voltages greater than 600 volts but must be able to operate |

< previous page | page_203 | next page >