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Page 216

TABLE 11-2 Troubleshooting Fuses And Cut-Outs

Trouble

Cause

Suggested Remedy

| Fuse blowing with no apparent cause |

| Fuse weakened as a result of being partially melted when another fuse blew at same installation |

| Adapt practice of always replacing all fuses at an installation where any one fuse blows. |

| Fuse blows during in-rush currents; motor starts; lightning |

| Fuse melts because of too much current for time applied |

| Improper fuse with too small a rating or wrong time-current characteristic must be replaced by an appropriate fuse. Also causes of high in-rushes should be checked. |

| Overheating of cut-out or mounting |

| Loose or dirty contacts in fuse carrier |

| Clean and repair contacts and cut-out of mounting. |

| Fuse carrier not properly aligned |

Realign fuse carrier.

| Current may exceed rating of cut-out or mounting; overload |

| Check and reduce load or replace with larger rated cut-out or mounting. |

| Oil leaking (oil fuse cut-out) |

| Oil level too high or cut-out mounted at angle that leaves bushing below oil level |

| Remove extra oil; straighten cut-out; check gaskets and replace if necessary. |

| Review |

| 1. Fuses serve to protect electric circuits and equipment, serving essentially as a one-shot circuit breaker. They are designed to melt at predetermined abnormal values of current flowing in a circuit. They are perhaps the most common protective device in use. |

| 2. All fuses consist of a metallic link that melts when more than rated current flows through it. The composition and dimensions determine the magnitude of the current and the minimum time to melt it; its mounting and enclosure also affect the melting time, and the arc-clearing time as well. The sum of these two values is known as the clearing time of the fuse, (Fig. 11-1). |

| 3. Power systems are divided into sections, each of which may be protected by a fuse, whose locations are known as coordinating points. If two or more are installed in a circuit, they must be coordinated so that only one section (faulted) becomes de-energized. Fuses at the coordinating points farthest from the source will have the lowest rating, while fuses at other coordinating points along the main line will increase in rating as they approach the source (Fig. 11-5). |

| 4. Fuses may be mounted in the open, or enclosed within a container that |

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