Nothing is more frustrating than having a distribution fuse link on a utility feeder blow for no apparent reason. The outage that results is both time consuming and expensive to repair and can lead to unhappy customers. Many people believe that nuisance fuse-link blowings are somehow caused by poor design or sloppy assembly. While there are indeed significant differences between manufacturers in the design and quality of their distribution fuse links, many nuisance outages can be eliminated simply by giving extra care to the installation of the fuse link in the cutout fuse tube.
Three common reasons for nuisance fuse-link blowings are:
- Improper tightening of the fuse link’s removable button head (for cutout fuse tubes without arc-shortening rods).
Solution: Fully tighten the fuse link’s removable button head. - Incomplete tightening of the fuse tube cap.
Solution: Fully tighten the fuse tube cap. In this case, tightening the cap by hand won’t cut it… you have to go back to your toolbox and grab a wrench. - Improper tightening of the fuse link into the arc-shortening rod (if the fuse tube is so equipped).
Solution: Fully tighten the fuse link into the arc-shortening rod.
In all three cases, a high-resistance current-carrying joint or “hot spot” is created. The excess heat will cause the fuse link to behave as if it is heavily preloaded which will result in a dramatic shift in the fuse link’s minimum melting curve. Under these conditions, a transient current that would not normally affect the fuse link will cause it to blow unexpectedly.
The thermal run-away described above can occur in virtually any fuse link application; it’s most common on lateral fuses and especially capacitor-bank fuses since the continuous currents are higher in these applications than for transformer-fusing applications.
A little extra time and attention to detail applied to installing fuse links can go a long way in minimizing nuisance outages due to fuse-link blowings.
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