Still Having Nuisance Fuse Operations? Examine Your Fuse Link Design

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Positrol Fuse Link Closeup

In a recent blog item, I described how utilities can eliminate many nuisance fuse link operations simply by doing three basic tasks. If after going through those and still some fuse link nuisance operations continue to occur on your system, it’s time to look at the design of the fuse links being used. Improperly designed fuse links’ Time-Current Characteristic (TCC) characteristics may change over time, causing the fuse links to operate when they’re not supposed to, thereby causing a nuisance operation. The operation and maintenance expense can rise quickly when having to replace the fuse links to address these nuisance operations.

To eliminate the unnecessary nuisance operation expense, the solution is to use a fuse link that does not change its TCC characteristics over time. Three major design aspects come into play to make the fuse links not change their TCC characteristics over time. They involve the fuse element’s material, the element’s shape, and the way the fuse element is attached to the rest of the fuse link components. Let’s review each below:

First is the element material. The best performing fuse link elements are made of high-content silver. Silver doesn’t change its physical characteristics until reaching 90% of its heating capacity. In addition, silver melts at a higher temperature. This enables the fuse link element to stay intact, even when carrying load currents close to its melting time.

If a metal other than silver is used, such as copper or tin, the fuse link element’s physical characteristics are susceptible to change more easily when the fuse link is carrying currents close to its melting time. A change in the physical characteristics of the fuse link element may cause a change in the fuse link’s TCC curve, causing the fuse link to operate unnecessarily.

The next aspect is the element shape. Fuse links, when in service, carry load currents that fluctuate on a regular basis. These load current fluctuations will have a mechanical stress on the fuse link element. A helically coiled fuse link element will ensure these mechanical stresses do not damage the element. A straight element cannot handle such mechanical stresses and eventually will become damaged, causing a change in the TCC characteristics. 

The last aspect is to ensure there are robust connections between various components of the fuse link. Soldering is a commonly used process to make electrical connections. The disadvantage is that solder melts at a low temperature and can be applied unevenly. Heat created by normal load currents carried by the fuse link may compromise a soldered electrical connection. This may cause the element to come apart, prompting a nuisance operation. Connections formed using mechanical pressure on the four sides, also known as swaging, eliminate this issue.

You should pick fuse links that stay intact–both physically and in terms of the TCC curve characteristics–over their course of use. The fuse links should only operate for fault currents and not for any normal load currents. That won’t occur with a copper or a tin element, a straight piece of wire, or an element connected using solder. It will occur with a silver, helically coiled element that uses swaging connections instead of solder.

Using fuse links complying with these aspects, plus doing the three basic tasks I described in my earlier blog item, should ensure your fuse links operate only when prompted by fault current, thereby potentially saving your utility millions from reduced truck rolls.

I’d be interested in learning your thoughts on the best ways to avoid nuisance fuse link operations in the Comments section below.

 

 

Spécialiste

Hemanth Jala

Date de publication

mai 22, 2018