Tips & News - September 2012

Photograph11

similar to a K link. The fuse operates when the fault current is to the right of the “knee” of the minimum melt curve (see Table II). In this mode of operation the element and strain wire will melt somewhere between the crimps. The crimped sections of the element, and strain wire will be undisturbed. The remains will look similar to the K link in Photograph 10 on page 14. A Slo-Fast link operating in the slow section as shown in Photograph 12 is sometimes mistakenly considered a “pull apart.” At fault or overload currents to the left of the knee of the minimum melt curve, the heater coil transmits enough heat to the solder junction to cause the solder to melt. Separation at this point (provided both crimp tubes are wetted with solder) is a normal operation. SLO-FAST LINKS - PULL APART When Slo-Fast fuse links aremechanically over stressed and pulled apart, they will separate in the lower (fast) section. This section is constructed similar to a K link. As shown in Photograph 13 below, the strain wire pulls out and the element breaks off a little way from the crimped connection. SUMMARY By knowing what to look for, you can spot the characteristics of a normal low-level fault interruption. When the auxiliary tube has not burst, look for the following signs: T links: Element and strain wire melt in mid-section. Strain wire crimps undisturbed. Smooth surface on remaining tin element. K links: Element and strain wire melt between crimps. Crimps undisturbed. Slo-Fast links: Element and strain wire melt between crimps. Crimps undisturbed. Separation at solder junction. Heater coil and strain pin intact.

Theselinkspulledapartatforcesabove20lb.Theelements brokeatthetopcrimpconnection.

TableII

Photograph13—7.0AmpSlo-FastLink

(Thislinkwasmechanicallypulledapartataforceabove20lbs.)

Photograph12

SOLDERMELTED

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