75 YEARS - Hubbell Power Systems

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 are mechanically
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.
Photograph 11
These links pulled apart at forces above 20 lb. The elements
broke at the top crimp connection.
Photograph 13—7.0 Amp Slo-Fast Link
(This link was mechanically pulled apart at a force above 20 lbs.)
Photograph 12
Table II
SOLDER MELTED
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