SPENT FUSE LINKS
A USED LINK HAS A STORY TO TELL
Finding cutouts open, but no signs of trouble on
the line, can be frustrating for line personnel. A link
removed from an open cutout and visually examined
is sometimes categorized as having “pulled apart.”
In reality, the link may be spent because there was a
fault and the link reacted to it. By looking for certain
characteristics on activated Chance Type T, K, and
Slo-Fast links, you can determine when links have
operated and at what general current levels.
FUSE CONSTRUCTION
To analyze the remains of a fuse link, one must first
know link construction. Different element materials,
strain wires, solder and crimp connections, heater
coils and solder junctions are all used in Chance
fuse links to control operating speeds. Those
performances are shown on published minimum
melt and total clear curves (Figure 1).
T LINK CONSTRUCTION
T links, 1 through 3 amps have stainless steel
elements mechanically crimped at the buttonhead
and cable adapter. The 6 through 100 amp links
have a similar construction except the stainless steel
wire acts mainly as a load-bearing strain member
(Photograph 1).
Figure 1
Current is carried primarily by a parallel pure tin
element which is soldered to the exterior of the
buttonhead and cable adapters at the crimps. 140
and 200 amp T links have a heavy copper element
which is soldered into the buttonhead at the top and
mechanically crimped into the cable adapter at the
bottom (Photograph 2).
K LINK CONSTRUCTION
The 1 through 3 amp K links are constructed the
same as T links. The 6 through 100 amp K links have
stainless steel strain wires to provide mechanical
strength and copper alloy elements to carry current.
These two wires are attached in parallel by crimps at
the buttonhead and cable adapter (Photograph 3).
The 140 and 200 amp K links have large silver/copper
elements which, unlike T links, are crimped both at
the cable adapter and buttonhead (Photograph 4).
SLOFAST LINK CONSTRUCTION
SloFast links have two sections (Figure 2). The slow
section has a current-carrying heater coil wrapped
around an insulated strain pin. The two in parallel are
crimped at one end in the buttonhead and at the
other in a small copper tube. The fast section has a
stainless steel strain wire and parallel copper element,
similar to a K link. These two are crimped at one end
in the cable adapter and at the other end in a small
copper tube. The two crimped copper tubes are
joined by a solder connection.
The element’s solder connections, and the strain
member’s crimped connections will be undisturbed.
In Photographs 6, 7 and 8 the auxiliary tubes did not
rupture, the tin elements and stainless steel strain
wires melted between connections, the remaining
elements have smooth surfaces, and the connections
are undisturbed. These are all signs of a normal fuse
operation at low current.
The 140 and 200 amp T links (Photograph 9) have a
solder connection at the buttonhead end. On lowlevel faults this connection will melt and pull out of
the buttonhead. This type of operation is
SMOOTH TIN ELEMENT
STRAIN WIRE
FUSE OPERATION
High level faults, 1,000 amps and up, are easy to
recognize. The fuse’s auxiliary tube is nearly gone
or totally destroyed, and the element is consumed.
When faults are in the 500 to 1,000 amp range, the
auxiliary tube will burst but remain intact. In such
cases most of the element is consumed depending
on the size of the fuse. Unlike high-level fault
incidents, lowlevel faults are sometimes hard to
accept as having occurred because of the limited
damage. In a low-level fault the auxiliary tube is not
damaged, and the element may be nearly whole. By
knowing the characteristics of a fuse link’s remains
after a low-current operation, one can recognize that
a fault has occurred. These characteristics vary with
the fuse type.
CRIMP BOTH ENDS
Photograph 1
SOLDER
COPPER ELEMENT
GENERAL STRENGTH
T, K, and SloFast links are designed to withstand
pull forces in excess of 20 lb. This is twice the
ANSI requirement of 10 lb. The actual withstand
capabilities vary from fuse to fuse. Generally, larger
links have greater capability, but type of construction
is also a big factor.
15 amp T link
SOLDER BOTH ENDS
140 amp T link
Photograph 2
CRIMP BOTH ENDS
COPPER ALLOY ELEMENT
STRAIN WIRE
Photograph 3 - 15 Amp K Link
CRIMP BOTH ENDS
SILVER COPPER ELEMENT
Photograph 4 - 140 Amp K Link
Figure 2
Photograph 5 —Whole LAuxiliary Tube
Photograph 6—10 Amp T Link
T LINK LOW-LEVEL OPERATION
T links, 1 through 100 amps, which have operated at
low fault or overload currents, have auxiliary tubes
(Photograph 5) that have not ruptured. The tin
element will melt somewhere between the soldered
connections. This transfers the full current to the
stainless steel strain member, which in turn melts in
its mid-section. After the interruption, the remaining
tin element will have its original smooth surface.
Page 2
Photograph 7 - 40 Amp T Link
Photograph 8 - 65 Amp T Link
SOLDER MELTED
Photograph 9
often mistakenly considered a “pull apart.” As long as
buttonhead and element are well wetted with solder,
this is a normal operation.
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 12.
T LINK PULL APART
A SloFast link operating in the slow section as shown
in Photograph 15 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.
T links, 6 through 100 amps, which have been
mechanically over stressed and pulled apart, will have
a stretched tin element. The surface of the element
will be rough and the element will be necked down
(Photograph 10) where it pulled apart. The strain wire
will either pull out of one of the crimps, recognized
by the bend at that end of the wire, or will break off
right at one of the crimps (Photographs 10 and 11).
T links, 1 through 3 amps, have only stainless steel
elements. If they pull apart, they will have elements
which pull out or break off like the strain wires.
The 140 and 200 amp T links are strong enough
that a pull apart (pulling out of the crimp or solder
connection) is unlikely.
SLOFAST LINKS - PULL APART
When SloFast 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 16 below,
the strain wire pulls out and the element breaks off a
little way from the crimped connection.
K LINK LOW-LEVEL OPERATION
“K” links that have operated at low fault currents
(Photograph 12) will have auxiliary tubes that have
not ruptured. Like T links, the strain wire and element
will melt somewhere between the connections,
and the crimps will be undisturbed. The amount of
element and strain wire consumed will be somewhat
proportional to the ratio of fault current to the fuse
rating, i.e., a fault of 15 times the fuse rating will
consume more element than one which is only three
times the rating.
K LINK PULL APART
K links, 6 through 100 amps, which have been
mechanically over stressed and pulled apart will have
strain wires which pull out of the crimp and/or break
off right at the crimp the same as T links (Photograph
13). The element, being much stronger than tin, will
not neck down much. Once the strain wire gives way,
the total stress is placed on the element. The element
will break at its weakest point anywhere along its
length, not necessarily at the crimp. K links, 1 through
3 amps, have only stainless steel elements. If they pull
apart, they will have elements which pull out or break
off like the strain wires shown at right (Photographs
13 and 14). The 140 and 200 amp K links are of such
strength that a pull apart is unlikely.
STRAIN WIRE
BROKEN
STRETCHED
ELEMENT
These links were mechanically pulled apart at forces above 20 lbs.
Photograph 11
STRAIN WIRE
PULLED OUT
ROUGH ELEMENT
SURFACE
Photograph 12 - 6 Amp K Link
(Fuse was operated at 22 amps, 15 kV)
Photograph 13
These links pulled apart at forces above 20 lb. The elements broke at the top crimp connection.
SLOFAST LINKS LOW-LEVEL
OPERATION
SloFast links have two sections that can operate at
currents of 500 amps and below. The “fast” section
is similar to a K link. The fuse operates when the fault
current is to the right of the “knee” of the minimum
melt curve (Figure 3). In this mode of operation the
element and strain wire will melt somewhere between
Photograph 10
Photograph 14
SOLDER MELTED
Photograph 15
Photograph 16 - 7.0 Amp SloFast Link
(This link was mechanically pulled apart at a force above 20 lbs.)
Page 3
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.
SloFast links: Element and strain wire melt between
crimps. Crimps undisturbed. Separation at solder
junction. Heater coil and strain pin intact.
Figure 3
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BR_11_002_E
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