A USED LINK HAS A STORY TO TELLSPENT FUSE LINKS

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).

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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.

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.

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.

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.

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 low-level faults this connection will melt and pull out of the buttonhead. This type of operation is

Figure 2

Photograph 3 - 15 Amp K Link

CRIMP BOTH ENDS

STRAIN WIRE

COPPER ALLOY ELEMENT

Photograph 5 —Whole LAuxiliary Tube Photograph 6—10 Amp T Link

Photograph 7 - 40 Amp T Link Photograph 8 - 65 Amp T Link

COPPER ELEMENT

SOLDER

140 amp T link

Photograph 2

15 amp T link

SMOOTH TIN ELEMENT

SOLDER BOTH ENDS

STRAIN WIRE CRIMP BOTH ENDSPhotograph 1

CRIMP BOTH ENDS SILVER COPPER ELEMENT

Photograph 4 - 140 Amp K Link

SOLDER MELTEDPhotograph 9

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often mistakenly considered a “pull apart.” As long as buttonhead and element are well wetted with solder, this is a normal operation.

T LINK PULL APART

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.

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.

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

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.

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.

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.

Figure 2

Photograph 12 - 6 Amp K Link

(Fuse was operated at 22 amps, 15 kV)

These links were mechanically pulled apart at forces above 20 lbs.

STRAIN WIRE BROKEN

STRETCHED ELEMENT

ROUGH ELEMENT SURFACE

STRAIN WIRE PULLED OUT

Photograph 13

These links pulled apart at forces above 20 lb. The elements broke at the top crimp connection.

Photograph 15

SOLDER MELTED

Photograph 16 - 7.0 Amp SloFast Link

(This link was mechanically pulled apart at a force above 20 lbs.)

Photograph 2

Photograph 1

Photograph 14

Photograph 11

Photograph 10

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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