fuse blows or melts because of excessive current
passing through it, the resultant arc attacks the walls of
the fiber tube, producing a gas which blows out the arc.
The melting of the fusible element of some cutouts
causes the door to drop open, signaling to the lineman
that the fuse has blown.
Each time the fuse blows, a small amount of the
vulcanized fiber of the expulsion tube is eroded away.
The larger the value of the current interrupted, the
more material is consumed. In general, a hundred or
more operations of average current values can be
performed successfully before the cutout fails.
Enclosed cutouts can be arranged to indicate when
the fuse link has blown by dropping the fuse holder.
The enclosed cutout is designed and manufactured for
operation on distribution circuits of 7,200 volts and
below. The standard current ratings of the cutouts are
50, 100, or 200 amps.
OPEN DISTRIBUTION CUTOUT.—Open
types of cutouts are similar to the enclosed types
except that the housing is omitted (fig. 4-33). The open
type of cutout is designed and manufactured for all
distribution system voltages. The open type is made for
100- or 200-amp operation. Some cutouts can be
uprated from 100 to 200 amps by using a fuse tube
rated for 200-amp operation.
PRIMARY FUSE LINKS.—A primary fuse link
consists of the button, upper terminal. fusible element,
lower terminal, leader, and sheath. The button is the
upper terminal and the leader is the lower terminal.
Fuse links for open-link cutouts are similar to the
primary fuse link except that open-link cutouts have
pull rings at each end. In either case, the sheath aids in
the interruption of low-value faults, and it provides
protection against damage during handling.
Figure 4-33.—Open-distribution cutout.
Standards specify the size of the fuse holder into
which the link must fit freely. Links rated 1 to 50 amps
must fit into a 5/16-inch diameter holder, 60- to 100-
amp links must fit into a 7/17-inch diameter holder,
and 125to 200-amp links must fit into a 3/4-inch
diameter holder. Links must withstand a 10-pound pull
while carrying no load current but are generally given a
25-pound test.
Fusible elements are made in a wide variety of
designs. Most silver-element fuse links use the
helically coiled construction. This construction
permits the fusible element to absorb vibration as well
as thermal shock due to current surges and heating and
cooling throughout the daily load cycle.
FUSE LINK OPERATION.—When a fault
occurs, the fusible element is melted by the excessive
current, and an arc forms across the open gap. The arc
is sustained temporarily in a conducting path of
gaseous ionized arc products. Gas pressure builds
rapidly; and this pressure, acting in conjunction with a
spring-loaded flipper at the lower end of the fuse tube,
rapidly ejects the fuse link leader, lengthening and
cooling the arc. For low values of fault current, the arc
acts on the fuse link sheath, generating considerable
amounts of deionizing gases. When the current passes
through the next zero value, as it changes the direction
of flow, the arc is interrupted (fig. 4-34). As the voltage
increases again across the opening in the fuse link, the
arc attempts to reestablish itself. A restrike, however,
is prevented by the deionizing gases which will have
rebuilt the dielectric strength of the open gap. For large
values of fault current, the sheath is rapidly destroyed,
and the arc erodes fiber from the inner wall of the
cutout tube. generating large amounts of deionizing
gas. During the fault interruption process, the cutout
expels large amounts of gas under high pressure as well
Figure 4-34.—Diagram of voltages, current, and timing
reference recorded with an oscillograph to show fuse
operation.
4-22
