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 4.5.4.2 Distribution Fuses. A distribution cutout consists of a fuse support, fuse holder,
and a fuse link. There are two types of expulsion fuses used in distribution cutouts, the
fiber-tube and the open-link fuse. The fiber-tube fuse consists of a replaceable fuse link inside a
tubular fuse holder that is lined with a commercial grade of hard fiber material. This type of fuse
is used in both the open and enclosed cutouts. In the open fuse cutout shown in Figure 4-5, the
fiber-type tube fuse is mounted between the ends of a single porcelain insulator having a
centrally located mounting bracket. In another design it may be mounted between two post-type
insulators attached to a channel base. The electrical connections at both ends of the fuse holder
are exposed. In the enclosed fuse cutout the fuse clips, fuse link, fuse holder, and all electrical
contacts are completely enclosed within a porcelain housing. The fuse tube is mounted on the
inside of the cutout door. In the open-link cutout shown in Figure 4-6, the open-link fuse
consists of a fusible element enclosed in a relatively small fiber tube with cable extensions of the
fusible element extending from both ends of the tube. These cable extensions are connected to
the spring-loaded,contacts of the fuse supports. The spring action insures separation of the open
ends of the fusible member upon operation of the fuse and it is used because of the relatively
limited interrupting capability of the small fiber tube. When an open-link fuse operates, it is
necessary to replace the fusible element and the holder. The open fiber-tube cutout and the
open-link cutout are either single-element cutouts or two- or three-element repeater-type cutouts.
Following the operation of the first fusible element of a three-element cutout, the second fusible
element is automatically put in series with the circuit. If the fault still persists, the third element
operates and isolates the fault. If the fault clears from the circuit in the time interval between the
operation of the first element and prior to the operation of the third element, however, the
repeater cutout will prevent an extended outage. The open fiber-tube fuse cutout and the
open-link cutout are dropout type and give visual indication that they have been operated. The
enclosed fiber-tube cutout may either be a dropout or nondropout device.
4.5.4.3 Oil Fuses. In an oil fuse (sometimes called oil-filled) cutout the fusible element is
immersed in oil in a sealed tank, and there is no external indication that the fuse has or has not
operated. The sealed tank permits the use of this fuse where damp, corrosive, or explosive
atmospheric conditions exist, where the fuse cutout may be subject to periodic submersion, or
where exposure of live electrical parts might be hazardous. Oil fuse cutouts are designed for
underground vault, pothead, cubicle, or pole-top installation and may be used for both indoor and
outdoor applications. The use of oil as the dielectric makes it possible to interrupt relatively high
fault currents as compared to the fault capability of a distribution cutout using a fiber-tube fuse.
Oil fuse cutouts can also be combined with current-limiting fuses in double compartment
enclosures. These oil fuse interrupter switches provide medium loadbreak and high fault current
interrupting capability.
4.5.5 Operation. A fuse protects electric circuits and equipment from damage within the
limits of their ratings. Successful protection depends not only upon the manufacturing quality
and correct application, but also on the regular monitoring of fuses. Failure to properly monitor
fuses may result in damage to costly equipment. It cannot be stressed too strongly that prescribed
safety rules should be adhered to at all times when operating fuses
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