Enclosed Distribution Cutout An enclosed distribution fuse cutout is one in which the fuse clips and fuse holder are mounted completely within an enclosure (fig. 4-34, view A). A typical enclosed cutout has a porcelain housing and a hinged door supporting the fuse holder. The fuse holder is a hollow vulcanized-fiber expulsion tube. The fuse link is placed inside the tube and connects with the upper and lower line terminals when the door is closed. When the fuse blows or melts because of excessive current passing through it, the resultant arc attacks the walls of the fiber tube, producing a gas that 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.

Open-link Distribution Cutout This type of cutout differs from the open cutout in that it does not use the fiber expulsion tube (fig. 4-34, view B). The fuse link is supported by spring terminal contacts. An arc-confining tube surrounds the fusible element of the link. During fault clearing, the spring contacts provide link separation and arc stretching. The arc-confining tube is incorporated as part of the fuse link.

Open Distribution Cutout Open cutouts are similar to the enclosed types, except that the housing is omitted (fig. 4-34, view C). The open type is made for 100-or 200-amp operation. Some cutouts can be up rated from 100 to 200 amps by using a fuse tube rated for 200-amp operation. 

Lightning Arresters Lightning arresters are designed to permit normal circuit operations at designed voltages, yet conduct any potentially destructive higher voltage, such as lightning

Figure 4-33.- Distribution cutout.

produces or transient currents, to ground without injury to line insulators, transformers, or other connected equipment. Use of lightning arresters is essential in all areas of power line construction. These include distribution, secondary, intermediate, and station distribution. The four different specifications of arresters, mentioned above, have different sparkover voltages, current discharge capabilities, and maximum surge discharge capabilities.

Figure 4-34.- Distribution cutouts (fused).

Secondary arresters are used on service and other low-voltage alternating-current circuits. Distribution arresters are used on primary distribution systems to protect insulators, distribution transformers, and other equipment. Intermediate type of lightning arresters are often used on substation exit cables and other locations on the distribution system, needing a high level of lightning and surge protection. Substation types of arresters are used in substations and generating stations to provide a high level of surge protection for the major pieces of equipment. Surge voltages can be generated by operating switches in the electric transmission system as well as by lightning.

Various types of lightning arresters are in use today. The valve, pellet, and air gap (fig. 4-35) are the most common and likely-to-be-seen types in the field. 

SWITCHES A switch is used to disconnect or close circuits that may be energized. High-voltage switches are operated remotely using a variety of mechanisms or manually. Depending on their purpose in the system and their physical makeup, switches are divided into three general classes: air, oil, and vacuum switches. These three classes can be further subdivided (depending on their function) into what is referred to as disconnects, circuit breakers, or reclosers.