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.