Fuses for both indoor and outdoor circuits of over 600 V are made in a variety of styles - granular-filled, solid material-filled, vented and oil-immersed.
Granular-Filled
The granular-filled fuse is an enclosed (non-expulsion) fuse in which the fuse link is surrounded by an inert granular material such as the silver-sand current-limiting fuse. This fuse has a silver wire or link surrounded by quartz sand. As the fuse link melts, the silver vaporizes and condenses on the quartz sand which melts and then solidifies, forming an insulation "rope" within the fuse unit.
Oil-Immersed
The oil-immersed fuse is an enclosed fuse in which all or part of the fuse is mounted in oil where the oil acts as the arc-quenching medium.
Solid Material-Filled
This fuse is designed to confine the arc inside a thick-walled tube of solid boric acid. The heat from the arc causes the boric acid to give up its water of crystallization which is turned to steam by the heat of the arc. The cooling effect of the arc by this steam causes deionization. Since the arc path has now become an insulator, the current flow is interrupted and the arc quenched. Attachments such as condensers, mufflers, and discharge filters are used to contain or reduce the exhaust discharge.
Vented
The vented fuse ejects arc gases, solid particles, liquid, and other materials. That is why it is only used outdoors. The arc formed by the parting of the link is channeled to the base of the fuse tube and gases are expelled to the atmosphere.
"E" Ratings
Continuous current ratings of power fuses are expressed in "E" amperes. The "E" rating means: (1) that the fuse will carry its rated current continuously without exceeding NEMA specified temperature rise; (2) that fuses with E
ratings of 100 A or less will melt on a current of 200 to 240% of rated amps in 300 seconds and (3) that fuses with E ratings greater than 100 A will melt in 600 seconds with a current of 220 to 264% of their ampere ratings.
