Electrical Expulsion Drop Out Fuse / Ceramic 100a Cut Out Fuse 61X34X11.5

two basic types of fuses

At the present time, two basic types of fuses are employed, the expulsion fuse and the current limiting fuse. Each type employs a fusible element designed to melt when a current of a predetermined magnitude and duration passes through the element.

Expulsion type and current-limiting type fuses effect interruption of overcurrents in a radically different manner. The expulsion type fuse interrupts overcurrents through the deionizing action of gases that are liberated when the fusible element melts. The current-limiting type interrupts overcurrents when the arc that is established by the melting of the fusible element is subjected to the mechanical restriction and cooling action of a sand filler that surrounds the fusible element.

The earliest of these two types of fuses was the expulsion fuse. An expulsion fuse typically employs a relatively short length of a fusible element (within what is commonly termed a "fuselink") contained within a tubular enclosure that is part of a larger assembly known as a "fuseholder". The enclosure used in the expulsion type fuse is lined with an organic material, such as bone fiber. Interruption of an overcurrent takes place within the fuse by the deionizing and explosive action of the gases which are liberated when the liner is exposed to the heat of the arc that is created when the fusible element melts in response to the overcurrent. The operation of the expulsion-type fuse is characterized by loud noise and violent emission of gases, flame and burning debris, all of which pose a danger to personnel who may be in close proximity to the fuse when it operates. Because of its violent mode of operation, this type of fuse has generally been restricted to outdoor usage only. Even when employed outdoors, the expulsion fuse must be mounted well away from the equipment it is intended to protect, as well as other equipment, due to the explosive nature of its operation and its tendency to inject ionized gases into insulating spaces. Further, expulsion fuses mounted on distribution system poles have been known to initiate grass fires resulting from the flaming debris which may be expelled.

Another inherent disadvantage of the expulsion type fuse is that it requires 1/2 or sometimes 1 full cycle of current before the fuse clears a high current fault. During this time, the equipment the fuse is designed to protect must endure the full available fault current that is allowed to pass through the fuse to the equipment. Potentially damaging energy that will be dissipated in the equipment will be proportional to the formula I2 T, where I is the magnitude of the overcurrent and T is the time that the current condition exists. Additionally, the high current that an expulsion fuse allows to flow prior to its interruption at a system current zero tends to cause bothersome voltage dips upon the network causing lights to flicker and sensitive computers and electronic equipment to suffer. Further, expulsion fuses may not clear the overcurrent condition soon enough to prevent sectionalizing fuses, reclosers or other protective relays and circuit devices from also sensing the overcurrent and responding by temporarily and sometimes permanently disconnecting other portions of the network.