A fuse is a device that provides protection to electrical circuits by melting a fusible link when current exceeding its rating flows through it. Line and service fuses are devices that protect distribution lines and equipment from damage due to overloads and short circuits. Excessive current, or amps, causes heat that can in some instances destroy equipment. Line and service fuses protect the equipment downstream from this excessive heat and are typically found at service transformers, or feeder and tap origination points, which are where distribution lines branch off from three-phase lines. These devices can also be used as a means of de-energizing a distribution line so that maintenance can be safely performed.
The photo below shows a line fuse that is part of a fused disconnect. The fuse or fuse link as it is formally called is located inside the little grey fuse barrel. The fuse barrel/link on the right side also serves as a switch to conveniently energize or de-energize the power going downstream.
When a service crew needs to de-energize a transformer for service or repair, they use a hotstick with a hook on the end and pull down the fuse barrel/link which is hinged near the bottom portion. After the work is completed, they can then reattach the hotstick and push the fuse barrel/link back up into location to re-energize power.
If a circuit overload occurs, the fuse will melt and a loud blast is heard. When this occurs, all power downstream from the fuse is then de-energized. To restore service, a crew must first check the line for damage, and then replace the blown fuse.