A fuse cutout is an electrical protection device utilized on overhead line systems to prevent distribution transformers from being damaged by power surges. These devices function by placing a fuse link in series with the transformer feed cables. The link passes through a hinged tubular fuse holder in such a way that it secures the fuse holder in position on the cutout body. Should the link melt and separate under overcurrent conditions, the fuse holder will drop free under its own weight, pivot down on the hinge and hang vertically, giving a clear visual indication of its status. Most fuse cutout designs also incorporate a metal ring on the fuse holder, which allows maintenance and repair personnel to manually disconnect the fuse should they be required to work on the transformer.
Pylon-mounted distribution transformers are an integral part of overhead power supply grids, feeding power tapped directly from the lines to consumer points on the ground. These transformers are particularly susceptible, as are the consumer points they feed, to overcurrent damage caused by the frequent power surges experienced on overhead power lines. The fuse cutout is a commonly-installed protective device on overhead line transformers, which not only prevents damage from power spikes, but can also be used as a switch to isolate the transformer and its distribution network. The fashion in which the fuses function also gives a clear visual indication of a fault condition from considerable distances facilitating rapid fault finding.
While there are many different variations of the basic fuse cutout concept, the core operating principles remain the same throughout. The devices typically consist of a pair of contacts separated by a heavy porcelain insulator. The lower contact is designed to accept the hinged end of a non-conductive, tubular fuse holder, which allows the holder to pivot freely on the contact assembly. The fuse holder's length allows it to make contact with the upper contact assembly, effectively forming a bridge between the two. The holder is secured to the upper contact by means of a fuse link, which passes through the fuse holder and is attached to both the upper and lower contact assemblies.
This arrangement sees the fuse link form a conductive path between the upper and lower contact assemblies while retaining the fuse holder in position between the two. The incoming power feed from the overhead line is connected to the upper contact and the output feed to the transformer to the lower contact. In this way, power is fed to the upper contact, through the fuse link to the lower contact and then to the transformer. Should a power surge occur on the overhead lines, the fuse link will melt, effectively cutting the power feed to the transformer and allowing the fuse holder to pivot on its hinge and drop away from the upper contact. Most fuse cutout assemblies are mounted horizontally or at an inclined angle to facilitate the free drop-away of the blown fuse.
The separated fuse holder then becomes a clearly visible indicator that the fuse has blown, allowing rapid response from repair crews. Most fuse cutout designs also include a brass ring mounted on the upper, non-hinged end of the fuse holder. This allows repair crews to manually disconnect the fuse and isolate the transformer if the need arises.