Drop Out Fuse
In the electrical distribution utility industry a cutout
is a combination fuse and switch used in primary overhead feeder lines and taps to protect stepdown transformers from current surges and overloads.
A cutout consists of three major components:
- The cutout body, an open "C"-shaped frame that supports the "fuse holder" and a porcelain insulator that electrically isolates the conductive portions of the assembly from the support to which the insulator is fastened.
- The fuse holder, often called the "fuse tube" or "door", which contains the interchangeable fuse element and also acts as a simple knife switch. When the contained fuse operates or blows, the fuse holder will drop open, disengaging the knife switch, and hang from a hinge assembly. This hanging fuse holder provides a visible indication that the fuse has operated and assurance that the down-stream circuit is electrically isolated.
- The fuse element, or "fuse link", is the replaceable portion of the assembly that operates due to high electrical currents.
The fuse elements, or fuse links used in most distribution cutouts are tin or silver alloy fuse links that melt (or operate) when exposed to high current conditions. Ampere ratings of fuse elements vary from 1 ampere to 200 amperes.
Cutouts are typically mounted about 20 degrees off vertical so that the center of gravity of the fuse holder is displaced and the fuse holder will rotate by its own weight and dangle when the fuse blows. Tension on the fuse link normally holds an ejector spring in a stable position. When the fuse blows, this tension is released which causes the ejector spring to rapidly pull the unmelted stub of the fuse link out of the fuse holder tube to reduce surge duration and damage to the transformer and fuse holder. This quenches any arc in the fuse holder.
Each fuse holder typically has an attached pull ring that can be hooked by a long fiberglass hookstick operated by a lineworker from the ground, to manually open the switch. While often used for switching, the standard cutout shown is not designed to be manually opened under load. For applications where the switch is likely to be used to interrupt power manually, a "load break" version is available that has an attachment to quench the arc.
Up until the mid 1990s each manufacturer used their own dimensional standards for cutout design; by the late 1990s most cutouts were of an "interchangeable design." This design allows for the interchangeable use of cutout bodies and fuse holders manufactured by different vendors.
The fuse holder may be replaced by a solid blade, which would allow the fuse holder assembly to be used as a switch only.
Cutouts may be used with circuit breakers called reclosers which sense and interrupt brief fault currents and then automatically restore service. After a preset number of operations (such as 3 or 4) the recloser remains open.