I got my hands of a old NEBB (Norsk Elektriske Brown Boveri) medium voltage fuse, rated for 7200 V, 200 A and 350 MVA. My best guess from the company name is that it originates from before 1988 where NEBB was merged with ASEA into ABB.
Medium voltage fuses are used at the secondary side of distribution transformers, which are often the link from the transmission grid to the distribution grid.
Here is the video of the teardown and below are some of the pictures, which also are used in the video.
The fuse implements a Striker system that is a series of trigger mechanisms to let a spring loaded pin fly out of one end, in case of a fuse failure, to activate a mechanical shutdown or electrical feedback when there is a blown fuse due to over-voltage, over-current or too high temperature.
Here the disassembled fuse can be seen, the blown fuse element and the single small parts of the chain that has blown apart nice and evenly. Typically a single wire in a fuse will cause a over-voltage surge. To resolve the over-voltage problem, it has been necessary to divide fuse elements into sections, thereby causing them to blow gradually. In technical terms, this is achieved by punching notches into a smooth strip. The fuse element is made from pure silver (999)
Silver is the preferred material for these fuse elements. For a number of reasons relating to physical chemistry, silver ensures the cleanest break. Its low resistance, due to its relative chemical stability, makes it the ideal material for carrying an increased current without the risk of aging (operating temperature of a strip: 180 to 250 °C).
This is usually sand (quartzite), which, by vitrifying, absorbs the high levels of energy developed by the arc and combines with the silver to form an insulating compound known as fulgurite.
The purity of the sand is essential to ensure reliable breaking in all areas, as is the absence of metallic compounds and humidity. Furthermore, its initial bulk ensures that the pressure (and therefore the voltage) of the arc channel is maintained.
Its granularity is selected according to the following data, which has been drawn from
- too fine a granularity (< 20 µ) is very
detrimental as its high density, which slows down
the diffusion of the fuse metal between the
grains of sand, makes the gradual elongation
and subsequent extinction of the arc difficult
- fine granularity facilitates breaks early in the fault current but also favours over-voltages
- coarse granularity facilitates breaks late in the fault current.
Modifying the granularity also enables the time/current characteristic in the zone 10 ms, 500 ms to be made more concave.
The still intact fulgurites from the melted fuse element can be seen here in greater detail. It is really interesting to see how the inside is now a insulated glass tunnel and the small silver beads have been pressed out and fused with the sand. The structure of the fuse element creates fulgurites that has alternating white stripes containing silver beads and yellow stribes that is only sand/glass.