Page 138 - Power Electronics Handbook

Overcurrent protection 131

End terminal

Figure 5.9 Construction of a high-rupturing capacity (HRC) fuse
from the fuse element to the case. The filler also extinguishes the arc by
fusing with the resultant silver vapour to form a non-conductive material
called fulgurite and distributes the high pressures generated in the fuse
link, during overload conditions, over the ceramic body.
The end cap and end terminals must give good electrical contact and
form a good fit with the inner cap. The material used is usually brass with a
high copper content. The end terminal is welded, soldered, or riveted and
soldered to the end cap. The construction used gives good electrical
contact and isolates the element from external mechanical shock and
vibration.
Power semiconductors and fast-acting fuses both exhibit the property of
withstanding almost constant values of i2f below about one cycle. This
greatly simplifies the design of protective fuses, since all that is basically
required is to ensure that the i2t rating of the fuse is less than that of the
semiconductor it is to protect, so that it will fail first. If the fuse is in the
primary of a transformer and the semiconductor cell in the secondary, then
the i2t of the fuse and semiconductor must be related by equation (5.2).