Mersen’s Effect of Harmonic Currents on Semiconductor Fuse Ratings white paper details a study calculates the current density distribution for typical arrangements of fuse strips as a function of frequency and discusses common methods of de-rating fuses at elevated frequencies.

Excerpt:

The continuous current ratings of semiconductor fuses are determined in standard “type” tests with specified conditions. In real applications the conditions are always different, and it is well known that adjustments have to be made to allow for ambient temperature, size of connecting cables and any aircooling. Standard type tests are done at 50 or 60Hz, but in modern power electronic applications the frequency may be much higher. It is not widely recognized that this may require de-rating of the fuses. A recent paper [1] described problems caused by premature opening of fuses in the a.c. line inputs to adjustable-speed drives which drew unusually high harmonic currents.

Semiconductor fuse elements are made from multiple thin silver strips, and at higher frequencies skin effect causes crowding of the current at the edges of the strips [2]. More importantly, proximity effect causes an additional shift of current towards the return bus. The resulting non-uniform
current distribution produces additional heating and a need for de-rating the fuse. The issue was first described by Howe & Jordan [3], who represented the fuse elements with an equivalent wire-grid array to calculate the magnitudes of skin and proximity effects. More recently Duong and others [4,5] used FEA and wire grid array methods to study the distribution of currents in strips with various orientations. Their calculations were also…>>Read More