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5.2 Series or Parallel Connection of SIBA U ltra- R apid Fuse Links 5.2.1 Series Connection of Fuse Links Rectifier circuits that operate at voltage levels exceeding, the rating of a single fuse, may have the possibility of being protected by the use of two standard voltage rated fuses that are series connected (see Figure 5.2a). This kind of protection requires that the series connected fuses are of the same voltage rating, and that the sum of the fuses, voltage ratings exceeds the rectifier’s operating voltage. Fuses of the same current rating utilized in series requires that the resistance tolerance be very small. Usually this can be achieved by the use of fuses from the same production batch. In addition, it has to be ensured that the size of the available short circuit current exceeds the fuse’s rated current by a minimum of a factor of 10. 5.2.2 Parallel Connection of Fuse Links To protect circuits with loading currents higher than the current range of the available fuse ratings, two or more fuse links can be used at parallel connection (see Fig. 5.2.b). This application requires an optimal equal current load for each fuse link group with identical rated current. In addition, the inner resistances of these fuse links have to be in close tolerance. It is also important to use fuses of the same production batch.
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Fig. 5.2 Series and Parallel Connection of Fuse Links
5.2.3 Semiconductors in Parallel Operation Parallel connection of semiconductors also requires equal path currents. In general, it is sufficient to make sure that none of the semiconductors have to take a too high let-through current. Some applications, especially when several semiconductors are connected in parallel, require measures against too high closing losses of the fastest switching thyristor. The series connection of resistances in the individual branches would be such a measure. Fuse links connected in series to thyristors can take over the function of resistances. Here it is certainly necessary that all fuse links have equal resistances. In such circuits (see Fig. 5.2.c) the same current flows through the fuse link and the semiconductor cell to be protected. The load current splits synchronously, half on each side. The integral values SIBA ultra-rapid fuse links are developed and rated according to standardized conditions. In practice, fuse links are often used for semiconductor protection under conditions differing from those stipulated and it may happen that the full rated current cannot be carried or that a higher load of the fuse link is possible. The conversion coefficients in the data sheets consider such deviating conditions. Therefore, it is necessary to consider the particular behavior of the fuses cooling, surrounding area, such as heat sources etc., and also to recognize cyclic load or peak currents. 6.1 Thermal Influences of the fuse link and the semiconductor cell are to be assigned directly to each other. 6 Consideration of Practical Behavior In Inverter Circuits A higher ambient temperature may reduce the current rating of the fuse link. As the different types of fuse links react differently to high temperatures, each type is given a constant, "a". To consider a surrounding temperature above 30°C it is necessary to calculate, on the basis of "a", the coefficient "A1":
a - T (ambient- a - 30
A1 = SQRT
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