URT
URT
The limited operating ranges of ultra-rapid fuse links are indicated by the dotted part of the curve. This is the range of load currents where the melting elements react, but the fuse links may not interrupt due to thermal reasons. From these broken line curves it is obvious that this group belongs to back-up fuses of operating class aR. Due to a self heating of semiconductor fuses at currents with melting times higher than 30 seconds, these over currents have to be limited with preconnected protection devices. A parallel curve with a value 80% of the dashed ones indicates the maximum acceptable interrupting current and response time of each protecting device. This is to limit the contact and insulation temperature of the cable and the fuse holder, if fitted. 4.2 Cut-Off Current Diagram The current limitation diagram is provided to determine the cut-off current I D and the maximum asymmetric short circuit current I s , depending on the initial short circuit alternating current I K" to be expected. The diagonal line in the diagram represents the possible peak value of the short circuit current to be expected according to the simple relation: I s =I K" * 2, should a fuse link not be used. Of greater importance for the user are the individual current limitation curves of the fuse link rated current. The value of the cut-off current in respect of the dynamic load in cases of short circuit is an important figure for the design of the device to be protected. 4.4 Switching Voltage Diagram Usually, the switching voltage is given at the fuse link rated voltage. For operation of ultra-rapid links with a load voltage smaller than the rated voltage, reduction of the switching voltage has to be fuse calculated. The diagram allows the determination of the arc voltage U L in relation to the loading voltage U B . 4.5 Values At high failure currents with melting times less than 5ms, an indication of the actual melting or switching times is no longer possible due to the adiabatic process inside the fuse link. In this case, the melting integral has to be used for the melting process and the value of the operating integral stands for the sum of the extinguishing and melting process. Here the impulse is defined I 2 t A , which is a constant for each rating. I 2 t A is composed of two parts; I 2 ts, the current impulse needed to melt all of the element's notches, and I 2 t L , the impulse current of the arc during the extinguishing process of the fuse. Note that impulse I 2 t A also passes through the semiconductor to be protected. In order to protect the semiconductor component from catastrophic failure, the rupture I 2 t of the device must be obtained. I 2 t A must be less than the I 2 t rupture value of the semiconductor component. As the energy impulse for melting is always constant for a certain rated current, this value is shown in the respective diagram as a statistic value. The correction curve diagram allows calculation of the operating integral at different operating voltages by giving a coefficient which is then multiplied by the operating integral given in the datasheet. 4.6 Operating Classes The respective range of currents, over which the fuse link can operate, determines the operating classes. aR - Back-Up Semiconductor Protection Partial range breaking capacity fuse links, which are able to carry continuous currents from the lowest up to their rated current. The fuses are able to interrupt currents above a certain multiple of their rated current and up to the breaking current. gR - Full Range Semiconductor Protection Full range breaking capacity fuse links, which are able to carry continuous currents from the lowest up to their rated currents and are able to interrupt currents from the minimum melting current up to the rated breaking current. gRL(gS) - Fuses For the Simultaneous Protection of the Semiconductors and Cables Full range SIBA fuse links with the same behavior as the above mentioned fuses of class gR. In addition, this fuse is suitable for the overload protection of cables. Instead of using a semiconductor protection fuse and a cable protection fuse, it may bepossible to use only one fuse of class gRL hence saving on space and cost.(See a special report on our website)
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