URT
URT
3. The Breaking Process 3.1 Breaking At Low and Medium Overload
The load current flowing through a fuse link results in a voltage drop according to its resistance. Multiplication of the voltage drop with the load current designates a power loss, which is in the form of heat, is transferred to the environment via the terminals and the cables. At load currents above the rated fuse current, temperatures rise at the melting element notches inside the fuse link, so that in time these notches will melt. At these interrupting spots individual arcs occur, allowing the current to flow until the arc is extinguished. In the range of low and medium fault currents where the melting time has the duration of some current half waves, an arcing time of less than 5 ms can be regarded as negligible. This melting time then corresponds to the total breaking time. 3.2 Breaking At Short Circuit Currents In the short circuit range the melting element notches melt and evaporate in nearly one millisecond due to the steep current rise. At the melted notches, arcs occur which remain active as long as a sufficient number of insulating bridges are created by the surrounding quenching medium. The arising arc voltage exceeds the value of the operating voltage however it is imited by the specific SIBA design of the melting element. The arcing time in this case can no longer be regarded as negligible, because the arcing time is greater than the melting time. Addition of melting time (t S ) and arcing time (t L ) results in the operating time (t A ). For protection of semiconductor components in this range the operating integral I 2 t A is decisive, this means the total of the melting integral and arc integral, I 2 t A must always be smaller than the limited load integral of the semiconductor element to be protected. 3.3 Voltage and Current Development At Short Circuit Currents Figure 3.3 shows the temporal development of a short circuit current at alternating and direct voltages. The voltage existing across the fuse link during the breaking process is indicated simultaneously with the current with regard to that time. The time t 0 is the beginning of the short circuit; a peak value as it is the maximum asymmetric short circuit current can be reached. After the time t s has passed, the fuse link limits the current to the value of the cut-off current I D . A breaking arc occurs, but is decreased rapidly during time t L by the influence of quartz sand. The breaking process is finished after the time t A .
Fig. 3.3 Short circuit interruption at AC and DC currents
4. Graphic Presentation of the Fuse Link Operating Behavior 4.1 Time-Current Curves(TCC)
Time-current curves demonstrates the dependence of the melting time on the fault current though the fuse link. The curves are a result of testing the melting time of non preloaded fuse links. The individual measured points are the arithmetic average value of at least three melting tests. If not otherwise noted, the tolerance of the time-current curve is ± 7% in direction of the current.
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