Time-current characteristic

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Current-time characteristic with tripping times, representation of the tolerance bands. The behavior described in this example is achieved by combining two tripping methods - thermal and magnetic release.

A time-current characteristic is a characteristic that shows the switch-off time of overcurrent protection devices ( fuses , miniature circuit breakers , self-resetting PTC fuse elements) at different current levels.

In order to be able to display both very short switch-off times at very high currents and long switch-off times at low currents, a double logarithmic grid is used for this. The abscissa typically defines the tripping current standardized to the nominal current from 1 to approx. 10,000 times the nominal current, the ordinate the switch-off time from 0.001  s to 10,000 s.

As an alternative to these diagrams, the product of the square of the current I 2 and the time t , also known as the melting integral , can be used to dimension fuses for consumers with high inrush currents . This information is provided by many fuse manufacturers. The value represents a measure of the specific energy that just does not lead to tripping, since the thermal output (current heat) of the fuse element depends on the square of the current. In contrast to the time-current characteristic, which in principle can be used to represent any curve shapes of the time dependency, the melting integral, which is only expressed from one value, cannot describe the behavior at long times, long in comparison to the thermal time constant of the fuse element. Additional information is then required in the description for this.

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