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Fuse short-circuit performance characteristics are published in the form of peak let-through (Ip) graphs and I 2t graphs. Ip (peak current) is simply the peak of the shaded triangular waveform, which increases as the fault current increases, as shown in Fig. 1.34(b). The electromagnetic forces, which can cause mechanical damage to equipment, are proportional to Ip2 . I 2t represents heat energy measured in units of A2 s (ampere squared seconds) and is documented on I 2t graphs. These I 2t graphs, as illustrated in Fig. 1.34(c), provide three values of I 2t: minimum melting I 2t, arcing I 2t,. | FIGURE 1.33 Time-current characteristic curves. Fuse short-circuit performance characteristics are published in the form of peak let-through Ip graphs and I2t graphs. Ip peak current is simply the peak of the shaded triangular waveform which increases as the fault current increases as shown in Fig. 1.34 b . The electromagnetic forces which can cause mechanical damage to equipment are proportional to I2. I21 represents heat energy measured in units of A2 s ampere squared seconds and is documented on I21 graphs. These I21 graphs as illustrated in Fig. 1.34 c provide three values of I21 minimum melting I21 arcing I2t and total clearing I2t. I21 and Ip short-circuit performance characteristics can be used to coordinate fuses and other equipment. In particular I21 values are often used to selectively coordinate fuses in a distribution system. 2000 by CRC Press LLC FIGURE 1.34 a Fuse short-circuit operation. b Variation of fuse peak let-through current Ip. c 12t graph. Selective Coordination In any power distribution system selective coordination exists when the fuse immediately upstream from a fault operates leaving all other fuses further upstream unaffected. This increases system reliability by isolating the faulted branch while maintaining power to all other branches. Selective coordination is easily assessed by 2000 by CRC Press LLC comparing the I2t characteristics for feeder and branch circuit fuses. The branch fuse should have a total clearing I2t value that is less than the melting I21 value of the feeder or upstream fuse. This ensures that the branch fuse will melt arc and clear the fault before the feeder fuse begins to melt. Standards Overload and short-circuit characteristics are well documented by fuse manufacturers. These characteristics are standardized by product standards written in most cases by safety organizations such as CSA Canadian Standards Association and UL Underwriters Laboratories . CSA standards and UL specify product designations dimensions
