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【正文】 a thermal heat capacity c. Moreover, suppose the current is I and that it flows for a period t that is typically less than 15 seconds. The heat generated in the conductor is given by Form ,we can calculate the temperature rise for a given value of :hence from which If follows that for a given conductor the temperature rise depends upon the I factor . It is well known that high temperature damage the insulation that covers a conductor. The I factor is ,therefore ,very important because it determines the temperature rise under shortcircuit conditions. For example , a copper conductor, initially at a temperature of 90,cannot endure an I factor in eccess of 22s if its temperature is to be limited to 250during a shortcircuit.In general, the I factor can be calculated knowing (a) the cross section of the conductor, (b) its position(copper or aluminum), and (c) the maximum temperature it can tolerate. The I factor for copper and aluminum conductors are given by the following equations:for copper conductors, for aluminum conductors, where I =shortcircuit current duration of the shortcircuit A = net crosssection of conductor without counting the empty spaces initial temperature of conductor final temperature of conductor Example 261___________________________________________An overhead line made of aluminum conductor AWG has a crosssection of . Under normal conditions this conductor can continuously carry a current of 160 A. Calculate the manximum permissible I factor during a shortcircuit, knowing that the initial temperature is 80and that the manximum temperature should not exceed 250. A manximum shortcircuit of 2000 A is foreseen on this overhead line. For how long can it circulate without exceeding the 250 temperature limit?Solution Using we find =) =7 The 2000 A current can flow for a time t given by I=7 2000 =7t = Example 261___________________________________________It is proposed to use a AWG copper wire as a temporary fuse. If its initial temperature is 50, calculate the following : The I needed to melt the wire ( copper melts at 1083)The time needed to melt the wire if the short circuit current is 30 A SolutionForm we have =197 A For a current of 30 A we obtain I=197 30 t=Thus, the fuse will blow in approximately 220ms. The role of fusesIn order to protect a conductor from excessive temperature during a shortcircuit, a fuse must be placed in series with the conductor. The fuse must be selected so that its I rating is less than that which will protect an excessive temperature rise of the conductor. In effect, we want the fuse to blow before the conductor attains a dangerous temperature, usually taken to be 250. In practice, the I
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