Download IEEE Recommended Practice for Calculating Short-Circuit by American National Standards Institute PDF

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The rate of decay differs for each source. In addition, the angles between machines begin to change as some accelerate and others slow down. The resistance and reactance of machines are fixed values based on the physical design of the equipment. Solving a system with many varying driving voltage sources becomes cumbersome. The same current can be determined by holding the voltage constant and varying the machine impedance. This interchange helps to simplify the mathematics. The value of the impedance that must be used in these calculations depends on the basis of rating for the protective device or equipment under consideration.

Consequently, simplified methods have been developed that yield conservative calculated short-circuit currents that may be compared with the assigned (tested) fault current ratings of various system overcurrent protective devices. Figure 2-6 provides a means of understanding the shape of the fault current waveform, and consequently the fault current magnitude at any point in time. The circuit consists of an ideal sinusoidal voltage source and a series combination of a resistance, an inductance, and a switch.

All rights reserved. 1 Introduction In order to calculate, with a reasonable degree of accuracy, the short-circuit current that can be expected to flow in a system, it is necessary to find an equivalent circuit for each system element that will adequately represent its performance under short-circuit conditions. Without the use of simplifying techniques, one is often faced with the necessity of solving complex differential equations to determine the short-circuit current. In this chapter, various calculating techniques will be discussed with particular emphasis placed on simplifying techniques and manipulations that will provide acceptable results using system conditions that are recognized and accepted.

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