A short circuit study determines the short circuit currents available at various points in the system. This information is necessary to evaluate the equipment being utilized in the system, to set the protective devices and to ensure that its ratings comply with the NEC 110.9, Interrupting Rating, and 110.10, Circuit Impedance and Other Characteristics. The available short circuit fault current is the key component in calculating incident energy and arc flash boundary. Hence, the calculated short circuit currents and a protective device evaluation should be included in the short circuit section of the arc flash hazard calculation study report.
After the short circuit calculations were performed, the calculated available fault currents are utilized to evaluate the new equipment Ampere Interrupting Capacity (AIC) ratings. This comparison of available fault and equipment AIC ratings is shown on the equipment evaluation forms in the short circuit study portion of the arc flash hazard calculation study report. As a result of this evaluation, all of the new equipment reviewed for the installation is adequately rated to withstand the calculated available fault currents at the respective locations in this system. Equipment evaluation forms and respective comments would also be included in the report for further discussion.
Short circuit calculations are performed on the system as defined on the accompanying one line diagram and the corresponding system data. An available fault current at the secondary of service entrance transformer(s) are used for these calculations. The calculations are performed using Short Circuit Analytic (SCA) software program for Windows PCs, developed by ARCAD INC. The SCA study performs comprehensive short circuit analysis taking into account both active and reactive parts of equipment. The program analyzes system data, converts it to per unit impedances, and simulates a bolted fault at each node to determine the equivalent impedance to the point of fault and the resultant fault current generated by the driving voltage at that point. The calculated fault currents are listed in the short circuit program summary report. The worst case values are also shown in the equipment evaluation portion of the study. The data used in the arc flash hazard calculation study report is included for verification and to serve as a record for the electrical system.
The report would include computer generated calculations of the available fault current for three phase bolted fault, unbalanced line to line and line to ground faults, and equipment evaluation table for the system. As a result of this evaluation, all of the new equipment reviewed for the installation would prove to be adequately rated to withstand the calculated available fault currents at the respective locations in the system.
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