Short Circuit Analysis

Using the available online diagram and data collection survey, the power system is modeled to determine the fault current that flows in the system under abnormal conditions. The source impedance and data for local generators, motors, and transformers is entered in the equipment data fields. Component database libraries are used to enter data for raceways, feeder cables, relays, breakers, fuses, panels and switchboard equipment assemblies. The system model is used to provide the fault analysis and solutions using the Comprehensive or ANSI methods to calculate the symmetrical, momentary and interrupting fault values as described in the standards for low and medium-voltage power systems.

The ANSI, UL and NEMA Standards outline the preferred method used to calculate the fault duty for electrical equipment. The equipment must withstand the thermal and mechanical stresses produced by the short-circuit current. The calculated fault duty is compared to the equipment ratings to verify that that the equipment will safely withstand the maximum available fault current. The calculated fault duty is required to adequately determine the short circuit withstand and interrupting ratings for the electrical apparatus. The results are also used to provide selective coordination the Time Current Characteristics (TCCs) of the protective devices throughout the electrical system.

In addition to the normal power mode, multiple operating scenarios can be created to determine the calculated fault duties based on the system configuration for closed-transition (utility and generator sources operate in parallel), main-tie-main (restoration of utility feeder) or UPS maintenance bypass mode and others. The calculated fault values for the different cases are analyzed and compared to determine the worst case results that are in turn compared to the equipment ratings and reported. The operating scenarios are also used in the arc flash evaluation.