Short Circuit Current Calculation
By Steve Hudgik
NEC 110 requires that short circuit current calculations, sometimes also called available fault current calculations, be done for all electrical equipment and panels. This is commonly called a short circuit study. In addition to NEC required labeling, the results of this study are used to specify interrupting rating requirements and to ensure electrical components and people are appropriately protected.
The two most common standards for short circuit current calculations are the ANSI/IEEE C37.010-1979 standards and the IEC 60909 standard (International Electrotechnical Commission).
The ANSI C37.010 Standard
The ANSI standard was originally created for power circuit breaker selection, which is one of the key purposes of a short circuit current calculations. However, the results of ANSI calculations also provide the information needed for NEC 110 required available fault current labeling.
The IEC 6090 Standard
The IEC 60909-3:2009 standard's origin is more generic. It was created to provide general guidelines for short-circuit current computations in any unbalanced short circuit in high-voltage three-phase 50 Hz or 60Hz A.C. electrical systems.. IEC 6090, is sometimes referred to just as IEC 909.
For example, the IEC 60909 short circuit current calculations are suitable for determining the currents during two separate simultaneous line-to-earth short circuits in an isolated neutral or resonant earthed neutral system. They are also suitable for calculating the partial short-circuit currents flowing through earth in case of single line-to-earth short circuit in a solidly earthed or low-impedance earthed neutral system.
Either the ANSI or the IEC short circuit calculation methods can be used. They have been compared and found to produce similar results. The ANSI method is commonly used in short circuit current calculation software. Some feel that the IEC method lends itself to manual calculations.
Short circuit current calculations must be done individually for all electrical equipment and panels. For example, the calculations take into consideration that rotating equipment, and in particular large motors, will impact the system impedance as a function of time. They must also take into consideration that faults may be balanced, un-balanced or open conductor (series) faults.
The results of short circuit current calculations are used to determine interrupting requirements for power circuit breakers, as well as for protective relaying system design. They are also used to provide the information for NEC required available fault current labeling.
Real-world advice and insight into preventing arc flash accidents.
NEC 110 Required Labels
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