Impact of Available Fault Current Variations on Arc-Flash Calculations

Prior to the arrival of the arc-flash hazard analysis and the incident energy calculations, it was common practice to perform short-circuit studies assuming an infinite source on the primary of the service transformer. With the main goal of a short-circuit study being to compare the maximum calculated short-circuit current to the short-circuit rating of protective devices, using an infinite source resulted in the most conservative short-circuit current. However, since the amount of energy available in an arc-flash incident is not only dependent on the available short-circuit current but also on the clearing time of the protective device, the assumption of an infinite source on the transformer primary will not guarantee the most conservative results for the incident energy calculations downstream of the transformer. This paper examines the effect of the utility available fault current on the incident energy calculations and proposes a new method to calculate the conservative arc-flash results in the event that the actual utility fault information is not available.