Impact of available fault current variations on arc-flash calculations

Prior to the arrival of the arc-flash hazard analysis and 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 incident energy calculations downstream of the transformer. This paper examines the effect of utility available fault current on incident energy calculations and provides the study engineer with a list of reasonable assumptions that can be made in the event that actual utility fault information is not available.