A Graphical Approach to Incident Energy Analysis

The goal of an incident energy analysis is to determine the largest incident energy that a worker could be exposed to at a piece of electrical equipment. This is most commonly done using the IEEE 1584 method. It is a common observation when performing this analysis that minor variations in fault current can result in large changes in a protective device's operating time and the resulting incident energy. This would not be a problem if it was possible to remove all errors from the bolted fault calculation and the arcing fault calculation. Unfortunately, the arcing fault current calculation has some margin of error and the data used in arc flash models are often estimated. A graphical approach to determining the incident energy circumvents these problems and ensures that the worst-case incident energy is determined. This graphical approach is derived from graphs of incident energy based on protective device time-current curves. An examination of the resultant graphs reveals a few maximums in the incident energy curve. These maximums are caused by the protective devices curve, the minimum and maximum arcing fault currents, and any time limitations that are applied. As a result, the locations of these maximums are easily predicted allowing the maximum incident energy to be determined by performing the IEEE 1584 calculation at a few key current values.