Arc Flash Hazard Calculations: Myths, Facts and Solutions

The scope of a typical arc flash hazard analysis often extends well beyond the conventional limits of a short circuit evaluation. When evaluating equipment such as motor control centers (MCCs) and bus ducts, each individual feeder circuit should be evaluated to determine the incident energy for the downstream protected locations. Depending on the impedance between the overcurrent protection and the downstream location considered, the arcing fault current may be reduced to the extent that the overcurrent device will respond relatively slowly. To evaluate every cable drop from a bus duct and every feeder circuit in an MCC requires extensive effort that is often avoided by implementing inaccurate assumptions and invalid shortcuts. As a result, this paper considers the possibility of limiting the required calculations by making accurate generalizations to identify a reduced number of circuits that merit a detailed analysis