A few years ago, the term “arc flash” crept into our electrical technical vocabulary. Since that time, performing arc flash calculations remains a challenge for many of us. Calculating incident energy levels and arc flash boundary distances for the purpose of estimating the hazard risk category (HRC) a worker would be exposed to while working on electrical equipment opens a window into the inner workings of the power distribution system. Arc flash calculations can tell us a great deal about how the system will behave during a fault condition. They also offer us a golden opportunity to optimize the system for safety and attempt to prevent the hazard from happening in the first place.

Arc flash regulations may be one of the best things that have ever happened to electrical designs, because they force engineers to look closer at details they might have otherwise overlooked in the past and put the power system calculations front and center in the design process. The very notion of considering arc flash early on in the design of a power distribution system is not only  prudent, but also economical.

The following two documents are the foundation for truly understanding arc flash calculations:

  • NFPA 70E, Standard for Electrical Safety in the Workplace, 2012 Edition
  • IEEE Std 1584, Guide for Performing Arc-Flash Hazard Calculations, 2002 Edition

In this article, we’ll concentrate on NFPA 70E instead of IEEE Std 1584. The calculations shown below will also focus on alternating current systems.