Calculating Arc Flash Energies for Circuits < 250V
The 2002 IEEE-1584 Guide for the calculation of Arc Flash Hazards provides equations for the calculation of arc flash incident energies for equipment rated 208V to 15 kV. The equations are used in power system analysis software when performing arc flash hazard studies. The guide has qualifiers for systems rated less than 240V served by transformers less than 125 kVA due to the lack of relevant test data in this area. NFPA 70E 2009 also provides qualifiers for determining arc flash hazards for systems rated <240V. Neither the Standard nor the Guide address key issues relevant to providing arc flash hazard incident energy calculations for circuits rated <250V served by transformers rated <125 kVA. Engineers performing arc flash hazard studies using power system analysis software are in need of detailed calculation methods for these circuits.
Recent 208V arc flash testing by a utility and an equipment manufacturer has provided significant new information and insights into how arcs at this voltage operate. Based on this information, modification of the arc duration time on circuits rated <250V allow the IEEE-1584 equations to provide more accurate 'real world' incident energy calculations. This has many advantages for both the consultant and the plant maintenance group in performing arc flash hazard studies with arc flash software. These include:
1. Allowing the entire one-line diagram of the electrical system to be modeled which provides compliance with NFPA 70E 205.2, and 120.2(F)(1).
2. The modified IEEE-1584 arc duration times yield more realistic results yet remain conservative as compared to 70E Table 130.7(C)(9).
3. A maintained software model provides the easiest way to update incident energy calculations and labeling when the new IEEE-1584 calculation procedures are released in 2012-2013.
4. Some arc flash software models provide work permits and safety program information directly from the one-line.
5. Easier labeling for all equipment modeled on the one-line diagram.
Selection of a proper arc duration time should provide a balance between conservative yet realistic incident energies. A comparison of the IEEE-1584 equations and various arc duration times requires more space than is available in this paper, however additional information can be found at the EasyPower Arc Flash Hazard Resource Center.
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About the Author:
Chet Davis, P.E., is the President of ESA, Inc., and the chief designer of EasyPowerâ electrical software for the design, analysis, and safety of industrial power systems. Mr. Davis is a member of IEEE-1584 (Arc-Flash Standard) and has written chapters in many of the IEEE Color Books related to analyzing electrical power systems. Get the full article Calculating Arc Flash Energies for Systems <250V from www.easypower.com.