What method to choose for arc flash calculations?

During the last two decades different formulas have been proposed to calculate incident energy at an assumed working distance, and the arc-flash boundary in order to determine arc rated personal protective equipment for qualified electrical workers. Among others, the IEEE Standard P 1584 Guide for Performing Arc-Flash Hazard Calculations and formulas provided in Annex D of NFPA 70E and CSA Z462 Workplace Electrical Safety Standard are the most often utilized in the industry to perform arc-flash hazard analysis.

Both methods are based on testing performed and calculations conducted for selected range of prospective fault currents, system voltages, physical configurations etc. The methods caution that they are suitable for estimating arc flash hazards only, and actual cases experienced in the field can be expected to vary from the predicted values.

Comparison of the above methods shows varying computed values for incident energy. The IEEE Standards method based on empirical equations developed through multiple tests of varying fault scenarios appears to be the most accurate one. It is applicable for systems with:

  • Voltages of 208 to 15,000V (three-phase)
  • Bolted fault current of 0.700 to 106 KA
  • Grounding of all types and ungrounded
  • Arc in open air or arc in equipment enclosures of commonly available sizes
  • Gaps between conductors of 13mm to 152 mm (0.5 to 6 inches)

The IEEE Std 1584 calculation procedure is limited to three phase arcing faults and therefore is not applicable for single phase and unbalanced arc fault analysis. IEEE paper [1] compared the measured three phase results with the calculated results given for single phase arcs. It reported that "three-phase test values of maximum incident energy for the open arcs were from 2.5 to 3 times the values predicted by the single-phase models. Three-phase test values of maximum incident energy for the arcs in the cubic box were 5.2 to 12.2 times the values predicted by the single-phase models". It was shown in [2], that the three-phase equations from IEEE 1584 can also be used for single phase arc flash analysis as long as the voltage of the single phase circuits is between 208V and 15kV, and they are likely to yield conservative results.

References

1. Predicting Incident Energy to Better Manage the Electric Arc Hazard on 600V Power Distribution Systems. IEEE Transactions on Industry Applications Vol. 36, No. 1, January/February 2000.
2. Assessing the hazards of high and low voltage single-phase arc-flash by Albert Marroquin. Electrical Safety Measures - Summer 2009.