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 in the range of 208 V to 15 000 V, three-phase (line-to-line).
  • Frequencies of 50 Hz or 60 Hz.
  • Bolted fault current (rms symmetrical):
    • 208 V - 600 V : 500 A to 106 000 A
    • 601 V - 15 000 V : 200 A to 65 000 A
  • Gaps between conductors:
    • 208 V - 600 V : 6.35 mm (0.25 in) to 76.2 mm (3 in.)
    • 601 V - 15 000 V : 19.05 mm (0.75 in) to 254 mm (10 in.)
  • Working distances of 305 mm (12 in) or larger
  • Fault clearing time: no limit
  • Enclosure dimension limits:
    • Maximum Height or Width: 1244.6 mm (49 in.)
    • Maximum Opening Area: 1.549 m2 (2401 in2)
    • Minimum Width: The width of the enclosure should be larger than four times the gap between conductors (electrodes).
  • Electrode Configurations:
    • VCB: Vertical conductors/electrodes inside a metal box/enclosure
    • VCBB: vertical conductors/electrodes terminated in an insulating barrier inside a metal box/enclosure
    • HCB: horizontal conductors/electrodes inside a metal box/enclosure
    • VOA: vertical conductors/electrodes in open air
    • HOA: horizontal conductors/electrodes in open air

IEEE 1584 Lee Equations

There are alternative calculation methods for system parameters which fall outside the range of the model. For cases with vertical electrodes in open air (VOA electrode configuration) where voltage is over 15 kV or gap between conductors is more than 254 mm (10 inches), the theoretically derived Lee method can be applied and it is implemented in Arc Flash Analytic version 6.0. The Lee method based upon Lee’s paper, is applicable for three-phase systems in open air substations, and open air transmission and distribution systems. This model is intended for applications where faults will escalate to three-phase faults. Where this is not possible or likely, this model will give a conservative result.

Single-phase systems

IEEE 1584 empirical model does not cover single-phase systems. Arc-flash incident energy testing for single-phase systems has not been researched with enough detail to determine a method for estimating the incident energy. Single-phase systems can be analyzed by using the single-phase bolted fault current to determine the single-phase arcing current. The voltage of the single-phase system, (i.e. line-to-line, line-to-ground, center tap voltage, etc.), can be used to determine the arcing current. The arcing current can then be used to find the protective device opening time and incident energy by using the three-phase equations provided in IEEE 1584 2nd edition and implemented in Arc Flash Analytic v6.0 software program. The incident energy result is expected to be conservative.