What is Incident Energy?
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Incident energy is a measure of thermal energy at a working distance from an arc fault. The unit of incident energy is cal/cm2. The working distance is the distance from where the worker stands to the flash location. The most common distance for which incident energy has been determined in tests is 18 inches. The incident energy is a function of system voltage, available short-circuit current, arc current, and the time required for circuit protective devices to open. Dr. Ralph Lee developed formulas for calculating incident energy and determining approach boundaries. Incident energy is inversely proportional to the working distance squared. It is directly proportional to the time duration of the arc and to the available bolted fault current. NFPA 70E 2000 includes Dr. Lee's calculation formulas, hazards risk assessment, standards for PPE, and similar information. The threshold value of incident energy for 2nd degree burn of human skin is assumedly equal to 1.2 cal/cm2 (5 Joules/cm2). One cal/cm2 is equivalent to the amount of energy produced by a cigarette lighter in one second. It is assumed that a second-degree burn will be curable and will not result in death.
Recently, it was shown (1) that using onset to second degree burn energy for bare skin exposure fixed to 1.2 cal/cm2 in calculating the arc flash boundary for arc durations other than one (1) second is open to dispute and heat flux rate should be factored-in when estimating skin damage imposed by an arc flash. Using the cal/cm2 energy for exposure times less than one second will result in undervalued arc flash boundaries while resulting in conservative but save arc flash boundaries for exposure times more than one (1) seconds.
After determining the incident energy, the value can be used to select the appropriate personal protective equipment. There are various types of PPE with distinct levels of thermal protection capabilities termed Arc Thermal Performance Exposure Values (ATPV) rated in cal/cm2.
1. "Evaluation of Onset to Second Degree Burn Energy in Arc Flash" by M. Furtak, L. Silecky. Electrical Safety Measures, March/April 2012.