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 calculated based on the available bolted fault current, system voltage, the clearing times of the associated over-current protective devices and the individual parameters of the electrical distribution system. Two calculations for incident energy are recommended by NFPA 70E-2015 Informative Annex D.5 - one at the predicted arcing current and another at the predicted arcing current reduced by 15%. These values are compared and the largest is used in the arc hazard analysis.
Incident energy analysis is required for selecting arc rated Personal Protective Equipment (PPE) when work is performing at or near a potential source of an arc flash. The selected PPE arc rating shall be based on the incident energy exposure. The incident energy analysis requirement stipulated in NFPA 70E Article 130.3(B)(1) is the basis for performing an arc flash hazard calculation study (AFHCS).
The threshold value of incident energy for 2nd degree burn of human skin is generally assumed to be 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 at 1 centimeter distance above the flame. It is assumed that a second-degree burn will be curable and will not result in death.
It was shown in Evaluation of Onset to Second Degree Burn Energy in Arc Flash 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 1.2 cal/cm2 threshold incident energy to 2nd degree burn 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. ARCAD Arc Flash Analytic (AFA) software program for arc flash hazard analysis and labeling calculates arc flash boundary based on 1.2 cal/cm2 onset energy to second degree burn for bare skin exposure as well as threshold incident energy calculated as a variable function of heat flux.
There are various types of PPE with distinct levels of thermal protection capabilities termed Arc Thermal Performance Exposure Values (ATPV) rated in cal/cm2. The main purpose of PPE is to reduce the level of burn injury to a worker to a level of curable burn.
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