![]() ![]() The plot below shows the median arc current variation in percent for each of the five electrode configurations. The arc current variation was determined from the median of the measured variation at each voltage level. The value of the arc current variation is no longer fixed to 15% but calculated continuously based on the equations provided in section 4.5 of IEEE 1584-2018. Almost half of the pages in the 2018 edition of NFPA 70E are devoted to 17 informative annexes. Based on the analysis done during the new arc flash model development phase, it was found that the variation in the arc current was higher at voltages below 480 Volts and far less at voltages such as 600 Volts and 2700 Volts ac. Further laboratory testing performed by IEEE and NFPA led to the development of a new model to represent the physical behavior of arc current and incident energy. The physical concept of arc current variation is not changed, however, it was improved. It includes only the average of all three-phase arc currents. The average current model in 2002 does not include the arc current measured during ignition or extinguishing periods of the arc. Also, the magnitude of the arc current can vary as the arc ignites, persists and extinguishes. In reality, the arc current can experience variations caused by the ac and dc components of the short-circuit current. The arc current predicted by the model is considered to be the average arc current for the duration of the arc. For single - phase systems above 250 volts, select the range that is equal to the systems maximum phase-to-ground voltage multiplied by 1.732. This is the reason why the new IEEE 1584-2018 model applied an enhanced arc current model. All dimensions are distance from exposed energized electrical conductors or circuit part to the employee. The arc current is the most important factor to determine the operation time of overcurrent protective devices.
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