Arc flash hazards can result from accidents or equipment deterioration such as dropping tools, accidental contact with electrical equipment, build up of conductive dust, corrosion, condensation, over-voltage stress, or insulation failure. An arc is produced when electric current passes through ionized air after an initial flash over or short circuit, resulting in a flash that could produce significant heat, with temperature in excess of 35,000°F. The extremely high temperature of an electric arc can cause major burns within ten feet and fatal burns within five feet of an arc flash. Recently enacted guidelines and regulations by OSHA and NFPA 70E regarding arc flash hazards have compelled many rail transit agencies to require that an arc flash hazard analysis be performed. The purpose of this analysis is to determine the potential risk of arc faults at every switchgear and electrical panel board to which a worker may be exposed. To comply with OSHA and NFPA, appropriate work practices and personal protective equipment (PPE) must be utilized to reduce the risks associated with arc flashes. Several methods for calculating the arc-flash hazard have been developed. This paper will examine and discuss the following three methods: a) the Ralph H. Lee’s theoretical model, b) the NFPA 70E equations and tables, and c) the IEEE Std 1584 methods. None of the above methods addresses arcing faults in DC switchgear. To date, there is no written standard for DC arc flash hazard analysis. DC arcing faults and calculation methods are discussed. Sample arc flash hazard analysis from a recent rail transit project is included.
Stability, spatial extension and extinction of an electric arc in aeronautical conditions of pressure under 540 V DC