ABSTRACT
An arc flash is a short circuit through the air. In an arc-flash incident, an enormous amount of concentrated radiant energy explodes outward. Arc blast or flash hazards include high temperatures (hotter than the surface temperature of the sun) over short periods of time (fractions of a second), hot gases, an intense pressure wave from the explosion (like having a hand grenade explode inches away), and shrapnel from vaporized and molten metal particles. Arc flashes are short circuits that flash from one exposed live conductor to another. Unfortunately, many times that conductor is a human being. The arc flash ionizes the air, creating electrically conductive, superheated plasma with temperatures that can reach over 5000°F. This explosion can take less than a fraction of a second and produces a bright flash, intense heat, and a blast wave equivalent to detonating dynamite. The combination can be lethal to anyone standing in the blast area. Arcing parts, including the parts of electric equipment that in ordinary operation produce arcs, sparks, flames, or molten metal, must be enclosed or separated and isolated from all combustible material. An arc flash can occur any time electrical equipment is disconnected, inspected, or serviced. Some examples include the following:
• Accessing a damaged circuit breaker panel • Accidentally contacting live (“hot”) equipment • Overvoltage conditions • Insulation failure
15.17 Class I Locations .......................................................................................209 15.17.1 Class I, Division 1 ...................................................................... 210 15.17.2 Class I, Division 2 ...................................................................... 210 15.17.3 Class I, Zone 0 ........................................................................... 210 15.17.4 Class I, Zone 1 ........................................................................... 210 15.17.5 Class I, Zone 2 ........................................................................... 211 15.17.6 Class II locations ........................................................................ 211
15.17.6.1 Class II, Division 1 ................................................. 211 15.17.6.2 Class II, Division 2 ................................................. 212
15.17.7 Class III Locations ..................................................................... 212 15.17.7.1 Class III, Division 1 ................................................ 212 15.17.7.2 Class III, Division 2 ................................................ 212
15.18 Electrical Installations .............................................................................. 212 15.19 Marking ..................................................................................................... 213 15.20 Conduits .................................................................................................... 213 15.21 Equipment in Division 2 Locations........................................................... 213 15.22 Protection Techniques for Hazardous Locations ...................................... 213 15.23 Class I, Zone 0, 1, and 2 Locations ........................................................... 214 15.24 Location and General Requirements ........................................................ 214 15.25 Protection Techniques and Zone Classification ........................................ 214 15.26 Special Precautions ................................................................................... 215 15.27 Listing and Marking.................................................................................. 215 References .............................................................................................................. 216
• Corrosion of terminals • High-amp current source close to a conducting medium • Internal and external sparks
De-energizing equipment does not absolve the facility from the responsibility of performing arc-flash analysis or providing the necessary personal protective equipment (PPE). Both the Occupational Health and Safety Administration (OSHA) and the National Fire Protection Association (NFPA) have basic rules that prohibit energized work. During hazardous energy control procedures, the circuit must be approached to verify that the circuit has been de-energized. Until the verification testing is completed, the circuit must be considered energized per NFPA 70E, Standard for Electrical Safety in the Workplace (NFPA, 2012); therefore, workers who approach a circuit for verification testing must wear full PPE (Tajali, 2012).
