Arc Flash and Blast Hazards per NFPA 70E
What is ARC FLASH and why is arc flash dangerous?
When an electric current passes through air between ungrounded conductors or between ungrounded conductors and grounded conductors, the temperatures can reach 35,000°F. Exposure to these extreme temperatures both burns the skin directly and causes ignition of clothing, which adds to the burn injury. The majority of hospital admissions due to electrical accidents are from arc flash burns, not from shocks. Each year more than 2000 people are admitted to burn centers with severe arc flash burns. Arc flashes can and do kill at distances of 3 m (10 ft).
What is ARC BLAST and why is arc blast dangerous?
The tremendous temperatures of the arc cause the explosive expansion of both the surrounding air and the metal in the arc path. For example, copper expands by a factor of 67,000 times when it turns from a solid to a vapor. The danger associated with this expansion is one of high pressures, sound, and shrapnel. The high pressures can easily exceed hundreds or even thousands of pounds per square foot, knocking workers off ladders, rupturing eardrums, and collapsing lungs. The sounds associated with these pressures can exceed 160 dB. Finally, material and molten metal is expelled away from the arc at speeds exceeding 1600 km/hr (700 mph), fast enough for shrapnel to completely penetrate the human body.
Protection against Arc Flash Hazards
Protection against the arc flash hazards described in Section K.3 is intended to protect employees from receiving incurable burns. Although arc-rated garments are available with ratings greater than 40 cal/cm2, the advice given in Informational Note No. 3 to 130.7(A) recognizes the extremely dangerous concussive forces, sound, and shrapnel that occur in arc blast events where the incident energy exceeds 40 cal/cm2. Arc-rated garments with higher ratings can be used by employees when they are testing for the absence of voltage or are performing some type of diagnostic task and are within the arc flash boundary of equipment where a high level of incident energy exists, but they are not intended to provide arc blast protection.