Arc Flash and Blast Explained

By R.W. Hurst, The Electricity Forum

arc flash
Electrical Blast Explosion and NFPA 70e

Arc flash, according to National Fire Protection Association (NFPA), is “a dangerous condition associated with the release of energy caused by an electric arc.” Arc flash typically occurs when the electrical insulation or isolation between live conductors is severed or can no longer withstand the applied voltage. How easy is it to cause an electrical explosion? More easy than you think. Arc faults can result from something as small as inserting a tool in the wrong place, or accidentally dropping a tool of some kind into a circuit breaker or electrical service area. In fact, an explosion can occur under any of the following conditions: a) for instance, an electrical worker is operating on or near energized conductors or circuits, or b) he/she moves near or contacts the equipment, or c) equipment failure occurs; any of these various conditions may cause a phase-to-ground and/or a phase-to-phase electrical fault and the result can be an arc flash/arc blast accident.

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The thermal temperature of an explosion can reach more than 5000 degrees. This energy is in the form of a blinding flash of light and a deafening noise. In this explosion, a giant amount of concentrated energy is forced outward from the electrical equipment toward the electrical worker, spreading hot gases and molten metal.

Every day in the United States, from five to 10 arc flash explosions happen. These explosions cause injuries (from the creation of pressure waves that can damage hearing or brain function and a flash that can damage eyesight and burn exposed and covered flesh) which can range from minor burn injuries to life threatening injuries and possible death. In addition to the electrical worker being injured, the fast-moving pressure wave can also injure other workers in the vacinity who can be struck by loose material such as pieces of equipment, metal tools, and other projectiles.

In addition to personal injury, arc flash can result in serious damage to electrical equipment from short circuit. which can cause disruption to electrical systems in manufacturing and process industry environments,office buildings, or institutions such as hospitals, airports, schools, etc. The cost from downtime can be considerable.

Accidents usually take place in situations above 120V, and often happens during the service or inspection for normal wear and tear of electrical equipment. In fact, some accidents occur when an electrical worker removes a cover from an electrical device. Many electrical service companies recommend that electrical equipment be de-energized before any service or maintenance work is started, although this is not always possible but it is always safest to work with some kind of protective device in place.

NFPA 70e recommends the establishment of a clearly defined “flash protection boundary” and then dictates a way to calculate such a boundary. This imaginary personal protection boundary, which surrounds the potential arc point, specifies what level of personal protective (ppe) clothing and equipment must be worn by a qualified electrical worker who enters within that flash protection boundary. NFPA 70e defines a “flash protection boundary within which a person could receive a second-degree burn if an electrical arc flash were to occur". It also gives a definition for incident energy as “the amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event.”

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