Arc flash blowout
C.G. Masi, Control Engineering -- Control Engineering, 12/1/2008
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Once an arc starts, the gap fills with electrically conducting plasma and grows rapidly. Source: Coastal Training Technologies www.coastal.com |
Arc flash is quite different from electric shock. When a person gets an electric shock, effects arise from the passage of electric current through sensitive tissues, mostly nerves. Arc flash occurs when electric current passes through air. Arcs generally begin when conductors in contact and carrying high current are pulled apart. Once an arc starts, the gap fills with electrically conducting plasma, and grows rapidly, especially when powered by high-voltage/low-impedance sources, such as powerlines. Any circuit energized by greater than 50 V is considered to have arc-flash potential.
Arc flash temperatures are limited by rapid energy loss through a number of mechanisms, including:
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Radiative emission in the form of EMI/RFI, infrared, and (the biggest sink) visible light (arc flash);
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Latent heat associated with changes of state (breaking chemical bonds, vaporizing and ionizing material in the blast zone);
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Adiabatic expansion (arc blast).
These energy-loss mechanisms limit arc flash temperatures to “merely” a few tens of thousands of degrees Kelvin, which is still hotter than anything else on Earth. They also carry the flash’s damage beyond the arc-flash core. Injuries from arc flashes include:
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Burns,
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Temporary blindness,
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Hearing disruption,
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Concussion trauma, and
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Shrapnel wounds.
Facing the danger
There are three strategies available to protect workers from arc flash. The first is to de-energize any electrical equipment before working on it.
The second is distance. The severity of possible injuries decreases with the square of the distance from the arc. If you can’t avoid working on live equipment, keep as much distance from the danger zone as possible.
Of course, keep non-essential personnel well away from the equipment you are working on. Establish a perimeter around the work area and keep everyone out that isn’t actively working on the equipment. That guy wearing the white shirt and tie with his hard hat shouldn’t be allowed within 15 feet of the operation — even if he is in charge of plant safety!
The third is protective clothing. Face shields, protective gloves, and outerwear is available from a number of industrial supply companies. Proper outerwear can prevent injury from nearly all arc flashes. For more information, do a Web search on “arc flash clothing.”
John Kay, engineering manager at Rockwell Automation points out that industries can address arc flash safety with both prevention and protection measures, including in-depth safety programs and new technology in electrical equipment that can redirect harmful arc flash energy away from personnel, such as relief vents in motor controllers. Other protection measures in motor control applications include NEMA low-voltage motor control centers (MCCs) with arc-containing features, such as arc-containment latches on all doors, insulation covers on the horizontal bus closing plates and automatic shutters for plug-in units.
Remote monitoring of motor control equipment offers uses a more sophisticated form of safety. Networked motor control devices can remotely monitor and isolate features to help prevent accidental exposure to energized parts. This technology can, for example, allow monitoring and troubleshooting without opening unit doors. Software permits real-time monitoring, configuring, and troubleshooting of both low- and medium-voltage equipment on the network from anywhere in the facility or even anywhere in the world.
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