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The Explosive Power of an Arc Flash
The many intense hazards created by arcing faults in today's industrial power systems arise from two factors: the tremendous amounts of power that can be delivered to such arcs and the workers' close proximity to them.
As you can see from Test Sequence 1, a frame-by-frame description of a test performed in Ferraz Shawmut's high-power lab, the development of a high-current arcing fault is like an explosion.
When an arcing fault is being established, current begins passing though ionized air. Large volumes of ionized gases, along with metal from the vaporized conductors, are rapidly expelled. As the arc runs its course, electrical energy continues to be converted into extremely hazardous forms of energy.
All of these events usually occur in less than 0.2 seconds, and can create:
Intense Heat
The electrical current flowing through the ionized air creates tremendously high levels of heat energy. This heat is transferred to the plasma, which rapidly expands away from the source of supply.
Tests have shown that heat densities at typical working distances can exceed 40 cal/cm². Even at lower levels, conventional clothing ignites, causing severe, often fatal, burns. At typical arc fault durations of less than one second, a heat density of only 1.2 cal/cm² on exposed flesh is enough to cause a second-degree burn. Even workers not in the plasma can be severely burned from the intense heat radiated beyond typical working distances.
Thermoacoustic Shock Wave
As the conductive element that caused the arc is vaporized, the power delivered to the arc fault rises rapidly. This corresponds to a rapid rise in surrounding pressure. The resultant shock wave can create impulse sound levels well beyond OSHA's allowable limits. Forces from the pressure wave can rupture ear drums, collapse lungs or cause fatal injuries.
Molten Metal
At high fault current levels, plasma jets are formed at the electrodes. Vaporized and molten electrode material is ejected at high velocity from these jets, reaching distances of several feet away. Since the molten metal is typically over 1000° C, it's a potential ignition source for conventional clothing.
Shrapnel
The force of the explosion also causes a significant amount of shrapnel to be accelerated away from the source. These particles can impact a nearby worker at high velocity, resulting in physical trauma.
Blinding Light
As the arc is established, an extremely bright flash of light occurs. The light can cause immediate vision damage and increase the potential for future vision deterioration.
Toxic Smoke
Also expelled into the atmosphere are toxic combustion byproducts and copper oxides formed when the cooling copper vapor combines with oxygen.
Contact with Energized Components
The explosive nature of an arc fault increases the possibility that an energized conductor or components will make contact with workers in the area.
For more information on arc flash hazards, see references.
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