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An arc faults happens when electric current flows through air gaps between
conductors. Insulation failure and accidents caused by touching a test probe to
the wrong surface or slipped tool are the most common causes of an arcing fault.
The fault current magnetic fields make conductors to separate producing an arc.
In other words, arc flash is caused by uncontrolled conduction of electrical
current from phase to ground, phase to neutral, and/or phase to phase
accompanied by ionization of the surrounding air. Because of the expansive
vaporization of conductive metal, a line-to-line or line-to-ground arcing fault
can escalate into a three phase arcing fault in less than a 1/1000 of a second.
The heat energy and intense light at the point of the arc is called arc flash.
Short circuits and arc faults are extremely dangerous and potentially fatal to
personnel. The product of arc fault current and voltage concentrated in one
place, results in enormous energy released in several forms. Arc fault generates
large amounts of heat that can severely burn human skin and set clothing on
fire. Temperatures at the arc can reach four times the temperature of the sun's
surface. The high arc temperature vaporizes the conductors in an explosive
change in state from solid to vapor. Copper vapor expands to 67,000 times the
volume of solid copper. Conductive vapors help sustain the arc and the duration
of the arc is primarily determined by the time it takes for overcurrent
protective devices to open the circuit. For example, fast acting fuses may open
the circuit in 8 ms or faster while other devices may take much longer to
operate and open. Metal is blasted and splattered from the fault location. The
arcing faults also produce large shock waves that can blow personnel off their
feet. The other exposure risks to arcing faults include flying debris, severe
sound waves, shock hazard due to touching energized conductors etc.
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