What causes an arc flash and what kind of injuries can arc flash cause?

An arc flash happens when electric current flows through an air gap between conductors. Accidents caused by touching a test probe to the wrong surface or slipped tool are the most common cause of an arcing fault. Arc flashes can also be caused by:

• Sparks due to breaks or gaps in the insulation
• Equipment failure due to use of substandard parts, improper installation, or even normal wear and tear
• Dust, corrosion or other impurities on the surface of the conductor

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 flash can cause the following injuries:

• Skin burns by direct heat exposure. Arc flash 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.
• High-intensity flash can also cause damage to eyesight

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 producing a considerable pressure wave and sound blast. In some cases, the pressure wave has sufficient energy to snap the heads of 3/8-inch steel bolts and to knock over construction walls. An arc blast can cause the following injuries:

• Large shock waves that can blow personnel off their feet
• Loss of memory or brain function from concussion
• Hearing loss from ruptured eardrums. The sound associated with the blast can greatly exceed the sound of a jet engine
• Exposure risks from flying debris. For example, shrapnel wounds from metal parts
• Shock hazard due to touching energized conductors
• Other physical injuries from being blown off ladders, into walls, etc.

Conductive vapors help sustain the arc and the duration of the arc is primarily determined by the time it takes for over-current protection 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.