Thanks to [user]starksk[/user] for the video link and to [user]church[/user] for a succinct, yet detailed definition which required only some minor editing.
Despite appearances to the contrary, an arc flash is not an explosion.
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.
Arc flash is extremely dangerous and potentially fatal to personnel. Arc flash is the product of arc fault current and voltage concentrated in one place, resulting in enormous energy released in several forms, including heat, light, sound and pressure. 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. In an arc flash incident, part of the copper wire instantly vaporizes, expanding to 67,000 times its original volume, resulting in a powerful pressure wave which propels droplets of molten metal up to 10 ft. from the electrical panel. This pressure wave has the potential to blow personnel off their feet. Additionally, super heated plasma (ionized vapor) is propelled a distance proportional to the arc power. The plasma and molten droplets in the arc stream instantly heat the air around it to about 35,000 degrees Fahrenheit. Conductive vapors help sustain the arc, the duration of which 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 miliseconds or faster while other devices may take much longer to operate and open. Exposure risks to arc flash include flying debris, severe sound waves and shock hazard due to touching energized conductors etc.
No two arc flash hazard levels are the same but a system for categorizing the level of hazard exists and this is the information provided on the arc flash warning label which should provide the following information:
- Flash Protection Boundary
- Incident energy at 18” expressed in cal/cm2
- PPE required
- Voltage shock hazard
- Limited shock approach boundary
- Restricted shock approach boundary
- Prohibited shock approach boundary
If you do not know what these mean then you must receive the appropriate training before working on this equipment. If you are working with this type of equipment then you must request the training and PPE. If you are an owner or supervisor you must ensure your workers receive the training and the PPE.
An arc flash can be caused by the following common causes and this is not an exhaustive list:
a) insulation failure in equipment (switch gear electrical distribution equipment etc.),
b) dust, moisture, other contamination,
c) people working on live equipment they incorrectly think has been de-energised,
d) people disconnecting electrical equipment without isolating the circuit,
e) loose connections that overheat, reach thermal runaway and fail
f) accidents caused by touching a test probe to the wrong surface or slipped tool while working on live equipment during fault finding
The following article is very informative about the causes and effects of arc flash:
Pressures Developed by Arcs, by Ralph H. Lee
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