|Posted on May 25, 2012 at 9:50 PM|
When a phase to ground or phase to phase short occurs at over 250 volts there is a potential for an arc flash/blast. The severity of the incident depends on a number of factors including the distance to the supply transformer, the rated fault current of that transformer, the size of the conductors coming from the transformer and the rating of the over current device. An arc flash/blast emits massive amounts of energy referred to as Incident Energy.
Incident Energy: The amount of energy impressed on a surface, a certain distance from the source, generated during an electrical arc event. One of the units used to measure incident energy is calories per centimeter squared. (cal/cm²)
An Arc flash can reach temperatures of 35,000˚F (four times hotter than the suns surface) at these temperatures copper will expand to 67,000 times its' original mass.
1.2 cal/cm² - 2nd degree burns
1.5 cal/cm² - 3rd degree burns
4 cal/cm² - Hazard Risk Category 1 (HRC1)
8 cal/cm² - HRC2
25 cal/cm² - HRC3
40 cal/cm² - HRC4