When we speak about arc flash, we focus on the thermal issues and rarely address the full scope of the hazard. The pressure wave developed by arc flashes have been documented collapsing a substation wall in Quebec while another case cites an employee being thrown 25 feet.
The pressure wave from an arc flash incident is comprised of two parts, the rapid expansion of vaporized metal and the heating of surrounding air. Copper expands by a factor of approximately 67000 times in vaporizing. To put this in perspective water only expands 1670 times during vaporization. This rapid expansion creates a pressure wave that can place as much as 600lbs per square inch on a exposed individual working within 2 feet of the arc flash source.
When we determine the Hazard/Risk Category for arc flash, anything above a 4 is considered prohibited due to the sound, pressure and concussive forces present. Above this level these forces are more significant than the thermal values.
Ralf E. Lee’s “Pressures Developed by Arcs” Is required reading for anyone dealing with Arc Flash hazards.
