What is an arc flash?
Arc Flash (incident energy or arc blast) is the result of a quick release of energy due to an arcing fault between a phase bus bar and another phase bus bar, neutral or a ground. During an arc fault the air acts as the conductor. Arc faults are generally limited to systems where the bus voltage is in excess of 120 volts. Lower voltage levels usually will not sustain an arc. An arc fault is similar to the arc obtained during electric welding and the fault has to be manually started by something creating the path of conduction or a failure such as a breakdown in insulation.
The cause of the short normally burns away during the initial flash and the arc fault is then sustained by the establishment of a highly-conductive plasma. The plasma will conduct as much energy as is available and is only limited by the impedance of the arc. This huge energy discharge burns the bus bars, vaporizing the copper and thus resulting in an explosive volumetric increase, the arc blast, conservatively estimated, as an expansion of 67,000 to 1 (meaning a pea-sized piece of copper, ends up blowing up to the size of a typical rail car in a fraction of a second). This fiery blast will destroy everything in its path, creating deadly shrapnel as it dissipates. This shrapnel can reach speeds exceeding 700 miles per hour. The core of the arc flash can reach temperatures of 35,000℉ (19427℃) in less than 1/1000th of a second; That’s roughly four times the temperature of the surface of the sun.
The arc fault current is normally much less than the available bolted fault current and below the rating of circuit breakers. Unless these devices have been selected to handle the arc fault condition, they will not trip and the full force of an arc flash will occur. The electrical equation for energy is volts x current x time. The transition from arc fault to arc flash takes less then half the blink of an eye, increasing in intensity as the pressure wave builds up. The challenge is to sense the arc fault current and shut off the voltage momentarily before it develops into a serious arc flash condition.
Field tests and surveys have shown that 22% of all breakers operate at less than 100% efficiency (slow trip), and more than 10% have been shown not to close at all. Even the slightest delay in the operation of a breaker or fuse, will double or triple the available incident energy in an arc flash event.
Injuries due to arc flash or arc blasts is among the top five leading causes of workplace fatalities.
2,000 workers are treated in specialized burn trauma centers each year as a result of arc flash injuries.
- -These high-tech facilities only treat the most devastated burn victims — those who have sustained incurable third-degree burns over more than half of their body.
- -Arc flash injuries are actually much higher than reported because workers receiving treatment for trauma and burns that do not require burn unit attention (i.e. second degree burns or third degree burns covering less than half their body) are admitted to standard hospitals which do not track the burn source.
The flash of light produced is intense enough to permanently damage eye-sight and can cause the early development of cataracts.
Even low voltage electrical shock has been shown to cause sustained “invisible” injury that manifests itself over the course of the days or weeks following the initial event. The injuries can take the form of numbness, muscle weakness, general or localized fatigue, and cognitive disfunction, including memory change or loss, concentration issues, and post-traumatic stress, among others. These side effects of shock have only recently become the subject research so that the medical community can begin to understand this phenomenon.
Medical costs for severe electrical burns can exceed $4M per person. Such injuries can cost businesses well over $30M in fines, medical costs, litigation, lost business and equipment costs.
References:
Bureau of Labor Statistics
ESFI (Electrical Safety Foundation International)
St. John’s Rehab Hospital (Dr. Joel Fish)
NFPA 70E
CapSchell, inc
NIOSH (National Institute of Occupational Safety & Health)
Electrical Safety Workshop Conference Proceedings, 2010; Update of Field Analysis of Arc Flash Incidents, PPE Protective Performance and Related Worker Injuries; Doan, Hoagland & Neal
National Safety Council
OSHA
Electricite de France
