The ensuing blast creates explosive pressures that are generated by the vaporization of metal and the thermal expansion of air as the arc passes through it. Cases have been documented where such explosions have knocked workers off ladders or thrown them across the room, or worse.
Laboratories measuring the concussive forces associated with arc flash have found sound levels of 1400 dB and pressure levels of 2160 lbs/ft2 (103 kPa) at 2 ft (0.6 m) from the flash on 480-V load center breakers. Eardrums rupture at around 720 lbs/ft2 (35 kPa), and lung damage occurs at 1728 lbs/ft2 to 2160 lbs/ft2 (83 kPa to 103 kPa).
Changes in Protection
For years workers have stood in front of the breaker cubicle with nothing to protect them. Then, as more was learned about arc flash blasts, personal protective equipment (PPE) became available. PPE has evolved from wearing leather gloves to wearing electrical-rated gloves and arc flash suits.
The National Fire Protection Association (NFPA) Standard 70E has been the bible of electrical safety for many years. Hazards were measured in calories per square centimeter. NFPA 70E ratings only went up to 40 cal/cm2 and workers were not to perform work on live buses above that rating. However, the original calculations did not take into account the multiple components required to cause a breaker to trip. A fault-detecting device (which adds its time), a trip relay (which adds its time) and the breaker itself must all be taken into account. Factoring these in and computing the energy values reveals that the calorie ratings can be double or even triple what the original calculations estimated the hazard to be.
In some cases, gloves and flash suits provide some measure of protection. However, gloves and suits protect workers only from burns; they provide little to no protection from the concussive effects of an arc flash blast. At an Institute of Electrical and Electronics Engineers (IEEE) conference, a doctor/scientist explained that in an arc flash blast above 40 cal/cm2, an arc flash suit would only provide your family with a body to bury. The blasts described earlier in this article show that just the pressure to lungs and ears would cause life-threatening injuries and permanent damage to any worker directly in the blast. Also, the vaporization of the copper and the fragments from the explosion could kill a worker, even if the person had been wearing the best arc flash suit on the market.
With five to 10 arc flash blasts occurring in the United States every day, more protection is clearly needed.
Distance is a Solution
One of the best solutions for preventing injuries in the event of an arc flash blast is to remove the worker from the blast zone right in front of the cubicle. Safe-T-Rack® is an innovative, patented tool for circuit breaker remote racking and control. This product family was specifically developed to provide users of low and medium voltage circuit breakers with a comprehensive alternative to arc flash protection garments.
The Safe-T-Rack® is a fully portable system that includes a color touch screen interface with true closed-door racking capability for increased operator safety. Redundant digital drives with battery backup provide failsafe racking in the event of a power failure, and real-time breaker travel indication and user controls include an emergency stop at any time during racking, manual start/stop, and automatic retrieval and recovery of a circuit breaker.
The Safe-T-Rack® system is the benchmark of remote racking tools, and National Field Services is proud to be an authorized dealer and certified installer of the Safe-T-Rack® system. For more information or to schedule a consultation with an NFS technician regarding the Safe-T-Rack System, contact National Field Services through our website.
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