What Is Arc Flash?
An arc flash (also known as arc blast) is a sudden, explosive electrical arc that results from a short circuit through air. The air in the vicinity of an arc flash is heated to between 5,000 and 35,000 degrees in no more than 1/1000 of a second, becoming an electrically-conductive plasma. The sudden heating can cause a shock wave blast equivalent to several sticks of dynamite and carrying vaporized metal and shrapnel.
An arc flash is not just an electrical arc. For example, although the electrical arc shown in this video is a spectacular arc, it is not an arc flash. Arc flash is perhaps the most dangerous hazard facing electrical workers. It does not require contact with a live conductor, and can cause severe injury or death.
What Causes Arc Flash?
An arc flash occurs when a conducting object approaches an exposed, high-amp electrical source. High voltage is not required for an arc flash. Most incidents occur on 120 to 240V equipment, and arc flashes can occur at voltages as low as 50V. Arc flash is a danger whenever energized equipment is being worked on, but there are certain conditions that make arc flash more likely. These conditions include:
- Dropped tools or other sources of sparks near energized equipment
- Gaps in insulating materials which expose conducting surfaces
- Corroded or improperly installed or maintained equipment
Arc flash most often occurs when performing the following tasks:
- Removing panels and opening electrical equipment doors
- Racking breakers
- Opening and closing breakers
- Taking voltage measurements
How Common Is Arc Flash?
In the United States, between 5 and 10 arc flash incidents causing severe injury occur every day. One or two result in death. In addition, there is an unknown number of arc flashes resulting in more minor injuries and damage to equipment. Although arc flash incidents are less common than electric shocks, more than 50% of burn-center admittances for electrical injuries are from arc flash.
What Are the Effects of Arc Flash?
The temperatures associated with arc flash can exceed the surface temperature of the sun. The accompanying shock wave may be capable of propelling shrapnel at speeds up to 700 mph. The worker closest to the blast is not the only one in danger. Many of the effects of arc flash, including flying metal and burns, may affect bystanders at some distance from the blast. Death is a very real possibility, but even if a worker survives the blast, they may be left with serious injuries, including:
- Burns from direct heat exposure or clothing ignition. Arc flash is capable of causing serious burns requiring skin grafts at distances of more than 10 feet.
- Hearing damage or total hearing loss from ruptured eardrums. The sound of a blast can exceed 160 dB. (Sounds above 140 dB cause hearing loss even with protective equipment.)
- Loss of eyesight from UV light emitted by vaporized metal
- Lung collapse or scarring from the shock wave and inhalation of vaporized metal
- Memory loss and other neurological damage from concussion
- Injuries from flying shrapnel
- Injuries from falls or collision with equipment. An arc fault of 50 kA can accelerate a nearby worker at speeds of up to 110 mph.
Even when personal protective equipment (PPE) is worn, arc flash can cause arc flash can cause injury or death. Workers may be left with painful and debilitating scars for the rest of their lives, and may be unable to return to work. Medical bills can easily exceed $1 million. The picture below was taken at a government laboratory that had an arc flash accident. The outline of the worker's knee in the arc flash shadow can be seen on the insulating mat.
Although personal injury is the most obvious effect of an arc flash incident, costs to employers don't stop with medical bills and lawsuits. Arc flash often results in damage to electrical equipment and other production facilities. Nearby structures and equipment are often abraded and corroded as their surfaces vaporize. Molten balls of metal carried on the blast wave act like buckshot, damaging delicate instruments. Depending on the incident, an arc flash may also force companies to shut down production lines for a time. If the employer is found to have failed in compliance with OSHA safety regulations related to the blast, fines are likely to be imposed in addition to the direct costs of repair.
A number of steps should be taken to minimize the risk of arc flash. Whenever possible, equipment should be de-energized before any work is done. De-energizing is not always possible, however, and in some cases may pose a greater hazard than working on live equipment. Whenever work must be done on energized equipment, a comprehensive safety program should be in place. Labeling arc flash hazards is an important part of an electrical safety program. In addition to arc flash labels, companies should perform an arc flash analysis, maintain a detailed safety plan, provide adequate personal protection equipment (PPE) for workers, and ensure that workers are properly trained in the procedures necessary to avoid arc flash injuries.
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