Electricity's dangerous downside
Few electrical phenomena are as dramatic as arc flash. When current leaves its assigned path and short-circuits through the air between conductors, or between a conductor and ground, temperatures in the resulting arc can soar well beyond that of the sun's surface (about 10,000F) in just a fraction of a second. The host of hazards produced includes electromagnetic radiation, superheated plasma, a sound pressure wave, and shrapnel.
Each arc flash event can cost millions of dollars in medical expenses, equipment replacement and repair, lost work hours and production, lawsuits, and higher insurance premiums. The human toll from arc flash burns and other injuries is especially acute.
A 10-year study by Électricité de France found that arcing caused 77% of all recorded electrical injuries. Similarly, one corporation noted that up to 80% of its electrical injuries involve thermal burns due to arcing faults. The U.S. National Institute of Occupational Safety and Health (NIOSH) tabulated 44,363 electrical-related lost-workday cases from 1992 to 2001. Arc flash burns (Figures 1 and 2) accounted for 17,101 (or about 39%) of those cases.
1. In your face. Flame-resistant shields and hoods protect the head, face, and eyes against the ultraviolet, infrared, radiant, and convective energies unleashed in an arc flash event. The damaged hood (bottom) protected a worker from serious injury during such an incident. The intense thermal energy produced by the flash caused the fabric to break open and char and the shield to bubble and blacken. Courtesy: Ferraz Shawmut High Power Test Laboratory
2. Hard head. A hard hat worn by an electrician exposed to an arc flash prevented injury from shrapnel propelled by the event. Flame-resistant clothing protected him from thermal burns. Courtesy: DuPont
Although arc flash phenomena have been of concern for a century or more, there are surprising gaps in what is known about them and how to protect workers from their effects. In an effort to fill this knowledge gap, the IEEE and NFPA have joined forces to explore the many aspects of arc flash and accompanying arc blast phenomena and determine how to ensure worker safety when arc flashes occur. This article first reviews these phenomena and then details what the IEEE/NFPA program hopes to accomplish.
Rogues’ gallery
The dramatic release of energy as electrons pass across a narrow arc through air takes many forms. Extreme heat is just one: The temperature at the point of flashover and in the arc can rise to between 5,000F and 30,000F, depending on the current level and other factors.
The energy packed into just a few tenths of a second also generates an explosion that sends pressure waves, debris, molten metal, and a plasma containing toxic gases and particulates expanding outward at hundreds of miles per hour (Figure 3). As a result, workers in the vicinity of an arc flash are at risk of multiple traumas, such as severe burns, smoke inhalation, eye damage, hearing loss, collapsed lungs, fractures, puncture wounds, loss of consciousness, and even death. Survivors may endure permanent disabilities and a reduced quality of life.
3. Bright as the sun. Photos taken at 1-microsecond intervals at the Ferraz Shawmut High Power Test Laboratory in Newburyport, Mass., illustrate the development of an arc flash as a jet of hot, ionized gas elongates from the sides of vertical conductors terminating in an insulator block. The conductors are part of a three-phase system energized at 480 V and 42 kA. The jet, which had its farthest reach in image "d" (shown impacting the vertical plates of a calorimeter), is shown collapsing in "e." Courtesy: Ferraz Shawmut High Power Test Laboratory
The sheer amount of thermal energy released in arc flashes and transferred via radiation, convection, and conduction makes burns the most frequent injury in these events. The extent of burns depends on the duration and temperature of the flash and its distance from the victim(s).
Laboratory measurements of the concussive forces associated with arc flash have found sound levels of 1,400 decibels and pressure levels of 2,160 psi 2 feet from the flash. As fans of heavy-metal music know, hearing is damaged by just 140 dB. Injuries associated with high sound pressure waves usually involve hearing loss but can also include burst eardrums and dislocated ossicles (bones). Elsewhere in the body, the waves can break alveoli in the lungs, producing respiratory distress, and/or cause profuse intestinal bleeding, a ruptured spleen or liver, and progressive peritonitis. They also can cause injuries if they are strong enough to knock personnel down or throw them for a distance.
The burst of energy in an arc flash expels hot gases containing, in part, copper vaporized from conductors, metal vaporized from enclosure walls, smoke from the incineration of insulation, paint and other materials, and copper oxides formed in an arc. The vaporized copper content can be significant: Just 2 to 3 inches of vaporized conductor can yield more than 75 ft3 of copper vapor. The toxic and corrosive plasma has its greatest impact on lungs, skin, eyes, and other mucus membranes.
The electromagnetic radiation produced by an arc flash is highly variable and can occur over a wide bandwidth that includes radio, microwave, infrared (IR), visible, ultraviolet (UV), and X-ray frequencies. Experience indicates that the IR, visible, and UV waves do the most damage.
IR radiation near the visible spectrum (just beyond red wavelengths) can injure the cornea and lens, whereas longer IR wavelengths are absorbed as heat. Excessive visible radiation can damage the skin and cause partial or total loss of sight. UV radiation also can harm the eyes. Ionizing radiation (shorter wavelengths, such as higher-frequency UV and X-rays) can strip electrons from atoms, creating waves of frequencies that can damage DNA and cause cancers.
As the blast wave expands outward at supersonic speed, it turns equipment, tools, and objects in its path into high-velocity projectiles. Equipment enclosures can affect the path and intensity of the energy released in arc flashes. For instance, the energy of an arc blast surging through an open enclosure door is focused through a relatively small opening. Many factors affect the impact of an arc flash event, including the size and shape of the enclosure, the size and placement of its door, and the equipment inside.
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Comments (1)
It may be over conservative, but is this the intent of the definition? There is concern for ventilated switchgear in particular because the path of the arc flash cannot be accurately predicted.