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 conductor 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 death or severe injury occur every day, of which 1 to 2 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 electricity-related injuries are from arc flash.
What Are the Effects of Arc Flash?
Arc Flash Injuries
The temperatures associated with arc flash can exceed the surface temperature of the sun. The accompanying shock wave is capable of propelling shrapnel at speeds up to 700 mph. 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, even 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.
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.
Even when personal protective equipment (PPE) is worn, 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.
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 the shut-down of production lines. 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.
What Regulations Govern Arc Flash Safety?
In the United States, arc flash safety is regulated by OSHA 29 CFR-1910, subpart S. This standard defines the qualifications needed to work on energized equipment, and states "Safety related work practices shall be employed to prevent electric shock or other injuries resulting from either direct or indirect electrical contacts...." It does not list specific work practices, but instead refers to the standards established by the National Fire Protection Association (NFPA).
Arc flash labeling in Canada is governed by CSA-Z462.
NFPA 70E - Arc Flash Labeling
NFPA 70E is the National Electric Code. Section 110.16
Warning Labels: "Switchboards, panel boards, industrial control panels, and motor control centers that are in other than dwelling occupancies and are likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked to warn qualified persons of potential electric arc flash hazards. The marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment."
In addition, NFPA 70-2000, "Standard for Electrical Safety Requirements for Employee Workplaces", provides the specific steps employers must take to be in compliance with OSHA regulations. These include:
- Establishing a safety program with defined responsibilities
- Calculations for the degree of electrical safety hazard
- Personal protective equipment (PPE) for workers
- Electrical safety training procedures
- Standards for tools and equipment for workers
- Warning labels on equipment
IEEE 1584 - Electrical Safety Calculations
IEEE Standard 1584-2002, "Guide for Electrical Safety Regulation," provides companies with the information and formulas necessary to calculate safety zones and necessary PPE for work with energized electrical equipment. This information will be required on arc flash labels by the 2008 version of NFPA 70E.
Techniques and devices such as passive ultrasonic analysis, current limiting fuses, or infrared scans are excellent tools that can help to prevent arc flash injuries, but they do not meet NFPA requirements or provide full protection against arc flash.
What Are the Requirements for Arc Flash Labels?
OSHA considers arc-flash labeling to be the responsibility of the equipment owners, rather than the manufacturer or installer. NFPA standards require that equipment be marked in the field, rather than at the factory, in order that actual operating specifications can be used in hazard calculations. But what equipment should be labeled?
NFPA 70E states that "Switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers in other than dwelling occupancies, which are likely to require examination, adjustment, servicing, or maintenance while energized, shall be field marked to warn qualified persons of potential electric arc flash hazards." In addition to the equipment required, many companies choose to mark other equipment that carries a risk of arc flash, such as disconnect switches.
Circumstances that require work on energized equipment may include:
- Circuits providing power to life-support equipment
- Circuits that are part of a larger process that cannot be completely shut down
- Circuits providing power to emergency lights and other safety equipment
- Any other cases in which de-energizing the equipment would pose a greater hazard than failing to de-energize
- Any other case in which de-energizing the equipment is not feasible
By law, only equipment installed or modified in any way after 2002 requires an arc flash label. A recent survey shows that only 14% of equipment installed before 2002 has arc flash labels. Safety-conscious companies will label all potentially hazardous equipment, regardless of when it was installed. By applying markings consistently throughout their facilities, companies emphasize the risks associated with arc flash and the need for appropriate work practices. Many accidents are caused by people not thinking before they act. Having equipment properly labeled helps prevent this from happening.
A final consideration in making labeling decisions is the NFPA requirement that labels should be easily visible to workers before they inspect or service equipment. Labels should be large enough to attract attention even in a cluttered visual field.
Currently, NPFA 70E Section 110.16 only requires that arc flash labels be posted on equipment that is likely to be serviced while energized. It does not specify what information the labels should include, or how they should be designed. However, the newest version of the document, to be released in 2009, will require labels to state either the available incident energy or the level of PPE required.
IEEE 1584 suggests that arc flash labels contain one of two signal words:
- The signal word DANGER indicates a hazardous situation that, if not avoided, will result in death or serious injury. This word should be used only in the most extreme situations. As a guideline, NFPA suggests using DANGER when incident energy is above 40 cal/cm2. The word DANGER should be in white letters on a red background.
- The signal word WARNING also indicates a hazardous situation that, if not avoided, will result in death or serious injury. It is appropriate for use in dangerous situations judged to be less hazardous than those requiring a DANGER signal. The word WARNING should be in black on an orange background.
Duke Power Heat Flux Calculator
Free software to help determine arc flash boundaries. Heat Flux Calculator.
Few power services companies will calculate the information required on arc flash labels. Software is available that can provide the incident energy available from a piece of equipment at an assumed working distance, as well as other information that companies may choose to include on arc flash labels. Click the link at left for a free arc flash boundary calculator.
These guidelines establish the minimum information required on arc flash labels by law. More information is necessary for maximum safety. We suggest that the following additional information be included on arc flash labels:
- Available incident energy
- Required PPE
- Assumed working distance for incident energy calculations
- Flash protection boundary, the distance at which PPE is needed to prevent severe burns in the even of an arc flash explosion.
- Limited approach boundary, the closest an unqualified person can approach unaccompanied by a qualified worker
- Restricted approach boundary, the closest to exposed, energized equipment a qualified worker can approach without appropriate PPE
- Prohibited approach boundary, the closest approach possible without the risk of arcing. Closer approach is equivalent to direct contact with energized equipment.
Try the free online Arc Flash Labeling Quiz
Links to other Arc Flash Resources including consultants, clothing, and PPE.