Electrical Safety: Arcing
An electric arc is a visible plasma discharge between two electrodes that is caused by electrical current ionizing gasses in the air. Electric arcs occur in nature in the form of lightning.
With proper control, electric arcs can be harnessed and used industrially for welding, plasma cutting and even certain types of lighting such as fluorescent lighting where a high voltage ionizes the inert gas within a glass tube; the flow of current across the ionized gas liberates visible light. However, for every harnessed electric arc there is an unwanted arc. For example, poorly installed (or poor quality) electrical switches, electrical circuit breakers and other electrical contact points are susceptible to these undesired arcs as contacts are opened and closed.
Implementing Hazard Control Measures for Arc Hazard Protection
Although the electric arc flash hazard only recently has garnered the attention equal to that of the long-recognized hazard of electric shock, the arc hazard is not new. Until the 1980s, occupational electrical hazards generally were described in terms of electric shock or electrocution, a fatal electric shock.
Electric shock entails passage of an electric current through the body. A shock victim generally must make contact with an energized conductor, or otherwise become part of the electrical circuit. Arc flash victims do not have to make physical contact with an energized conductor or be a part of the electrical circuit. The victim may be several feet away from energized conductors or equipment and be severely injured by the intense thermal energy transfer produced by an electric arc. The burn injuries can be radiant burns to bare exposed skin, large-area body burns from ignition or melting of clothing or burns caused by heat transfer through clothing, including flame-resistant clothing.
ABOUT ARC FLASH
Arc flash events usually are very short occurrences — typically less than 0.5 seconds in duration. They can be initiated by a wide range of factors and complicated by other contributing factors. There can be human errors, such as touching an energized conductor the employee thought was de-energized, or accidentally dropping a tool onto an energized conductor.
In addition, there may be environmental causes, such as roof leaks or dirt accumulation in electrical switch gear. There may be management system failures in critical aspects of training, maintenance programs, design specifications or tool requirements, such as allowing use of voltage testing devices not rated for industrial and commercial electrical systems. Switch
gear or other equipment failures during switching or operating interaction also can expose workers to the hazards of electric arc flash.
Most arc flash events occur faster than the unaided human eye can perceive. High-speed photography of laboratory simulations of arcing faults have provided images of how these events can engulf workers in a ball of fire. Electric arcs are very hot; next to the laser, arc flashes create the most intense heat source on earth. Temperatures in the arc can reach 35,000 degrees F. Arc flash events actually are multiple energy events, with intense blast, mechanical and acoustic energy accompanying the intense thermal energy. People within several feet of an arc can be severely burned.
As the body of knowledge and understanding of the arc flash phenomena grew, leadership emerged to change federal regulations; update building codes; improve the design of electrical equipment; increase the application of circuit protection; create safe work practices; train personnel in utility, industrial and commercial work environments; and develop personal protective equipment (PPE). Technologies to further reduce or mitigate arc flash hazards were brought to market, including current limitation, metal cladding, venting to redirect arc blast forces and “arc resistant” designs.
In 2004, the National Fire Protection Association (NFPA) and the Institute of Electrical and Electronics Engineers Inc. (IEEE) established a collaborative research project to further study the phenomena of electric arcs. This collaboration helped advance the protection of workers from heat, pressure, sound, toxicity and other medical effects of exposure to electric arcs.
HAZARD CONTROL MEASURES
Below are a few examples of applying hazard control measures to the unique hazard of electric arc flash. The examples are not all-inclusive, and only serve to illustrate concepts useful in helping assure long-term effectiveness and sustainability. The most effective design and application of an arc flash mitigation program incorporating these control measures usually can best be achieved through a collaboration involving electrical subject matter experts, safety professionals knowledgeable in safety management systems and management resources that can help assure financial and other resources are allocated to the program.
Electrical experts may be knowledgeable in all things electrical, but not in the subtleties of safety management systems. Safety professionals, on the other hand, may be expert in safety management, but have only a general or limited knowledge of electrical equipment and work practices. Bringing experts from different competencies together can produce high quality results in fleshing out details of an effective and sustainable electrical safety program.
Eliminating the hazard — With a high degree of certainty, the best way to protect people from an arc flash exposure is to completely eliminate the arc hazard. It is easier to design new facilities with the intent to eliminate arc flash hazards than it is to retrofit existing electrical installations.
However, an organization that asks the question, “Do we have any exposures that are unnecessary and could be eliminated?” may discover some opportunities. One example is the discovery that a long-established employee break area, located in an electrical control room, was within the calculated arc flash boundary. While the individuals in the break area may not have interacted directly with the electrical equipment, the routine congregation of people within the arc flash boundary created an unnecessary risk. Relocating the break area to a new location eliminated the hazard.
Substituting less hazardous equipment or materials — With increased understanding of the need to reduce worker exposures to arc hazards, equipment manufacturers and system designers are bringing innovative solutions to market to help employers reduce arc flash exposures to their workers. The design of new installations and modifications to existing electrical systems should be analyzed for arc flash hazards, potential exposures and their severity identified and options to reduce severity or frequency of exposures considered.
Design choices that tend to reduce the severity and/or frequency of exposure to arc hazards include high-resistance grounding for industrial power systems, arc-resistant switchgear that direct thermal energy from an arc away from personnel interacting with the gear, current limiting protective devices that reduce the exposure by shortening the arc duration and “smart” switchgear and motor control centers that can reduce exposures by changing how people interact with the equipment during troubleshooting and other maintenance tasks.
Implementing engineering controls to reduce exposure or severity — Engineering controls impacting arc flash exposure span a wide range of consideration. Engineering analysis to identify and quantify potential arc hazard exposures is one very important engineering control measure in arc hazard mitigation. Remote switching and remote racking of power circuit breakers are examples of equipment options that allow personnel to work outside of the arc flash zone. Other engineering functions very critical to arc flash mitigation include maintenance and reliability improvement programs.
It is important that workers responsible for operating and maintaining the electrical system are familiar with the effects of their work on the arc flash incident energy. For example, if there is a process upset and they change out a fuse to a larger size (no fuse of the existing size was available quickly), then they need to understand that the arc flash energy of the equipment has been changed and may be higher. Protective devices including protective relays, circuit breakers and switchgear must be maintained, inspected and tested to help assure designed functionality when operating during an arc fault.
Given that some of the highest frequency and severity of exposures to arc hazards involve interaction with 600-volt-class motor control centers, programs to increase mean time between the failure of motors serve to reduce maintenance and operations personnel interaction with motor control centers. Consider these tasks that occur every time a motor fails mechanically or electrically: the motor starter disconnect switch is operated at least twice (to disconnect and eventually to re-energize), voltage testing is performed to verify electrical isolation, motor leads are disconnected and then reconnected and fuses may be removed and reinstalled. Each one of these interactions has some risk for an arc flash incident. Electrical equipment and systems reliability improvement is an important component of an arc flash hazards mitigation program.
Warnings, signs and other communications — Labels and signage ensure personnel understand their proximity to potential hazards. Signs and labels may be temporary or permanent in nature, depending on the work activity or duration of the potential hazard. The warning could be a sign on switchgear, or a boundary marked on the floor. It could be a temporary barricade during certain work activity.
Because signage and labeling practices may not be consistent industry-wide, contractors working in multiple facilities need to be aware of each facility’s standards. One important consideration is consistency and uniformity, at least within the site operations, to help assure common understanding by the people potentially at risk.
Administrative controls, including safe work practices — Administrative controls include training and qualification requirements, job procedures, planning tools, lock out practices and auditing systems. These administrative controls are well addressed in NFPA 70E. However, some circumstances may call for additional procedures not specifically described in the standard.
Personal protective equipment — The use of PPE, including flame-resistant clothing, face shields and other accessories, is a critical-control measure of any arc flash hazards mitigation program. However, it should not be the only control measure.
Arc flash PPE serves to minimize the injury severity in the event of an arc flash incident. In order for the PPE to perform effectively, its arc thermal performance rating (ATPV) must meet or exceed the thermal energy transfer during the arc flash incident. The best way to predict the thermal energy transfer, or incident energy, is to have performed an arc flash hazard analysis. PPE clothing and accessories then can be selected on performance rating (i.e Hazard Risk Category 1-4 from NFPA70E) and matched to the predicted energy exposure.
There are many flame-resistant clothing products designed for arc flash application on the market today, but all are based on two technologies: fabric made from (a) inherently or (b) chemically treated, flame-resistant fibers. “Inherent,” as it relates to flame-resistant garments, means that the flame-resistant properties always have been a part of the fibers used in the fabric. The protection is intrinsic, permanent and cannot be washed out or worn away, no matter how the garment is used or laundered. The terms “treated” or “topically treated” refer to a manufacturing process whereby a mixture of chemicals is added to a naturally flammable fabric, such as cotton or cotton/nylon blends. Unlike inherent FR fibers, treated fibers may have their flame-resistant properties diminished or removed, depending on how the garment is laundered or the chemicals to which it may be exposed to in the work environment.
In selecting protective garments, the most important criteria are that it is from a reputable manufacturer and is labeled with the Hazard Risk Category that meets or exceeds the potential incident energy exposure. The selection of fabric technology may depend on frequency of use, environmental conditions, worker feedback from wear trials, garment durability and an evaluation of total costs that takes the initial purchase price, garment life expectancy and laundering and maintenance costs into consideration.
At-risk workers need to be educated on when, where and how to properly use PPE garments and accessories. PPE garments and accessories need to cleaned, inspected and maintained in accordance with manufactures’ recommendations in order to preserve the designed protection performance.
CREATE A PROGRAM
An effective arc flash hazards mitigation and protection program is more than buying flame-resistant garments and making them available for potentially exposed personnel to wear. An effective program involves management commitment to designing and implementing a comprehensive set of proven control measures consistent with occupational safety and health management systems standards, such as ANSI Z10. An effective program should include, but is not limited to:
·Project engineering practices that include analysis for opportunities to eliminate or reduce arc flash exposure through wise evaluation of engineering operations in equipment and systems design.
·Maintenance programs that help assure electrical equipment is kept in proper condition to ensure the safety features and functionality critical to prevention and/or mitigation of arc flash hazards maintains or exceeds design intent.
·Warnings, labels, signs and other means to help assure personnel are informed of identified hazards.
·Administrative/management controls to help ensure personnel are trained and qualified fro their roles and responsibilities, and that proper tools and resources are available to perform work safely and that all elements of the program periodically are audited to monitor and control drift from designed expectations.
·Flame-resistant personal protective garments and accessories that are engineered and manufactured to recognized industry standards, selection of PPE based on engineering analysis to determine predicted thermal incident energy, PPE rated performance meets or exceeds the exposure potential and that personnel at risk know when, where, what and how to wear PPE appropriate for the task and exposure.
Plant engineers can manage arc flash hazards through understanding and application of appropriate regulations and standards, implementing hazard assessments, evaluating mitigation options, reducing risks and designing and implementing control measures to help assure an effective and sustainable program.
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