Arc flash

Published on January 2017 | Categories: Documents | Downloads: 16 | Comments: 0 | Views: 104
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It's time for a new approach to a difficult issue when working near open-air substations, switchyards and transmission and distribution lines. For more than 100 years, we've worked hard to avoid contact with energized electrical equipment. In the recent past, arc flash (the inadvertent, unintentional, instantaneous conversion of electric energy to thermal energy in free air in the vicinity of workers) has become an issue within the electrical power industry. The National Fire Protection Association addressed arc flash in NFPA 70E — Standard for Electrical Safety in the Workplace. IEEE has revised National Electrical Safety Code C2 to include arc flash considerations (effective Jan. 1, 2009) and developed Standard 1584 — IEEE Guide for Performing Arc Flash Hazard Calculations. The energy released during an arc flash can be incredibly damaging. Hair and clothing can burn instantaneously. Skin can be severely damaged or completely destroyed. As little as 2 calories (approximately 8 W/sec) in contact with one square centimeter of skin can destroy skin tissue. There are textbooks that describe the blistering, charring and full-thickness burns that can be caused by highvoltage arcing. Understanding the risk is key. Alerting everyone to potential incident-energy levels due to an arc flash should be the responsibility of every manager and every work group leader. Selecting appropriate personal protective equipment (PPE) is essential. Incident-energy contour maps (similar to topographical contour maps, but marked to show possible arc flash energy in calories per square centimeter of exposed skin surface at predetermined working distances) should be developed to provide a means for workers to select areas for on-site meetings and other activities. The protection engineer who selects fuses, circuit breakers and relaying schemes must be required to provide incident-energy contour maps so that dwell time, protective equipment, working distance and incident-energy levels can be coordinated to reduce risk. It goes without saying that walking through a substation is a much lower risk activity than opening a disconnect switch. We should borrow from NFPA 70E and prepare tables that provide a means of classifying the hazard/risk based on work activity. These tables could be used to coordinate work activity and PPE requirements. Although arc flash incidents are unlikely, they can have severe consequences. However, technical committees sponsored by NFPA, IEEE and other concerned parties have not reached consensus as to which methodology best addresses arc flash concerns. Unfortunately, we have yet to develop PPE withstand capability curves (time-current curves) for arc flash scenarios. (Presently, we develop calculations for specific maximum short-circuit current values and need to repeat the calculations when maximum short-circuit current or fault-clearing time increases or decreases.) This may be due to the fact that NFPA 70E, IEEE 1584 and NESC C2 each approach arc flash from different perspectives. It may also be due to the fact that different time-current curves are needed to evaluate different voltage levels, different working distances and different incident-energy levels. We need to develop families of time-current curves that assure compliance over a wide range of time and current values, and soon. In the meantime, the ultraconservative methodology from Annex D,

Section D.7 of NFPA 70E, 2004 should be used as it provides an easy-to-understand technique for evaluating system voltage, short-circuit current, arcing time and working distance. Incident-energy contour maps and hazard/risk category tables, displayed at access points to substations, should be used to increase awareness of arc flash hazards and emphasize the fact that only workers with appropriate PPE can be in a substation switchyard when work is in progress near energized electrical equipment. Everyone agrees that safety is a core value and that our industry has a remarkable safety record. Going forward, we need to enhance our safety awareness by implementing simple, easy-to-use techniques that assure everyone understands potential arc flash dangers when they approach energized electrical equipment. The concepts discussed in this article are a step in that direction. Recognizing the importance of arc flash issues, I'd like to see IEEE, NFPA and other industry groups get together to develop one standard method (that includes the use of time-current withstand curves) to select appropriate PPE. The same techniques used to determine transformer withstand and fuse clearing time should be used to determine PPE requirements. If you would like to get involved, send me an email and I'll keep you informed as this issue continues to evolve.

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