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Arc Flash Energy Underestimated __________________________________________________________________________________________________________

Electrical Engineering & Computer Science Article – November 2016 Authors – Jay Prigmore, Ph.D., P.E. and Justin Bishop, Ph.D., P.E., CFEI, CFVI According to the National Fire Protection Agency (NFPA), arc flash injuries have surpassed electrical shock injuries as the most common form of electrical workplace injuries. Approximately every 30 minutes a worker is injured in an arc flash incident. Over 2,000 injuries occur every year in the United States alone, more than five a day on average [Arc Flash Phenomena]. Standards set by organizations such as NFPA and the Institute of Electrical and Electronics Engineers (IEEE) help inform workers about dangers they are exposed to on a daily basis and provide methods for protection and equations for determining the incident energy and its corresponding hazard exposure level at a particular piece of energized equipment. NFPA 70E and IEEE 1584 are “guidelines and recommended practices” that are not mandated by the Occupational Safety and Health Association (OSHA) for compliance but are generally accepted and recognized as industry best practices. The forthcoming update for IEEE 1584 is expected to revise calculations of incident energy based on recent testing. The current equations adopted in 2002 have a discontinuous region around 1,000 V where the results may begin to underestimate the incident energy for higher voltages. The updated equations will take into account new empirical data and provide a much improved estimation based on different working conditions and electrical system information. The new IEEE 1584 equations will be continuous with improved accuracy from 208 V up to at least 38 kV. These proposed changes are being evaluated within the IEEE 1584 working group with a vote to go to a ballot near the end of 2016. If the update is passed (which it is expected to be), publication of the update is expected in 2017 or 2018. Whether or not the update to the existing 2002 equations will result in changes in Personal Protective Equipment (PPE) and/or procedure requirements remains to be seen. Once the update is released, it may be recommended to re-perform arc flash studies using the updated equations and re-assess any changes in PPE and/or procedure requirements. Should there be recommendations for updating PPE or procedures, energized devices, consequently, would require new calculations of incident energy and corresponding hazard exposure. Protecting Workers A worker’s recommended PPE is based on the calculated incident energy and its corresponding arc flash hazard exposure level for a particular piece of equipment being worked on in an energized state. Unfortunately, even with proper PPE, exposure to an arc flash event may still result in injury or possibly fatality. Examples of required PPE are shown below based on NFPA 70E – 2015 edition and the different incident energy levels [NFPA70E, PowerHawke].

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Figure 1. PPE Requirements for each Hazard Level. The goal for any arc flash mitigation technique is to reduce the incident energy level as much as reasonably possible. In practical terms, the aim of acceptable industry practice is to reduce the hazard to level 2 or lower, which allows workers to wear more comfortable clothing that maximizes mobility, vision, and dexterity compared to level 3 and above. The PPE shown for hazard levels 3 and 4 in Figure 1 is known as an arc flash suit but is also referred to as, for example, a flame suit, moon suit, flash suit, or arc suit. Each level of PPE is rated based on the incident energy protection level it provides. Arc flash suits are rated by the Arc Thermal Performance Value (ATPV). The ATPV is defined as the maximum incident heat energy a fabric can absorb to reduce the injury to a 2nd degree burn (1.2 cal/cm2). Even if a worker is wearing his properly-rated PPE, he may still experience 2nd degree burns [ATC]. While the arc suit ratings reach 40 cal/cm2 (level 4 maximum), there are arc flash suits that are rated up to 100 cal/cm2 (see Figure 1). Beyond 100 cal/cm2 there is no known commercial PPE, as far as the authors are aware, that can protect a worker. Where possible, the incident energy should be reduced by engineering the hazard out or by procedural changes which remove the operator from or limit exposure to hazardous conditions, since PPE is generally considered a last resort defense against an arc flash. In addition, NFPA 70E – 2015 edition has recently removed hazard level 0 (<1.2 cal/cm2), although it is still widely understood in practice.

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Figure 2. An example of an arc flash event using a dummy [ArcWear].

Updates to IEEE 1584-2002 Calculations Engineers perform arc flash calculations by gathering electrical system data from the particular site and typically entering it into commercial software to obtain the incident energy level at the particular piece of equipment. The results of the simulations yield the level of PPE the worker must wear when approaching the arc flash boundary limit. An arc flash boundary limit is defined as the distance from the piece of equipment that would have an incident energy level of less than or equal to 1.2 cal/cm2 (2nd degree burn). Each piece of electrical equipment is required to have an “arc flash label” which informs workers of the degree of hazard exposure at that particular piece of equipment. The basic parameters needed for calculating the incident energy are listed below:

• Equipment Class • System Grounding Method • Conductor Gap • Voltage Class • Available 3-phase Short-circuit current • Working Distance • Arc Duration (Protective device clearing time)

There are two methods of calculating incident energy; one is the IEEE 1584 method and the other is known as Lee’s method. Presently, Lee’s method is mostly used in the medium voltage range 15 kV class and above, while the IEEE 1584 method is mostly used below 15 kV. At the time the 2002 version of IEEE 1584 was published, industry practitioners considered the IEEE 1584 equations adequately accurate below 1,000 V, but the equations began underestimating incident energy above 1000 V [EC&M]. As the voltage levels increased, the underestimation of the IEEE 1584 equations grew significantly which is why Lee’s method is typically used for higher voltages. The IEEE 1584 working group recognized the need to improve upon the 2002 equations as those equations only took into account a small subset of test points and system configurations. The calculation method was considered state-of-the-art but is now being re-evaluated to account for more configurations.

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Over the past 14 years, the working group was tasked with performing subsequent tests at multiple voltage levels, current levels, conductor spacing, conductor orientation, usage of enclosures, and other system configurations with the aim to provide updated equations that specifically take into account the end user’s individual system configurations for improved estimations [footnote]. Some of the approximations and measured test points of the original test data can be seen in the 3 figures below. The new test data is not published yet and, therefore, cannot be included in this article, but changes are expected.

Figure 3. 600V Empirical Data Comparison [EC&M].

Figure 4. 4.16 kV Empirical Data Comparison [EC&M].

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Figure 5. 13.8 kV Empirical Data Comparison [EC&M].

In conclusion, the IEEE 1584-2002 equations are recognized as benefiting from improvement as those equations only took into account a small subset of test points and system configurations. Consequently, the IEEE 1584-2002 arc flash equations are being revised to better coincide with the most recent experimental data which covers a number of system conditions and physical layouts. The IEEE 1584 revision is expected to be released in 2017 or 2018. Once the standard update is released, it may be recommended to re-perform arc flash analyses using the newly updated equations to determine if any changes in PPE or procedural changes are required. Expanding our knowledge to continuously improve industry best practices will also result in continuous improvement in adequate selection of PPE during hazardous work conditions. References [Dupont] - Dupont - http://www2.dupont.com/Electrical_Arc_Protection/en_GB/arc-flash/arc-flash-definition.html [ArcWear] - Arc Flash Event - http://www.arcwear.com/ [EC&M] - Arc flash calculations/figures - http://ecmweb.com/content/putting-arc-flash-calculations-perspective [PowerHawke] - Power Hawke Arc flash table - http://powerhawke.com/our-solutions/electrical-safety/arc-flash/ppe/ [NFPA] - NFPA 70E 2015 – Table 130.7 (C)(16) [Fire Protection Agency] - “Occupational Injuries From Electrical Shock and Arc Flash Events,” The Fire Protection Research Foundation – NFPA – March 2015. www.nfpa.org/.../electrical/rfarcflashoccdata.pdf? [Arc Flash Phenomena] - “Arc Flash Phenomena” – IEEE, NFPA, https://standards.ieee.org/about/arcflash/prospectus.pdf [ATC] - “ARC Frequently Asked Questions’ – American Transmission Company. http://www.atcllc.com/wp-content/uploads/2011/06/ARCFAQs.pdf