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Stanley Arc-Flash Power Point

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Stanley Arc-Flash Power Point

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    3. Goals & Objectives What Is Arc Flash? Historical Context Rules & Regulations NFPA 70E and NESC 41 Arc Physics & Computation Approaches

    4. What Is It? Need Basic Definition for Common Understanding Need to Understand Context of Issues Need to See What It Is

    5. Why of Concern?

    6. Electrical Hazards Fire – First hazard to be recognized due to economic losses Formed basis of first NEC in 1897 Shock/Electrocution – Second hazard to be generally recognized and addressed Arc Flash/Blast – Last hazard to be recognized First addressed in NFPA 70E-1995

    7. Potential Consequences of an Arc-Flash Incident Injury Death (in the case of burn injuries, often a slow and painful death) Monetary Cost to Employer Damage to Equipment, Down Time Personnel Injury: ~$25,000,000/person Death of Personnel: ~$15,000,000/person 2% of workplace injuries are electrical in nature, but they account for 28% of the costs of injuries

    8. What Is An Arc Flash? Defined as “a dangerous condition associated with the release of energy caused by an electric arc” Created by such things as: Equipment failure (misaligned contacts, ferroresonance, insulation failure) Human error (dropping a tool, body part getting too close to energized parts, stupidity)

    9. Classic Arc Flash

    10. What Is It?

    11. Staged Test

    12. When Do Arcs Occur? Primarily from physical movement: Switch or Circuit Breaker is opened/closed Contactors/starters pick up or drop out Door or Cover is opened/closed Test Equip. and Safety Grounds are installed Equipment is inserted/withdrawn from energized bus

    13. Electric Arc History Arcs have been studied since Aristotle did experiments with sheep’s wool First studies were static electricity and lightning Mid-1800s began to experiment and understand electric arcs Discharges studied by physicists as well as engineers

    14. Arc History Partial discharges along with arcs studied in early 1900s – Peek’s Gradient, Paschens Law, Etc Arc physics still under study as mathematical models still have limits

    15. Anatomy of An Electrical Arc Flash

    16. Components of Arc Flashes Two basic components of arc flashes: Heat Radiation (quantified as “Incident Energy Level”) Pressure Wave (also known as “Arc Blast”)

    17. Heat Electric arc temperatures are considered to be approximately 4 times hotter than the sun’s surface. Temperature at arc terminals can reach 35,000°F (for reference, the surface temperature of the sun is ~9,000°F, and the temperature of a wood fire is ~900°F)

    18. Heat Radiation Heat radiation exposure is a function of: Distance to arc Available fault current Fault clearing time Equipment type Gap between conductors (determined by equipment) Vaporizes metals Ignites clothing 2030F - skin not curable (cell death) Possible to become fatally burned or seriously injured when working at a distance of 10 feet or more from an electrical arc.

    19. Personnel Reaction to Heat Some potential effects on personnel due to Heat Radiation: External burns, potentially very severe Internal burns, such as to the lungs due to ingestion of vaporized metal and superheated air Health effects due to inhalation of toxic gases and heavy smoke due to the burning of paint, insulators, and other components Partial or total loss of sight Disability Death

    20. Pressure Wave Electrical blast (or explosion) is the result of the rapid expansion of air caused by an electric arc.

    21. Pressure Wave Peaks in the first ˝ cycle of fault (~ 9msec) Pressure levels of 2,160 pounds per square foot (psf) in the immediate vicinity of the blast have been detected Caused by superheating of air and vaporizing of conductors (air expands to roughly 1670 times and copper expands to roughly 67,000 times its volume – i.e. 1 in3 becomes 1.4 yd3) Function of arc fault gap and available fault current

    22. Pressure Wave During an arc blast metal droplets travel 10 ft or more and faster than the speed of sound! Expansion produces explosion that results in: Molten metal Fragmented metal High temperatures Pressure on the body

    23. Pressure Wave (cont.) Some potential effects on personnel due to Pressure Wave Injury due to blast Collapsed eardrums leading to partial or near-total loss of hearing and possibly tinnitus – Sound levels of 141.5 decibels at 2 feet from the blast have been detected Collapsed lungs Injuries due to shrapnel being ejected from equipment One positive benefit: can lessen effects of Heat Radiation due to personnel being thrown away from equipment, but this can also lead to other injuries

    24. Arc Study Studied in high voltage labs Voltage Steady State Dynamic Transient Use Gaps Spheres Rods Planes

    25. Applicable Testing Standards International Electrotechnical Committee (IEC) Canadian Standards Association (CSA) IEEE - IEEE 4 European Union Standards – BS, DIN,etc Japanese Standards (JIS)

    26. Arc Physics Multiple Parameters Research is Ongoing

    27. Rules & Regulations OSHA NFPA NESC RUS

    28. Which Applies? Type of employer Risk Manager and Legal Counsel

    29. OSHA Federal Standard 1910.269 – Employers must supply appropriate clothing General Duty Clause – Employer must identify and quantify risks OSHA May 15, 2008 Ruling – Employer must provide PPE at no cost State of Colorado – See Regulations

    30. NFPA 70 and 70E National Electrical Code - NFPA 70 NFPA 70E – Standard for Electrical Safety in the Workplace Original 2000, Updated 2004 to compute flah energy Flash boundaries Flash identification with “stickers” Select PPE based on energy

    31. Flash Protection Boundaries

    32. Arc-Flash Hazard Warning Label

    33. NESC 2007 Section 41 410 (A) Effective January 1, 2009 Employer must do assessment to determine potential exposure to arc flash Clothing provided for > 2 cal/cm2

    34. NESC 41 Computations must be performed Include Available fault current Arc duration Distance from arc to employee Table 410-1

    35. RUS 7 CFR Ch. XVII Subpart E – Electric System Design, 1724.40 Compliance with National Electrical Safety Code (a) “Borrowers shall ensure that its electric system…is designed, constructed, operated, and maintained in accordance with all applicable provisions of the mist current and accepted criteria of the National Electrical Safety Code” and requirements of State and local governmental entity. RUS has initiated a task force to review its position on arc flash

    36. NFPA 70E vs NESC 41 Regardless of legal situation, computations must be performed Arc energy estimated Appropriate PPE selected NFPA 70E contains guidance on computations NESC 41 provides no guidance except that listed above

    37. Electric Arcs Are Functions Of … Weather – Temperature and Humidity Atmospheric Pressure – Altitude Gap Size & Type Terminal Metals / Characteristics Voltage – Determines voltage gradient (V/cm) and flashover level Fault Current – Power follow current magnitude Determines plasma temperature Heat Flux => Heat Energy = cal/cm2 Time – Fault duration

    38. Arc Effects – Function Of …. Electric Arc Parameters (Previous Slide) Working Distance – Distance from arc source to person

    39. Arc Physics and Computation Approaches IEEE 1584 Industry Research Computer Simulations

    40. Flash Boundary and Incident Energy Level Calculation – IEEE 1584 Formulas based on testing are applicable for systems with: Voltages from 208V to 15kV Bolted fault current from 700A to 106kA Gaps between conductors of 13 - 152 mm Faults involving three phases Formulas also based on statistical analysis to provide results that covers 95% of the cases (i.e. 5% of the arc flashes statistically will be worse than calculated) Also have theoretical formulas for systems where the testing-based formulas do not apply

    41. Arc Calculations & Research All have limitations Assumptions for atmospheric conditions 3LG vs 1LG Arc behavior Choice of computation approach is based on what results are desired in what environment None Currently Address Pressure Wave

    42. Approach May Be Different Based on Type of Equipment Metal Clad, Metal-Enclosed Padmounts Gas (SF6) Substations Air Bus Substations Underground Lines Overhead Lines

    43. Example Theoretical Computation 230kV Air Bus / Overhead Line Operating Voltage = 230kV 1LG Fault = 15kA Symmetrical rms Total Clearing Time = 9 Cycles Distances (NESC 410.1 Does Not Specify) Gap = 15.6 in Working Distance = 30 in

    44. 230kV Air Bus / Overhead Line

    45. 230kV Air Bus / Overhead Line

    46. Example Theoretical Computation 12.47kV Overhead Line Operating Voltage = 12.47kV 1LG Fault = 600A Symmetrical rms Fault Location = Midpoint of line Distances (NESC 410.1) Gap = 2 in Working Distance = 15 in Fault Interrupter – 50-4H Recloser

    47. 50-4H Recloser Sequence of Operation Fast Trip Setting Trip = 0.047 sec or ~ 3 cycles Open Time Delay = 1 ˝ sec delay Reclose Fault Still Exists Time Delay Trip = 0.2257 sec or ~ 14 cycles

    48. 50-4H Recloser First Trip =0.047 sec (~ 3 cycles)

    49. 50-4H Recloser Trip = 0.2257 sec (~ 14 cycles)

    50. Example Theoretical Computation 24.9kV Overhead Line Operating Voltage = 24.9kV 1LG Fault = 600A Symmetrical rms Fault Location = Midpoint of line Distances (NESC 410.1) Gap = 4 in Working Distance = 15 in Fault Interrupter – 50-4H Recloser

    51. 50-4H Recloser Sequence of Operation Fast Trip Setting Trip = 0.047 sec or ~ 3 cycles Open Time Delay = 1 ˝ sec delay Reclose Fault Still Exists Time Delay Trip = 0.2257 sec or ~ 14 cycles

    52. 50-4H Recloser First Trip =0.047 sec (~ 3 cycles)

    53. 50-4H Recloser Trip = 0.2257 sec (~ 14 cycles)

    54. Theoretical Computation 24.9kV Overhead Line Example 2 Operating Voltage = 24.9kV 1LG Fault = 300A Symmetrical rms Fault Location = Midpoint of line Distances (NESC 410.1) Gap = 4 in Working Distance = 15 in Fault Interrupter – 50-4H Recloser

    55. 50-4H Recloser First Trip =0.047 sec (~ 3 cycles)

    56. 50-4H Recloser Trip = 0.2257 sec (~ 14 cycles)

    57. Means of Mitigating Arc Flash Hazards Work only on electrically safe (i.e. de-energized) equipment – still exposure to hazard in making equipment electrically safe Increase distance to possible arc Remote mounting of control equipment or SCADA Remote racking of equipment – move operators away from flash zone Hookstick operation Reduce fault current level Use current-limiting devices – need to cover minimum arc current

    58. Means of Mitigating Arc Flash Hazards (cont.) Reduce fault clearing times (does not affect pressure wave component): Bus differential Temporarily change relay settings while work is being performed (e.g. 50 device with “safety switch”) Non-coordinated overcurrent protection Zone-selective interlocking Arc detection systems such as ABB Arc-Guard or similar system in metal clad or metal enclosed switchgear Fuses – need to cover minimum arc current Use “Arc-Resistant” equipment Will likely be other means developed

    59. Summary Arc Flashes Components – Heat Radiation + Pressure Wave, Both pose serious dangers to personnel Regulations vary with industry Differing computations for differentr equipment Must mitigate Change working conditions PPE (No pressure defense)

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