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Electrical Hazards

Electrical Hazards. Electrical Hazards. What are the hazards as you approach electrical equipment to perform work?. Electrical Hazards. Shock Arc flash Heat Fire Arc blast Pressure Shrapnel Sound. Example of an arcing fault. Basic Electrical Theory. I = V / Z

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Electrical Hazards

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  1. Electrical Hazards

  2. Electrical Hazards What are the hazards as you approach electrical equipment to perform work?

  3. Electrical Hazards • Shock • Arc flash • Heat • Fire • Arc blast • Pressure • Shrapnel • Sound Example of an arcing fault

  4. Basic Electrical Theory I = V / Z • What happens with shock? • What happens when there is a fault? • What is the difference between a short-circuit and an arcing fault?

  5. Electric Shock • Over 30,000 non-fatal electrical shock accidents occur each year • Over 600 people die from electrocution each year • Electrocution remains the fourth (4th) highest cause of industrial fatalities • Most injuries and deaths could be avoided

  6. Human Resistance Values

  7. Electric Shock Human body resistance (hand to hand) across the body is about 1000 W Ohms law: I = V / R amps = 480 volts / 1000 W = 0.48 amps (480 mA) Product safety standardsconsider 5 mA to be the safe upper limit for children and adults

  8. Electric Shock mAAffect on person 0.5 - 3 - Tingling sensations 3 - 10 - Muscle contractions and pain 10 - 40 - “Let-go” threshold 30 - 75 - Respiratory paralysis 100 - 200 - Ventricular fibrillation 200 - 500 - Heart clamps tight 1500 + - Tissue and organs start to burn

  9. (A) Touch Potential (B) Step Potential (C and D) Touch / Step Potential Current passing through the heart and lungs is the most serious Electric Current Pathways

  10. Electric Shock Injury

  11. Arc Flash As much as 80% of all electrical injuries are burns resulting from an arc-flash and ignition of flammable clothing Arc temperature can reach 35,000°F - this is four times hotter than the surface of the sun Fatal burns can occur at distances over 10 ft Over 2000 people are admitted into burn centers each year with severe electrical burns

  12. Arc Blast An arc fault develops a “pressure wave” Sources of this blast include: • Copper expands 67,000 times its original volume when vaporized • Heat from the arc, causes air to expand, in the same way that thunder is created from a lightning strike This may result in a violent explosion of circuit components and thrown shrapnel The blast can destroy structures, knock workers from ladders, or across the room

  13. A B A B BoltedShort Circuit Arcing Fault Current Thru Air

  14. Electric Arc Molten Metal 35,000 °F Pressure Waves Sound Waves Shrapnel Copper Vapor: Solid to Vapor Expands by 67,000 times Hot Air-Rapid Expansion Intense Light

  15. Personnel Hazards Associated With Arc Flash & Arc Blast • Heat – burns & ignition of material • Arc temperature of 35,000oF • Molten metal, copper vapor, heated air • Second degree burn threshold: • 80oC / 175oF (0.1 sec), 2nd degree burn • Third degree burn threshold: • 96oC / 205oF (0.1 sec), 3rd degree burn • Intense light • Eye damage, cataracts

  16. Personnel Hazards Associated With Arc Flash & Arc Blast • Pressures from expansion of metals & air • Eardrum rupture threshold: • 720 lbs/ft2 • Lung damage threshold: • 1728 - 2160 lbs/ft2 • Shrapnel • Flung across room or from ladder/bucket

  17. Overcurrent Protection Role Flash protection boundaries and incident energy exposure calculations both dependent upon: Duration of arc-fault or time to clear • Speed of the overcurrent protective device Arc-fault current magnitude • Available fault current • Current-limitation can reduce

  18. IEEE / PCIC & NFPA 70E • Ad Hoc Safety Subcommittee • Users • Consultants • Manufacturers • Medical experts • Following are some of the tests run • All of the devices used for this testing were applied according to their listed ratings

  19. IEEE / PCIC Staged Arc Flash Test Set-up

  20. 22.6 KA Symmetrical Available Fault Current @ 480V, 3 Phase Test No. 4 6 cycle STD 640A OCPD Non Current Limiting with Short Time Delay Set @ 6 cycle opening Fault Initiated on Line Side of 30A Fuse 30A RK-1 Current Limiting Fuse Size 1 Starter

  21. Test 4 Still Photo

  22. Test 4 Still Photo

  23. Test 4 Still Photo

  24. Test 4 Still Photo

  25. Test 4 Still Photo

  26. Test 4 Still Photo

  27. Test 4 Still Photo

  28. Results: Test No.4 Sound 141.5 db @ 2 ft. P1 T2 >2160 lbs/ft2 >225oC/437oF T1 >225oC/ 437oF T3 50oC/122oF > Indicates Meter Pegged

  29. 22.6 KA Symmetrical Available Fault Current @ 480V, 3 Phase Test No. 3 601A. Class L Current Limiting Fuse Fault Initiated on Line Side of 30A Fuse 30A RK-1 Current Limiting Fuse Size 1 Starter

  30. Test 3 Still Photo

  31. Test 3 Still Photo

  32. Test 3 Still Photo

  33. Test 3 Still Photo

  34. Results: Test No.3 Sound 133 db @ 2 ft. P1 504 lbs/ft2 T2 62oC/143.6oF T1 > 175oC/ 347oF T3 (No Change From Ambient) > Indicates Meter Pegged

  35. 22.6 KA Symmetrical Available Fault Current @ 480V, 3 Phase Test No. 1 601A. Class L Current Limiting Fuse 30A RK-1 Current Limiting Fuse Fault Initiated on Load Side of 30A Fuse Size 1 Starter

  36. Test 1 Still Photo

  37. Test 1 Still Photo

  38. Test 1 Still Photo

  39. Test 1 Still Photo

  40. Results: Test No.1 Sound (No Change From Ambient) P1 (No Change From Ambient) T2 (No Change From Ambient) T1 T3 (No Change From Ambient) (No Change From Ambient)

  41. Current-Limitation: Arc Energy Reduction Test 4 Non-Current Limiting Test 3 Reduced Fault Current via Current-Limitation Test 1

  42. Summary • Shock, arc flash and arc blast are the three recognized electrical hazards • Shock injuries result from electrical current flowing through the body • Arcing faults can generate enormous amounts of energy • Injuries from arcing faults are a result of the tremendous heat and pressure generated

  43. Summary • Overcurrent protective devices have an impact on the two most important variables of arc flash hazards: • Time (speed of the OCPD) • Fault current magnitude (current-limitation may help reduce) • Current-limitation may be able to significantly reduce the energy released during arcing faults

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