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TM 650 – Electrical Hazards

TM 650 – Electrical Hazards. Carter J. Kerk , PhD, PE, CSP, CPE Industrial Engineering Department South Dakota School of Mines. Assignment. Read Asfahl, Chapter 17 HW14, Review Questions, p. 414-5 1-37, divisible by three. Is Electrical Safety Important?.

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TM 650 – Electrical Hazards

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  1. TM 650 – Electrical Hazards Carter J. Kerk, PhD, PE, CSP, CPE Industrial Engineering Department South Dakota School of Mines

  2. Assignment • Read Asfahl, Chapter 17 • HW14, Review Questions, p. 414-5 • 1-37, divisible by three

  3. Is Electrical Safety Important? • 11% of OSHA general industry citations address electrical hazards • Improper grounding • Ungrounded tools • Portable tools in damp/wet locations • Ground pin broken off • Exposed live parts • Sloppy electrical installations • Breaker box open

  4. Raise your hand • Have you ever received a shock from a 110V circuit? • Does anyone have any good stories to tell? • Is there anyone present who has not received such a shock?

  5. Can a 110V circuit kill? • Myth: Ordinary 110V circuits are safe. • Reality: 110V circuits kill many more people than 220 or 440V circuits (which people do respect) • Why? Almost everyone has received a shock from a 110V circuit and lived to tell about it.

  6. Useful equations • Ohm’s Law • I = V / R; V = IR • Current [amps] = voltage [V] / resistance [ohms] • Watts (measure of power) • W = (V) (I) • If there is more than one path, the primary flow will be through the path of least resistance

  7. Example • Take a 60W bulb, I = W/V = 60 / 110 = 0.55 ampere (550 m-amps) • Dry human skin may have a resistance of 100,000 ohms • I = V/R = 110 / 100,000 = 0.0011 (about 1 m-amp) • See Figure 12-1, hardly noticeable • Perspiration on skin brings resistance to about 500 ohms • I = V/R = 110 / 500 = 0.22 amps (220 m-amps) • At fibrillation threshold, possible asphyxiation • A possible fatal dose

  8. Path of Least Resistance • If there is more than one path, the primary flow will be through the path of least resistance • If that path leads through critical organs (especially the heart), more likely to be fatal

  9. Physiological Effects • Mild shock • Painful shock • Muscular paralysis (no-let-go phenomenon) • Asphyxiation • Fibrillation (60 Hz is one of the most dangerous frequencies for the heart) • Heart paralysis • Tissue burning • See next two slides

  10. Effects of 60 Hz AC electrical current on a 150 lb man.

  11. Effects of Electricity on the Human Body

  12. Electrical Hazards • Electric shock, heat, fire, explosion • Electricity energizes mechanical equipment • Some electrical devices produce harmful levels of X rays, microwaves, laser light, magnetic fields

  13. Control of Electrical Hazards • Physical: materials used, design of components, placement of electrical equipment, shielding, enclosures, double insulated tools • Overcurrent Devices: fuses, circuit breakers • Switching Devices: lockouts, interlocks, thermal & overspeed cutouts • Grounding & Bonding • GFCI • Low Voltage Tools • Smart Power Integrated Circuits • Warnings • Procedures

  14. Will a circuit breaker protect humans? • An ordinary house circuit breaker of 20 or 30 amperes will not trip the circuit breaker until there is a current flow of 20,000 to 30,000 m-amps, respectively • This is about 100 to 1000 time as much as the lethal dose • This is where GFCI can save the day!!

  15. GFCI • Ground Fault Circuit Interrupter • Protects people from electrical shock • Very fast-acting circuit breaker that senses very low current levels • Can sense as little as 2 mA and shut off current in as little as 0.02 s

  16. Low Voltage Tools • Select low voltage tools for use in confined spaces and wet areas to reduce hazard. • Operate at less than 24 V

  17. First Aid • Train personnel for special considerations for accidents from electrical hazards • Rescuers often become victims also • Respiratory arrest and fibrillation are common • Knowledge of CPR is essential • Must have immediate response • AED – Automated External Defibrillator

  18. Test Equipment • Simple outlet tester • Branch circuit analyzer

  19. Static Electricity • Sparks can ignite certain dusts and vapors • Damage sensitive electrical and computer equipment • E = 0.5 C V2 • E is energy in joules • C is capacitance in farads • A material property • People, ~100 pF • V is potential in volts • A walk over carpet in dry air can produce 50kV

  20. Static Electricity Controls • Minimize charge buildup • Proper selection of materials • Bonding and grounding • Humidification • Fueling operations • Grounding and bonding • Conductive floors and clothing • Training

  21. Other Issues • Lightning • AED (Automatic External Defibrillators) • One in eight workplace fatalities is due to cardiac arrest • About half of sudden cardiac arrest victims could be saved with CPR and defibrillation • OSHA recommends but does not mandate • http://www.osha.gov/SLTC/aed/solutions.html • A complete AED program includes risk assessment, training, maintenance, and recordkeeping

  22. Battery Charging • Lead-Acid Batteries • Used in cars, trucks, industrial vehicles • Can explode during charging • Spraying battery acid and particles to eyes and skin • Have emergency eye wash stations & shower nearby • Wear protective eyewear and clothing • Two types of explosions • Flammability of hydrogen gas (4 – 75% by vol) • Use proper ventilation • Electrical • Make sure batteries have same voltage • Train to use proper order of cable connection

  23. Fire Hazards –Hazardous Locations • What industrial locations may require special wiring and equipment to prevent explosions? • Switches, motors, broken light bulbs can produce momentary arcs • Ignition mechanisms are different for different materials • Ignitable dusts versus ignitable vapors • National Electrical Code has established a classification system • Hazardous Location Classifications: I, II, and III • Divisions I and II

  24. Decision chart for classifying hazardous locations that are dangerous from the standpoint of ignition of materials in the air. National Electrical Code.

  25. Ordinary weatherproof outlets, not approved for hazardous locations! Explosion-proof electrical equipment approved for Class I, Division I hazardous locations. Note heavy-duty, machined components. Electrical outlet plug and receptacle; switch.

  26. Lockout/Tagout (LOTO) • 29 CFR 1910.147 • The Control of Hazardous Energy • 29 CFR 1910.147, Appendix A • Typical Minimal Lockout Procedures • Lockout/Tagout Interactive Training Program • http://www.osha.gov/dts/osta/lototraining/index.htm • LOTO Plus – Expert Advisor • http://www.osha.gov/dts/osta/oshasoft/lotoplus.html

  27. Examples of Lockout/Tagout (LOTO) devices.

  28. Assignment • Read Asfahl, Chapter 17 • HW14, Review Questions, p. 414-5 • 1-37, divisible by three

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