IE 331 Electrical Safety

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IE 331 Electrical Safety

• Carter J. Kerk, PhD, PE, CSP, CPE
• Industrial Engineering Department
• South Dakota School of Mines

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Homework

• Read Brauer, Chapter 12
• HW12, Review Questions, p. 175
• Divisible by three
• Due ??

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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

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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?

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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.

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Useful equations

• Ohms 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

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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

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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

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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

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Effects of 60 Hz AC electrical current on a 150
lb man.
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Effects of Electricity on the Human Body
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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

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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

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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!!

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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

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Low Voltage Tools

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

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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

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Test Equipment

• Simple outlet tester
• Branch circuit analyzer

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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

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Static Electricity Controls

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

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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

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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

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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

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Decision chart for classifying hazardous
locations that are dangerous from the standpoint
of ignition of materials in the air. National
Electrical Code.
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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.
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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.ht
  m
• LOTO Plus Expert Advisor
• http//www.osha.gov/dts/osta/oshasoft/lotoplus.htm
  l

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Examples of Lockout/Tagout (LOTO) devices.
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Homework

• Read Brauer, Chapter 12
• HW12, Review Questions, p. 175
• Divisible by three
• Due ??