TM 650

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

• 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

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
(No Transcript)
17
Low Voltage Tools

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

18
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

19
Test Equipment

• Simple outlet tester
• Branch circuit analyzer

20
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

21
Static Electricity Controls

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

22
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

23
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

24
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

25
Decision chart for classifying hazardous
locations that are dangerous from the standpoint
of ignition of materials in the air. National
Electrical Code.
26
(No Transcript)
27
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.
28
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

29
Examples of Lockout/Tagout (LOTO) devices.
30
Assignment

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