IE 331

1
IE 331 Electrical Safety

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

2
Introduction

• Read Chapter 12
• 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

3
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?

4
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.

5
Useful equations

• Ohms Law
• I V / R
• Current voltage / resistance
• 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

6
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

7
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

8
Physiological Effects

• See Figure 12-1 and Table 12-2
• 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

9
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

10
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

11
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!!

12
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

13
Low Voltage Tools

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

14
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
• See upcoming slide.

15
Test Equipment

• Two types to show

16
Static Electricity

• Sparks can ignite certain dusts and vapors
• Damage electrical equipment

17
Other Issues

• Lightning
• Battery Charging
• 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 defib
• OSHA recommends but does not mandate
• A complete AED program includes risk assessment,
  training, maintenance, and recordkeeping