Download Chapter 7

Chapter 7
Safety
Introduction
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This chapter covers the following topics:
Dangers of electricity
Preventive measures
Electrostatic discharge
Safety practices
Dangers of Electricity
• Technician’s responsibility
– Prevent electric shock
– Work smart and safely when dealing with
electricity
• Static electricity
– Electrical charge at rest on a surface
– Discharge takes place during contact with a
neutral surface
Dangers of Electricity (cont’d.)
• Flow of current through the body causes
pain or even death
• Dry skin
– Offers some insulation
• Wet skin
– Body resistance drops
– Allows more current to pass through the body
Figure 7-1 As body resistance decreases, the current through the body will increase.
As body resistance increases, the current through the body will decrease.
© 2014 Cengage Learning.
Dangers of Electricity (cont’d.)
• Electric shock
– Current flows through the body when a
complete circuit exists
Figure 7-2 Results from different levels of current flowing through the body.
© 2014 Cengage Learning.
Dangers of Electricity (cont’d.)
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Factors influencing electric shock :
Intensity of the current
Frequency of the current
Current path through the body
Length of time current passes through the
body
Dangers of Electricity (cont’d.)
• Actions to take with severe electrical
shock :
• First, send for help
• Remove the source of power
• Do not attempt to touch victim without
removing power source
• If power source cannot be secured, use
nonconducting material to move victim
away
Dangers of Electricity (cont’d.)
• Actions to take with severe electrical
shock (cont’d.) :
• When victim is removed from the circuit,
check for signs of breathing and pulse
• Begin CPR if necessary and trained
Preventive Measures
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Work in only clean, dry areas
Do not wear loose or flapping clothing
Wear only nonconductive shoes
Remove metal jewelry and watches
Do not use bare hands to remove hot
parts
Preventive Measures (cont’d.)
• Use a shorting stick to remove highvoltage charges on capacitors
• Ensure equipment is properly grounded
with polarized plugs
• Remove power to circuit prior to attaching
alligator clips
• When measuring voltages over 300V, do
not hold the test prods
Figure 7-3 All equipment should have polarized plugs.
© 2014 Cengage Learning.
Preventive Measures (cont’d.)
• Methods to prevent electrical shock
– Insulation
– Grounding
• Copper wire conductors are normally
insulated
– Also power tools and appliances
• Grounding provides a path to carry the
current to ground
Preventive Measures (cont’d.)
• Ground fault interrupter (GFI) :
• Fast-acting circuit breaker
• Sensitive to very low levels of leakage to
ground
• Designed to limit electrical shock to
prevent serious injury
• Operates only on line to ground fault
currents
Figure 7-5 A ground fault interrupter (GFI) is a fast-acting circuit breaker.
© 2014 Cengage Learning.
Preventive Measures (cont’d.)
• Underwriters Laboratories (UL) label
– Implies product is safe for use as intended
– Product has been tested for compliance with
safety standards
• CSA certification mark
– Indicates product has been tested and meets
requirements of recognized standards
Preventive Measures (cont’d.)
• National Fire Protection Association
– Produces electrical wiring codes under
National Electrical Code (NEC)®
• Requirement in many states
– All wiring must be done or approved by
master electrician
Electrostatic Discharge
• Static electricity created when two
substances rubbed together or separated :
• Substances can be solid or fluid
• Electrons transfer from one to the other
• When either substance contacts a
conductor, current flows
– Event known as electrostatic discharge
Electrostatic Discharge (cont’d.)
• For a person to feel a shock:
– Voltage potential must be about 3,500-4,000V
• Integrated circuits
– Can be damaged or destroyed by static
discharge
– Damage is known as ESD latent defect
• May not be apparent and causes failure at a later
time
Figure 7-9 Potential
electrostatic sources.
© 2014 Cengage
Learning.
Figure 7-10 Electrostatic charges in the work environment.
© 2014 Cengage Learning.
Figure 7-11 Semiconductor devices that can
be damaged by electrostatic discharge.
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Figure 7-12 Antistatic workstation.
© 2014 Cengage Learning.
Safety Practices
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Safe work environment guidelines :
Keep area neat and orderly
Be alert and attentive at all times
Know correct operation and safety
procedures of test equipment, tools, and
machinery
– Know actions to take if an accident occurs
– If a chemical is involved, check the MSDS
Safety Practices (cont’d.)
• Personal safety when working with electricity :
• Keep one hand behind back when working on
live circuits
• Use an isolation transformer when working on
AC powered equipment
• Discharge capacitors before troubleshooting
• Use grounded line cords and polarized plugs
Safety Practices (cont’d.)
• Personal safety when working with
electricity (cont’d.) :
• Keep hands off live circuits
• Read chemical labels
• Work in well-ventilated areas
• Wear safety glasses and use personal
protective equipment
Safety Practices (cont’d.) :
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Safety guidelines for soldering
Wear safety glasses
Never touch soldering tip
Be aware of fingers slipping over the handle and
touching the heating element
• Be aware of molten solder
• Use care when handling printed circuit boards
• Avoid breathing fumes or vapors
Safety Practices (cont’d.)
• Preventing damage to electrical
components
• Keep components in antistatic bags,
boxes, or wraps when not in use
• Remove components from antistatic bags
only in the antistatic work area
• Keep antistatic workstations free of staticgenerating material