Chapter 7

Safety

Introduction

• 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.)

• 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

• 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 high-

voltage 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.© 2014 Cengage Learning.

Figure 7-12 Antistatic workstation.© 2014 Cengage Learning.

Safety Practices• 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.) :• 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 static-

generating material