ELECTRICAL SAFETY HANDBOOK docx

Flash and Thermal Protection / 2.2A Note on When to Use Thermal Protective Clothing / 2.2 Thermal Performance Evaluation / 2.3 Head, Eye, and Hand Protection / 2.13 Head and Eye Protecti

ELECTRICAL SAFETY HANDBOOK

SAFETY HANDBOOK

Cadick Corporation, Garland, Texas

CapSchell, Inc., Chicago, Illinois

AVO Training Institute, Inc., Dallas, Texas

Third Edition

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Professional

Want to learn more?

John Cadick

To my wife, Brenda Neitzel, who always believed in

me and who encouraged me to continue my education and strive to be the best that I could be; to the U.S Air Force for giving me my start in an electrical career; to all of my employers, who gave me

countless opportunities to learn and progress; and

to John Cadick, who believed in me enough to ask

me to contribute to this book

Dennis Neitzel

In dedication especially to Michael Allen, the father

of Sarah, Benjamin, Amelia, and Natalie, with my hope that each will continue to learn the most they can from books.

Mary Capelli-Schellpfeffer

Definition and Description / 1.8

Arc Energy Release / 1.9

The Pulmonary System / 1.23

Summary of Causes—Injury and Death / 1.23

Flash and Thermal Protection / 2.2

A Note on When to Use Thermal Protective Clothing / 2.2

Thermal Performance Evaluation / 2.3

Head, Eye, and Hand Protection / 2.13

Head and Eye Protection / 2.14

When and How to Use / 2.50

Safety Tags, Locks, and Locking Devices / 2.51

Low Voltage Voltmeter Safety Standards / 2.60

Three-Step Voltage Measurement Process / 2.60

General Considerations for Low-Voltage Measuring Instruments / 2.62 Safety Grounding Equipment / 2.63

The Need for Safety Grounding / 2.63

Safety Grounding Switches / 2.64

Safety Grounding Jumpers / 2.65

Selecting Safety Grounding Jumpers / 2.70

Installation and Location / 2.74

Ground Fault Circuit Interrupters / 2.74

Operating Principles / 2.74

Applications / 2.75

Safety Electrical One-Line Diagram / 2.78

The Electrician’s Safety Kit / 2.78

References / 2.79

Chapter 3 Safety Procedures and Methods 3.1

Introduction / 3.1

The Six-Step Safety Method / 3.1

Think—Be Aware / 3.2

Understand Your Procedures / 3.2

Follow Your Procedures / 3.2

Use Appropriate Safety Equipment / 3.2

Ask If You Are Unsure, and Do Not Assume / 3.2

Do Not Answer If You Do Not Know / 3.3

Pre-Job Briefings / 3.3

Definition / 3.3

What Should Be Included? / 3.3

When Should Pre-Job Briefings Be Held? / 3.3

Energized or De-Energized? / 3.3

The Fundamental Rules / 3.3

A Hot-Work Decision Tree / 3.5

After the Decision Is Made / 3.6

Safe Switching of Power Systems / 3.6

Introduction / 3.6

Remote Operation / 3.7

Operating Medium-Voltage Switchgear / 3.7

Operating Low-Voltage Switchgear / 3.11

Operating Molded-Case Breakers and Panelboards / 3.15

Operating Enclosed Switches and Disconnects / 3.17

Operating Open-Air Disconnects / 3.18

Operating Motor Starters / 3.20

Energy Control Programs / 3.23

General Energy Control Programs / 3.23

Specific Energy Control Programs / 3.24

Basic Energy Control Rules / 3.24

Lockout-Tagout / 3.26

Definition and Description / 3.26

When to Use Locks and Tags / 3.26

Locks without Tags or Tags without Locks / 3.26

Rules for Using Locks and Tags / 3.27

Responsibilities of Employees / 3.27

Sequence / 3.28

Lock and Tag Application / 3.28

Isolation Verification / 3.28

Removal of Locks and Tags / 3.28

Safety Ground Application / 3.29

Placement of Safety Grounds / 3.37

Safety Grounding Principles / 3.37

Safety Grounding Location / 3.38

Application of Safety Grounds / 3.38

The Equipotential Zone / 3.43

Removal of Safety Grounds / 3.44

Control of Safety Grounds / 3.44

Flash Hazard Calculations and Approach Distances / 3.46

Introduction / 3.46

Approach Distance Definitions / 3.46

Determining Shock Hazard Approach Distances / 3.46

Calculating the Flash Hazard Minimum Approach Distance

(Flash Protection Boundary) / 3.49

Calculating the Required Level of Arc Protection (Flash Hazard Calculations) / 3.51

Introduction / 3.51

The Lee Method / 3.52

Methods Outlined in NFPA 70E / 3.52

IEEE Standard Std 1584-2002 / 3.53

Software Solutions / 3.55

Required PPE for Crossing the Flash Hazard Boundary / 3.55

A Simplified Approach to the Selection of Protective Clothing / 3.56

Barriers and Warning Signs / 3.56

Illumination / 3.61

Conductive Clothing and Materials / 3.61

Confined Work Spaces / 3.62

Tools and Test Equipment / 3.62

General / 3.62

Authorized Users / 3.62

Visual Inspections / 3.63

Electrical Tests / 3.63

Wet and Hazardous Environments / 3.63

Field Marking of Potential Hazards / 3.65

The One-Minute Safety Audit / 3.65

References / 3.66

Chapter 4 Grounding of Electrical Systems and Equipment 4.1

Introduction / 4.1

Electric Shock Hazard / 4.1

General Requirements for Grounding and Bonding / 4.2

Definitions / 4.2

Grounding of Electrical Systems / 4.3

Grounding of Electrical Equipment / 4.6

Bonding of Electrically Conductive Materials and Other Equipment / 4.6

Performance of Fault Path / 4.8

Arrangement to Prevent Objectionable Current / 4.8

Alterations to Stop Objectionable Current / 4.8

Temporary Currents Not Classified as Objectionable Current / 4.8

Connection of Grounding and Bonding Equipment / 4.8

Protection of Ground Clamps and Fittings / 4.9

Clean Surfaces / 4.9

System Grounding / 4.9

Purposes of System Grounding / 4.9

Grounding Service-Supplied Alternating-Current Systems / 4.9

Conductors to Be Grounded—Alternating-Current Systems / 4.11

Main Bonding Jumper / 4.11

Grounding Electrode System / 4.12

Grounding Electrode System Resistance / 4.14

Grounding Electrode Conductor / 4.14

Grounding Conductor Connection to Electrodes / 4.16

Bonding / 4.18

Equipment Grounding / 4.19

Equipment to Be Grounded / 4.19

Grounding Cord- and Plug-Connected Equipment / 4.19

Equipment Grounding Conductors / 4.21

Sizing Equipment Grounding Conductors / 4.22

Use of Grounded Circuit Conductor for Grounding Equipment / 4.22

Hazards Associated with Electrical Maintenance / 5.3

The Economic Case for Electrical Maintenance / 5.3

Reliability Centered Maintenance (RCM) / 5.4

What is Reliability Centered Maintenance? / 5.5

A Brief History of RCM / 5.5

RCM in the Industrial and Utility Arena / 5.5

The Primary RCM Principles / 5.6

Failure / 5.8

Maintenance Actions in an RCM Program / 5.8

Impact of RCM on a Facilities Life Cycle / 5.9

Maintenance Requirements for Specific Equipment and Locations / 5.14

General Maintenance Requirements / 5.14

Substations, Switchgear, Panel Boards, Motor Control Centers, and

Disconnect Switches / 5.15

Fuse Maintenance Requirements / 5.16

Molded-Case Circuit Breakers / 5.16

Low-Voltage Power Circuit Breakers / 5.18

Medium Voltage Circuit Breakers / 5.20

Protective Relays / 5.21

Rotating Equipment / 5.23

Portable Electric Tools and Equipment / 5.23

Personal Safety and Protective Equipment / 5.24

Conclusion / 5.24

References / 5.24

Chapter 6 Regulatory and Legal Safety Requirements

Introduction / 6.1

The Regulatory Bodies / 6.1

The American National Standards Institute (ANSI) / 6.1

The Institute of Electrical and Electronic Engineers (IEEE) / 6.3

National Fire Protection Association (NFPA) / 6.3

American Society for Testing and Materials (ASTM) / 6.4

American Society of Safety Engineers (ASSE) / 6.5

The Occupational Safety and Health Administration (OSHA) / 6.6

Other Electrical Safety Organizations / 6.12

The National Electrical Safety Code (NESC)—ANSI C-2 / 6.12

General Description / 6.12

Industries and Facilities Covered / 6.13

Technical/Safety Items Covered / 6.13

The National Electrical Code (NEC)—ANSI/NFPA 70 / 6.14

General Description / 6.14

Industries and Facilities Covered / 6.15

Technical and Safety Items Included / 6.15

Electrical Equipment Maintenance—ANSI/NFPA 70B / 6.15

General Description / 6.15

Industries and Facilities Covered / 6.16

Technical and Safety Items Covered / 6.16

Standard for Electrical Safety in the Workplace—ANSI/NFPA 70E / 6.16

General Description / 6.16

Industries and Facilities Covered / 6.17

Technical Safety Items Covered / 6.18

The American Society for Testing and Materials (ASTM) Standards / 6.19

Occupational Safety and Health Administration (OSHA) Standards / 6.19

Overview / 6.19

General Industry / 6.19

Construction Industry / 6.22

Chapter 7 Accident Prevention, Accident Investigation,

General First Aid / 7.8

Resuscitation (Artificial Respiration) / 7.12

Chapter 8 Medical Aspects of Electrical Trauma 8.1

Introduction / 8.1

Statistical Survey / 8.1

Non-Occupational Electrical Trauma / 8.4

Fatality and Injury Related Costs / 8.4

Electrical Events / 8.6

Electrocution and Electrical Fatalities / 8.7

Medical Aspects / 8.8

Non-Electrical Effects in Electrical Events / 8.10

Stabilization and Initial Evaluation / 8.13

Medical and Surgical Intervention / 8.14

Rehabilitation Focus and Return to Work Planning / 8.16

Reentry to Employment Settings / 8.16

Locking and Tagging / 9.21

Closing Protective Devices After Operation / 9.21

Electrical Safety Around Electronic Circuits / 9.21

The Nature of the Hazard / 9.21

Special Safety Precautions / 9.22

Stationary Battery Safety / 9.23

Introduction / 9.23

Basic Battery Construction / 9.24

Safety Hazards of Stationary Batteries / 9.25

Battery Safety Procedures / 9.25

Chapter 10 Medium- and High-Voltage Safety Synopsis 10.1

Introduction / 10.1

High-Voltage Equipment / 10.1

Current Transformers / 10.1

Grounding Systems of Over 1000 V / 10.3

Locking and Tagging / 10.13

Closing Protective Devices After Operation / 10.13

Electrical Safety Program Structure / 12.1

Electrical Safety Program Development / 12.2

The Company Electrical Safety Team / 12.2

Company Safety Policy / 12.4

Assessing the Need / 12.4

Problems and Solutions / 12.4

What Material Should Be Covered / 12.9

When Meetings Should Be Held / 12.10

Where Meetings Should Be Held / 12.10

How Long Meetings Should Be / 12.10

Evaluation of Safety Meetings / 12.10

Outage Reports / 12.11

Safety Audits / 12.11

Internal versus External Audits / 12.14

Chapter 13 Safety Training Methods and Systems 13.1

Training Consultants and Vendors / 13.9

Canned Programs and Materials / 13.9

Electricity has been recognized as dangerous since it began to be used for street lighting inlarge cities in the northeastern United States in the late 1800s People received severe shocksand were frequently electrocuted as they made contact with energized electrical equipment.Inadequate electrical installation often resulted in fires, and standard installation methods didnot exist

At that time, even though fires and electrocutions were occurring, the use of electricityquickly expanded to other parts of the United States Electrical designs and installationsvaried widely from one facility to another Injury data and economic losses illustrated thatboth fire and electrocution were hazards, and insurance companies recognized the impor-tance of standardization If an installation standard could be developed, both electrocutionand fire could be reduced

The system of voluntary electrical standards that currently exists was developed after fire,and electrocution became recognized electrical hazards The system of codes and standardsthat guides installations consists of documents generated by standards-developing organizations,third-party inspection of electrical equipment, and enforcement by inspecting organizations.The system does a good job of keeping the public safe from both electrocution and fires, pro-vided the electrical equipment doors are latched, adequately maintained, and the equipment

is operating normally

In recent years, the community has begun to recognize that in addition to fire and cution, arc-flash and arc-blast hazards also result in injury The knowledge base about thesehazards is expanding but is not yet complete The community knows that as the distancebetween a worker and an electrical hazard decreases, the degree of exposure increases.Workers must understand that they are exposed to these hazards until an electrically safework condition has been established, as explained in this handbook

electro-Before an electrically safe work condition exists, workers are exposed to all hazards

asso-ciated with electrical energy, in many different ways:

● Electrical equipment, devices, and components have a life expectancy, and control devicessometimes malfunction When a failure occurs, a worker is expected to identify the problem,repair the problem, and restore the equipment to normal service

● To extend the life of electrical equipment, the equipment must be maintained Although theelectrical energy sometimes is removed before a worker begins a maintenance task, thosetasks often are executed while the source of electricity is energized

● Equipment and circuits sometimes are modified to add new devices or circuits Short-termemployees may be expected to work in an environment that includes exposure to energizedelectrical circuits and components Consultant and service employees are frequentlyexposed to energized electrical equipment and circuits

● When a problem exists that causes equipment to operate other than normally, a workermight open a door or remove a cover and expose an energized electrical conductor orcomponent In many cases, the worker might troubleshoot while the circuit is energized.Components and conductors might be added within a piece of equipment while the equip-ment or parts of the equipment remain energized

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● After correcting a problem, workers sometimes create further hazardous conditions: leave anequipment door ajar, with latches open; replace covers with a minimum number of screws;and remove devices, leaving penetrations through a door.

In recent years, the electrical community has begun to understand that workers areexposed to many different electrical hazards Workers certainly should not be unnecessarilyexposed to hazards Workers should understand how and when they could be exposed to ahazard and how to assess the hazard and risk of injury They must also understand how toselect and use work practices that minimize or eliminate risk of injury and how to select andwear protective equipment that will minimize or eliminate that risk

Employees must be trained to understand the concepts discussed in each chapter Theauthors of this handbook are warriors in the fight for electrical safe workplaces This newedition provides critical, up-to-date information for employers about how to avoid injury All

of the information is here in the handbook, and work practices, procedures, and the overallelectrical safety program should embody the ideas discussed herein I highly recommend thisvaluable resource to employers as they join the growing effort to ensure that their workersremain safe Providing a safe work place is not only an economical asset, it is the right thing

to do

Ray A Jones, P.E.

NFPA 70E Technical Committee Chair

It seems like only a few days since the second edition of this handbook was completed;however, in the five years that have passed, amazing things have happened in the world ofsafety First, the 2000 edition of NFPA 70E has become the 2004 edition with a multitude

of changes Second, the electrical world seems to have become much more aware of thehazards of electricity and has started to embrace many, if not all, of the more modernsafety requirements, such as flash-hazard evaluations Finally, companies throughout theworld are starting to gear up to provide enhanced safety programs for their personnel andwork in teams to achieve that goal

The Electrical Safety Handbook has continued to receive remarkably broad

accep-tance in the electrical safety world, perhaps because it is the only independent ence source for all of the various aspects of electrical safety With this in mind, wehave expanded virtually all of the previous chapters in this edition as well as added abrand new chapter covering the safety aspects of electrical maintenance We trulyhope that the increased detail in the old chapters and the addition of the new chapterwill be met with the same enthusiasm as the previous edition

refer-Chapters 1, 2, and 3 continue to serve as the central core of the book, by presentingthe case for electrical safety (Chapter 1), a broad coverage of electrical safety equip-ment (Chapter 2), and a detailed coverage of electrical safety procedures (Chapter 3).The changes in Chapter 3 should be of special interest to the reader since we haveupdated the arc energy calculations to be consistent with new industry innovations asintroduced in IEEE Standard Std 1584 and NFPA 70E

Chapter 4 has been revised to include references to the 2005 edition of the NationalElectrical Code (NEC) Additional information has also been added that includes fer-roresonance Chapter 4 continues to provide a detailed overview of the general require-ments for grounding and bonding electrical systems and equipment This chapter alsoprovides some needed explanations, illustrations, and calculations necessary for applyingthe requirements of NEC Article 250 as well as OSHA 29 CFR 1910.304(f) It should beemphasized, however that this chapter is not intended to replace or be a substitute for therequirements of the current NEC or OSHA regulations Always utilize the most currentstandards and regulations when designing, installing, and maintaining the grounding sys-tems within a facility

Chapter 5, new to this edition, introduces a broad coverage of safety-related nance concepts The chapter is not intended to be a maintenance reference, rather it intro-duces the economic and safety-related reasons for performing maintenance on an electri-cal power system We also introduce the current philosophies on good maintenanceincluding discussions of topics such as reliability-centered, predictive, preventive, andcondition-based maintenance Finally, the chapter covers the eight broad steps of a goodmaintenance program and introduces a cross section of maintenance and testing proce-dures for a variety of electrical equipment

mainte-Chapter 6 (mainte-Chapter 5 in the second edition), updates the previous coverage on theconsensus and mandatory standards and regulations in the workplace Also included areexplanations of several of the general industry OSHA regulations, which are quoted

xix

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directly from the Federal Registers preamble as well as the Directorate of Compliancedocuments.

The specific information reprinted from OSHA has been updated to the most recentversions as of the date of this publication As before, the reader should always refer tothe OSHA publications (available at www.osha.gov) for the most recent information Chapter 7 (Chapter 6 in the second edition) has been expanded and rewritten in severalkey ways New information, addressing the relationship of proper engineering and safety,has been added showing the reader how they can engineer safety hazards out of existence.Also, the section on first aid and CPR has been brought more up-to-date

In Chapter 8, a statistical survey of electrical injuries and fatalities is added.Details about the possible medical scenarios following exposure to electrical hazardsare included to assist the reader in appreciating how survivors of a multivictim electri-cal incident may share an event, but not necessarily the same effects to their bodies.Graphs and tables, new to the handbook’s third edition, give quick summaries of enhancedinformation Figures are presented to explain the basic principle that injury results with thetransformation and transfer of energy

Chapters 9 and 10 (Chapters 8 and 9 in the second edition) have been updated withnew information taken from Chapters 2 and 3 As before, these two chapters serve as aquick reference to the reader for low-, medium-, and high-voltage safety

Added to the topic of human factors now found in Chapter 11 is a historical spective Insights regarding human factors engineering developed within the civiliannuclear power industry are also included

per-Chapter 12 (per-Chapter 11 in the second edition) has been updated to include tion garnered from the collective experience of the authors since the second editionwas published Of special importance is the material covering the interrelationshipsamong management, labor, and legal counsel

informa-Chapter 13 (informa-Chapter 12 in the second edition) has a significant new section covering amethod to develop a training that will provide a more consistent and compliant safety pro-gram By using tried and tested educational methods, the new sections show the reader in astep-by-step manner how to create their training program

John Cadick, P.E Mary Capelli-Schellpfeffer, M.D., M.P.A.

Dennis K Neitzel, C.P.E.

M Irfan, Square D Company, A.B Chance Co., AVO Multi-Amp Institute; Dr BrianStevens, Ph.D., Phelps Dodge Mining Company; Jason Saunders, Millennium InorganicChemicals; Cindy Chatman, Bulwark; Alan Mark Franks; Sandy Young; Bruce McClung;

Dr Raphael C Lee, M.D., Sc.D., and Zoe G Foundotos

John Cadick

AVO Training Institute, Inc (a Subsidiary of Megger); Erico, Inc (Cadweld); Ronald P

O’Riley, Electrical Grounding (Delmar Publishers); National Fire Protection Association,

2005 National Electrical Code Handbook (NFPA).

Dennis Neitzel

Dr Capelli-Schellpfeffer’s research was supported in part by grant R01 OH04136-02from the U.S Center for Disease Control and Prevention (CDC) and the NationalInstitute of Occupational Safety and Health (NIOSH) Her comments do not representofficial agency views

Mary Capelli-Schellpfeffer

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ELECTRICAL SAFETY HANDBOOK

electric-Understanding the steps and procedures employed in a good electrical safety programrequires an understanding of the nature of electrical hazards Although they may have trou-ble writing a concise definition, most people are familiar with electric shock This oftenpainful experience leaves its memory indelibly etched on the human mind However, shock

is only one of the electrical hazards There are two others—arc and blast This chapterdescribes each of the three hazards and explains how each affects the human body.Understanding the nature of the hazards is useless unless protective strategies are devel-oped to protect the worker This chapter also includes a synopsis of the types of protectivestrategies that should be used to protect the worker

GLOSSARY

Arc (electric) The heat and light energy release that is caused by the electrical

breakdown of and subsequent electrical discharge through anelectrical insulator, such as air

Arc energy input The total amount of energy delivered by the power system to the

arc This energy will be manifested in many forms includinglight, heat, and mechanical (pressure) energy

Arc incident energy The amount of energy delivered by an electric arc to the clothing

or body of a worker This amount of energy will be somewhatless than the arc energy based on factors in the workplace

Arc-resistant Metal-clad switchgear which features strengthened mechanicalswitchgear construction as well as pressure relief systems Arc-resistant

switchgear is designed to minimize the probability of an

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arc-flash as well as contain the energy in the event that oneoccurs.

Blast (electric) The explosive effect caused by the rapid expansion of air and

other vaporized materials that are a superheated by the suddenpresence of an electric arc

Contractor muscle A muscle whose contraction bends or closes a joint The bicep is a

flexor muscle

Electrocution Death caused by the passage of electricity through the body

Death caused by electric shock

Extensor muscle A muscle whose contraction extends or stretches a body part The

tricep is an extensor muscle

Fibrillation Rapid and inefficient contraction of muscle fibers of the heart

caused by disruption of nerve impulses

Horny layer The commonly applied name for the stratum corneum layer of the

epidermis The stratum corneum is called the horny layerbecause its cells are toughened like an animal’s horn

Plasma A high-temperature, electrically ionized gas Because of the high

temperatures and electrical characteristics of a plasma, it isusually identified as a fourth state of matter The othersincluding solid, liquid, and gas

Shock circuit The path that electric current takes through the body If the shock

circuit includes critical organs, severe trauma is more likelythan if it does not

Shock (electric) The physical stimulation or trauma that occurs as a result of

electric current passing through the body

HAZARD ANALYSIS

The division of the electrical power hazard into three components is a classic approach used

to simplify the selection of protective strategies The worker should always be aware thatelectricity is the single root cause of all of the injuries described in this and subsequentchapters That is, the worker should treat electricity as the hazard and select protectionaccordingly

SHOCK

Description

Electric shock is the physical stimulation that occurs when electric current flows throughthe human body The distribution of current flow through the body is a function of the resis-tance of the various paths through which the current flows The final trauma associated with

the electric shock is usually determined by the most critical path called the shock circuit.

The symptoms may include a mild tingling sensation, violent muscle contractions, heartarrhythmia, or tissue damage Detailed descriptions of electric current trauma are included

in Chap 8 For the purposes of this chapter, tissue damage may be attributed to at least twomajor causes

Burning. Burns caused by electric current are almost always third-degree because theburning occurs from the inside of the body This means that the growth centers aredestroyed Electric-current burns can be especially severe when they involve vital internalorgans.

Cell Wall Damage. Research funded by the Electric Power Research Institute (EPRI) hasshown that cell death can result from the enlargement of cellular pores due to high-intensityelectric fields.1

This research has been performed primarily by Dr Raphael C Lee and hiscolleagues at the University of Chicago This trauma called electroporation allows ions toflow freely through the cell membranes, causing cell death

Influencing Factors

Several factors influence the severity of electrical shock These factors include the physicalcondition and responses of the victim, the path of the current flow, the duration of the cur-rent flow, the magnitude of the current, the frequency of the current, and the voltage mag-nitude causing the shock

Physical Condition and Physical Response. The physical condition of the individualgreatly influences the effects of current flow A given amount of current flow will usuallycause less trauma to a person in good physical condition Moreover, if the victim of theshock has any specific medical problems such as heart or lung ailments, these parts of thebody will be severely affected by relatively low currents A diseased heart, for example, ismore likely to suffer ventricular fibrillation than a healthy heart

Current Duration. The amount of energy delivered to the body is directly proportional tothe length of time that the current flows; consequently, the degree of trauma is also directlyproportional to the duration of the current Three examples illustrate this concept:

1 Current flow through body tissues delivers energy in the form of heat The magnitude

of energy may be approximated by

J = I2

where J= energy, joules

I= current, amperes

R= resistance of the current path through the body, ohms

t= time of current flow, seconds

If sufficient heat is delivered, tissue burning and/or organ shutdown can occur Note that the

amount of heat that is delivered is directly proportional to the duration of the current (t).

2 Some portion of the externally caused current flow will tend to follow the current paths

used by the body’s central nervous system Since the external current is much larger thanthe normal current flow, damage can occur to the nervous system Note that nervous sys-tem damage can be fatal even with relatively short durations of current; however,increased duration heightens the chance that damage will occur

3 Generally, a longer duration of current through the heart is more likely to cause

ven-tricular fibrillation Fibrillation seems to occur when the externally applied electric fieldoverlaps with the body’s cardiac cycle The likelihood of this event increases with time

Frequency. Table 1.1 lists the broad relationships between frequency and the harmfuleffects of current flow through the body Note that at higher frequencies, the effects of

Joule (I2t) heating become less significant This decrease is related to the increased

capac-itive current flow at higher frequencies

It should be noted that some differences are apparent even between DC (zero Hz) andstandard power line frequencies (50 to 60 Hz) When equal current magnitudes are com-pared (DC to AC rms), DC seems to exhibit two significant behavioral differences:

1 Victims of DC shock have indicated that they feel greater heating from DC than from

AC The reason for this phenomenon is not totally understood; however, it has beenreported on many occasions

2 The DC current “let-go” threshold seems to be higher than the AC “let-go” threshold.

In spite of the slight differences, personnel should work on or near DC power supplieswith the same level of respect that they use when working on or near AC power supplies.This includes the use of appropriate protective equipment and procedures

Note: Unless otherwise specifically noted, the equipment and procedures suggested in

this handbook should be used for all power frequencies up to and including 400 Hz

Voltage Magnitude. Historically, little attention was paid to the effect that voltage nitude has on an electrical trauma It was assumed that a 200-V source would create thesame amount of physical trauma that a 2000-V source would—assuming that the currentmagnitude is the same In fact, higher voltages can be more lethal for at least three reasons:

mag-1 At voltages above 400 V the electrical pressure may be sufficient to puncture the

epi-dermis Since the epidermis provides the only significant resistance to current flow, thecurrent magnitude can increase dramatically

2 The degree of electroporation is higher for greater cellular voltage gradients That is, the

higher voltages cause more intense fields, which in turn increase the severity of the troporation

elec-3 Higher voltages are more likely to create electrical arcing While this is not a shock

trauma per se, it is related to the shock hazard since arcing may occur at the point of tact with the electrical conductor

con-TABLE 1.1 Important Frequency Ranges of Electrical Injury

DC–10 kHz Low Commercial electrical power, Joule heating; destructive cell

frequency soft tissue healing; trans- membrane potentials

cutaneous electrical stimulation

100 kHz– Radio Diathermy; electrocautery Joule heating; dielectric

100 MHz– Microwave Microwave ovens Dielectric heating of water

100 GHz

1013–1014Hz Infrared Heating; CO2lasers Dielectric heating of water

1014–1015Hz Visible light Optical lasers Retinal injury; photochemical

reactions

1015Hz and Ionizing Radiotherapy; x-ray imaging; Generation of free radicalshigher radiation UV therapy

Current Magnitude. The magnitude of the current that flows through the body obeysOhm’s law, that is,

(1.2)

where I= current magnitude, amperes (A)

E= applied voltage, volts (V)

R= resistance of path through which current flows, ohms (Ω)

In Fig 1.1 the worker has contacted a 120-V circuit when an electric drill short-circuitsinternally The internal short circuit impresses 120 V across the body of the worker fromthe hand to the feet This creates a current flow through the worker to the ground and back

to the source The total current flow in this case is given by the formula

Electric drill

Current path

Physical circuit(a)

resistance, the internal body resistance, and the resistance of the shoes where they tact the earth.

con-Typical values for the various components can be found in Tables 1.2 and 1.3 Assume, forexample, that a worker shown in Fig 1.1 is wearing leather shoes and is standing in wet soil.This person is perspiring heavily and has an internal resistance of 200 Ω From Tables 1.2and 1.3 the total resistance can be calculated as

500Ω (drill handle) + 200 Ω (internal) + 5000 Ω (wet shoes) = 5700 ΩFrom this information the total current flow through the body for a 120-V circuit is calcu-lated as

* Resistances shown are for 130-cm 2 areas.

TABLE 1.2 Nominal Resistance Values for Various Parts of the Human Body

Hand around 1 -inch (in) pipe (or drill handle) 1–3 kΩ 0.5–1.5 kΩ

an “electrical hold.” This is a condition wherein the muscles are contracted and held by thepassage of the electric current—the worker cannot let go Under these circumstances, theelectric shock would continue until the current was interrupted or until someone intervenedand freed the worker from the contact Unless the worker is freed quickly, tissue and mate-rial heating will cause the resistances to drop, resulting in an increase in the current Suchcases are frequently fatal.

The reader should note that the values given in this example are for illustration only.Much lower values can and do occur, and many workers have been electrocuted in exactlythis same scenario

Parts of the Body. Current flow affects the various bodily organs in different manners.For example, the heart can be caused to fibrillate with as little as 75 mA The diaphragmand the breathing system can be paralyzed, which possibly may be fatal without outsideintervention, with less than 30 mA of current flow The specific responses of the variousbody parts to current flow are covered in later sections

ARC

Caution: The calculation and formulas in this section are shown to illustrate the basic

con-cepts involved in the calculation of arc parameters including current, voltage, and energy.The calculation of actual values for specific field conditions is a complex, safety-relatedprocedure, and should be done only under the direction of experienced engineers

TABLE 1.4 Nominal Human Response to Current Magnitudes

Current (60 Hz) Physiological phenomena Feeling or lethal incidence

3–10 mA

10 mA Paralysis threshold of arms Cannot release hand grip; if no grip,

victim may be thrown clear (mayprogress to higher current and be fatal)

30 mA Respiratory paralysis Stoppage of breathing (frequently

4 A Heart paralysis threshold Heart stops for duration of current

(no fibrillation) passage For short shocks, may restart

on interruption of current (usually notfatal from heart dysfunction)

≥5 A Tissue burning Not fatal unless vital organs are

burned

Notes: (1) This data is approximate and based on a 68-kg (150-lb) person (2) Information for higher current

lev-els is obtained from data derived from accident victims (3) Responses are nominal and will vary widely by individual.

Source: Courtesy of Ralph Lee.

Definition and Description

ANSI/IEEE Std 100-1988 defines arc as: “A discharge of electricity through a gas, mally characterized by a voltage drop in the immediate vicinity of the cathode approxi-mately equal to the ionization potential of the gas.”2

nor-A similar definition, perhaps more useful in the discussion of electrical safety is given in theglossary of this handbook as: “The heat and light energy release that is caused by the electricalbreakdown of and subsequent electrical discharge through an electrical insulator such as air.”Electric arcing occurs when a substantial amount of electric current flows through what pre-viously had been air Since air is a poor conductor, most of the current flow is actually occur-ring through the vapor of the arc terminal material and the ionized particles of air This mixture

of super-heated, ionized materials, through which the arc current flows, is called a plasma.

Arcs can be initiated in several ways:

● When the voltage between two points exceeds the dielectric strength of the air This canhappen when overvoltages due to lightning strikes or switching surges occur

● When the air becomes superheated with the passage of current through some conductor.For example, if a very fine wire is subjected to excessive current, the wire will melt,superheating the air and causing an arc to start

● When two contacts part while carrying a very high current In this case, the last point ofcontact is superheated and an arc is created because of the inductive flywheel effect.Electric arcs are extremely hot Temperatures at the terminal points of the arcs can reach

as high as 50,000 kelvin (K) Temperatures away from the terminal points are somewhatcooler but can still reach 20,000 K Although the specific results of such temperatures willvary depending on things such as distance from the arc, ambient environmental conditions,and arc energy; anecdotal evidence supported by experimental results developed by theInstitute of Electrical and Electronics Engineers (IEEE) clearly shows the following:

● The heat energy of an electrical arc can kill and injure personnel at surprisingly largedistances For example, second-degree burns have been caused on exposed skin at dis-tances of up to 12 feet (ft) or (3.6 meters [m]) and more

● Virtually all types of clothing fibers can be ignited by the temperatures of electrical arcs.Clothing made of non-flame resistant fibers will continue to burn after the arc source hasbeen removed and will continue to cause serious physical trauma Table 1.5 shows theignition temperature of various fabrics and identifies those that will support combustionafter the arc energy is gone

TABLE 1.5 Ignition Temperatures and Characteristics of Clothing Fibers

* FR treatment of cotton of rayon does not affect ignition temperatures.

All temperatures are expressed in °F Please note that polyester ignites at a higher temperature and burns at a lower temperature than cotton This shows the fallacy of using untreated cotton as an FR garment.

The amount of energy, and therefore heat, in an arc is proportional to the maximum able short circuit volt-amperes in the system at the point of the arc Calculations by RalphLee indicate that maximum arc energy is equal to one-half the available fault volt-amperes

avail-at any given point.3

Later research by Neal, Bingham, and Doughty show that while the imum may be 50 percent, the actual value will usually be somewhat different depending onthe degree of distortion of the waveform, the available system voltage, and the actual arcpower factor.4

max-The same research also shows that enclosing the arc to create a so-called “arc

in the box” focuses the incident arc energy and increases its effect by as much as threefold.4,5

The arc energy determines the amount of radiated energy and, therefore, the degree ofinjury The arc energy will be determined by the arc voltage drop and the arcing current.After the arc is established, the arc voltage tends to be a function of arc length; conse-quently, the arc energy is less dependent on the system voltage and more dependent on the

magnitude of the fault current This means that even low voltage systems have significant

arc hazard and appropriate precautions must be taken Figures 1.2 and 1.3 show the results

of two experiments that were conducted with manikins exposed to electric arcs As can beseen, both high and low voltages can create significant burns

Arc Energy Release

Arc energy is released in at least three forms—light, heat, and mechanical Table 1.6describes the nature of these energy releases and the injuries that they cause Note that lightand heat tend to cause similar injuries and will, therefore, be treated as one injury source inlater calculations Also note that mechanical injuries are usually categorized as blastinjuries, even though the ultimate cause is the electric arc

To be conservative in arc energy release calculations, two assumptions must be made:

1 All arc energy is released in the form of heat measured in cal/cm2

TABLE 1.6 Factors That Affect the Amount of Trauma Caused by an Electric Arc

Distance The amount of damage done to the recipient diminishes by approximately

square of the distance from the arc Twice as far means one-fourth thedamage (Empirical evidence suggests that the actual value may besomewhat different because of the focusing effect of the surroundings.)Temperature The amount of energy received is proportional to the difference

between the fourth power of the arc temperature and the body

temperature (T a − T b)

Absorption coefficient The ratio of energy received to the energy absorbed by the body.Time Energy received is proportional to the amount of time that the arc

is present

Arc length The amount of energy transmitted is a function of the arc length For

example, a zero length arc will transmit zero energy Note that for anygiven system, there will be an optimum arc length for energy transfer.Cross-sectional area of The greater the area exposed, the greater the amount of energy received.body exposed to the arc

Angle of incidence of Energy is proportional to the sine of the angle of incidence Thus, energythe arc energy impinging at 90° is maximum

FIGURE 1.2 Electric arc damage caused by a medium voltage arc (Courtesy

Brosz and Associates.)

(b)

1.10

Arc Energy

Several major factors determine the amount of energy created and/or delivered by an tric arc Table 1.7 lists the major factors and their qualitative effect The quantitative effects

elec-of electric arc are the subject elec-of many on-going studies

An individual’s exposure to arc energy is a function of the total arc energy, the distance

of the subject from the arc, and the cross-sectional area of the individual exposed to the arc

Arc Energy Input

The energy supplied to an electric arc by the electrical system, called the arc input energysmay be calculated using the formula

(1.5)

where J arc= arc energy, joules

V arc= arc voltage, volts

I arc= arc current, amperes

Equation1.7 is the formula used for electrical arcs in systems with voltages less than 1000 V and

Eq 1.8 is used for systems with voltages equal to or greater than 1000 V

log10(I a)= K + 0.662 log10(I bf)+ 0.0966V + 0.000526G

+ 0.5588V log10(I bf)− 0.00304G log10(I bf) (1.7)log10(I a)= 0.00402 + 0.9831og10(I bf) (1.8)

J arc V arc I arc dt o

t

=∫ × ×cos (θ)×

TABLE 1.7 Electrical Arc Injury, Energy Sources

Light Principally eye injuries, although severe burns can also be caused if the ultra-violet

component is strong enough and lasts long enough

Heat Severe burns caused by radiation and/or impact of hot objects such as molten metal.Mechanical Flying objects as well as concussion pressures

where I a= arcing current (kA)

K= a constant: (−0.153 for open configurations or −0.097 for box configurations)

I bf= the bolted, RMS symmetrical, three-phase fault current (kA)

V= system phase-to-phase voltage (kV)

G= the gap between the arcing conductors (mm)

Note that these equations are based on a specific model as developed for Std 1584-2002 The model includes the following:

● Three-phase voltages in the range of 208 to 15,000 V phase-to-phase

● Frequencies of 50 or 60 Hz

● Bolted fault current in the range of 700 to 106,000 A

● Grounding of all types and ungrounded

● Equipment enclosures of commonly available sizes

● Gaps between conductors of 13 to 152 mm

● Faults involving all three phases

Arcing Voltage

Arc voltage is somewhat more difficult to determine Values used in power system tion calculations vary from highs of 700 V/ft (214.4 V/m) to as low as 300 V/ft (91.4 V/m).Two things are well understood:

protec-1 Arc voltages start low and tend to rise Periodically, the arc voltage will drop if the arc

lasts long enough.6

2 Arc voltage is proportional to arc length Therefore, from Eq 1.6, arc power and energy

are proportional to arc length

Modern software programs used to calculate incident arc energy take differentapproaches to determine arc voltage It should be noted that, at best, arc voltage calculation

is only approximate for any given scenario

Arc Surface Area

While the actual shape of an electrical arc may vary, all classic, realistic solutions start by

assuming that an arc causes an approximately cylindrical plasma cloud with length L and radius r This cylindrical structure will have a lateral surface area equal to 2 πrL The area

of the ends of the cylinder are ignored in this calculation since they are so small relative tothe side of the arc To simplify the calculation of energy density, the arc is assumed to form

a sphere with a surface area equal to the cylinder, Fig 1.4 Thus, the arc sphere will have aradius of

(1.9)

where r s= radius of equivalent sphere

r= radius of arc cylinder

L= length of arc

r s=1 2rL

Incident Energy

The single most important of all arc energy calculations is the one that determines theenergy transfer from the arc to the nearby body This is called the incident energy Thisinformation can be used to determine the necessary level of protective clothing required,and can also be used in the performance of a risk analysis

The ultimate measure of tissue injury from electrical arc is the temperature to which thetissue rises during exposure However, calculation of these temperatures starts with the cal-culation of the amount of energy, or heat flux (measured in calories per square centimeter),delivered to the skin Many methods have been developed to calculate the incidentenergy—some more conservative than others The following sections describe some of themethods that have been determined either empirically or theoretically The reader should beaware that research into these areas is continuing at a frantic pace Always refer to the mostrecent industry literature for the most up-to-date information

The Lee Method. Ralph Lee has predicted that the heat energy received by an object (orworker) can be calculated using Eq 1.10

(1.10)

where Q o= heat flux received by the object (cal/cm2)

Q s= heat flux generated by source (cal/s/cm2)

A s= surface area of arc sphere

r= distance from center of source to object (cm)

t= length of arc exposure

Using Eq 1.8 as a starting point, Lee determined that the energy received by the worker

is calculated using Eq 1.11

(1.11)

where E= incident energy in J/cm2

V= system voltage (phase-to-phase)

D bf