ELECTRICAL SAFETY HAZARDS HANDBOOK

Littelfuse is the global leader in circuit protection

Companies around the world have come to rely on

Littelfuse’s commitment to providing the most advanced

circuit protection solutions and technical expertise. It’s this

focus that has enabled Littelfuse to become the world’s

leading provider of circuit protection solutions.

For over 75 years, Littelfuse has maintained its focus on

circuit protection. As we expand in global reach and technical

sophistication, you can continue to count on us for solid circuit

protection solutions, innovative technologies, and industry

leading technical expertise. It is a commitment that only a

world class leader with staying power can support.

A comprehensive approach to circuit protection

Littelfuse goes well beyond efﬁcient and comprehensive

product delivery. We offer an integrated approach to circuit

protection that includes:

A very broad, yet deep selection of products

and technologies from a single source, so you

beneﬁt from a greater range of solutions and

make fewer compromises.

Products that comply with applicable industry

and government standards, as well as our own

uncompromising quality and reliability criteria.

Forward thinking, application-speciﬁc solutions

that provide the assurance your most demanding

requirements will be met.

Dedicated global, customer-focused and

application-speciﬁc technical support services.

•

The World’s Leading Provider of

Circuit Protection Solutions

LITTELFUSE ELECTRICAL SAFETY HAZARDS HANDBOOK

This Electrical Safety Hazards Handbook was developed for general education purposes only and is not intended

to replace an electrical safety-training program or to serve as a sole source of reference. The information herein is

also not intended to serve as recommendations or advice for speciﬁc situations. It is the responsibility of the user to

comply with all applicable safety standards, including the requirements of the U.S. Occupational Safety and Health

Administration (OSHA), the National Fire Protection Association (NFPA), and other appropriate governmental and

industry accepted guidelines, codes, and standards. Use the information within this Handbook at your own risk.

Littelfuse is Committed to Safety

Littelfuse has a continuing commitment to improved

electrical safety and system protection. As the leader in

circuit protection, Littelfuse offers a variety of products and

services designed to help you increase safety in your facility.

For assistance with Arc-Flash, products and services, or

application information, call 1-800-TEC-FUSE (832-3873).

Electrical Safety is a Serious Issue

Electrical Safety in the workplace is the most important

job of an electrical worker. No matter how much training

one has received or how much employers try to safeguard

their workers, Electrical Safety is ultimately the responsibil-

ity of the electrical worker. The human factor associated

with electrical accidents can be immeasurable. No one can

replace a worker or loved one that has died or suffered the

irreparable consequences of an electrical accident.

Table of Contents

Introduction 6

Why is Electrical Safety so Important? 6

Electricity Basics 8

Ohm’s Law 8

Types of Electrical Faults

Overloads 9

Short Circuits

Overcurrent Protective Devices

Interrupting Rating 1

Current Limitation 1

Fuses 12

Circuit Breakers 1

Circuit Protection Checklist 1

History of Electrical Safety 16

Electrical Safety Organizations 19

OSHA 19

The General Duty Clause 1

OSHA Regulations

NFPA 2

IEEE 2

NRTL 20

NEMA 2

ANSI 21

ASTM 21

NECA 21

Electrical Safety Codes and Standards 22

Working on Deenergized Equipment 22

Establish a Safe Work Condition 22

Working on Energized Equipment 2

Who is Qualiﬁed? 24

Energized Electrical Work Permit 2

Employer and Employee Responsibilities 2

Arc-Flash and Other Electrical Safety Hazards 27

Electrical Safety Hazards 27

Electric Shock 2

Arc-Flash and Arc Blasts 2

Arc-Flash Metrics 2

Arc-Blast Effect 3

Light and Sound Effects 3

Electrical Hazard Analysis 32

Shock Hazard Analysis 32

Approach Boundaries 3

Flash Hazard Analysis 3

Arc-Flash Calculations

Arc-Flash Hazard Calculation Examples 3

IEEE 1584 Arc-Flash Hazard Calculation 3

NFPA 70E Table Method 4

Steps Required to Use the

NFPA 70E Table Method 4

Minimizing Arc-Flash and Other

Electrical Hazards 42

1. Design a Safer System 42

2. Use and Upgrade to Current-limiting

Overcurrent Protective Devices 43

3. Implement an Electrical Safety Program 45

4. Observe Safe Work Practices 45

5. Use Personal Protective Equipment (PPE) 47

6. Use Warning Labels 49

7. Use an Energized Electrical Work Permit 49

8. Avoid Hazards of Improperly Selected or

Maintained Overcurrent Protective Devices 50

9. Achieve or Increase Selective Coordination 51

Electrical Safety Summary 53

Annex A 5

Electrical Safety Terms and Deﬁnitions 54

Annex B 61

Electrical Safety Codes and Standards 61

Annex C 63

Energized Electrical Work Permit 63

Annex D 65

Arc-Flash Calculation Steps 65

Annex E 67

Arc Flash Calculator Tables 67

Annex F 71

Resources for Electrical Safety 71

Annex G 73

References 73

Annex H 74

Electrical Safety Quiz 74

97%

of all

electricians

have been

shocked or

injured

on the job.

Safety in the workplace is job number one for

employer and employee alike. It is especially

important for those who install and service

electrical systems. Nothing can replace a

worker or loved one that has died or suffered

the irreparable consequences of an electrical

accident. No matter how much an employer

tries to safeguard its workers or how much

safety training is provided; the ultimate

responsibility lies with the worker. The human

factor is part of every accident or injury.

The purpose of this handbook is to identify

electrical safety hazards and present ways

to minimize or avoid their consequences. It

is a guide for improving electrical safety and

contains information about governmental

regulations, industry-accepted standards

and work practices. It presents ways to

meet the standards and reduce the hazards.

While parts of the standards, regulations,

and codes especially relating to electrical

safety are quoted or summarized herein, it is

the responsibility of the user to comply with

all applicable standards in their entirety.

Why is Electrical Safety so Important?

Electrical hazards have always been recognized,

yet serious injuries, deaths, and property

damage occur daily. Organizations like the US

Department of Labor and the National Safety

Council compile statistics and facts on a

regular basis. The following table demonstrates

the importance of electrical safety.

Introduction

Electrical Safety Hazards Overview

FACTS...

97% of all electricians have been shocked or injured on the job.

Approximately 30,000 workers receive electrical shocks yearly.

Over 3600 disabling electrical contact injuries occur annually.

Electrocutions are the 4th leading cause of traumatic occupational fatalities.

Over 2000 workers are sent to burn centers each year with severe Arc-Flash burns.

Estimates show that 10 Arc-Flash incidents occur every day in the US.

60% of workplace accident deaths are caused by burn injuries.

Over 1000 electrical workers die each year from workplace accidents.

Medical costs per person can exceed $4 million for severe electrical burns.

Total costs per electrical incident can exceed $15 million.

In the year 2002, work injuries cost Americans $14.6 billion.

•

For more information:

800-TEC-FUSE

www.littelfuse.com

Information derived from Industry Surveys, the NFPA, The National Safety Council, Bureau of Labor Statistics, and CapSchell, Inc.

The moral obligation to protect workers

who may be exposed to electrical hazards is

fundamental, but there are legal and other

factors that require every facility to establish

a comprehensive Electrical Safety Program.

Meeting OSHA regulations, reducing insurance

costs, and minimizing downtime and repair

costs are additional beneﬁts of Electrical Safety

programs. When electrical faults occur, the

electrical system is subjected to both thermal

and magnetic forces. These forces can severely

damage equipment and are accompanied

by ﬁres, explosions and severe arcing. Such

violent damage often causes death or severe

injury to personnel. Costs of repairs, equipment

replacements, and medical treatment can run

into millions of dollars. Loss of production

and damaged goods are also important

considerations. Other major factors include

the cost of OSHA ﬁnes and litigation. Severe

electrical faults may shut down a complete

process or assembly plant, sending hundreds

or thousands of workers home for weeks while

repairs are being made. It is also possible that

one tragic event could close a plant permanently.

Implementing and following a well designed

Electrical Safety Program will protect employees

and employers against:

Injury to personnel

OSHA citations and ﬁnes

Increased costs for insurance

and workman compensation

Lost or unusable materials

Unplanned equipment

repair or replacement costs

Multi-million dollar lawsuits

Possible bankruptcy

Electrical Safety is not an option — it is absolutely

necessary for workers and employers alike.

•

Littelfuse offers a variety

of products and services

designed to help you

increase safety in your

facility, such as:

•

Current-Limiting Fuses

•

Fuse Holders and

Accessories

•

Training Seminars

& Presentations

•

Arc-Flash Calculators

•

Electrical Safety

Literature

•

Electrical Safety Video

•

Warning Labels

•

Electrical Designers

Reference (EDR) Software

•

Technical Papers

•

MRO

plus Fuse

Inventory Analysis

•

Technical Support &

Engineering Services

For more information:

800-TEC-FUSE

www.littelfuse.com

Even the simplest electrical system can

become dangerous. Unless proper procedures

are instituted, personnel installing or servicing

these systems are frequently exposed

to the hazards of shock, arc ﬂash and arc

blast. Eliminating and/or reducing these

hazards require a basic knowledge of electric

circuits. The following is a brief overview.

Electricity can be deﬁned as the ﬂow of electrons

through a conductor. This is similar to the

ﬂow of water through a pipe. Electromotive

force, measured in volts, causes the current

to ﬂow similar to a pump moving water. The

higher the water pressure and the larger the

pipes, the greater the water ﬂow. In electrical

circuits the rate of current ﬂow is measured

in amperes, similar to gallons of water per

second. Figure 1 illustrates a simple circuit.

Ohm’s Law

In 1827, George Simon Ohm discovered that the

ﬂow of electric current was directly proportional

to the applied voltage and inversely proportional

to the “resistance” of the wires or cables

(conductors) and the load. This discovery became

known as Ohm’s Law.

Ohm’s Law:

The Current in Amperes (I) is equal to the

electromotive force, or Voltage (V) divided

by the Resistance (R) in “ohms”.

Current (I) =

Voltage (V)

Resistance (R)

I =

Figure 1

Ohm’s Law:

The Current (I) in Amperes

is equal to the electromotive

force, or Voltage (V) divided

by the Resistance (R)

in “ohms.”

Current (I) =

Voltage (V)

Resistance (R)

I =

Electricity Basics

CURRENT FLOW

SHORT CIRCUIT

GEN.

LOAD

Accidental

Connection

Creates Fault

GEN.

LOAD

System voltage and load resistance

determine the ﬂow of current.

During a short circuit, only the resistance of

the fault path limits current. Current may

increase to many times the load current.

(red lines indicate increased current)

CURRENT FLOW

SHORT CIRCUIT

GEN.

LOAD

Accidental

Connection

Creates Fault

GEN.

LOAD

System voltage and load resistance

determine the ﬂow of current.

During a short circuit, only the resistance of

the fault path limits current. Current may

increase to many times the load current.

(red lines indicate increased current)

When two of the variables are known, the third

can be easily determined using mathematical

equations as shown above. Current seeks

the path of least resistance; whether it is a

conductor, the ground, or a human body. For

example, at a given voltage, the higher the

resistance is the lower the current will be. The

lower the resistance is, the higher the current

will be. Materials that have very low resistance

such as metals like copper and aluminum

are termed conductors, while non-metallic

materials like rubber, plastics, or ceramics

that have very high resistance are termed

insulators. Conductors are usually insulated to

conﬁne current to its intended path and to help

prevent electrical shock and ﬁres. Conductor

cross-section and material determine its

resistance. Current produces heat as a function

of current squared X resistance (I

R). The NEC

publishes tables that show the rated current

carrying capacity of various sizes and types of

conductors (wire and cables). Currents that

exceed the rating of the conductor increase

temperature and decrease insulation life.

Types of Electrical Faults

Together, current and voltage supply the

power we use every day. Any electric current

that exceeds the rating of the circuit is an

Overcurrent. When the current exceeds

the rated current carrying capacity of the

conductor, it generates excess heat that can

damage insulation. If insulation becomes

damaged, personnel may be severely injured

and equipment or property compromised or

destroyed. Overcurrents can be divided into

two categories: Overloads and Short Circuits.

Overloads

An Overload is deﬁned as an overcurrent that is

conﬁned to the normal current path. Excessive

connected loads, stalled motors, overloaded

machine tools, etc. can overload a circuit. Most

conductors can carry a moderate overload for a

short duration without damage. In fact, transient

moderate overloads are part of normal operation.

Startup or temporary surge currents for motors,

pumps, or transformers are common examples.

Overcurrent protection must be selected that will

carry these currents. However, if the overload

persists for too long, excessive heat will be

generated ultimately causing insulation failure.

This may result in ﬁres or lead to a short circuit.

Short Circuits

A Short Circuit is any current not conﬁned to

the normal path. The term comes from the

fact that such currents bypass the normal load

(i.e., it ﬁnds a “short” path around the load).

Usually, when a current is greater than 6 times

(600%) the normal current, it should be removed

as quickly as possible from the circuit. Short

Circuits are usually caused by accidental contact

or worn insulation and are more serious than

overloads. Damage occurs almost instantly.

Examples of Short Circuits include two or more

conductors accidentally touching, someone

touching or dropping tools across energized

conductors or accidental connection between

energized conductors and ground. Such ground

faults may vary from a few amperes to the

maximum available short circuit fault current.

Overcurrent Protective Devices

Overcurrent protective devices (fuses and

circuit breakers) are used to protect circuits

and equipment against overloads and

Types of

Electrical

Faults:

• Overloads

• Short Circuits

Current

Time

Current ﬂow during an overload condition. Figure 2

Current

Time

Current ﬂow during a short circuit condition. Figure 3

For more information:

800-TEC-FUSE

www.littelfuse.com

short circuits (faults). These devices vary in

characteristic, design and function. Fuses

and circuit breakers are designed to sense

abnormal overloads and short circuits and

open the circuit before catastrophic events

occur. Each device, however, has different

time characteristics and must be used and

applied according to the appropriate standards

and manufacturer’s recommendations

for the individual application.

Fuses and circuit breakers must be able

to discern the difference between normal

current variations that pose no threat to

equipment, and dangerous overloads or short

circuits that can cause extensive damage to

equipment and compromise safety. Not all

devices are designed to protect against both

overloads and short circuits. Most motor

starters provide only overload protection,

while some circuit breakers provide only

short-circuit protection. Overcurrent protective

devices should be selected carefully to make

sure they will open the circuit safely under any

abnormal overcurrent condition. Interrupting

ratings and opening times, especially

under short-circuit conditions, must also

be carefully observed. Failure to select the

properly rated overcurrent protective device

can result in ﬁres, explosions, and death.

UL CLASS RK 1

Interrupting Rating

Interrupting Rating (sometimes called

Interrupting Capacity) is the highest available

symmetrical rms alternating current (for DC

fuses the highest DC current) at which the

protective device has been tested, and which it

has interrupted safely under standardized test

conditions. Fuses and circuit breakers often

have very different interrupting ratings. Current-

limiting fuses have interrupting ratings up to

300,000 Amperes. UL Class H fuses and most

common molded case circuit breakers have

interrupting ratings of only 10,000 Amperes. If

an overcurrent protective device with 10,000

AIR (Amperes Interrupting Rating) is used in

a circuit that is capable of delivering a short

circuit over 10,000 amperes, a violent explosion

or ﬂash ﬁre can occur. Always use overcurrent

protective devices that have interrupting

ratings greater than the maximum available

fault current of your electrical system.

ELECTRICITY BASICS

Always use

overcurrent

protective

devices that

have interrupting

ratings greater

than the maximum

available fault

current of your

electrical system.

Current Limitation:

A current-limiting

device is one that

opens and clears

a fault within the

ﬁrst half cycle.

One half cycle of

standard 60 hz cur-

rent is equivalent

to .00833 seconds.

Article 240.2 of the

National Electrical

Code (NEC) further

states that a

current-limiting

device will reduce

the peak let-thru

current to a value

substantially less

than the potential

peak which would

occur if the

current-limiting

device were not in

the circuit.

Current Limitation with a Current-limiting Fuse

Current

Time

NOTE:

Total Clearing I

t =

Melting I

t + Arcing I

Fault Occurs

Arc is Extinguished

Peak Let-Thru / Current (l

peak

)

Fuse Elements Melt

Arcing Energy (l

Available Peak Current

Melting Energy (l

Melting

Time

Arcing

Time

Fuse Total Clearing Time

(less than ½ cycle)

Current Limitation

What exactly is “Current Limitation” and why

is it important? Article 240.2 of the National

Electrical Code

(NEC

)

deﬁnes a Current-

Limiting Overcurrent Protective Device as: “A

device that, when interrupting currents in its

interrupting range, reduces the current ﬂowing

in the faulted circuit to a magnitude substantially

less than that obtainable in the same circuit if

the device were replaced with a solid conductor

having comparable impedance.” What this really

means is that a current-limiting device is one that

opens and clears a fault before the ﬁrst current

zero after the fault occurs, and limits the peak

fault current. In most cases the current-limiting

device will clear a fault in less than one half cycle

1. National Electrical Code

and NEC

are registered trademarks of the

National Fire Protection Association, Quincy, MA.

of standard 60 Hz current (8.33 milliseconds).

Figure 4 is a graphical representation of the

effect of current limitation on a faulted circuit.

As seen above, the total clearing time “t” occurs

before the ﬁrst zero. The I

t energy is the area

under the curves. It is clear that I

t through

the fuse is much less than would otherwise

occur. Heating is a direct function of current

squared x time (I

t). Reducing current in half

reduces heat by 75%. Generally, the lower the

peak instantaneous current is, the lower the

t energy will be. The square of peak current

determines the amount of magnetic stress.

For a given circuit, cutting the peak current

in half reduces magnetic stress by 75%.

Figure 4

ELECTRICITY BASICS

For more information:

800-TEC-FUSE

www.littelfuse.com

Fuses

A fuse is an intentional weak link in a

circuit. It is a thermally responsive device

designed to provide overcurrent protection.

The main function of a fuse is to protect

conductors and equipment from damaging

overcurrents and quickly deenergize faulted

circuits minimizing hazards to personnel.

Fuses may be classiﬁed as fast-acting or time-

delay and as current-limiting or non-current-

limiting. Fast-acting fuses are designed to

respond quickly to overload currents, while time-

delay fuses are required to carry an overload

current for a predetermined amount of time. This

permits time-delay fuses to carry starting current

and other temporary overloads. Fuses that limit

the maximum peak current (Ip) that could ﬂow

during a short circuit are classiﬁed as current-

limiting fuses. Whether the fuse is classiﬁed as

fast-acting or time-delay, current-limiting fuses

will open quickly during short-circuit conditions.

Standard electrical fuses are available in

current ratings from 1/10 to 6000 Amperes

and for voltages up to 600 Volts. Underwriters

Laboratories (UL) and CANENA (Council

for the Harmonization of Electrotechnical

Standards of the Americas) classify low

voltage fuses (600VAC and less) into several

main classes such as R, J, CC, CD, L, T, G,

H, K and Plug, as well as Semiconductor or

Supplemental fuses. Each class is deﬁned

by its performance characteristics, size, and

function. Low voltage cartridge fuses are

further classiﬁed as either current-limiting or

non-current-limiting types. Cartridge fuses

have ferrules, blades, or screw type methods

of installation. They are generally intended

for and suitable for branch circuit, feeder, and

service entrance overcurrent protection in

accordance with ANSI/NFPA 70, commonly

known as the National Electrical Code®.

Inside a typical fuse, the current ﬂows through

the fuse elements, or “links”. When enough

heat is generated, the fuse element will melt

and open (blow). Most power fuses incorporate

a silica sand “ﬁller” material that safely

quenches the arc and stops the current ﬂow.

Figure 5 illustrates the components of a

Littelfuse LLSRK_ID current-limiting dual

element time-delay fuse with blown fuse

indication. It consists of two current sensing

elements in series with each other. The

ﬁrst element is made with a very precise

elastomeric silicone overload section that

protects against sustained overloads. The

second element opens quickly under short

circuit conditions, limiting the damaging heat

energy during short circuits and Arc-Flash

events. Finally and perhaps just as important,

the blown fuse indication makes trouble-

shooting and replacement safe, fast, and easy.

A fuse is designed to safely open the circuit

only once. Therefore, it must be carefully

selected to keep the equipment operating unless

there is danger of severe overheating or if a

short circuit or arcing fault occurs. Selecting

the right fuse for the application is critical to

overall safety and reliability. At the same time,

fuses are fail-safe. Unlike mechanical devices,

nothing can happen to a fuse that will prevent

it from opening or increase its opening time.

Circuit Breakers

Like fuses, circuit breakers are designed to

protect circuits from overload and short circuit

conditions when applied within their ratings.

Most circuit breakers utilize a mechanical

latching, spring assisted switching mechanism

and a thermal, thermal-magnetic, hydraulic-

magnetic, or electronic current sensing circuit

that causes the switching mechanism to

unlatch and open the circuit. Typical circuit

breakers are not current-limiting. However,

current-limiting circuit breakers are available

in some ratings, but at a higher cost.

Standard circuit breakers are available with

current ratings up to 6300A and voltage ratings

up to 1000V. As current levels increase, the

type of circuit breaker may vary from Molded

Low Voltage

UL Fuse Classes:

Class R

Class J

Class CC

Class CD

Class L

Class T

Class G

Class H

Class K

Plug

Refer to UL 248 for

more information.

•

Current-limiting

fuses usually have

much higher inter-

rupting ratings and

react much faster

to short circuits and

Arc-Flash events,

making them safer

and more reliable

to use than most

circuit breakers.

Case Circuit Breakers (MCCB) to Insulated-

Case Circuit Breakers (ICCB) to Low-Voltage

Power Circuit Breakers (LVPCB) types. Some

circuit breakers have magnetic only trip units

or electronic trip sensors that can be adjusted

for long, short, or instantaneous delays.

In all cases, the sensing circuit causes the

switching circuit within the circuit breaker

to operate (open). Due to the mass of the

contacts and mechanical switching components

and other factors, opening times of non-

current-limiting circuit breakers under short

circuit conditions can vary from ¾ cycles (13

msec.) to 8 cycles (130 msec.) or more.

Common Molded Case Circuit Breakers

(MCCB’s) such as the one shown in Figure 6

usually have “Thermal-Magnetic” trip units.

This means they have two sensing circuits in

series with a spring assisted latching switch.

The ﬁrst sensing circuit uses a “thermal”

sensing element that reacts to overloads.

The second sensing circuit is a “magnetic”

coil that reacts to short circuits. Either the

thermal sensing circuit or the magnetic sensing

circuit can cause the mechanically latched

switching circuit to open the circuit. This

provides time-current characteristics similar

to dual-element fuses. However, most fuses

have much higher interrupting ratings and

react much faster to short circuits and Arc-

Flash events, making them safer and more

reliable to use than most circuit breakers.

Plated

End Caps

AFTER OPENING (blowing)

BEFORE OPENING (blowing)

Precision Formed

Short Circuit Element

Granular

Quartz Filling

Blown Fuse

Indicator Assembly

Elastomeric Silicone

Overload Section

Figure 5

For more information:

800-TEC-FUSE

www.littelfuse.com

Figure 6

(Drawing courtesy of AVO Training Institute, Dallas, TX)

Circuit breaker manufacturers typically

recommend that their circuit breakers be cycled

ON and OFF at least once each year to keep

the tripping mechanism from seizing under

certain environmental conditions. Most

manufacturers of industrial and commercial

circuit breakers publish field-testing and

maintenance instructions. This often includes

annual testing and recalibration that requires

special equipment and qualiﬁed personnel.

Instructions for thermal-magnetic breakers

require many of these tests to be performed at

room temperature that can take breakers out of

service for several hours. After a circuit breaker

has opened, it is very important to examine the

circuit to determine if the cause was a short

circuit or an overload. Article 225.3 of NFPA 70E

requires that if a circuit breaker interrupts a

fault at or near its interrupting rating, it must be

inspected by a trained technician and tested,

repaired or replaced in accordance with the

manufacturer’s speciﬁcations.

Circuit breakers must be carefully selected

according to the application and NEC

requirements. Current ratings that are too low

will cause nuisance tripping and excessive

downtime. Current ratings that are too

high can cause excessive overheating or

higher arc-ﬂash hazards. Failure to follow

NFPA standards and guidelines and the

manufacturers’ recommendations can

result in catastrophic consequences.

Whether you use fuses or circuit breakers,

both types of overcurrent protective devices

must be tested and approved by a nationally

recognized safety agency, such as Underwriters

Laboratories. The device must also be applied

in accordance with the National Electrical Code

or other codes and standards required by the

Authority Having Jurisdiction over the facility. It

is also important to remember that even if a fuse

or circuit breaker is approved by a recognized

safety agency like UL, it must be installed

and used in accordance with any instructions

included with its labeling or listing. There are

differences, for example, in UL standards used

to qualify fuses and circuit breakers such as UL

248, UL489, and UL1077. Always check the

applicable standards and the manufacturer to

determine if their devices meet the required

interrupting ratings, voltage ratings, current

limitation, etc. for each application. Failure to

apply overcurrent protective devices within their

ratings can result in ﬁres, explosions, and deaths.

Short Circuit Current Rating (SCCR)

With all of the advances in engineering and

safety, why is it that every day 1 maintenance

person is either killed or injured in electricity

related accidents? Is it possible the majority

of effort that has gone into engineering and

inspecting for safe electrical systems has

ended when the electricity reaches the line

side terminals of the equipment? The 2005

National Electrical Code addresses this

situation with the advent of required labels on

equipment that clearly state the equipment’s

Short Circuit Current Rating (SCCR). The NEC

speciﬁcally addresses this for industrial control

panels [Article 409], industrial machinery

electrical panels [670], multiple motor HVAC

equipment [440], meter disconnect switches

[230] and multiple motor controllers [430].

The most dangerous and common

misconception of SCCR by equipment

manufacturers is that the interrupting capacity

or rating of a circuit protection device is also

the SCCR of the end use equipment in which

it is installed. Meaning, the manufacturer

ELECTRICITY BASICS

Failure to follow

NFPA and all

applicable standards

and guidelines along

with the the manu-

facturers’ recommen-

dations can result in a

catastrophy.

Unlike fuses,

circuit breakers

require annual

maintenance to

meet manufacturer’s

speciﬁcations.

that labels the equipment with a 22kA SCCR,

solely because the main circuit breaker or

fuse has an interrupting capacity of 22kA,

is mislabeling its equipment and creating a

potentially dangerous condition in your plant.

In order to build and label a safe piece of

equipment, the manufacturer must determine

the component in the primary electrical path

with the lowest SCCR or withstand rating.

The SCCR of the equipment then must match

the rating of that component with the lowest

SCCR. Just as every device within the electrical

distribution system of your facility must be

rated to handle a worst-case scenario in order

to completely protect the people and equipment

within your facility, every component within your

equipment must be designed to handle a worst-

case scenario for exactly the same reason.

The NEC

recognizes and speciﬁcally requires

equipment to have accurate SCCR labels.

These labels will allow you and inspectors to

compare fault current studies to the SCCR and

minimize potential hazards in your facilities.

Circuit Protection Checklist

Before a system is designed or when

unexpected events may occur, circuit designers

should ask themselves the following questions:

What is the normal or average

current expected?

What is the maximum continuous (three

hours or more) current expected?

What inrush or temporary surge

currents can be expected?

Are the overcurrent protective

devices able to distinguish

between expected inrush and surge

currents and open under sustained

overloads and fault conditions?

What kind of environmental

extremes are possible? Dust,

humidity, temperature extremes

and other factors need to be

considered.

What is the maximum available

fault current the protective device

may have to interrupt?

Is the overcurrent protective device

rated for the system voltage?

Will the overcurrent protective

device provide the safest and most

reliable protection for the speciﬁc

equipment?

Under short-circuit conditions, will

the overcurrent protective device

minimize the possibility of a ﬁre or

explosion?

Does the overcurrent protective

device meet all the applicable

safety standards and installation

requirements?

Answers to these questions and other criteria will

help to determine the type of overcurrent protective

device to use for optimum safety and reliability.

For more information:

800-TEC-FUSE

www.littelfuse.com

Contrary to popular belief, Benjamin Franklin did

not “discover” or “invent” electricity. The ﬂow

of electricity and its effects have been known

for centuries, especially when traveling through

air in the form of lightning. It wasn’t until the

late 18

and early 19

centuries, however,

that scientists began to discover and analyze

what electricity really is and how to harness

it for man’s beneﬁt. Thus began the need to

regulate electrical installations to protect people

and equipment from its unintended effects.

With the advent of the electric light bulb and

electric motors in the late 19

Century, it was

soon discovered that electricity could also cause

ﬁres and kill people. Thomas Edison is said to

have developed the ﬁrst “fuse” by using a wire

between two terminals that would melt if too

much current ﬂowed through it. In 1882, Edison

opened the world’s ﬁrst central electric light

power station in New York City. It produced

enough DC current to power 7200 electric

lamps. In 1887, Edison was issued the ﬁrst

fuse patent. Ever since, controlling electricity

and protecting wires from ﬁre has become

more and more complex. In an effort to increase

electrical safety, Thomas Edison and George

Westinghouse confronted each other on the

relative beneﬁts and dangers of Direct Current

(DC) vs. Alternating Current (AC). Concerned

with electrical safety, Thomas Edison tried to

establish DC current as the standard in the US.

He argued that DC current was not as dangerous

as AC, which George Westinghouse was

promoting. In 1889, the state of New York

commissioned the development of the electric

chair for their capital punishment program. Even

though Edison was not a proponent of capital

punishment, he was asked to design the electric

chair and assumed Westinghouse would be

approached if he refused. Edison viewed this as

an opportunity to prove that AC was more

dangerous than DC and designed the “chair”

using AC. In 1893, George Westinghouse

received the contract to design the “Palace of

Electricity” at the World’s Columbian Exposition

in Chicago. AC was used and shown to be

safely applied. Obviously, Edison was proven

wrong regarding the safe application of AC.

Westinghouse also had a better plan for

generating and distributing electrical energy

over long distances at higher voltages and then

transforming it to lower useable voltages. Thus

began the need for increased electrical

construction and safety standards.

1860s

1889 1897 1913 1970 1980 1990 1995 2002

2005

1880s 1890s 1897 1940s 1979 1982 1995 2000 2004

First fuses

developed

Electric chair

was developed

First NEC

released

Edition

Electrician’s Handbook

OSHA is formed

First burn

centers opened

OSHA Subpart S

updated

Arc-Resistant

switchgear introduced

NEC requires

warning labels

NEC is updated with

new safety definitions

AC/DC Electrical systems

were expanded

Circuit breakers

developed

UL is formed

Current-limiting fuses

were developed

NFPA 70E is

Released

Ralph Lee’s

Arc-Flash paper

NFPA 70E recognizes

Arc-Flash

NFPA 70E expands

on Arc-Flash

NFPA 70E is expanded

and revised

Thomas Edison

is said to have

developed the ﬁrst

“fuse” by using a

wire between two

terminals that

would melt if too

much current

ﬂowed through it.

History of Electrical Safety

Because insurance companies were concerned

about ﬁre safety and electricity, the Underwriters

Electrical Bureau (later to become UL) was

established in 1894 to review various electrical

safety standards and building codes that were

quickly being developed. In the 1890’s, the ﬁrst

crude circuit breakers were also developed. In

1896, the National Fire Protection Association

was formed in New York City. Because electricity

was viewed as a ﬁre hazard, the National Board

of Fire Underwriters unanimously approved the

ﬁrst “National Electrical Code” in June of 1897.

Thus, the “NEC” was born.

Many electric generating plants and transmission

lines were built and installed in the US in the

early 20

Century. Construction and safety

standards were quickly developed. In 1904

Underwriters Laboratories published the ﬁrst

fuse standard. In 1913, the ﬁrst edition of the

“American Electricians’ Handbook” was issued. In

the 1930’s, the Wiggington Voltage Tester

(a.k.a. the “Wiggie”) was developed for testing

the presence of voltage, etc. In June of 1940,

UL published the ﬁrst circuit breaker standard,

UL489, entitled “Branch-Circuit and Service

Circuit-Breakers.” It was later in the 1940’s when

the ﬁrst current-limiting fuses were developed.

Despite advances in technology and as

hard as it may be to believe, the American

Electricians Handbook of 1942 had the

following to say about Electrical Safety:

“158.

Electricians often test for

the presence of voltage by

touching the conductors with the ﬁngers.

This method is safe where the voltage does

not exceed 250 and is often convenient to

locating a blown-out fuse or for ascertaining

whether or not a circuit is alive. Some men

can endure the electric shock that results

without discomfort whereas others cannot.

Therefore, the method is not feasible in

some cases. Which are the outside wires

and which is the neutral of a 115/230-volt,

three-wire system can be determined in

this way by noting the intensity of the shock

that results by touching different pairs of

wires with the ﬁngers. Use the method with

caution and be certain the voltage of the

circuit does not exceed 250 before touching

the conductors.

159.

The presence of low voltages

can be determined by tasting.

The method is feasible only where the

pressure is but a few volts and hence is

used only in bell and signal work. Where the

voltage is very low, the bared ends of the

conductors constituting the circuit are held a

short distance apart on the tongue. If voltage

is present a peculiar mildly burning sensation

results, which will never be forgotten after

one has experienced it. The taste is due to

the electrolytic decomposition of the liquids

on the tongue, which produces a salt having

a taste. With voltages of 4 or 5 volts, due to

as many cells of a battery, it is best to test

for the presence of voltages by holding one

of the bared conductors in the hand and

touching the other to the tongue. Where a

terminal of a battery is grounded, often a

taste can be detected by standing on moist

ground and touching a conductor from the

other battery terminal to the tongue. Care

should be exercised to prevent the two

1860s

1889 1897 1913 1970 1980 1990 1995 2002

2005

1880s 1890s 1897 1940s 1979 1982 1995 2000 2004

First fuses

developed

Electric chair

was developed

First NEC

released

Edition

Electrician’s Handbook

OSHA is formed

First burn

centers opened

OSHA Subpart S

updated

Arc-Resistant

switchgear introduced

NEC requires

warning labels

NEC is updated with

new safety definitions

AC/DC Electrical systems

were expanded

Circuit breakers

developed

UL is formed

Current-limiting fuses

were developed

NFPA 70E is

Released

Ralph Lee’s

Arc-Flash paper

NFPA 70E recognizes

Arc-Flash

NFPA 70E expands

on Arc-Flash

NFPA 70E is expanded

and revised

For more information:

800-TEC-FUSE

www.littelfuse.com

conductor ends from touching each other at

the tongue, for if they do a spark can result

that may burn.“

After World War II, the demand for electric

power increased for new construction and

advances in productivity created the need for

circuit protection devices with higher current

ratings and interrupting capacities. Electrical

safety standards and practices needed to

keep pace with the ever-increasing growth

of electrical power use and generation.

In 1970, when the Williams-Steiger Act was

signed into law, the Occupational Safety and

Health Administration (OSHA) was created. It

took OSHA several years before they issued

comprehensive regulations that governed

aspects of all workers safety. At OSHA’s request,

the National Fire Protection Association, which

issues the National Electrical Code

, (NFPA 70),

was asked to research and provide guidelines for

electrical safety in the workplace. In 1979, the

NFPA issued the ﬁrst edition of NFPA 70E,

entitled “Standard for Electrical Safety

Requirements for Employee Workplaces” (since

renamed the “Standard for Electrical Safety in

the Workplace.”) This was the ﬁrst nationally

accepted standard that addressed electrical

safety requirements for employee workplaces.

In the 1970’s, in addition to the known shock

hazards associated with electricity, researchers

began to address the phenomena of arcing

faults that released large amounts of heat and

light energy as well as pressure and sound

energy. In 1980, Dr. Raphael Lee opened

the ﬁrst burn center in Chicago dedicated to

the care and treatment of electrical burns.

In 1982, Mr. Ralph Lee (no relation) wrote

an IEEE technical paper entitled “The Other

Electrical Hazard: Electric Arc Blast Burns.”

This paper introduced methods to determine

and calculate the severity of electrical arc-

ﬂash hazards. It remains today as one of the

1. Croft, Terrell, American Electricians’ Handbook, 5th edition,

McGraw-Hill, New York, NY, 1942

most comprehensive dissertations on the

causes and effects of Arc-Flash hazards. It

was also the ﬁrst notable publication that

attempted to analyze and quantify the potential

energy released during an Arc-Flash event.

In 1990, OSHA updated subpart S of the Code

of Federal Regulations, CFR 29 Section 1910,

which deals speciﬁcally with the practical

safeguarding of electrical workers at their

workplaces. In 1995, NFPA 70E was revised

to include formulas to establish shock and

ﬂash protection boundaries. Also in the mid

1990’s, equipment makers began to design their

equipment to be more arc resistant. In the year

2000, NFPA 70E was again revised to include

an expanded section on Arc-Flash hazards. In

2002, the National Electrical Code (NEC)

was

updated to include the requirement of shock

and Arc-Flash hazard warning labels on all

equipment that is likely to be worked on while

energized. Also in 2002, the IEEE (Institute of

Electronic and Electrical Engineers) published

IEEE 1584 “Guide for Performing Arc-Flash

Hazard Calculation”. The latest edition of

NFPA 70E recognizes IEEE 1584 as a preferred

method of calculating Arc-Flash hazards.

In addition to OSHA, NFPA, and the IEEE, there

are several other safety organizations and

standards such as American National Standards

Institute (ANSI), American Society of Testing

and Materials (ASTM) and the International

Electrotechincal Commission (IEC) that have

developed practices and have set standards

for materials and the testing of products to

protect workers from electrical hazards.

HISTORY OF ELECTRICAL SAFETY

At OSHA’s request,

the National Fire

Protection Associa-

tion was asked to

research and pro-

vide guidelines for

electrical safety in

the workplace.

As a result the

NFPA 70E “Standard

for Electrical Safety

in the Workplace.”

was issued.

The primary goal

of OSHA is “to

ensure safe and

healthful condi-

tions for every

American worker.”

Electrical Safety Organizations

Several organizations have developed and

continue to revise standards to address the

numerous concerns involving electrical power.

Standards and safety organizations include:

OSHA

Occupational Safety &

Health Administration

NFPA

National Fire

Protection Association

IEEE

Institute of Electrical and

Electronic Engineers

Underwriters Laboratories

NEMA

National Electrical

Manufacturers Association

ANSI

American National

Standards Institute

ASTM

American Society for

Testing and Materials

NECA

National Electrical

Contractors Association

OSHA

The primary goal of the Occupational Safety

and Health Administration (OSHA) is “to ensure

safe and healthful conditions for every American

worker.” OSHA currently has thousands of rules

and regulations that cover workplace safety.

Federal and state OSHA programs enforce

regulations through workplace inspections,

voluntary assistance programs, and training

activities. Citations and ﬁnes are also levied

for violations found during inspections.

•

The General Duty Clause

Section 5(a)(1) of the Occupational

Safety and Health Act of 1970 reads,

“ 5. Duties

(a) Each Employer

(1) Shall furnish to each of his

employees employment and a

place of employment which are

free from recognized hazards

that are causing or are likely

to cause death or serious

physical harm to his employees;”

The “General Duty Clause” is essentially the

mission that OSHA strives to enforce. It is

also often cited when OSHA investigates a

workplace accident. Many OSHA regulations

are prescriptive in nature like the “General

Duty Clause”. In other words, OSHA is

the “shall” or the reason for addressing an

issue. In some cases, OSHA will also provide

detailed information on how to meet the

requirements. In other instances, OSHA

refers to national safety organizations such as

NFPA to provide the required level of detail to

meet the regulations. In either case, OSHA

covers all employees and all employers.

OSHA Regulations

Published by the U.S. Federal Register,

OSHA regulations can be found in the Code

of Federal Regulations (CFR) under Title

29. More speciﬁcally, and legally enforced

by OSHA, Subpart S (Parts 1910.301 to

1910.399) addresses “Electrical” safety

standards and covers the practical

safeguarding of electrical workers. Subpart

S is divided into four major divisions:

Design safety standards

Safety-related work practices

Safety-related maintenance

requirements

Safety requirements for special

equipment

•

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800-TEC-FUSE

www.littelfuse.com

Other OSHA standards outline some of

the general requirements for electrical

installations and general safe work practices:

29 CFR 1910.132

Personal Protective Equipment

General Requirements

29 CFR 1910.335

Electrical Personal Protective Clothing

29 CFR 1910.147

Control of Hazardous Energy

(Lockout / tagout)

29 CFR 1910.269

Power Generation, Transmission,

& Distribution

OSHA and NFPA have worked with each other

to establish standards and codes that ensure

employee safety in the workplace. One of

their objectives is to minimize the hazards of

electricity through standards that specify safe

design characteristics and work practices for

electrical equipment and systems. Many of the

standards and codes are not only accepted in

the United States, but throughout the world.

NFPA

The primary organization in the U.S. for ﬁre and

electrical safety standards is the NFPA. Their

document, NFPA 70E, Standard for Electrical

Safety in the Workplace

, has been adopted by

the American National Standards Institute (ANSI)

as an American National Standard. This standard

covers safety related work practices, deﬁnes

qualiﬁed and unqualiﬁed workers and provides

guidance to establish an electrical safety

program. It also requires an electrical hazard

analysis for shock and ﬂash, discusses energized

work permits, and proper Lockout/tagout

procedures. NFPA 70E deﬁnes and establishes

shock and Arc-Flash approach boundaries to

energized equipment and addresses how to

select appropriate PPE (personal protective

equipment) and protective clothing.

In order to help meet the required OSHA

regulations for electrical safety and training,

OSHA refers to NFPA 70E as a national

consensus standard for electrical safety in

the workplace. NFPA also publishes NFPA 70,

otherwise known as the National Electrical

Code

, and other standards that address

public safety and practices. Together, OSHA

and the NFPA continue to work to improve

workplace safety. To ensure the safety of

your plant and personnel, OSHA and NFPA

standards should always be followed.

IEEE

The Institute of Electrical and Electronic

Engineers, Inc. (IEEE) is an association of

electrical and electronic engineers established

to advance the theory and application of

electro-technology and allied sciences. The

Industry Application Society (IAS) of the IEEE

is the group that addresses power distribution

in industrial and similar facilities. There are

numerous sub-committees that meet regularly

to research, publish, and update standards

and guidelines for the testing, evaluation,

and application of their particular industry

or specialty. In 2002, the Petroleum and

Chemical Industry Committee IAS published

IEEE1584, entitled, IEEE Guide for Performing

Arc-Flash Hazard Calculations

. Although there

are other methods of determining Arc-Flash

hazards, IEEE 1584 has quickly become

the de facto standard for determining the

extent of potential Arc-Flash Hazards.

NATIONALLY RECOGNIZED TESTING

LABORATORIES (NRTL)

The best-known NRTL is Underwriters Labora-

tories, Inc. (UL). UL is an independent, not-for-

proﬁt product safety testing and certiﬁcation

organization that lists and labels products for

conformance to accepted standards. Work-

ing with industry associations, manufacturers,

experts, insurance companies, and government

agencies, UL publishes various standards and

minimum test requirements for all types of

electrical equipment. Manufacturers submit

HISTORY OF ELECTRICAL SAFETY

OSHA and NFPA

have worked with

each other to estab-

lish standards and

codes that ensure

employee safety in

the workplace.

OSHA commonly

is referred to as the

“Shall” and NFPA

70E as the “How

to” with regards to

electrical safety.

their products to be evaluated for conformance

to one or more of these standards. If the

product meets or exceeds the standards, UL

lists the product in their guides and permits

manufacturers to display the UL label on the

product. Protective devices such as fuses and

circuit breakers must meet rigid standards such

as UL248, UL489, or UL1077. There are other

Nationally Recognized Testing Laboratories

such as Canadian Standards Association (CSA),

Electrical Testing Laboratories (ETL) that test

and evaluate products to UL or other industry

standards. Equipment that has been modiﬁed

may require new evaluation and manufacturers

routinely submit their products to UL for re-

evaluation to maintain their listing.

NEMA

The National Electrical Manufacturers

Association (NEMA) has over 400 member

companies including large, medium, and small

businesses that manufacture products used in

the generation, transmission and distribution,

control, and end-use of electricity. NEMA has

developed and published hundreds of standards

jointly developed by its member companies.

The standards have been established in the

best interests of the industry and users

of its products. NEMA works closely with

the American National Standards Institute

(ANSI) and the International Electrotechnical

Commission (IEC) to be an advocacy group to

UL and governmental agencies. Many NEMA

publications have been adopted by ANSI as

American National Standards. Some address the

use and application of overcurrent protective

devices including AB3-2001 Molded Case

Circuit Breakers and their Application

; AB4-

2003 Guidelines for Inspection and Preventive

Maintenance of Molded Case Circuit Breakers

Used in Commercial and Industrial Applications

;

and FU1-2002 Low-voltage Cartridge Fuses,

while others address safety issues such

as safety signs, tags, and barricades.

ANSI

The American National Standards Institute

(ANSI) is a private, non-proﬁt organization

that administers and coordinates the U.S.

voluntary standardization and conformity

assessment system. Working in conjunction

with organizations such as NFPA, IEEE, NEMA,

ASME (American Society of Mechanical

Engineers), ASCE (American Society of Civil

Engineers), AIMME (American Institute of

Mining and Metallurgical Engineers), and

ASTM (American Society of Testing and

Materials), ANSI coordinates and adopts

these various industry consensus standards

and publishes standards to promote US and

Global conformity. ANSI has adopted many

NFPA, NEMA, and ASTM standards for

procedures, materials, and personal protective

equipment used by electrical workers.

ASTM

ASTM International, formerly known as the

American Society for Testing and Materials, is a

voluntary standards development organization

primarily involved with establishing standards

for the testing and analysis of materials.

The ASTM has published several standards

accepted by ANSI and other organizations that

govern the manufacturing, testing methods,

and ratings of personal protective equipment

used by electrical and other workers.

NECA

NECA, the National Electrical Contractors

Association, is in the process of developing

installation standards for electrical

construction work. They have also developed

electrical safety standards with emphasis

on their members. In many cases, these

standards are being adopted by ANSI.

21 21 21

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Over 20,000 standards have been developed

to reduce the risk of electrical hazards. Except

for OSHA regulations most standards do not

automatically become law. However, they are often

adopted by governmental bodies and become law;

enforced by the Authority Having Jurisdiction (AHJ).

Other standards are written into manufacturing

and construction speciﬁcations. Whether law or

not, applicable standards should be followed to

improve safety and reduce potential hazards.

Working on deenergized equipment

OSHA 29 Part 1910.333 covers selection and

use of (electrical) work practices. It deﬁnes and

regulates such things as working on or near

energized or deenergized parts, Lockout / tagout

procedures, who is or is not considered qualiﬁed

to work on live circuits, approach distances, use

of personal protective equipment, and other

requirements. Paragraph 1910.333 (a)(1) reads:

“Deenergized parts.

Live parts to which an employee may

be exposed shall be deenergized before

the employee works on or near them,

unless the employer can demonstrate

that deenergizing introduces additional

or increased hazards or is infeasible

due to equipment design or operational

limitations. Live parts that operate

at less than 50 volts to ground need

not be deenergized if there will be no

increased exposure to electrical burns

or to explosion due to electric arcs.”

To demonstrate the close relationship between

OSHA and NFPA 70E, here is what NFPA

70E Article 130.1 has to say regarding the

need for equipment to be deenergized:

.“Justiﬁcation for Work.

Live parts to which an employee

might be exposed shall be put into an

electrically safe work condition before

an employee works on or near them,

unless the employer can demonstrate

that deenergizating introduces

additional or increased hazards or is

infeasible due to equipment design

or operational limitations. Energized

parts that operate at less than 50

volts to ground shall not be required

to be deenergized if there will be no

increased exposure to electrical burns

or to explosion due to electric arcs...”

When electrical equipment has been

deenergized, OSHA Part 1910.147 (c) and

1910.333 (b)(2) requires Lockout/tagout

procedures be followed. Failure to follow

Lockout/tagout procedures is also consistently

listed as one of the top ten OSHA violations.

How to establish an electrically safe

work condition

Equipment that has been deenergized and

veriﬁed as such is said to be in an electrically

safe work condition. Article 120.1 of NFPA

70E outlines 6 steps that must be followed

to insure that employees are working in an

electrically safe work condition. They are:

1. Reprinted with permission from NFPA 70E-2004, Standard for

Electrical Safety in the Workplace, Copyright

2004, National Fire

Protection Association, Quincy, MA. This reprinted material is not the

complete and ofﬁcial position of the NFPA on the referenced subject,

which is represented only by the standard in its entirety.

Electrical Safety Codes and Standards

20,000

standards have

been developed to

reduce the risk of

electrical hazards.

Establish a “safe

work condition”

and work on

system components

deenergized

when possible.

1. “Determine all possible sources

of electrical supply to the speciﬁc

equipment. Check applicable

up-to-date drawings, diagrams,

and identiﬁcation tags.

2. After properly interrupting the load

current, open the disconnecting

device(s) for each source.

3. Wherever possible, visually verify

that all blades of the disconnecting

devices are fully open or that

drawout-type circuit breakers

are withdrawn to the fully

disconnected position.

4. Apply Lockout / tagout devices in

accordance with a documented

and established policy.

5. Use an adequately rated voltage

detector to test each phase

conductor or circuit part to verify

they are deenergized. Test each

phase conductor or circuit part

both phase-to-phase and phase-

to-ground. Before and after each

test, determine that the voltage

detector is operating satisfactorily.

6. Where the possibility of induced

voltages or stored electrical

energy exists, ground the phase

conductors or circuit parts before

touching them. Where it could be

reasonably anticipated that the

conductors or circuit parts being

deenergized could contact other

exposed energized or circuit parts,

apply ground connecting devices

rated for the available fault duty.”

It is important to note that a safe work

condition does not exist until all 6 steps are

complete. During the process of creating

the electrically safe work condition, the

appropriate PPE must also be utilized.

1. Reprinted with permission from NFPA 70E-2004, Standard for

Electrical Safety in the Workplace, Copyright

2004, National Fire

Protection Association, Quincy, MA. This reprinted material is not the

complete and ofﬁcial position of the NFPA on the referenced subject,

which is represented only by the standard in its entirety

Working on energized equipment

Although the best practice is to always

work on deenergized equipment, OSHA

and NFPA do recognize that in some

circumstances it may create an additional

hazard or be infeasible to deenergize.

OSHA 29 CFR 1910.333 (a)(2) states:

“Energized parts.

If the exposed live parts are not

deenergized (i.e., for reasons of

increased or additional hazards or

infeasibility), other safety-related work

practices shall be used to protect

employees who may be exposed to

the electrical hazards involved. Such

work practices shall protect employees

against contact with energized

circuit parts directly with any part

of their body or indirectly through

some other conductive object….”

Electrical tasks such as troubleshooting and

testing for the presence of voltage, current,

etc., can only be done while equipment

is energized. In these instances, work on

energized equipment is allowed, but workers

must follow safe work practices and use the

appropriate PPE. Other exceptions that allow

work on energized equipment include:

Life-support equipment

Emergency alarm systems

Hazardous area ventilation

equipment

Deenergizing these types of equipment

could increase or create additional hazards. A

mistake often made is confusing infeasibility

with inconvenience. For example, meeting

a manufacturing production schedule does

not qualify as infeasible. It may be very

inconvenient but it still does not authorize

working on energized equipment. OSHA 29

CFR 1910.331-335 outlines the conditions

for working on energized circuits in much

greater detail. When work is to be performed

on energized equipment, extra care must

be used and all applicable OSHA and NFPA

•

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codes and standards followed. Electrical

workers must also be trained and specially

“qualiﬁed” to work on energized equipment,

and the speciﬁc equipment to be serviced.

Who is Qualiﬁed?

The deﬁnition of a “Qualiﬁed” person continues

to change and evolve. As a worker, you may

be qualiﬁed for some tasks and unqualiﬁed for

others. Knowing the difference may even save

your life. It is no longer sufﬁcient for those who

will install and/or maintain electrical systems and

equipment to be just “familiar” with the hazards

involved. Training is the key in determining who

is considered a qualiﬁed worker. All personnel

who may be exposed to electrical hazards MUST

receive documented training in order to become

qualiﬁed. OSHA 29 CFR 1910.333 (c)(2) states;

“Work on energized equipment.

Only qualiﬁed persons may work on

electric circuit parts or equipment

that have not been deenergized under

the procedures of paragraph (b) of

this section. Such persons shall be

capable of working safely on energized

circuits and shall be familiar with the

proper use of special precautionary

techniques, personal protective

equipment, insulating and shielding

materials, and insulated tools.”

Article 100 of the National Electrical Code

and

NFPA 70E also deﬁnes a Qualiﬁed Person as:

“Qualiﬁed Person

One who has skills and knowledge

related to the construction and operation

of the electrical equipment and

installations and has received safety

training on the hazards involved.”

1. Reprinted with permission from NFPA 70-2005, National Electrical

Quincy, MA. This reprinted material is not the complete and ofﬁcial

position of the NFPA on the referenced subject, which is represented

only by the standard in its entirety.

NFPA 70E Article 110.6 (D) Employee Training

covers the requirements for “Qualiﬁed” persons

in more detail. In addition to being trained

and knowledgeable, qualiﬁed persons must

also be familiar with emergency procedures,

special precautionary techniques, personal

protective equipment, Arc-Flash, insulating

materials and tools, and testing equipment. In

some instances, employees receiving on-the-

job training may be considered “Qualiﬁed”

for speciﬁc duties under supervision.

Ultimately, a person can be considered

qualiﬁed with respect to certain equipment and

methods but still be considered unqualiﬁed

for others. Unqualiﬁed persons must also

be trained in the risks they are exposed to

and the procedures that are necessary to

ensure their safety, however, they may not

be considered “qualiﬁed” to work on speciﬁc

equipment. It is vital that Unqualiﬁed workers

have an understanding of what tasks can

only be performed by Qualiﬁed workers.

Energized Electrical Work Permit

Before work is performed on energized

equipment, NFPA 70E states:

Article 130 (A)(1)

“If live parts are not placed in an

electrically safe work condition (i.e., for

the reasons of increased or additional

hazards or infeasibility per 130.1), work

to be performed shall be considered

energized electrical work and shall be

performed by written permit only.”

The intent of an Energized Electrical Work

Permit is to discourage the practice of working

on energized equipment. The objective is to

get the supervisor or manager to recognize

2. Reprinted with permission from NFPA 70E-2004, Standard for

Electrical Safety in the Workplace, Copyright ® 2004, National Fire

Protection Association, Quincy, MA. This reprinted material is not the

complete and ofﬁcial position of the NFPA on the referenced subject,

which is represented only by the standard in its entirety.

ELECTRICAL SAFETY CODES AND STANDARDS

As a worker, you

may be qualiﬁed for

some tasks and un-

qualiﬁed for other.

Knowing the

difference between

the two can save

your life.

and fully understand the additional risks

involved so they will be less likely to approve

work on energized components. In essence,

this shifts the decision to work on energized

equipment from the worker to management.

According to the NFPA 70E Handbook, work

permits can also be written to cover a certain

length of time for routine tasks provided the

worker is trained and qualiﬁed. Other tasks that

are not routine should generate a work permit

as needed to insure the worker is trained and

qualiﬁed for the task. Exceptions to the written

work permit include testing, troubleshooting,

and voltage measuring by qualiﬁed workers.

NFPA 70E does not require a speciﬁc format for

an Energized Electrical Work Permit. However,

it should contain the following 11 elements:

1. The location and description of

equipment to be serviced

2. Justiﬁcation why circuit

cannot be deenergized

3. Description of safe work

practices employed

4. Results of the shock hazard analysis

5. Determination of the shock

protection boundaries

6. Results of the ﬂash hazard analysis

7. The Flash Protection Boundary

8. Description of PPE to be used

9. Description of barriers used

to restrict access

10. Evidence of job brieﬁng

11. Signature of responsible management

The intent of

an Energized

Electrical Work

Permit is to

discourage

the practice

of working

on energized

equipment.

HOW WILL ACCESS TO THE WORK AREA BE RESTRICTED FROM UNQUALIFIED PERSONNEL?

HAS A JOB BRIEFING BEEN COMPLETED?

WHAT EVIDENCE IS AVAILABLE?

WERE THERE ANY JOB SPECIFIC HAZARDS?

IN YOUR OPINION, CAN THIS JOB BE COMPLETED SAFELY?

Signature of Qualiﬁed Person Date

Section 3 - Approval to Perform Work on Energized Equipment

(To be completed by Management)

IS WORK ON ENERGIZED EQUIPMENT APPROVED?

Signature of Manufacturing Manager Date

Signature of Plant Manager Date

Signature of Safety Manager Date

Signature of Electrical Maintenance Manager Date

Signature of Qualiﬁed Person Date

YES NO YES NO

XYZ COMPANY ENERGIZED ELECTRICAL WORK PERMIT

Section 1 - Work Request

(To be completed by person requesting the permit)

DESCRIPTION OF TASK:

DESCRIPTION OF EQUIPMENT:

SYSTEM VOLTAGE:

AVAILABLE FAULT CURRENT:

Section 2 - Justiﬁcation of Work

(To be completed by Qualiﬁed Person performing the work)

WHY IS TASK BEING PERFORMED IN ENERGIZED CONDITION?

WHAT WORK PRACTICES WILL BE UTILIZED TO INSURE SAFETY?

WHAT WERE THE RESULTS OF THE SHOCK ANALYSIS?

LIMITED: RESTRICTED: PROHIBITED:

WHAT WERE THE RESULTS OF THE FLASH HAZARD ANALYSIS?

WHAT IS THE REQUIRED PERSONNEL PROTECTIVE EQUIPMENT (PPE) FOR THIS TASK ?

HARD HAT

SAFETY GLASSES

SAFETY GOGGLES

FACE SHIELD

FLASH HOOD

EAR PROTECTION

T-SHIRT

LONG SLEEVE SHIRT

FR SHIRT

VOLTAGE RATED GLOVES

LEATHER GLOVES

COTTON UNDERWEAR

LONG PANTS

FR PANTS

FR COVERALL

FLASH SUIT

LEATHER SHOES

WORK ORDER NO:WORK ORDER NO:

LOCATION

EQUIPMENT

LOCATION

EQUIPMENT

START DATE: TIME: TIME REQUIRED: TIME REQUIRED:START DATE: TIME: TIME REQUIRED: TIME REQUIRED:

HAZARD RISK

CATEGORY:

INCIDENT

ENERGY:

FLASH PROTECTION

BOUNDARY:

HAZARD RISK

CATEGORY:

INCIDENT

ENERGY:

FLASH PROTECTION

BOUNDARY:

Figure 7

See Appendix C for Sample Work Permit Energized Electrical Work Permit

SAMPLE

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www.littelfuse.com

The implementation and proper use of Energized

Work Permits has forced employers and

employees to perform hazard risk assessments

and justify working on potentially hazardous

energized equipment. At this time, OSHA

does not speciﬁcally require the written

Energized Electrical Work Permit. However, it

is implied within current OSHA regulations and

will most likely be enforced in future OSHA

revisions. For an example of an Energized

Electrical Work Permit refer to Annex C of

this handbook or Annex J of NFPA 70E.

Employer and Employee Responsibilities

According to OSHA and NFPA 70E, if work

is planned or performed on energized

equipment, employers must:

Justify why work must be per-

formed on energized equipment.

Perform an electrical hazard

assessment.

Inform and train employees

of the potential hazards

and how to avoid them.

Test and verify that employees

are “qualiﬁed” to work on

speciﬁc equipment.

Select and provide proper personal

protective equipment for employees.

Train employees how to

use and care for PPE.

Provide their employees with a job

brieﬁng and written Energized Work

Permit signed by management.

•

Employees are expected to:

Be trained and “qualiﬁed”

Use the PPE provided

by their employer

Inform their employers of the

need to repair or replace PPE

At the end of the day, safety is the

responsibility of both the employer and

employee. Together they must develop

and implement safe work practices and

procedures and an Electrical Safety Program.

•

ELECTRICAL SAFETY CODES AND STANDARDS

Safety is the

responsibility of both

the employer and

employee. Together

they must develop

and implement

safe work practices

and procedures

and an Electrical

Safety Program.

Electrical Safety Hazards

When electrical systems break down

what are the primary hazards and what

are the consequences to personnel?

Electric shock

Exposure to Arc-Flash

Exposure to Arc-Blast

Exposure to excessive light

and sound energies

Secondary hazards may include burns, the

release of toxic gases, molten metal, airborne

debris and shrapnel. Unexpected events can

cause startled workers to lose their balance

and fall from ladders or jerk their muscles

possibly causing whiplash or other injuries.

•

Electric Shock

When personnel come in contact with energized

conductors they receive a shock with current

ﬂowing through their skin, muscles and vital

organs. The severity of the shock depends on

the current’s path through the body, the current

intensity, and the duration of the contact.

They may only experience a mild tingling

sensation or it could result in serious injury or

death. As voltage levels increase, the effects

of electric shock escalate. Current may also

cause an erratic heartbeat known as ventricular

ﬁbrillation. If ﬁbrillation occurs even brieﬂy and

goes untreated, the effects are usually fatal.

A clear understanding of how electric current

travels through the body can help minimize

injury if such contact occurs. The table below

outlines the effects that various values of

electrical current have on the human body.

Arc-Flash and Other Electrical Safety Hazards

When personnel

come in contact

with energized

conductors they

receive a shock

with current

ﬂowing through

their skin, muscles

and vital organs.

The shock may

result in a serious

and sometimes

fatal injury.

1-3mA of current

10mA of current

30mA of current

30-75mA of current

100-200mA of current

50-300mA of current

Over 1500mA of current

150˚ F

200˚ F

CONDITION EFFECTS

Mild sensation

Muscles contract, releasing grip may be difﬁcult

Breathing difﬁcult, possible loss of consciousness

Respiratory paralysis

Ventricular ﬁbrillation

Shock (potentially fatal)

Tissue and organ burn

Cell destruction

Skin experiences “third degree” burns

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As little

as 50 mA

of current

can be fatal.

There are three basic pathways electric

current travels through the body;

1) Touch Potential (hand/hand path)

2) Step Potential (foot/foot path)

3) Touch/Step Potential (hand/foot path)

Figure 8 illustrates these groups and the path of

current through the body.

1) In a touch potential contact, current

travels from one hand through the

heart and out through the other hand.

Because the heart and lungs are in

the path of current, ventricular ﬁbril-

lation, difﬁculty in breathing, uncon-

sciousness, or death may occur.

2) In a step potential contact, current travels

from one foot through the legs, and out

of the other foot. The heart is not in the

direct path of current but the leg muscles

may contract, causing the victim to col-

lapse or be momentarily paralyzed.

3) In a touch/step potential contact, cur-

rent travels from one hand, through the

heart, down the leg, and out of the foot.

The heart and lungs are in the direct

path of current so ventricular ﬁbrilla-

tion, difﬁculty in breathing, collapse,

unconsciousness, or death may occur.

Even though there may be no external signs

from the electrical shock, internal tissue or organ

damage may have occurred. Signs of internal

damage may not surface immediately; and

when it does, it may be too late. Any person

experiencing any kind of electrical shock should

seek immediate medical attention. Using

the correct personal protective equipment

(PPE) and following safe work practices will

minimize risk of electrical shock hazards.

Arc-Flash and Arc Blasts

An Arc-Flash is an unexpected sudden release

of heat and light energy produced by electricity

traveling through air, usually caused by

accidental contact between live conductors.

Temperatures at the arc terminals can reach or

exceed 35,000 degrees Fahrenheit (F), or four

TOUCH POTENTIAL

(hand/hand path)

STEP POTENTIAL

(foot/foot path)

TOUCH/STEP POTENTIAL

(hand/foot path)

Source

Ground

Source Ground Ground

Figure 8

ARC-FLASH AND OTHER ELECTRICAL SAFETY HAZARDS

Incident energy is

the instantaneous

energy released by

an Arc-Flash and is

usually expressed

in calories per

square centimeter

(cal/cm

times the temperature of the sun’s surface. The

air and gases surrounding the arc are instantly

heated and the conductors are vaporized

causing a pressure wave called an Arc Blast.

Personnel directly exposed to an Arc-Flash

and Arc-Blast events are subject to third

degree burns, possible blindness, shock, blast

effects and hearing loss. Even relatively small

arcs can cause severe injury. The secondary

effect of arcs includes toxic gases, airborne

debris, and potential damage to electrical

equipment, enclosures and raceways. The high

temperatures of the arc and the molten and

vaporized metals quickly ignite any ﬂammable

materials. While these ﬁres may cause extensive

property damage and loss of production, the

hazards to personnel are even greater.

Any energized electrical conductor that makes

accidental contact with another conductor or

with ground will produce an Arc-Flash. The arcing

current will continue to ﬂow until the overcurrent

protective device used upstream opens the

circuit or until something else causes the current

to stop ﬂowing. The arc current can vary up to

the maximum available bolted fault current.

Arc-Flash Metrics

In order to determine the potential effects of an

Arc-Flash, we need to understand some basic

terms. An Arc-Flash produces intense heat at

the point of the arc. Heat energy is measured

in units such as BTU’s, joules, and calories. The

following data provides a basis for measuring

heat energy:

A Calorie is the amount of heat energy

needed to raise the temperature of one

gram of water by one degree Celsius.

Since energy equals power multiplied by time,

and power (wattage) is volts X amps, we can

see that calories are directly related to amperes,

voltage, and time. The higher the current,

voltage and time, the more calories produced.

To deﬁne the magnitude of an Arc-Flash and

the associated hazards, some basic terms have

been established: The amount of instantaneous

heat energy released by an Arc-Flash is generally

called incident energy. It is usually expressed

in calories per square centimeter (cal/cm

) and

deﬁned as the heat energy impressed on an

area measuring one square centimeter (cm

However, some calculation methods express

the heat energy in Joules/cm

and can be

converted to calories/cm

by dividing by 4.1868.

If we place instruments that measure incident

energy at varying distances from a controlled

Arc-Flash, we would learn that the amount

of incident energy varies with the distance

from the arc. It decreases approximately as

the square of the distance in feet. Just like

walking into a room with a ﬁreplace, the

closer we are, the greater the heat energy.

Tests have indicated that an incident energy

of only 1.2 cal/cm

will cause a second-degree

burn to unprotected skin. A second-degree

burn can be deﬁned as “just” curable.

For the purpose of understanding the potential

effects of an Arc-Flash, you must determine

the working distance from an exposed “live”

part. Most measurements or calculations are

made at a working distance of 18 inches. This

distance is used because it is the approximate

distance a worker’s face or upper body torso

may be away from an arc, should one occur.

Some parts of a worker may be less than 18

inches away, but other work may be performed

at greater distances. The working distance

is used to determine the degree of risk and

the type of personal protection equipment

necessary to protect against the hazard.

NFPA 70E, Standard for Electrical Safety in the

Workplace categorizes Arc-Flash Hazards into

ﬁve Hazard Risk Categories (HRC 0 through 4)

Energy (E) = Power (P) × Time (t)

Power (P) = Volts (V) × Amps (I)

Calories (E) = Volts (V) × Amps (I) × Time (t)

1 Calorie = 4.1868 watt-seconds

1 Joule = 1 watt-second

ARC-FLASH METRICS

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based on the amount of energy that can be

released at a certain working distance during an

Arc-Flash event. They are:

Studies show that many industrial Arc-Flash

events produce 8 cal/cm

(HRC 2) or less, but

other accidents can produce 100 cal/cm

or more

(exceeding all HRC). It is important to remember

that it only takes 1.2 cal/cm

(HRC 0) to cause a

second degree burn to unprotected skin.

What determines the severity of

an Arc Flash?

Several groups and organizations have

developed formulas to determine the

incident energy available at various working

distances from an Arc-Flash. In all cases,

the severity of the Arc-Flash depends on

one or more of the following criteria:

Available short circuit current

System voltage

Arc gap

Distance from the arc

Opening time of overcurrent

protective device (OCPD)

•

When a severe enough Arc-Flash occurs, the

overcurrent protective device (fuse or circuit

breaker) upstream of the fault interrupts the

current. The amount of incident energy a worker

may be exposed to during an Arc-Flash is directly

proportional to the total clearing ampere-squared

seconds (I²t) of the overcurrent protective

device during the fault. High current and longer

exposure time produces greater incident energy.

The only variable that can be positively and

effectively controlled is the time it takes for the

overcurrent protective device to extinguish the

arc. A practical and signiﬁcant way to reduce the

duration of an Arc-Flash and thereby the incident

energy is to use the most current-limiting

OCPD’s throughout the electrical system.

Current-limiting devices such as Littelfuse type

LLSRK_ID or JTD_ID fuses will open in ½ AC

cycle (8.33 milliseconds) or less under short

circuit conditions. Studies have shown that many

existing molded case circuit breakers take up

to 6 AC cycles (100 milliseconds) or longer to

open under short circuit conditions. Refer to the

table on page 31 showing the typical opening

times for various overcurrent protective devices.

Arc Blast Effect

During an Arc-Flash, the rapidly expanding

gases and heated air may cause blasts, pressure

waves, or explosions rivaling that of TNT.

The gases expelled from the blast also carry

the products of the arc with them including

droplets of molten metal similar to buckshot. For

example, the high temperatures will vaporize

copper, which expands at the rate of 67,000

ARC-FLASH AND OTHER ELECTRICAL SAFETY HAZARDS

The amount of

incident energy a

worker may be

exposed to during

an Arc-Flash is

directly proportional

to the clearing time

of the overcurrent

protective device.

In general, a

current-limiting

fuse will clear a

fault much quicker

than a standard

circuit breaker.

INCIDENT ENERGY

(cal/cm

)

RESULTS/EXAMPLE

0.0033

Amount of energy the sun produces in 0.1sec. on the ground’s surface at the equator.

Equivalent to a finger tip exposed to a cigarette lighter flame for one second

1.2

Amount of energy that will instantly cause 2

degree burns to bare skin

Amount of energy that will instantly ignite a cotton shirt

Amount of energy that will instantly cause incurable 3

degree burns to bare skin

HAZARD RISK CATEGORY

0 to 1.2

1.21 to 4

4 .1 to 8

8.1 to 25

25.1 to 40

INCIDENT ENERGY

(cal/cm

)

Circuit breakers

can take up to

12 times longer

to open under

short circuit

conditions

than current-

limiting fuses.

times its mass when it changes from solid to

vapor. Even large objects such as switchboard

doors, bus bars, or other components can

be propelled several feet at extremely high

velocities. In some cases, bus bars have

been expelled from switchboard enclosures

entirely through walls. Blast pressures may

exceed 2000 pounds per square foot, knocking

workers off ladders or collapsing workers’

lungs. These events occur very rapidly with

speeds exceeding 700 miles per hour making it

impossible for a worker to get out of the way.

Light and Sound Effects

The intense light generated by the Arc-Flash

emits dangerous ultraviolet frequencies, which

may cause temporary or permanent blindness

unless proper protection is provided. The

sound energy from blasts and pressure waves

can reach 160 dB, exceeding the sound of an

airplane taking off, easily rupturing eardrums

and causing permanent hearing loss. For

comparison, OSHA states that decibel levels

exceeding 85 dB require hearing protection.

Common Causes

The most common cause of Arc-Flash and other

electrical accidents is carelessness. No matter

how well a person may be trained, distractions,

weariness, pressure to restore power, or over-

conﬁdence can cause an electrical worker to

bypass safety procedures, work unprotected,

drop a tool or make contact between energized

conductors. Faulty electrical equipment can

also produce a hazard while being operated.

Electrical safety hazards such as exposure to

shock and Arc-Flash can also be caused by:

Worn or broken conductor insulation

Exposed live parts

Loose wire connections

Improperly maintained switches

and circuit breakers

Obstructed disconnect panels

Water or liquid near electrical

equipment

High voltage cables

Static electricity

Damaged tools and equipment

The severity and causes of electrical hazards are

varied, but the best protection is to deenergize

equipment before working on it. No one has

ever been killed or injured from an Arc-Flash

while working on deenergized equipment. If

equipment cannot be deenergized, electrical

workers must be “qualiﬁed”, trained, wear

appropriate personal protective equipment

(PPE), and follow all applicable OSHA and

NFPA standards. It is important to remember

that proper selection and application of

overcurrent protective devices (OCPD) will

also substantially reduce the hazards.

•

Current-limiting fuses or

current-limiting circuit breakers

0.1 to 1 second < ½ cycle = 8.3 milliseconds

Molded case circuit breakers without adj. trip

5 to 8 seconds 1.5 cycles = 25 milliseconds

Molded case circuit breakers with adj. trip

1 to 20 seconds 1.5 cycles = 25 milliseconds

Large air power breakers with electronic trip

5 to 20 seconds 3 cycles = 50 milliseconds

Medium voltage breakers with electronic trip

5 to 20 seconds 5 to 6 cycles = 100 milliseconds

OVERCURRENT PROTECTIVE DEVICE

TYP. OPENING TIME

AT 8

× RATING

TYPICAL OPENING TIME

AT 20

× RATING

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Both OSHA and NFPA 70E require an Electrical

Hazard Analysis prior to beginning work on

or near electrical conductors that are or may

become energized. The analysis must include all

electrical hazards: shock, Arc-Flash, Arc-Blast,

and burns. NFPA 70E Article 110.8(B)(1)

speciﬁcally requires Electrical Hazard Analysis

within all areas of the electrical system that

operate at 50 volts or greater. The results of the

Electrical Hazard Analysis will determine the

work practices, protection boundaries, personal

protective equipment, and other procedures

required to protect employees from Arc-Flash

or contact with energized conductors.

Shock Hazard Analysis

NFPA 70E Articles 110.8(B)(1) and 130.2(A)

require a Shock Hazard Analysis. The Shock

Hazard Analysis determines the system

voltage to which personnel can be exposed,

the protection boundary requirements as

established in NFPA 70E Table 130.2(C), and

identiﬁes personal protective equipment

(PPE) required to minimize shock hazards.

Approach Boundaries

NFPA 70E has established three

shock protection boundaries:

1) Limited Approach Boundary

2) Restricted Approach Boundary

3) Prohibited Approach Boundary

Limited Approach Boundary

The Limited Approach Boundary is an

approach boundary to protect personnel

from shock. A boundary distance is

established from an energized part

based on system voltage. To enter this

boundary, unqualiﬁed persons must be

accompanied by a qualiﬁed person and

use PPE.

Restricted Approach Boundary

The Restricted Approach Boundary is an

approach boundary to protect personnel

from shock. A boundary distance is

established from an energized part

based on system voltage. Only qualiﬁed

persons are allowed in this boundary

and they must use PPE.

Prohibited Approach Boundary

The Prohibited Approach Boundary is an

approach boundary to protect personnel

from shock. Work in this boundary is

considered the same as making direct

contact with an energized part. Only

qualiﬁed persons are allowed to enter

this boundary and they must use PPE.

Electric Hazard

Analysis is required

for all areas of the

electrical system

that operate at 50

volts or higher.

Electrical Hazard Analysis

Shock protection boundaries are based on

system voltage and whether the exposed

energized components are ﬁxed or movable.

NFPA 70E Table 130.2(C) defines these

boundary distances for nominal phase-to-

phase system voltages from 50 Volts to 800kV.

Approach Boundary distances may range from

an inch to several feet. Please refer to NFPA

70E Table 130.2(C) for more information.

In summary, a Shock Hazard Analysis is

performed to reduce the potential for direct

shock. It will establish shock protection

boundaries and determine PPE required for

protecting workers against shock hazards.

Completing a shock

hazard analysis

establishes the

system voltage,

shock protection

boundaries and type

of personal protec-

tion equipment

required to protect

workers against

shock hazards.

Limited Approach Boundary

Restricted Approach Boundary

Prohibited Approach Boundary

Energized Part

Figure 9

Shock Protection Boundaries

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Flash Hazard Analysis

A complete electrical hazard analysis must also

contain a Flash Hazard Analysis. NFPA 70E Article

130.3 requires this analysis to be performed:

“A Flash Hazard Analysis

shall be done in order to protect

personnel from the possibility of being

injured by an Arc-Flash. The analysis

shall determine the Flash Protection

Boundary and the personal protective

equipment that people within the Flash

Protection Boundary shall use.”

The analysis requires the available fault current

to be calculated and documented at every

point in the electrical system. This includes all

components contained in the electrical system.

The end result of this research will be an accurate,

documented one-line diagram, which will provide

the data for a short circuit analysis, and the other

calculations that determine the Flash Protection

Boundary and required level of PPE. In part,

Arc-Flash hazard calculations are based on the

available fault current and the opening time of

overcurrent protective devices. NFPA 70E has also

assigned Hazard Risk Categories based on the

estimated incident energy (typically expressed in

cal/cm²), from an Arc-Flash.

The Flash Protection Boundary (FPB)

The Flash Protection Boundary is the

distance in feet (D

) from a given arc

source that will produce a second-

degree burn on exposed bare skin.

Unlike the Shock Hazard Protection Boundaries

that are based solely on system voltage, the

Flash Protection Boundary is not ﬁxed. In order

to determine the potential Arc-Flash hazard,

Flash Protection Boundaries must be calculated

1. Reprinted with permission from NFPA 70E-2004, Standard for

Electrical Safety in the Workplace, Copyright ® 2004, National Fire

Protection Association, Quincy, MA. This reprinted material is not the

complete and ofﬁcial position of the NFPA on the referenced subject,

which is represented only by the standard in its entirety.

at every point where service on energized

equipment, devices, or conductors may be

required. The discussion and examples that

follow are intended to introduce readers to the

required data and some of the methods for

performing an electrical ﬂash hazard analysis at

600 volts and below. Readers are cautioned that

calculations for systems with different voltages,

equipment, devices, and a wider range of fault

currents require the more complete methods

contained in NFPA 70E Article 130 and Annex D.

According to NFPA 70E, the default Flash

Protection Boundary is four feet (48”) based

on an OCPD clearing time of 6 cycles (0.1

sec) and an available fault current of 50 kA

or other combinations not exceeding 5,000-

ampere seconds. For other conditions or

under engineering supervision, calculations

are permitted to determine the Flash

Protection Boundary. Complete formulas

for varying conditions are given in NFPA

70E Article 130 and NFPA 70E Annex D.

The following data is required to

complete the Flash Hazards Analysis:

Up-to-date one-line circuit diagram

of the electrical distribution system

Available fault current from

the utility or generator

Maximum available bolted fault

currents at each location

Minimum self-sustaining

arcing current at each location

Clearing times of all

overcurrent protective devices

As power is distributed throughout your facility, it

is important to remember that although voltage

levels may be higher at the service entrance,

secondary power distribution transformers can

produce much higher current levels and Arc-

Flash energy levels. Power utilities should be

consulted regularly to establish the maximum

available fault current at the service entrance

location of your building. Hand calculations or

•

ELECTRICAL HAZARD ANALYSIS

In part, Arc-Flash

hazard calcula-

tions are based on

the available fault

current and the

opening time of

overcurrent protec-

tive devices.

commercial software can be used to estimate the

maximum available short circuit current at every

access point in your electrical system.

Arc-Flash Calculations

If the maximum available fault current at a

particular location is known, then an analysis of the

upstream overcurrent protective device (OCPD)

will determine how fast the device will clear the

circuit at the fault current. If these two factors

are known, the amount of incident energy and

the Flash Protection Boundary can be calculated.

The Flash Protection Boundary (D

) measured

in feet is based on the bolted fault mega volt-

amperes (MVA

) and the clearing time (t) of

the OCPD. If the bolted fault current (I

) is not

known, it can be calculated based on the MVA

rating and impedance of the source transformer.

An alternate method of determining the Flash

Protection Boundary based on the MVA rating of

a source transformer with an impedance of 5%

and the clearing time (t) of the OCPD is supplied

in NFPA 70E. Table A provides a basic formula

for calculating the Flash Protection Boundary.

Multiple methods are also provided in NFPA

70E Annex D for estimating incident energy

under varying conditions. The results can vary

drastically depending on the speciﬁc system

parameters. An arcing fault will also produce very

different incident energy levels depending on if

the arc is in open air or conﬁned in a cubic box.

The formula in Table B estimates the incident

energy for a fault occurring in a 20 inch cubic

box with one side open. This estimate simulates

the potential effect of an arc-ﬂash while working

in equipment and switchgear enclosures.

During arcing faults the arc impedance

(resistance) reduces arc current. Because

the opening times of OCPD increase as the

short-circuit current (I

) decreases, lower

arc fault currents may greatly increase the

total arc energy. Studies have shown that

the minimum self-sustainable arc in 480 volt

systems is 38% of the available bolted fault

current. Because of the increased time at this

reduced fault level, the incident energy may be

higher than under bolted fault conditions. Each

point in the system needs to be evaluated for

both maximum and minimum fault currents.

NFPA 70E refers

to three methods

to determine

the incident

energy or Hazard

Risk Category.

= FPB distance in ft. from the Arc

MVA

= Available bolted fault MVA in mega volt-amps at point of fault

MVA = Transformer capacity in mega volt-amps

t = Clearing time of overcurrent protective device in seconds

= [2.65 x MVA

x t]

OR D

= [53 x MVA x t]

Table A

Note:

The formula in Table B only applies to systems where the available

short circuit current is in the range of 16kA to 50kA.

= Maximum 20 in. cubic box incident energy

= Distance from arc electrodes, (usually 18 in.)

= Arc duration, seconds

F = Bolted fault current in kA

= 1038.7 D

-1.4738

[0.0093F

-0.3453F + 5.9675] cal/cm

Table B

ELECTRICAL HAZARD ANALYSIS

For more information:

800-TEC-FUSE

www.littelfuse.com

Arc-Flash Hazard Calculation Examples

The following examples break down the

calculations and compare the Hazard Risk

Category (HRC), Incident Energy (cal/cm²), Flash

Protection Boundary (FPB) and PPE (Personal

Protective Equipment) required to work on an

energized 480V system protected by either

2500 Amp Class L fuses or a 2500 Amp low

voltage power circuit breaker.

Example 1

Calculation for energized work in the

transformer metering section of a 2000 kVA

substation. Transformer secondary

protected with current-limiting fuses.

V A

FUSE:

Littelfuse KLPC2500

CLEARING TIME:

0.01 sec @ 43.7kA

2000 kVA

4160V/480V

5.5%Z

Example 2

Calculation for energized work in the

transformer metering section of a 2000 kVA

substation. Transformer secondary

protected with a circuit breaker.

V A

CIRCUIT BREAKER

CLEARING TIME:

0.083 sec @ 43.7 kA

2000 kVA

4160V/480V

5.5%Z

The following

example illustrates

the difference in

incident energy

between a current-

limiting fuse and a

circuit breaker.

Refer to Annex D for Step by step instructions of this example.

Example 1 (continued)

NFPA 70E Article 130.3 (A)

Flash Protection Boundary Distance

Calculate MVA

Note: 2000 kVA = 2 MVA

MVA

100

MVA

100

%Z 5.5

MVA

= 36.4 MVA

Calculate D

t = 0.01 sec

= (2.65 × MVA

× t)

= (2.65 × 36.4 × 0.01) = 0.98 ft ≅ 12 inches

NFPA 70E Annex D.6.2(a)

Incident Energy Exposure

Calculate I

MVA × 10

100

3 × V

2 × 10

100

= 43,738 Amps

3 × 480 5.5

Calculate F:

F =

Isc

43,738 A

= 43.7 kA

1000 1000

Calculate E

For this calculation, D

= 18 inches

= 1038.7 × D

-1.4738

× t

× (0.0093 F

– 0.3453 F + 5.9675)

= 1038.7 × (18)

–1.4738

× (.01) × [0.0093 (43.7)

– 0.3453 (43.7)

+5.9675]

= 1.27 cal/cm

Example 2 (continued)

NFPA 70E Article 130.3 (A)

Flash Protection Boundary Distance

Calculate MVA

Note: 2000 kVA = 2 MVA

MVA

100

MVA

100

%Z 5.5

MVA

= 36.4 MVA

Calculate D

t = 0.083 sec

= (2.65 × MVA

× t)

= (2.65 × 36.4 × 0.083) = 2.83 ft ≅ 34 inches

NFPA 70E Annex D.6.2(a)

Incident Energy Exposure

Calculate I

MVA × 10

100

3 × V

2 × 10

100

= 43,738 Amps

3 × 480 5.5

Calculate F:

F =

Isc

43,738 A

= 43.7 kA

1000 1000

Calculate E

For this calculation, D

= 18 inches

= 1038.7 × D

-1.4738

× t

× (0.0093 F

– 0.3453 F + 5.9675)

= 1038.7 × (18)

–1.4738

× (.083) × [0.0093 (43.7)

– 0.3453 (43.7)

+ 5.9675]

= 10.54 cal/cm

In this example,

the incident en-

ergy for a current-

limiting fuse is

eight

times

lower

than the incident

energy for a

circuit breaker.

In this example the incident energy is much less when

a current-limiting fuse is used to provide protection.

For more information:

800-TEC-FUSE

www.littelfuse.com

Example Results Comparison

As the examples show, the Flash Protection

Boundary, Incident Energy, and Hazard Risk

Category can vary greatly depending on the

overcurrent protective device being used. In

this particular comparison, the required level of

PPE would also be quite different between the

fuse and circuit breaker. The above calculations

can also be performed using commercially

available software programs. Refer to Annex D

of this handbook for more details on the steps

required to complete the hand calculations.

IEEE 1584 Arc-Flash Hazard Calculation

The Institute of Electrical and Electronic

Engineers (IEEE) publishes the IEEE 1584

“Guide for Performing Arc-Flash Hazard

Calculations.” It contains detailed methods and

data that can be used to calculate Arc-Flash

Hazards for the simplest to the most complex

systems. The Petroleum and Chemical Industry

committee of the IEEE spent many years

developing these methods. They are based on

empirical testing of Class RK1 and Class L fuses,

Low Voltage Molded Case Circuit Breakers,

Insulated Case Circuit Breakers and Low Voltage

Power Circuit Breakers as well as theoretical

modeling. Included in 1584 are spreadsheet

programs that simplify the calculation of

incident energy and ﬂash-protection boundaries.

IEEE 1584 does not address the Safety-

related Work Pratices in the same manner as

NFPA 70E. IEEE 1584 concerns itself primarily

with performing the calculations that may

be necessary to determine safe practices.

The calculation methods in Annex D of

NFPA 70E are based on IEEE 1584 but do

not contain all the data or descriptions of

how these methods were developed. IEEE

1584 outlines 9 steps necessary to properly

perform an Arc-Flash hazard calculation.

Step 1

Collect the system and installation data

Depending on whether you are doing a

complete site analysis or looking at one

individual portion, this step can take a few

minutes or several weeks to perform. Begin

by reviewing the latest up-to-date single line

diagram(s) of the equipment or system you

are analyzing. If single line diagrams are not

available, you must create them. The utility

can provide you with the available fault MVA

and X/R ratio at the entrance to your facility.

If you generate your own electricity, or if you

have emergency or standby generators and

large motors, a more detailed analysis must be

performed. In order to calculate the bolted fault

current available at the point of your application,

you must record on your one line diagram all

transformers and their ratings, circuit breakers

or fusible distribution circuits and their ratings,

MCC’s, and all other equipment between the

power source and the area you are concerned

with. Next, you must record the size, type,

length, and number of cables or busbars, etc.

used between the utility and the distribution and

control equipment being analyzed. The type of

conduit or raceway must also be recorded. All

transformer data must be recorded including

MVA ratings and impedance, and all overcurrent

protective devices must be identiﬁed with their

speciﬁc characteristics or trip ratings recorded.

ELECTRICAL HAZARD ANALYSIS

According to the

previous example

the required level

of PPE needed

while working

on the equipment

protected by

the circuit breaker

would be much

greater than the

level of PPE needed

while working on

the equipment pro-

tected by the fuse.

DATA EX. 1 - FUSE EX. 2 - CIRCUIT BREAKER

Step 2

Determine the system modes of operation

Most installations have only one mode of

operation with one utility connection. However,

larger industrial or commercial buildings or

manufacturing plants may have two or more

utility feeders with tie switching of two or

more transformers, or co-generators running

in parallel. Each mode can be very complex

and require a detailed hazard analysis.

Step 3

Determine the bolted fault currents

You can perform hand calculations or use

commercially available software programs such

as the Littelfuse EDR software to calculate the

bolted fault currents at all points between the

utility and the distribution or control equipment

you are analyzing. It will be necessary to plug

in all of the data you have recorded about the

transformers, cable sizes and lengths, and

type of conduit, etc. used in each installation

to determine the bolted fault currents.

Step 4

Determine the arc fault currents

After determining the bolted fault currents, IEEE

1584 provides a formula to calculate the predicted

arc fault current due to typical arc impedance

and other factors. The predicted arc fault current

for system voltages under 1kV depends on the

bolted fault current, system voltage, arc gap,

and whether the arc would most likely occur in

the open air or in an enclosed box conﬁguration.

Step 5

Find the protective device characteristics and

the duration of the arcs

From the data collected in Step 1 and the

predicted arc fault current determined in Step

4, the next step is to establish the total clearing

time of the overcurrent protective device

immediately on the LINE side of the equipment

you are analyzing. If the fuse manufacturer

or circuit breaker manufacturer publishes

maximum and minimum clearing times, it is

important to use the maximum clearing time

possible for the predicted arc fault current.

NOTE: This step can be omitted if the

overcurrent protective devices are those

already tested and listed in the IEEE 1584

document. See Section 4.6 of IEEE 1584.

Step 6

Document the system voltages and classes

of equipment

Make sure you document the system voltages

and class of equipment such as 15kV switchgear,

5kV switchgear, low-voltage switchgear, low-

voltage MCCs and panelboards, or cable runs.

Step 7

Select the working distances

IEEE 1584 has established three typical working

distances for different classes of equipment.

As previously discussed, incident energy

calculations and Hazard Risk Categories will

depend on the working distances selected.

Step 8

Determine the incident energy for

all equipment

You can use formulas included in the IEEE 1584

document or commercially available software

to calculate the incident energy possible in

cal/cm

at the working distance selected.

Step 9

Determine the ﬂash protection boundary

for all equipment

The formulas given within IEEE 1584 can

IEEE 1584 is

one method

of determining

incident energy

and Flash Protection

Boundaries (FPB).

Another method

will be to use NFPA

70E equations and

calculations.

be used to determine the distance from

the arc at which the onset of a second-

degree burn will occur to unprotected

skin. This distance must be established

and will vary based on system parameters.

Software programs automatically calculate

the distance based on the arc fault current,

system voltage, arc gap, and arc duration.

If the overcurrent protective devices (OCPD)

are something other than those covered

by IEEE 1584, or if the voltage levels and

short circuit currents exceed the IEEE 1584

limitations, then the opening times of

the overcurrent protective devices must

be analyzed and the corresponding Flash

Protection Boundary and incident energy

must be calculated by another method.

NFPA 70E Table Method

Although NFPA 70E (Article 130.3) requires

a Flash Hazard Analysis, it also provides an

alternate method for determining Hazard

Risk Categories and required PPE. This is

commonly called the “Table Method” and

is based on various tasks to be performed

on energized equipment (see NFPA 70E

Table 130.(C)(9)(a)). The Table Method may

be used in lieu of a complete Flash Hazard

Analysis in some cases. However, a complete

analysis provides more accurate results.

Caution is advised when using the Table

Method. All footnotes listed at the end of

NFPA 70E Table 130.7(C)(9)(a) and in any

applicable Tentative Interim Amendments

must be observed and all prescribed conditions

veriﬁed. If a task is not listed in NFPA 70E

Table 130.7(C)(9)(a) or cannot be veriﬁed,

then NFPA 70E leaves no other alternative

but to do a complete hazard risk assessment

using one of the other calculation methods.

Steps Required to Use the NFPA 70E

Table Method

Step 1

Once the equipment is identiﬁed where work

is to be performed, review the up-to-date one

line drawing for information about the available

short circuit current and other details about

the location of the equipment. If the one line

drawing is not up to date or the available short

circuit is not known, it must be determined.

Step 2

Consult NFPA 70E Table 130.7(C)(9)(a) and ﬁnd

the task to be performed. If the desired task to

be performed is not listed, the Table Method

cannot be used and a complete Flash Hazard

Analysis is required.

Step 3

Once you ﬁnd your task in the table, identify the

Hazard Risk Category and determine if voltage

rated gloves or tools are required.

Step 4

Verify that the conditions stated in the footnotes

for NFPA 70E Table 130.7(C)(9)(a), and any

Tentative Interim Amendments such as those

stated in NFPA 70E, are applicable to the task.

Step 5

Using NFPA 70E Tables 130.7(C)(10-11) and

the corresponding notes in Table 130.7(C)(9)(a),

identify the required PPE for the task.

Step 6

The NFPA 70E Table Method does not provide

the Flash Protection Boundary, but it must be

determined. For systems 600V and below,

NFPA 70E deﬁnes the FPB as 4 feet. See NFPA

70E for more information on calculating the FPB.

ELECTRICAL HAZARD ANALYSIS

The Table Method

may be used in

lieu of a complete

Flash Hazard

Analysis. However,

a complete analysis

will provide more

accurate results.

For more information:

800-TEC-FUSE

www.littelfuse.com

Whether calculations are made or NFPA 70E

Table 130.7(C)(9)(a) is used, the results of an

Electrical Hazard Analysis (Shock and Flash

Hazard Analysis) will determine the following:

The Limited Approach Boundary

The Restricted Approach Boundary

The Prohibited Approach Boundary

Incident Energy possible at

each location

Flash Protection Boundary

Hazard Risk Category

PPE required to work on

energized equipment

•

OSHA regulations

must be followed

to perform a hazard

assessment, and

to determine the

PPE required for

properly protecting

electrical workers.

For more information:

800-TEC-FUSE

www.littelfuse.com

NFPA 70E guidelines and practices are generally

considered the “How to” of conforming to the

OSHA regulations when performing a hazard

assessment, and determining the required PPE.

There are many practices that will help reduce

Arc-Flash and other electrical hazards while

conforming to OSHA and NFPA 70E regulations

and guidelines. Circuit designers and electrical

maintenance engineers should carefully consider

each of the following recommendations:

1. Design a safer system.

2. Use and upgrade to current-limiting

overcurrent protective devices.

3. Implement an Electrical Safety Program.

4. Observe safe work practices.

5. Use Personal Protective Equipment (PPE).

6. Use Warning Labels.

7. Use an Energized Electrical Work Permit.

8. Avoid hazards of improperly selected or

maintained overcurrent protective devices.

9. Achieve or Increase Selective Coordination.

1. Design a safer system.

Goals

When designing a safer system the following

goals and factors should be considered:

Provide maximum protection to

personnel, equipment, and property.

Meet all applicable code require-

ments (OSHA, NFPA, Building

and Insurance codes, etc.)

Utilize current-limiting overcurrent

protective devices to minimize

Arc-Flash hazards.

Utilize “touch-safe” components

to minimize exposure to energized

components

Utilize fuses with blown fuse

indication to minimize exposure

to energized components while

trouble-shooting the circuit.

Provide selective coordination

(only the area where the

fault occurs is shut-off)

Provide a system that is safe to

service and maintain.

•

Estimates show that

10 Arc-Flash

incidents

occur every day

in the U.S.

Minimizing Arc-Flash and Other Electrical Hazards

System Requirements

Once the goals for your system are established,

the selection of the overcurrent protective

devices that best meet those goals can be

determined. What is the best choice for your

application; fuses or circuit breakers? Fuses offer

many safety and performance advantages over

circuit breakers. Factors to consider include:

System voltage

Voltage ratings for fuses are stan-

dardized at 250, 300, and 600

volts. In comparison, some circuit

breakers are rated for dual volt-

ages and are often mis-applied.

Interrupting rating

Most fuses have standard ratings of

200kA at full rated voltage. Circuit

breaker interrupting ratings may range

from 10kA to 100kA, but the interrupt-

ing ratings of many breakers vary with

system voltage and type of trip unit.

System changes resulting in increased

available fault current

If your facility grows or the utility makes

changes, fault currents have been

known to more than double. Interrupting

ratings of overcurrent protective devices

must be regularly reviewed to insure

the device will still protect the system.

Load current characteristics

Inductive loads such as motors and

transformers and even large incandes-

cent lamps have large inrush currents

that require circuit breakers to be

oversized so that overload protection is

sacriﬁced. Properly selected time-delay

fuses can be sized close to load currents

and will offer better overload protection.

•

Current-limitation

Current-limiting OCPDs reduce

damage from major faults. Often

devices or equipment can be easily

repaired rather than face time-con-

suming and costly replacement.

Are your sensitive control devices such as

motor starters truly protected?

After a fault, will the units be usable or will

they require replacement? Only current-

limiting fuses can provide Type 2 Protec-

tion. That means you are up and running

once the cause of the fault is removed.

2. Use and upgrade to current-limiting

overcurrent protective devices.

The incident energy from an Arc-Flash depends

on the magnitude of the current and the time it

is allowed to ﬂow. Within their current-limiting

range, current-limiting devices reduce the peak

fault current. Current-limiting fuses have much

faster clearing times when operating within

their current-limiting range than standard circuit

breakers. The faster the overcurrent protective

device clears the fault, the lower the I²t and

incident energy will be. If current-limiting fuses

are used, the incident energy and the Hazard

Risk Category may be reduced signiﬁcantly.

Upgrade to Class RK1 or Class J current-

limiting fuses

One of the quickest and easiest ways to reduce

potential incident energy, lower the Hazard Risk

Categories and reduce the required PPE, is to

replace UL Class H, K5 or Class RK5 fuses with

current-limiting UL Class RK1 or Class J fuses.

Upgrading to time-delay Class J fuses affords the

best solution by providing the best current limitation

while assuring non-interchangeability with non-

current-limiting fuses. If an equipment manufacturer

•

Upgrading to

Class RK1 or Class J

current- limiting

fuses is the

easiest way to:

Reduce potential

Arc-Flash hazards

Reduce Hazard

Risk Categories

Reduce the

amount of

required PPE

•

For more information:

800-TEC-FUSE

www.littelfuse.com

has speciﬁed a non-time delay fuse, standard Class

J fuses are available. If your equipment already has

UL Class H fuse clips, it is very easy to replace the

Class H or K5 fuses being used with Class RK1

fuses. For a given current and voltage rating, UL

Class H, K5, RK5, and RK1 fuses are the same

physical size, therefore, it is easy and strongly

recommended to upgrade to better fuse protection.

To assure that only current-limiting fuses are used,

consider changing to Class J clips or to rejection

type clips that will accept only Class R fuses.

Current-limiting fuses that also offer blown

fuse indication such as the Littelfuse Class

J JTD_ID and Class RK1 LLSRK_ID can help

reduce exposure to electrical hazards. The

unique blown fuse indicator decreases

downtime by immediately indicating the opened

circuit and maximizes safety by minimizing

exposure to energized components when

trouble-shooting. Replacing non-current-

limiting fuses with Littelfuse current-limiting

Indicator

fuses can signiﬁcantly reduce the:

Incident energy from an Arc-Flash

The Hazard Risk Category

The level and type of PPE necessary

Trouble-shooting and downtime.

•

MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS

Current-limiting fuses

that also offer blown

fuse indication such

as the Littelfuse Class

RK1 LLSRK_ID series

can help:

reduce exposure to

electrical hazards

decrease downtime

maximize safety

•

* Consult Article 430 of the NEC

when substituting for loads with motors, or call 800-TEC-FUSE

Class L

KLPC

KLLU

KRPC

KLU

KTU

A4BQ

A4BY

A4BT

LCL

LCU

Class RK1

(600 Volts)

LLSRK_ID

FLSR

FLSR_ID

NLS*

RLS*

LLSRK

KLSR*

FRSR

NOS*

RES*

LPSRK

KTSR*

TRS

OTS*

RFS*

A6DR

A6KR*

GF6B

ECSR

LESRK

KOS*

ERS*

Class RK1

(250 Volts)

LLNRK

FLNR

NLN*

RLN*

KLNR*

FRNR

NON*

REN*

LPNRK

KTNR*

OT*

RF*

A2DR

A2KR*

GF6A

ECNR

LENRK*

KON*

ERN*

Class J

JTD_ID

JTD

JLS*

LPJ

JHC

JKS*

AJT

A4J*

JDL

JFL*

Class CC

CCMR

FLM

FLQ

KLDR

KLK

BLS

BLF

BLN

KLKR

LPCC

FNM

FNQ

FNQR

KTKR

KTK

BBS

BAF

BAN

ATDR

TRM

ATQ

ATQR

ATMR

SBS

OTM

ATM

EDCC

MEN

MEQ

HCTR

HCLR

MCL

EBS

MOL

You Should Use This Fuse....

If You Have This Fuse....

Use the table below to consolidate your

fuse inventory and eliminate unsafe or

unnecessary fuses.

3. Implement an Electrical Safety Program.

Electrical Safety Programs protect both

employees and employers and provide goals,

procedures and work practices to insure safety.

NFPA 70E Article 110.7 requires employers

to establish an Electrical Safety Program that

must be documented and include the minimum

following components:

Scope of the Program

Company Philosophy

Responsibilities

Establishment of a Safety

Team or Committee

Written Procedures

Work Instructions

Identiﬁcation of Industry Codes

& Standards to be adhered to

Establishment of a Documented

Training Program

Establishment of Assessment

and Audit Requirements

Company Policies and Enforcement

Increased safety will be possible with the

implementation and vigorous enforcement of

a well-designed and documented Electrical

Safety Program. These programs should

be in accordance with all OSHA regulations

and nationally recognized safety standards

such as NFPA 70E and NEC

. For more

information on establishing an Electrical

Safety Program, refer to NFPA 70E Annex E

or NFPA’s Electrical Safety Program Book.

4. Observe safe work practices

Maintenance

Safe maintenance practices and procedures

include properly training employees in the

knowledge of the equipment and tools

necessary for maintenance and repair. NFPA

70E states that employees “shall be trained

in and familiar with the speciﬁc maintenance

procedures and tests required.” Test equipment

•

as well as hand tools are often overlooked and

must be insulated and rated for the voltage

of the circuits where they will be used. All

tools and equipment used for maintenance

must also be periodically inspected to ensure

they are not damaged (i.e. torn insulation)

and are still in good working condition.

Disconnect Operation

Operating a damaged disconnect switch,

whether it’s a fusible switch or circuit breaker,

can be dangerous. Serious injury could occur if

someone is standing in front of a faulty switch

or circuit breaker while opening or closing

the device. If the handle is on the right hand

side of the device, stand to the right, use

your left hand to grasp the handle, turn your

face away and then operate it. If the handle

is on the left side, reverse the procedure.

Use special caution while operating circuit

breakers. If closed into a fault, circuit breakers

will trip, drawing an internal arc. The gases

from the arc are very hot, and vent through

openings in the breaker. These hot gases often

vent around the handle and can cause burns

unless proper protective equipment is used.

Proper Service or Repair of All

Equipment or Devices

a) Locate the equipment where work is to be

performed. If equipment is running, follow

manufacturer’s shutdown procedures being

sure that all unit switches are off. Do not

open any enclosures. Determine if there is

adequate working space and that it is clear

of obstructions.

b) Locate all disconnecting means providing

power to the equipment, including all

sources of emergency, alternate, and control

power. This must include discharging

capacitors and other sources of stored

energy. Turn all disconnecting devices to the

OFF position and apply lockout/tagout

devices as required by OSHA and the

company’s Electrical Safety Program.

c) While wearing proper personal protective

equipment, open the enclosure door or

access panels. Test the voltage meter to be

The implementation

and enforcement

of a well-designed

Electrical Safety

Program in accor-

dance with OSHA

and NFPA 70E will

increase safety in

your facility.

MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS

For more information:

800-TEC-FUSE

www.littelfuse.com

used on a known energized source to be

sure it is working properly. Test all exposed

wires, contacts and other components likely

to be energized insuring that the equipment

is in an electrically safe work condition.

Equipment containing fuses

d) If it is suspected there is one or more

opened fuses, remove fuses from the circuit

using the proper size fuse puller.

Note: The use of Littelfuse Indicator

Fuses will minimize time required to

locate opened fuses, and help avoid

mixing them with good fuses.

e) Place fuses on a non-conductive surface and

measure fuse resistance across the ends

(endcaps/blades) of the fuse with a meter. If

the fuses have knife blades be sure to test

from blade to blade since some types of

fuses have insulated end caps and will give a

false reading. High resistance indicates that

the fuse may be open.

f) Investigate the circuit to identify the cause of

any blown fuses. Look for loose connections

or signs of overheating. Correct the problem.

g) Verify the proper fuse class, voltage, ampere,

and interrupting ratings before installing

replacement fuses. (Caution: because fuse

characteristics may vary between manufac-

turers and fuse classes, fuses should be of

the same manufacturer and class for each

application.)

h) Examine fuse clips or mountings for signs of

corrosion, overheating, or loss of tension.

Service if necessary. Install the replacement

fuse with the proper size fuse puller.

Equipment containing circuit breakers

i) After following steps 1 through 3 above,

look for circuit breakers and examine to

see if any are tripped. Examine the circuit

breaker(s) to see if the case or surround-

ing area shows signs of severe venting

indicating a serious fault.

j) Investigate the circuit for the causes of

circuit breaker tripping. Correct the problem.

If breaker is protecting motor starters,

especially IEC or single-purpose type, test

the motor starters to be sure they are still

functional. If the motor starters have

heaters (resistance coils) in the overloads,

test the resistance across the heaters to

insure they are still functional.

k) Test resistance across the poles of the

open circuit breaker to be sure all poles are

open and there are no shorts between

poles. Close the circuit breaker and

measure resistance across the closed

poles to insure resistances are within

tolerances and are equal from pole to pole.

Placing equipment in service

l) Following manufacturer’s instructions, close

all internal switches and circuit breakers and

other procedures necessary for start-up.

m) Close enclosure door(s) and access panels

and check the area for other personnel.

Remove lockout/tagout devices following

OSHA and safety program procedures.

n) Restore power standing to the side of the

switch enclosures.

o) Restart equipment following manufacturer’s

instructions and exercising caution until

satisfactory operation is insured.

Lockout/tagout Procedures

OSHA requires that energy sources to

machines or equipment must be turned off and

It is estimated that

Lockout/Tagout

prevents about

120 fatalities and

50,000 workday

injuries annually.

Source:

Occupational Safety and Health Administration

disconnected isolating them from the energy

source. The isolating or disconnecting means

must be either locked or tagged with a warning

label. While lockout is the more reliable and

preferred method, OSHA accepts tagout to

be a suitable replacement in limited situations.

It is estimated that Lockout/tagout prevents

about 120 fatalities and 50,000 workday injuries

annually. Approximately 39 million workers are

protected by Lockout/tagout practices. Failure to

comply with Lockout/tagout safety regulations

is frequently one of the top ﬁve OSHA violations.

In 2004 alone, there were over 4,300 violations

cited by OSHA. NFPA 70E Article 120 contains

detailed instructions for lockout/tagout and placing

equipment in an Electrically Safe Work Condition.

Application of Lockout/tagout Devices

a) Make necessary preparations

for shutdown

b) Shut down the machine or equipment

c) Turn OFF (open) the energy-

isolating device (fuse/circuit breaker)

d) Apply the lockout or tagout device

e) Render safe all stored or

residual energy

f) Verify the isolation and deenergiza-

tion of the machine or equipment

Removal of Lockout/tagout Devices

a) Inspect the work area to ensure that non-

essential items have been removed and

that machine or equipment components

are intact and capable of operating

properly. Especially look for tools or

pieces of conductors that may not have

been removed.

b) Check the area around the machine

or equipment to ensure that all

employees have been safely

positioned or removed.

c) Make sure that only the employees who

attached the locks or tags are the ones

that are removing them.

d) After removing locks or tags, notify

affected employees before starting

equipment or machines.

5. Use Personal Protective Equipment (PPE)

The proper selection and use of Personal

Protective Equipment will signiﬁcantly reduce

the risk of Arc-Flash and other electrical

hazards to personnel working on energized

equipment. OSHA Part 1910.335 (a) states:

“…Employees working in areas where

there are potential electrical hazards

shall be provided with, and shall

use, electrical protective equipment

that is appropriate for the speciﬁc

parts of the body to be protected

and for the work to be performed.”

A variety of PPE is available from numerous

manufacturers. The most common

types of protective gear include:

Nonconductive ﬂame-resis-

tant head, face, and chin pro-

tection (hard hats, full face

shields, switching hoods, etc.)

Eye protection (face shields,

safety glasses, goggles)

Body protection resistant to

ﬂash ﬂame (shirts, pants,

jackets, coveralls)

Hand and arms protection

(insulating gloves and sleeves

with leather protectors)

Foot and leg protection (insulated

leg and footwear)

Insulating blankets or mats

•

The proper

selection and use

of PPE will greatly

reduce the risk

of Arc-Flash and

other electrical

hazards.

Selection of PPE is dependant on the task to be

performed. NFPA 70E Tables 130.7(C)(9), (10),

and (11) provide guidance for the selection of

personal protective equipment to be used for

speciﬁc tasks and hazard levels. The Table of PPE

requirements below provides typical clothing

requirements for Hazard Risk Categories from

0 through 4. Note: Hazard Risk Category 0 still

requires some level of protective clothing or

equipment. Manufacturers have also developed

tables and selection guides based on NFPA 70E

recommendations. It is important to note that

the level of PPE recommended by NFPA 70E is:

“intended to protect a person from arc-ﬂash and

shock hazards”. Even with PPE, some arc-ﬂash

conditions may result in burns to the skin or include

arc blast pressures, toxic vapors, and propelled

particles and materials. PPE that is selected should

be rated for, or greater than, the minimum Arc-Flash

rating required for each Hazard Risk Category.

Common Personal Protective

Equipment Terms and Deﬁnitions

Arc Thermal Performance

Exposure Value (ATPV)

The incident energy level (in cal/cm²) that can

cause the onset of a second-degree burn

as deﬁned in ASTM F 1959 Standard Test

Method for Determining the Arc Thermal

Performance Value of Materials for Clothing.

Personal Protective Equipment will be labeled

with a calorie rating (Example: 11 cal/cm²).

V-rated

Tools and gloves rated and tested

for the line-to-line voltage at the area

where the work is to be performed.

Flame Resistant (FR)

“The property of a material whereby combustion

is prevented, terminated, or inhibited

following the application of a ﬂaming or non-

ﬂaming source of ignition, with or without

susequent removal of the ignition source.”

Breakopen Threshold Energy (E

)

The incident energy level which does not

cause ﬂame resistant (FR) fabric breakopen

and does not exceed second-degree burn

criteria, as deﬁned in ASTM F 1959.

Standards such as OSHA 1910.137 also specify

that protective gear must be maintained and

periodically inspected to ensure that it remains in

a safe and reliable condition. NFPA also supports

this in NFPA 70E Articles 130.7(B), 130.7(C)(8) and

130.7(F). NFPA requirements state that PPE should

be inspected before and after each use, and be

repaired, cleaned or laundered according to the

manufacturer’s instructions prior to use. It is also

extremely important to avoid contamination of

PPE material. Contact with grease, solvents, and

ﬂammable liquids may destroy the protection.

1. Reprinted with permission from NFPA 70E, Standard for Electrical

Safety in the Workplace, Quincy, MA: National Fire Protection

Association, 2004. This reprinted material is not the complete and

ofﬁcial position of the NFPA on the referenced subject, which is

represented only by the standard in its entirety.

PPE that is selected

should be rated for,

or greater than, the

minimum Arc-Flash

rating required for

each Hazard

Risk Category.

MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS

For more information:

800-TEC-FUSE

www.littelfuse.com

Hazard Risk

Category

Required

Minimum

Arc Rating of

PPE (cal/cm

)

Typical Protective Clothing Systems

Clothing Description

Minimum

Flash

Protection

Boundary

(in.)

0 N/A

1 layer of non-melting, flammable fabric with weight of at least 4.5 oz/yd

1 4

1 layer of a FR shirt and FR pants or FR coverall

2 8

1 or 2 layers of FR shirt and FR pants with conventional cotton underwear

3 25

2 or 3 layers of FR shirt, FR pants plus FR coverall cotton underwear

4 40

3 or more layers of FR shirt, FR pants plus multi-layer flash suit

~120

PPE REQUIREMENTS

Derived from NFPA 70E Table 130.7(C)(11)

6. Use Warning Labels.

The National Electrical Code

recently recognized

Arc-Flash hazards and developed a warning

label requirement. NEC

Article 110.16 states:

“110.16 Flash Protection:

Switchboards, panelboards, industrial

control panels, meter socket enclosures,

and motor control centers that are

in other than dwelling occupancies

and are likely to require examination,

adjustment, servicing, or maintenance

while energized shall be ﬁeld marked

to warn qualiﬁed persons of potential

electric Arc-Flash hazards. The marking

shall be located so as to be clearly

visible to qualiﬁed persons before

examination, adjustment, servicing, or

maintenance of the equipment.”

While the overall requirement is very

comprehensive, the required label format

can be very generic. However, if a complete

electrical hazard analysis is performed, the

preferred approach would be to include

the Hazard Risk Category, Flash Protection

Boundary, Incident Energy available, level

of PPE required, system voltage, and shock

protection boundaries on labels. See Figures 10

and 11 for examples of typical warning labels:

Minimum Label Requirements

Bus: SERVICE4 05/12/05

42 inches 12 inches 1 inch

1 480 VAC

35 kA

32 inches

2.77 cal/cm

Figure 10

1. Reprinted with permission from NFPA 70-2005, National Electrical

Code

Quincy, MA. This reprinted material is not the complete and ofﬁcial

position of the NFPA on the referenced subject, which is represented

only by the standard in its entirety.

Preferred Label Approach

Bus: SERVICE4 05/12/05

42 inches 12 inches 1 inch

1 480 VAC

35 kA

32 inches

2.77 cal/cm

Figure 11

The use of detailed warning labels not only

increases safety, but also minimizes the time

required to identify minimum levels of PPE.

Other types of warning labels should also be

used to include information about proper fuse

replacements, location of disconnects and

other sources of power, etc. Warning labels

can be applied directly to pieces of equipment

or on enclosure doors. Computer programs

and adhesive blank labels make it easy to

create labels for almost every purpose.

7. Use an Energized Electrical Work Permit.

NFPA 70E requires that a detailed written

Energized Electrical Work Permit must

be used and signed by responsible

management whenever work is performed

on live energized equipment.

See Annex C of this handbook for an

example of an Energized Electrical

Work Permit. For additional information

on Energized Electrical Work Permits,

refer to NFPA 70E Article 130.1(A).

The use of detailed

warning labels will

increase safety as

well as minimize

the time required to

identify minimum

levels of PPE.

For more information:

800-TEC-FUSE

www.littelfuse.com

8. Avoid Hazards of Improperly Selected or

Maintained Overcurrent Protective Devices.

Whether in the design or maintenance of

an electrical system, hazards exist if the

proper overcurrent device is not selected and

applied. Circuit breakers and other electrical

equipment must be maintained and serviced

regularly to ensure that they will operate

properly when needed. Unfortunately, in many

industries and especially during economic

turndowns, the tendency is to limit or eliminate

regularly scheduled maintenance on circuit

breakers and other electrical equipment.

However, the potential costs associated with

OSHA violations, liability lawsuits, workers

compensation, equipment replacement, and

lost production far exceeds the costs of

regular testing and maintenance of circuit

breakers and other electrical equipment.

OSHA 29 CFR 1910.334(b)(2)

“Reclosing circuits after protective

device operation. After a circuit is

deenergized by a circuit protective

device, the circuit may NOT be

manually reenergized until it has been

determined that the equipment and

circuit can be safely reenergized. The

repetitive manual reclosing of circuit

breakers or reenergizing circuits

through replaced fuses is prohibited.

NOTE:

When it can be determined from the

design of the circuit and the overcurrent

devices involved that the automatic

operation of a device was caused

by an overload rather than a fault

condition, no examination of the circuit

or connected equipment is needed

before the circuit is reenergized.”

In this section of the regulations, OSHA

recognizes the importance of knowing why

the overcurrent protective device has opened.

If the fuse or circuit breaker opened due to

an overload, no examination of the circuit or

connected equipment is necessary. However, if

the overcurrent protective device opened due

to a short circuit fault, catastrophic results can

occur if the fuse or circuit breaker is replaced or

closed on the short circuit before it is corrected.

This is especially important for circuit breakers

and switches because short circuit currents can

permanently damage the equipment to the point

that it will not operate safely when reenergized.

Circuit Breakers

Circuit breakers, like fuses, are rated to safely

interrupt their maximum interrupting current

only once. Molded Case Circuit Breakers

(commonly referred to as MCCB’s) must meet

the requirements of UL489, “Standard for

Safety,” Molded-Case Circuit Breakers, Molded

Case Switches and Circuit Breaker Enclosures.

This standard allows manufacturers to list their

circuit breakers at varying degrees of available

fault currents, current-limiting ability and other

characteristics. They must be applied within

the maximum limitations of their ratings.

Circuit breaker manufacturers typically

recommend that their circuit breakers be cycled

ON and OFF at least once each year to keep

the tripping mechanism from seizing under

certain environmental conditions. Cycling circuit

breakers ON and OFF manually may help keep the

switching mechanism from seizing, but may not

guarantee that the tripping mechanism will operate

properly. Some manufacturers also recommend

that their circuit breakers be periodically tested and

recalibrated under carefully controlled conditions.

When testing time-current characteristics,

recommendations state the circuit breaker

being tested must be at room temperature. This

practice would increase equipment downtime

while the circuit breaker to be tested cools

down after it is removed from service.

The National Electrical Manufacturers Association

(NEMA) has published standard AB 4-2003

entitled, “Guidelines for Inspection and Preventive

Maintenance of Molded Case Circuit Breakers

Used in Commercial and Industrial Applications.”

It deals exclusively with the maintenance and

care of Molded Case Circuit Breakers to provide

reliable protection. The expected lifetime of a

circuit breaker, however, depends on circuit

conditions and its’ environment. Standard AB

4-2003 emphasizes that safe work practices

MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS

Circuit breakers trip-

ping mechanisms

could seize up and

not operate properly

if not maintained

to manufacturer’s

speciﬁcations.

include regular periodic maintenance, and

investigating what caused the circuit breakers to

operate prior to reenergizing the circuit — similar

to OSHA 29 CFR 1910.334(b)(2). There are other

published industry standards for maintenance

of large Air Power Circuit Breakers. Preventive

maintenance of these circuit breakers should be

performed at least annually, and after interruption

of a fault some 20 or more steps are required

before placing the circuit breaker back in service.

The Institute of Electrical and Electronic

Engineers (IEEE) has also published Standard

493-1997, otherwise known as the “Gold Book,”

entitled, Recommended Practice For the Design

Of Reliable Industrial And Commercial Power

Systems. The IEEE studied failure statistics of

typical industrial and commercial electrical

distribution systems and components over

several years prior to 1974 and more recently in

1996. The results of the 1996 study concluded

nearly

of all circuit breakers that failed while in

service could have been avoided if proper

maintenance and testing was performed.

Article 225.3 of NFPA 70E

requires that if a circuit breaker

interrupts a fault at or near its

interrupting rating, it must be inspected

by a trained technician and tested,

repaired or replaced in accordance with

the manufacturer’s speciﬁcations.

If proper maintenance and repair is neglected,

circuit breakers may fail to open or open more

slowly than when ﬁrst calibrated. The IEEE

study noted that circuit breaker failures caused

excessive equipment damage, blackouts,

unexpected repair and replacement costs, lost

production, scrap production, and most

importantly, severe blast and burn injuries to

personnel. Proper care and maintenance of

circuit breakers must be part of any Electrical

Safety Program.

Other common safety hazards involve using

overcurrent protective devices with improper

interrupting ratings. Standard circuit breakers

have relatively low interrupting ratings (typically

10,000 to 100,000 AIC) when compared to

fuses (200,000 AIR). The circuit breaker’s

low interrupting rating may not be an initial

hazard, but as available fault currents rise from

the utility, a dangerous situation is created.

During service upgrades, circuit breakers with

low interrupting capacities may have to be

replaced by higher rated devices or protected

by fuses in order to lower fault currents.

Non-current-limiting fuses

Another potential electrical hazard is the

use of non-current-limiting fuses including

“renewable” fuses. Although fuse standards

and fuse technology have changed greatly,

many older machines and equipment may

contain Class H (one-time or renewable) or

K5 one-time fuses. The continued use of

these fuses especially where available fault

currents exceed their interrupting ratings

can be catastrophic. In addition to being

non-current-limiting, Class H and K5 fuses

have lower interrupting ratings than the Class

R or J fuses. Just like non-current-limiting

circuit breakers, the Class H and K5 fuses

may permit much higher current to ﬂow for a

much longer time, increasing risk to workers

and the equipment. The incident energy of an

Arc-Flash could be deadly to an unsuspecting

worker who is not properly protected.

9. Achieve or Increase Selective Coordination.

When an overcurrent occurs in a system only

the overcurrent protective device immediately

on the line side of the overcurrent should

open. This reduces unnecessary shutdown

of other equipment and simpliﬁes locating

the problem. Such systems are deﬁned as

“selectively coordinated.” Refer to Figure 12.

If a system is not selectively coordinated, a fault

at point A can cause the fuses or circuit breakers

at points B, C, and D to open, needlessly

shutting off power to two or more unaffected

areas. In a selectively coordinated system, a

fault at point A will cause only the fuse or circuit

breaker immediately before the fault to open,

keeping power supplied to the rest of the feeder

and branch circuits throughout the facility.

For more information:

800-TEC-FUSE

www.littelfuse.com

MINIMIZING ARC-FLASH AND OTHER ELECTRICAL HAZARDS

Achieving a selec-

tively coordinated

system not only

reduces downtime

and the risk of

Arc-Flash exposure,

but the National

Electrical Code

requires it.

Feeder circuit breakers or fuses are typically

rated at least twice that of the downstream

devices. An Arc-Flash that opens the upstream

devices means that the total I

t heat energy and

consequently, incident energy, is determined by

the largest upstream device. In this situation, the

electrical system is not selectively coordinated,

and the incident energy increases as a result

of the time elapsed before the upstream

overcurrent protective device clears the fault.

Achieving a selectively coordinated system

not only reduces downtime and the risk of

Arc-Flash exposure, but Articles 240.12, 700.27,

701.18 and 620.62 of the National Electrical

Code require it. These code speciﬁcations refer

to emergency circuits or potential life saving

circuits such as alarm circuits, emergency

lighting, and elevator circuits. For example,

during an emergency or in a building with an

elevator, an overcurrent on one elevator motor

must not cause the feeder circuit to open all

other elevator circuits, or alarm systems.

It is also unsafe to replace blown fuses with

slightly higher ampere ratings in order to

compensate for nuisance openings. Doing

so will defeat selective coordination and can

dramatically increase the amount of risk to

workers if an Arc-Flash occurs. In order to

decrease downtime and reduce the risk of

Arc-Flash exposure to unsuspecting workers,

it is best to replace non-current-limiting fuses

and circuit breakers with more accurately

rated time-delay current-limiting fuses such as

the Littelfuse Class RK1 LLSRK_ID series.

Electrical safety is important for everyone.

Employees working on electrical systems

are at risk every day, but with the

properly designed overcurrent protection

system, the implementation of safe

work practices and the utilization of the

appropriate PPE, risk can be minimized.

C C

LOAD

Fuse Opens

Circuit Breaker Opens

Fuse Not Opened

Unnecessary

Power Outage

LOAD

Without Selective

Coordination

With Selective

Coordination

LOAD

Examples of loads

include: motors,

elevators, lighting, etc.

FAULT FAULT

Legend:

Figure 12

Selective Coordination

Electrical Safety Summary

Employees working

on electrical systems

are at risk every day,

but with the properly

designed overcurrent

protection system,

the implementation

of safe work practices

and the utilization

of the appropriate

PPE, risk can be

minimized.

Here is a brief review of some basic

electrical safety concepts.

1) Unless there is a compelling safety

issue such as life-support equipment,

alarm systems, hazardous location

ventilation, or lighting required for

safety, OSHA requires that circuits be

deenergized and the system be placed

in an Electrically Safe Work Condition

before any work is performed.

2) When placing equipment in an

Electrically Safe Work Condition,

always follow proper Lockout/tagout

procedures.

3) An Electrical Hazard Analysis must

be performed on all circuits 50 volts

and higher that may be worked on

while energized.

4) The Hazards must be identiﬁed and

warning labels must be applied to all

equipment that may be worked on

while energized.

5) Workers must be trained on the

equipment, hazards and safety

precautions, and be certified as

“qualified” to work on energized

equipment. Training and certiﬁcation

must be documented.

6) All work performed on energized

equipment must be preceded by a

job brieﬁng and a signed Energized

Electrical Work Permit.

7) When working on or approaching

energized circuits, proper protective

clothing must be worn. The minimum

ﬂame retardant clothing, safety

glasses, and protective gloves and

equipment must meet OSHA and

NFPA 70E guidelines. Protective

insulating blankets and mats are

also used to minimize exposure.

8) Be certain there is adequate lighting

for the tasks to be performed. Por-

table lighting must be fully insulated

so that it will not accidentally cause

short circuits when used near

energized components.

9) Use barricades or barriers to warn

unqualiﬁed individuals from entering

the area.

10) Be prepared for the unexpected. Make

sure emergency communications and

trained medical personnel are avail-

able if something goes wrong.

11) Use current-limiting overcurrent

protective devices wherever possible

to reduce the potential electrical

hazards.

Electrical Safety in the workplace can only be

attained when workers and employers diligently

follow OSHA and industry accepted standards

and regulations. It is our sincere hope and desire

that this handbook has been helpful in informing

the reader of the importance of Electrical Safety

while providing methods and information on how

to effectively and safely reduce electrical hazards.

For more information:

800-TEC-FUSE

www.littelfuse.com

A.I.C. or A.I.R.:

See Interrupting Capacity.

Ambient Temperature:

The air temperature surrounding a device.

For fuses or circuit breakers in an enclosure,

the air temperature within the enclosure.

Ampacity:

The current in amperes that a conductor can

carry continuously under the conditions

of use without exceeding its temperature

rating. It is sometimes informally applied to

switches or other devices. These are more

properly referred to by their ampere rating.

Ampere Rating:

The current rating, in amperes, that is

marked on fuses, circuit breakers, or other

equipment. It is not to be inferred that

equipment or devices can continuously carry

rated amperes. Various derating factors may

apply. Refer to NEC

for further information.

Ampere-Squared-Seconds (I²t):

The heat energy passed by a fuse or circuit

breaker from the instant the fuse links melt

or circuit breaker contacts part (arcing

time). It is equal to the rms arcing current

squared multiplied by the arcing time.

Approach Boundaries:

Protection boundaries established

to protect personnel from shock.

Arcing I²t:

The heat energy passed by a fuse during

its arcing time. It is equal to the rms arcing

current squared multiplied by the arcing time.

Arcing Current (See Figure 13):

The current that ﬂows through the

fuse after the fuse link has melted

and until the circuit is interrupted.

Peak Let-through Current

Peak Current which would occur

without current limitation

Current

Time

Melting

Time

Arcing

Time

Arcing Energy (l

Melting Energy (l

Figure 13

Current Limitation

Arcing-fault:

A short circuit that arcs at the point of fault.

The arc impedance (resistance) tends to

reduce the short-circuit current. Arcing

faults may turn into bolted faults by welding

of the faulted components. Arcing faults

may be phase-to phase or phase-to-ground.

Arc-Blast:

A pressure wave created by the

heating, melting, vaporization, and

expansion of conducting material

and surrounding gases or air.

Arc-Flash:

The sudden release of heat energy and

intense light at the point of an arc. Can

be considered a short circuit through

the air, usually created by accidental

contact between live conductors.

54 54

Annex A

Electrical Safety Terms and Deﬁnitions

Arc Gap:

The distance between energized conductors

or between energized conductors and

ground. Shorter arc gaps result in

less energy being expended in the

arc, while longer gaps reduce arc

current. For 600 volts and below, arc

gaps of 1.25 inches (32 mm) typically

produce the maximum incident energy.

Arc Rating:

A rating assigned to material(s) that relates

to the maximum incident energy the material

can resist before breakopen of the material

or onset of a second-degree burn. The

arc rating is typically shown in cal/cm².

Arcing Time:

(See Figure 13): The time between

the melting of a fuse link or parting

of circuit breaker contacts, until

the overcurrent is interrupted.

Arc Voltage:

A transient voltage that occurs across

an overcurrent protection device during

the arcing time. It is usually expressed

as peak instantaneous voltage (Vpeak or

Epeak), but sometimes as rms voltage.

Asymmetrical Current:

AC current that is not symmetrical around

the zero axis. Usually caused by a fault

in circuits with low power factors. (See

Power Factor and Symmetrical Current).

Available Short Circuit Current:

(also Available or Prospective Fault Current):

The maximum rms Symmetrical Current

that would ﬂow at a given point in a system

under bolted-fault conditions. Short-circuit

current is maximum during the ﬁrst half-

cycle after the fault occurs. (See deﬁnitions

of Bolted Fault and Symmetrical Current.)

Bolted Fault:

A short circuit that has no electrical

resistance at the point of the fault. It

results from a ﬁrm mechanical connection

between two conductors, or a conductor

and ground. Bolted faults are characterized

by a lack of arcing. Examples of bolted

faults are a heavy wrench lying across

two bare bus bars, or a crossed-phase

condition due to incorrect wiring.

Boundaries of Approach:

Protection boundaries established to protect

personnel from shock and Arc-Flash hazards.

Calorie:

The amount of heat needed to raise the

temperature of one gram of water by one

degree Celsius. 1 cal/ cm² is equivalent

to the exposure on the tip of a ﬁnger

by a cigarette lighter for one second.

Clearing I²t (Also Total Clearing I²t):

The ampere-squared seconds (I²t) through an

overcurrent device from the inception of the

overcurrent until the current is completely

interrupted. Clearing I²t is the sum of the

Melting I²t and the Arcing I²t.

Coordination or Coordinated System:

See Selective Coordination.

Current-Limiting Fuse (Figure 14):

A fuse which, when interrupting currents

within its current-limiting range, reduces the

current in the faulted circuit to a magnitude

substantially less than that obtainable in the

same circuit if the device was replaced with

a solid conductor having comparable

impedance. To be labeled “current-limiting,”

a fuse must mate with a fuse block or fuse

holder that has either a rejection feature or

dimensions that will reject non-current-

limiting fuses.

Current

Time

Fault occurs

Arc is extinguished

Current which

would flow if

not interrupted

Current before fault

Fuse opens and

clears short circuit

in less than ½ cycle

Figure 14

Current Limiting Fuse

For more information:

800-TEC-FUSE

www.littelfuse.com

Current-Limiting Range:

For an individual overcurrent protective

device, the current-limiting range begins at

the lowest value of rms symmetrical current

at which the device becomes current-limiting

(the Threshold Current) and extends to the

maximum interrupting capacity of the device.

Current Rating:

See Ampere Rating.

Deenergized:

Equipment or components that have

had all energy sources removed.

Device Rating:

Refers to the standard ampere rating

of a device as deﬁned by NEC

Article

240.6. Standard ampere ratings

ranges from 1 to 6000 amperes.

Distance to Arc:

Refers to the distance from the receiving

surface to the arc center. The value used

for most calculations is typically 18 inches.

Electrical Hazard:

A dangerous condition caused by

equipment failure or contact with an

energized conductor. Hazards include

shock, Arc-Flash, burns and arc blasts.

Electrical Hazard Analysis:

A study to identify the potential electrical

hazards that may be exposed to

personnel. The analysis should address

both shock and Arc-Flash hazards.

Electrically Safe Work Condition:

Condition where the equipment and

or circuit components have been

disconnected from electrical energy

sources, locked/tagged out, and tested to

verify all sources of power are removed.

Energized:

Refers to components within a system

being connected to a “live” voltage source.

Fault:

Same as Short-Circuit and

used interchangeably.

Fault Current:

Same as Short-Circuit Current.

Flash Hazard Analysis:

A study that analyzes potential exposure

to Arc-Flash hazards. The outcome of

the study establishes Incident Energy

levels, Hazard Risk Categories, Flash

Protection Boundaries, and required PPE.

It also helps deﬁne safe work practices.

Flash Protection Boundary:

A protection boundary established to protect

personnel from Arc-Flash hazards. The

Flash Protection Boundary is the distance

at which an unprotected worker can receive

a second-degree burn to bare skin.

Fuse:

An overcurrent protective device consisting

of one or more current-carrying elements

enclosed in a body ﬁtted with contacts

so that the fuse may be readily inserted

into or removed from an electrical

circuit. The elements are heated by the

current passing through them, thus

interrupting current flow by melting

during speciﬁed overcurrent conditions.

Ground-fault:

A short circuit caused by insulation

breakdown between a phase conductor

and a grounded object or conductor.

Hazard Risk Category:

A classiﬁcation of risks (from 0 to 4) deﬁned

by NFPA 70E. Each category requires PPE

and is related to incident energy levels.

Incident Energy:

The amount of thermal energy impressed

on a surface generated during an

electrical arc at a certain distance from

the arc. Typically measured in cal/cm

I²t:

Symbol for Ampere-Squared-Seconds. A means

of describing the thermal energy generated by

current ﬂow. When a fuse or current-limiting

circuit breaker are interrupting currents

within their current-limiting range, the term is

expressed as melting, arcing, or total clearing

I²t. (See Melting I²t, Arcing I²t, and Clearing I²t)

ANNEX A

Instantaneous Peak Current

or I

peak

The maximum instantaneous current

value developed during the ﬁrst half-

cycle (180 electrical degrees) after fault

inception. The peak current determines

magnetic stress within the circuit.

Interrupting Capacity (AIC):

The highest available symmetrical rms

alternating current (for DC the highest direct

current) at which the protective device has

been tested, and which it has interrupted

safely under standardized test conditions.

The device must interrupt all available

overcurrents up to its interrupting capacity.

Also commonly called Interrupting Rating.

Interrupting Rating (IR, I.R., AIR or

A.I.R.):

The highest rms symmetrical current,

at speciﬁed test conditions, which

the device is rated to interrupt. The

difference between Interrupting Capacity

and Interrupting Rating is in the test

circuits used to establish the ratings.

Limited Approach Boundary:

An approach boundary to protect personnel

from shock. A boundary distance is

established from an energized part based

on system voltage. To enter this boundary,

unqualiﬁed persons must be accompanied

with a qualiﬁed person and use PPE.

Melting I²t:

The heat energy created by an overcurrent

required to melt the fuse link(s). It equals

the rms current (or DC current) squared,

multiplied by the melting time in seconds.

For times less than 0.004 seconds, melting I²t

approaches a constant value for a given fuse.

Overcurrent:

Any current larger than the equipment,

conductor, or devices are rated to

carry under speciﬁed conditions.

Overload:

An overcurrent that is conﬁned to the

normal current path (e.g., not a short

circuit), which, if allowed to persist, will

cause damage to equipment and/or wiring.

Peak Let-through Current

(See Figure 15):

The maximum instantaneous current

that passes through an overcurrent

protective device during its total clearing

time when the available current is

within its current-limiting range.

Peak Let-through Current

Peak Current which would occur

without current limitation

Current

Time

Melting

Time

Arcing

Time

Arcing Energy (l

Melting Energy (l

Figure 15

Peak let-through

Power Factor (X/R):

As used in overcurrent protection, power

factor is the relationship between the inductive

reactance (X) and the resistance (R) in the

system during a fault. Under normal conditions

a system may be operating at a 0.85 power

factor (85%). When a fault occurs, much of the

system resistance is shorted out and the power

factor may drop to 25% or less. This may cause

the current to become asymmetrical. See

deﬁnition of Symmetrical Current.

PPE:

An acronym for Personnel Protective

Equipment. It can include clothing,

tools, and equipment.

Prohibited Approach Boundary:

An approach boundary to protect personnel

from shock. Work in this boundary is

considered the same as making direct

contact with an energized part. Only

qualiﬁed persons are allowed to enter

this boundary and they must use PPE.

Protection Boundaries:

Boundaries established to protect

personnel from electrical hazards.

Qualiﬁed Person:

A person who is trained and knowledgeable

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on the construction and operation of the

equipment and can recognize and avoid

electrical hazards that may be encountered.

Rating:

A designated limit of operating

characteristics based on deﬁnite

conditions, such as current rating,

voltage rating and interrupting rating.

Renewable Fuse:

A fuse that may be readily restored

to service by replacing the renewable

element after operation.

Restricted Approach Boundary:

An approach boundary to protect

personnel from shock. A boundary

distance is established from an energized

part based on system voltage. Only

qualiﬁed persons are allowed in the

boundary and they must use PPE.

Selective Coordination

(See Figure 16):

In a selectively coordinated system, only

the protective device immediately on the

line side of an overcurrent opens. Upstream

protective devices remain closed. All other

equipment remains in service, which

simpliﬁes the identiﬁcation and location of

overloaded equipment or short circuits.

Shock:

A trauma subjected to the body by electrical

current. When personnel come in contact

with energized conductors, it can result

in current ﬂowing through their body

often causing serious injury or death.

In a selective system:

For a fault at "X" only fuse "C" will open.

For a fault at "Y" only fuse "F" will open.

B C D E

F G H J

Figure 16

Short Circuit (See Figure 17):

A current ﬂowing outside its normal path.

It is caused by a breakdown of insulation

or by faulty equipment connections. In a

short circuit, current bypasses the normal

load. The amount of current is determined

by the system impedance (AC resistance)

rather than the load impedance. Short-circuit

currents may vary from fractions of an

ampere to 200,000 amperes or more.

ANNEX A

CURRENT FLOW

SHORT CIRCUIT

GEN.

LOAD

Accidental

Connection

Creates Fault

GEN.

LOAD

System voltage and load resistance

determine the ﬂow of current.

During a short circuit, only the resistance of

the fault path limits current. Current may

increase to many times the load current.

(red lines indicate increased current)

Figure 17

Short-Circuit Rating:

The maximum rms symmetrical short-circuit

current (for DC equipment DC current) at which

a given piece of equipment has been tested

under speciﬁed conditions, and which at the

end of the test, is in essentially the same

condition as prior to the test. Short-circuit

ratings (also called withstand ratings) apply to

equipment that will be subjected to fault

currents, but which are not required to interrupt

them. This includes switches, busway (bus

duct), switchgear and switchboard structures,

motor control centers and transformers. Most

short-circuit ratings are based on tests which

last three complete electrical cycles (0.05

seconds). Some equipment may have reduced

short-circuit rating for times longer than 3

cycles. Refer to manufacturers literature. If the

equipment is protected during the test by fuses

or by a circuit breaker with instantaneous trips,

the test duration is the time required for the

overcurrent protective device to open the circuit.

When so protected during testing, the

equipment instructions and labels must

indicate that the equipment shall be

protected by a given fuse class and rating, or

by a speciﬁc make, type, and rating of circuit

breaker. Certain circuit breakers equipped

with short-delay trip elements instead of

instantaneous trip elements may have

withstand (short-circuit) ratings in addition to

their interrupting rating. The withstand rating

is the fault current the breaker must be able to

withstand for a speciﬁed time or if it is

protected by a fuse or another circuit breaker

in series with it. They may also have reduced

interrupting ratings.

Symmetrical Current:

The terms “Symmetrical Current” and

“Asymmetrical Current” describe an

AC wave’s symmetry around the zero

axis. The current is symmetrical when

the peak currents above and below the

zero axis are equal in value, as shown

in Figure 18. If the peak currents are not

equal, as shown in Figure 19, the current

Equal

Peaks

Symmertrical Current

Zero Axis

Unequal

Peaks

Asymmertrical Current

Zero Axis

Figure 18 Figure 19

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is asymmetrical. The degree of asymmetry

during a fault is determined by the change

in power factor (X/R) and the point in the

voltage wave when the fault occurs.

System Voltage:

The phase-to-phase or three-phase

voltage(s) at the point being evaluated.

Threshold Current:

The minimum current for a given fuse size

and type at which the fuse becomes current-

limiting. It is the lowest value of available

rms symmetrical current that will cause the

device to begin opening within the ﬁrst 1/4

cycle (90 electrical degrees) and completely

clear the circuit within 1/2 cycle (180 electrical

degrees). The approximate threshold

current can be determined from the fuse’s

peak let-through charts. See Figure 20.

3500

8000

Fuse approximate

threshold current = 3500

Peak let-through current

Available Fault Current Symmetrical RMS Amperes

Peak Let-Through in Amperes

Figure 20

Threshold Ratio:

The threshold current divided by the

ampere rating of a speciﬁc type or class

of overcurrent device. A fuse with a

threshold ratio of 15 becomes current-

limiting at 15 times its current rating.

Unqualiﬁed Person:

A person that does not possess all

the skills and knowledge or has not

been trained for a particular task.

Voltage Rating:

The maximum rms AC voltage and/or the

maximum DC voltage at which the device

is designed to operate. For example,

fuses rated 600 volts may be applied at

any system voltage less than or equal to

their rating. [There are no speciﬁc rules

for applying AC fuses in DC circuits.]

Fuses used on DC circuits must have

proper DC voltage ratings. Circuit breaker

ratings are more complex since some

breakers may have dual voltage ratings.

Withstand Rating:

See Short-Circuit Rating.

ANNEX A

National Electrical Code

(NEC

)

In 1896 members of the industry met in New

York City to develop a single electrical installation

code from the ﬁve then in use. After review

by over 1200 individuals, it was published in

1897 and has become known as the National

Electrical Code. In 1911 the NFPA became the

sponsor of the NEC and continues the tradition

of wide spread consensus. The purpose of

the National Electrical Code “is the practical

safeguarding of persons and property from

hazards arising from the use of electricity. The

NEC contains provisions considered necessary

for safety.” The NEC is updated and revised

every three years. The NEC, also known as

NFPA 70, is the nationally accepted standard for

safe electrical installation methods and practices.

Although the NEC is regarded as the “Bible”

for electrical construction practices, it does not

provide comprehensive details for workplace

safety when servicing electrical systems.

While the NEC is not a design manual, following

its provisions help ensure that electrical

systems are reasonably safe. Some of the NEC

provisions speciﬁcally addressing application

of overcurrent protective devices are listed

herein, however users are cautioned the

NEC must be considered in its entirety.

NEC Articles

The following NEC paragraphs are important

when designing and servicing electrical systems:

“110.4 Voltages.

Throughout this Code, the voltage

considered shall be that at which the

circuit operates. The voltage rating of

the electrical equipment shall not be

less than the nominal voltage of the

circuit to which it is connected.”

“110.9 Interrupting Rating:

Equipment intended to interrupt

current at fault levels shall have an

interrupting rating sufﬁcient for the

nominal circuit voltage and the current

that is available at the line terminals of

the equipment. Equipment intended

to interrupt current at other than fault

levels shall have an interrupting rating

at nominal circuit voltage sufﬁcient for

the current that must be interrupted.”

1, 2. Reprinted with permission from NFPA 70-2005, National

Association, Quincy, MA. This reprinted material is not the complete

and ofﬁcial position of the NFPA on the referenced subject, which is

represented only by the standard in its entirety.

61 61

Annex B

Electrical Safety Codes and Standards

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“110.10 Circuit Impedance and

Other Characteristics:

The overcurrent protective devices,

the total impedance, the component

short-circuit current ratings, and

other characteristics of the circuit to

be protected shall be selected and

coordinated to permit the circuit-

protective devices used to clear a fault

to do so without extensive damage

to the electrical components of the

circuit. This fault shall be assumed

to be either between two or more of

the circuit conductors or between any

circuit conductor and the grounding

conductor or enclosing metal raceway.

Listed products applied in accordance

with their listing shall be considered to

meet the requirements of this section.”

“110.16

Flash Protection: Switchboards,

panelboards, industrial control panels,

meter socket enclosures, and motor

control centers that are in other than

dwelling occupancies and are likely

to require examination, adjustment,

servicing, or maintenance while

energized shall be ﬁeld marked to

warn qualiﬁed persons of potential

electric Arc-Flash hazards. The marking

shall be located so as to be clearly

visible to qualiﬁed persons before

examination, adjustment, servicing,

or maintenance of the equipment.”

“240.2 Deﬁnition: Current-Limiting

Overcurrent Protective Device:

A device that when interrupting

currents in its current-limiting

range, reduces the current ﬂowing

in the faulted circuit to a magnitude

substantially less than that obtainable

in the same circuit if the device were

replaced with a solid conductor

having comparable impedance.”

“240.12 Electrical

System Coordination:

Where an orderly shutdown is

required to minimize the hazard(s) to

personnel and equipment, a system of

coordination based on the following

two conditions shall be permitted:

1. Coordinated short-circuit protection

2. Overload indication based on

monitoring systems or devices.

FPN:

The monitoring system may cause

the conditions to go to alarm, allowing

corrective action or an orderly shutdown,

thereby minimizing personnel hazard

and equipment damage.”