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Protecting AgainstArc Flash

By Daniel Roberts and George Gregory,

Schneider Electric

Electric shock is the hazard most people associate

with electricity, yet the majority of hospital

admissions stemming from electrical accidents are

due to arc-flash burns, not shocks . In his

landmark 1982 paper Ralph H. Lee calls arc

flash “the other electrical hazard.”

“The other electrical hazard”

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Safe work practices designed to prevent arc flash incidents arecontained in NFPA 70E – Standard for Electrical Safety in the Workplace– 2004 Edition. NFPA 70E is published by the National Fire ProtectionAssociation (NFPA), an international codes and standards organizationbased in the United States. The Canadian Standards Association (CSA)currently does not have a standard equivalent to NFPA 70E. In theabsence of a Canadian Standard, many companies in Canada areadopting the NFPA 70E standard and modifying it to suit their needs.

In the United States the Occupational Safety and Health Administration(OSHA) has begun to aggressively monitor compliance with NFPA 70E.What does Canadian occupational health and safety legislation have tosay about arc flash?

Canadian Occupational Health and Safetylaw and arc flash hazards

Arc flash is addressed legislatively at both the provincial and federallevels.

All provincial occupational health and safety acts have a general dutyclause requiring employers to take reasonable precautions to ensuretheir employees’ health and safety. Additionally, some provincesspecifically mandate protection against arc flash.

For example, Ontario’s Construction and Industrial Regulations requireworkers to be protected against electrical shock and burns while workingon or near the live, exposed parts of equipment or conductors. Alberta’sOccupational Health and Safety Code requires workers exposed toelectrical equipment flashover to wear flame-resistant clothing. Adoptingthe safe work practices found in NFPA 70E is certainly a reasonablemeasure for an employer to follow to protect his employees.

Federally, as of 31 March 2004, Bill C-45 established a duty under theCriminal Code of Canada for employers, managers and supervisors toensure workplace health and safety. Under the Criminal Code of Canadaas amended by Bill C-45, there is no specific limit on fines against acorporation that’s found guilty, and individual representatives of acorporation can receive a maximum sentence of life imprisonment ifconvicted of criminal negligence causing death.

Conclusion? Employers in Canada have a legal duty to protect theirworkers against arc flash, the other electrical hazard.

So what exactly is arc flash and how canworkers be protected?

Defining arc flashAn arc flash occurs when electric current passes through air (arcs)between conductors. An electric arc is one of the hottest things on earth– temperatures in the arc can reach close to 20,000°C. The incredibletemperature of the arc causes an explosive expansion of air and metalaround the arc. Concentrated energy radiates outward from the arc inthe form of plasma, molten metal and pressure waves. The effects onthe human body can range from traumatic – severe burns, damagedeyesight, ruptured eardrums, collapsed lungs – to fatal.

There are two key practices outlined in NFPA 70E that, when followed,protect against arc flash hazards: (1) establishing an electrically safework condition, and (2) performing a flash hazard analysis.

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Establishing an electrically safe work conditionNeither an electric shock nor an arc flash event can occur if theequipment is at a zero-energy-state or de-energized. Generallyspeaking, other than for the purposes of testing or troubleshooting,Canadian Occupational Health and Safety legislation requires the powersupply to electrical equipment be disconnected and locked out ofservice before any work is done on or near live exposed conductors.NFPA 70E calls this "establishing an electrically safe work condition."

Appropriate personal protective equipment (PPE) must be used whenestablishing an electrically safe work condition, or when testing ortroubleshooting. The electrical industry has done a good job ofidentifying and providing the required PPE for shock hazards – e.g.class of gloves, safe limits of approach, etc. The next step is to take arcflash to the same level of awareness. This would involve performingwhat NFPA 70E terms a "flash hazard analysis – a study investigating aworker’s potential exposure to arc flash energy, conducted for thepurpose of injury prevention and the determination of safe workpractices and the appropriate levels of PPE."

Performing a flash hazard analysisA flash hazard analysis determines two important things: the flashprotection boundary distance and the possible arc flash incident energy.NFPA 70E defines these two terms as:

"Incident energy – the amount of energy impressed on a surface, acertain distance from the source, generated during an electrical arcevent." This energy is generally expressed in calories/cm2. The"surface" of concern "impressed on" is a worker’s body – particularly thehead and trunk. Incident energy is calculated using variables such asavailable fault current, system voltage, expected arcing fault durationand the worker’s distance from the arc. The data obtained from thecalculations is used to select the appropriate flame resistant (FR) PPE,just as voltage level would be used to select a class of rubber gloves.

"The flash protection boundary is an approach limit at a distance fromexposed live parts within which a person could receive a second-degree burn if an electrical arc flash were to occur." FR PPE must beworn by anyone that is within the flash protection boundary. SeeFigure 1 – Approach Boundaries.

NFPA 70E does not specifically require or exclude any method toperform the flash hazard analysis. IEEE 1584, Guide for Performing ArcFlash Hazard Calculations, provides fairly accurate methods of analysis,as they are based on a large number of tests. When installation data isknown, the IEEE spreadsheet calculations are relatively straightforward.Results can be obtained for a variety of circuits or working conditions,but they require detailed information about the installation and acomputer, or good calculating skills.

Facilities with neither the time nor the expertise can contract thirdparties to calculate flash protection boundaries and incident energylevels for each piece of electrical distribution equipment in their facility,label the equipment, and train their staff in electrical safety and arcflash hazards.

A simpler approach is to use the hazard analysis and PPE selectiontables and guidelines within NFPA 70E. They provide enoughinformation to perform the analysis and select the appropriate PPE withno other source material.

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Figure 1 – Approach Boundaries

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Use of the three tables can be broken down into a five-step procedure:

From Table 130.7(C)(9)(a) – Hazard / Risk Category Classification:1. Select the voltage class of equipment to be worked on and the task to

be performed.2. Identify the hazard category.3. Identify whether or not voltage rated tools and gloves are required.

From Table 130.7(C)(10) – Protective Clothing and Personal ProtectiveEquipment Matrix:4. Identify the required personal protective equipment, including FR

clothing.

From Table130.7(C)(11) – Protective Clothing Characteristics:5. Ensure the FR clothing selected meets the minimum arc rating for the

hazard category identified in step two.

This relatively simple approach may be satisfactory when either thecapability or the time to perform a more focused analysis is lacking (seeexample).

The primary drawback of using the NFPA 70E tables is that values arebased on a given level of fault current and duration of the arc event. Forvalues different from that condition, PPE category values can be higheror lower. An important point not generally understood is that incidentenergy may actually increase in a low fault current situation, which is notreflected in the tables.

No substitute for safety

The importance of your electrical safety program addressing arc flashhazards cannot be overstated. Arc flash is a serious hazard that ispotentially fatal to those exposed to it. It can also cause lengthydowntime to repair or replace severely damaged equipment.

Until such a time as a Canadian Standard exists, the requirements ofNFPA 70E can help users of electricity to reduce the probability of an arcflash event and eliminate the resulting injuries.

Daniel Roberts is the safety manager for the Canadian Services Divisionof Schneider Electric and a subcommittee member for Sections 2 and 24of the Canadian Electrical Code. He can be contacted [email protected].

George Gregory is manager of industry standards-circuit protectionproducts for Schneider Electric and a member of the NFPA 70E committee.He can be contacted at [email protected].

Typical protective clothing systems

Hazard risk category

Clothing description (Typical number ofclothing layers is given in parentheses)

Arc rating ofPPE (cal/cm2)

NFPA 70E Table130.7(C)(11) – Protective Clothing Characteristics

0

1

2

3

4

Non-melting, flammable materials (i.e.,untreated cotton, wool, rayon, or silk, orblends of these materials) with a fabricweight at least 4.5 oz/yd2 (1)

FR shirt and FR pants or FR coverall (1)

Cotton underwear — conventional shortsleeve and brief/shorts, plus FR shirtand FR pants (1 or 2)

Cotton underwear plus FR shirt andFR pants plus FR coverall, or cottonunderwear plus two FR coveralls (2 or 3)

Cotton underwear plus FR shirt andFR pants plus multilayer flash suit (3 or more)

N/A

4

8

25

40

Step 5 – Arc Rating of PPE for Category 0 = N/A(ensure clothing fabric weight is at least 4.5 oz/yd2)

NFPA 70E is a registered trademark of the National fire Protection Association,Quincy, MA 02169

Hazard/risk categorynumber

Untreated natural fiber

a. T-shirt (short-sleeve)

b. Shirt (long-sleeve)

c. Pants (long)

FR Clothing (Note 1)

a. Long-sleeve Shirt

b. Pants

c. Coverall

Eye protection

a. Safety glasses

Protective clothing and equipment

Step 4 – Required personal protective equipment for Category 0 = Cotton long sleeve shirt and long pants + safety glasses

NFPA 70E Table 130.7(C)(10) – Protective Clothing and PersonalProtective Equipment Matrix

2

–

X

X (note 6)

–

X

X (note 6)

(note 7)

–

AL

1

–

X (note 4)

–

X

X (note 4)

(note 5)

–

X

0

–

X

X

–

–

X

Protective systems for hazard/risk category

3

–

X

X

–

X (note 9)

X (note 9)

X (note 9)

–

AL

4

–

X

X

–

X

X

(note 5)

–

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Task (assumes equipment isenergized, and work is done withinthe flash protection boundary)

Panelboards rated 240 V andbelow – notes 1 and 3

Circuit breaker (CB) or fused switch operation with covers on

Step 1 – Voltage class = 240; Activity = Switch operation, covers onStep 2 – Hazard/risk category = 0Step 3 – Voltage rated gloves and tools = not required

NFPA 70E Table 130.7(C)(9)(a) – Hazard Risk Category Classification

Hazard/risk

category

–

0

V-ratedgloves

–

N

V-ratedtools

–

N

Example:Determine what PPE is required when operating a240 Volt fused switch with the cover on.