White-Paper-Arc-Flash-Compliance.pdf

7/27/2019 White-Paper-Arc-Flash-Compliance.pdf

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When the employer has reason to believe that any affected employee who has already been

trained does not have the understanding and skill required by paragraph (f)(2) of this section, the

employer shall retrain each such employee. Circumstances where retraining is required include,

but are not limited to, situations where: Changes in the workplace render previous training

obsolete; or Changes in the types of PPE to be used render previous training obsolete; or

Inadequacies in an affected employee's knowledge or use of assigned PPE indicate that the

employee has not retained the requisite understanding or skill.

The employer shall verify that each affected employee has received and understood the required

training through a written certification that contains the name of each employee trained, the

date(s) of training, and that identifies the subject of the certification.

The NPFA 70, National Electrical Code (NEC) included a section (110-16) requiring the labeling of

panels with an arc flash warning beginning with the 2002 edition. The 2005 edition of the NEC made

minor changes and now reads;

110-16 Flash Protection

Switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control

centers in commercial and industrial occupancies that are likely to require examination,adjustment, servicing, or maintenance while energized must be field marked to warn qualified

persons of the danger of electric arc flash. The marking must be clearly visible to qualified

persons before they examine, adjust, service, or perform maintenance on the equipment.

The NEC does not provide specific direction regarding label content. Consequently the information

shown in Figure 1 could comply.

2006 Doble Engineering Company-73rdAnnual International Doble Client Conference

All Rights Reserved3

! WARNINGAppropriate PPE Required

Arc Flash and Shock Hazard

Basic Arc Flash Warning Label

FIGURE 1

However, for personnel assigned to perform energized work this is not enough information since it does

not quantify the hazard in a way that appropriate PPE can be selected. Fine print notes in the NEC

reference NFPA 70E-2004 as a guide to quantifying the hazard. The NEC warning label is an interim

step for work on energized equipment.

NFPA released an update to NFPA-70E in 2004 that adopted the IEEE Std. 1584-2002 methods for

determining the incident energy (a simplified explanation of this calculation method in included in

Appendix 1). The standard was renamed to NFPA 70E Standard for Employee Safety in the Workplace2004 Edition. This standard details methods and procedures that meet OSHA requirements for safe work

practices. All new arc rated PPE include the Arc Thermal Performance Value (ATPV) with units in

cal/cm2. The required PPE at specific locations is determined by comparing the calculated incident

energy to the ratings for specific combinations of PPE. An example is given in NPFA 70E as follows:


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TABLE 1

Protective Clothing Characteristics

Hazard/Risk Category

Typical Protective Clothing Systems

Required

Minimum Arc

Rating of PPE

(cal/cm2)

0Non-melting, flammable materials (natural or treated

materials with at least 4.5 oz/yd2)N/A (1.2)

1 FR pants and FR shirt, or FR coverall 4

2 Cotton Underwear, plus FR shirt and FR pants 8

3Cotton Underwear, plus FR shirt and FR pants and FR

coverall25

4Cotton Underwear, plus FR shirt and FR pants and

multiplayer flash suit40

This example should NOT be used for final calculations. For actualapplications, the calculated incident

energy must be compared to specific PPE combinations used at the facility being evaluated. Theexception to this is the upper limit of 40 cal/cm2. While PPE is available in ATPV values of 100 cal/cm2

or more, values above 40 cal/cm2 are considered prohibited due to the sound, pressure and concussive

forces present. Above 40 cal/cm2 these forces are more significant than the thermal values. It may be

unfortunate that some PPE manufacturers have adopted these categories when promoting their PPE. In

doing so these manufacturers label the PPE for any value within the category range. Therefore a garment

with an 8.2 calorie/cm2 rating can be labeled HRC2 (Hazard Risk Category 2) even though the Category 2

can go up to 25 calories.

NFPA 70E also contains an example set of Hazard/Risk Category Classifications that defines the PPE

classification based on voltage and task being performed. These are not recommended for use since they

assume the same combinations of PPE used in the example table. Additionally, some of the

task/categories recommended do not make logical sense (i.e. removing a cover is determined a higher riskcategory task than directly working on the energized equipment). Also, the fine print notes and other

notes indicate that the actual incident energy might be higher or lower based on higher or lower fault

currents or clearing times.

An alternate calculation method is included in NFPA 70E Annex D (D.1 through D.7). This method is

based on earlier testing that differentiated the effect on incident energy when an arc in contained in a box

versus open air. However this method is limited to low voltage (600V and less) applications.

In addition to flash protection, NFPA 70E also defines requirements for shock protection and safe

distances for qualified and unqualified personnel. These include:

Flash Protection Boundary. An approach limit at a distance from exposed live parts within

which a person could receive a second-degree burn if an electric arc flash were to occur.

Appropriate flash-flame protection equipment must be utilized for persons entering the flash

protection region. This distance may be outside or inside the following shock protection

distances.

Limited Approach Boundary. An approach limit at a distance from an exposed live part within

which a shock hazard exists. A person crossing the limited approach boundary and entering the

limited region must be qualified to perform the job/task.


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Restricted Approach Boundary. An approach limit at a distance from an exposed live part

within which there is an increased risk of shock, due to electrical arc over combined with

inadvertent movement, for personnel working in close proximity to the live part. The person

crossing the Restricted approach boundary and entering the restricted space must have a

documented work plan approved by authorized management, use PPE that is appropriate for the

work being performed and is rated for voltage and energy level involved.

Prohibited Approach Boundary. An approach limit at a distance from an exposed live part

within which work is considered the same as making contact with the live part. The person

entering the prohibited space must have specified training to work on energized conductors or

live parts. Any tools used in the prohibited space must be rated for direct contact at the voltage

and energy level involved.

Distances ApproachFIGURE 2

NFPA specifies the approach boundary distances based on the voltage at the point being evaluated as

follows:

TABLE 2

Approach Distances versus Voltage



In order to meet the requirements of the standards referenced here, more than just a warning is necessary.

The following are two examples of labels generated using the arc flash module in a power systems

analysis software package.

72660ANALYSIS REQD15kV

72660ANALYSIS REQD5kV

11242ANALYSIS REQD480V

AVOID CONTACTAVOID CONTACT42ANALYSIS REQD208V

PROHIBITED

APPROACH

RESTRICTED

APPROACH

LIMITED

APPROACH

FLASH HAZARDSYSTEM

VOLTAGE


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Complete Arc Flash Labels

FIGURE 3

The left hand label indicates a Class 0 protection (using the NPFA 70E example categories) and the label

on the right indicates prohibited work (incident energy is far above the 40 cal/cm2 limit for energized

work). Note that in the right hand label, the text and the color of the label changes to indicate the

prohibited status of location. The labels show the calculated flash protection boundary, incident energyand PPE category (with description). In addition to the incident energy information, the label also

includes required glove classification and the shock protection boundaries required by NFPA 70E. Since

these labels are part of a certified process, they include who performed the work and on what date.

Summarizing the various standards discussed here yields the following requirements.

1. The arc flash hazardmust be quantified2. Appropriate PPE must be selected for non-prohibited work3. The arc flash assessment results must be documented4. Personnel must be trained, understand the hazards, and take appropriate action.5. Analysis should be re-evaluated if the standards, PPE types, or system configuration changes.

IV. QUANTIFYING THE HAZARD

The majority of electrical facilities have yet to implement an Arc Flash protection program or perform

analysis on their systems. This presents a quandary for the personnel assigned to perform energized

work. How to perform the work safely?


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The first question to ask is does the work qualify as energized work? Does the location require hardware

to be removed or will the plane of the equipment be passed, and is the voltage present above 50 Volts. If

so, then it is considered energized work.

The next step is to determine if the area of work has sufficient energy to require arc rated PPE. If the

equipment is rated 240V or less and is served by a 125kVA transformer or smaller, then it is considered

Category 0 (does not require arc-rated PPE).

If it is determined that energized work needs to be performed and the location may require arc-rated PPE

then a hazard assessment must be completed.

If sufficient lead-time exists before the start of a project the locations where energized work is to be

performed can be evaluated and labeled with the appropriate incident energy/shock protection information

(This would be a complete Arc Flash study). The alternative is for the worker to do a field assessment to

determine what PPE is required. For low voltage equipment this can be done using the simplified

equations in IEEE 1584 in either a hand calculation or a spreadsheet program. Medium voltage

applications are more difficult since the available fault current may be harder to determine from the

installed equipment (the transformer impedance is not the dominant impedance for medium voltage

systems as it is for low voltage).

In order to quantify the arc flash hazard using the IEEE methodology for a specific location the following

items are required.

1. Available fault current at the location.2. Clearing time for the source-side protective device(s) at the calculated arcing fault current.3. Location type (open air, cable, switchgear)4. Working distance for energized work.5. APTV values for PPE combinations used at the site.6. Site specific issues and limitations (egress, process)

The first two items are generally obtained from short-circuit and protective device coordination studies.

In order for the results to be accurate, the study must be complete and up to date. However, unlike most

short-circuit and coordination studies, accurate installed source information instead of worst-case

information is required. Using worst case fault duties could result in using a clearing time that is too

short, yielding an incident energy value that is much lower than that with an accurate short circuit duty.

The third through sixth items are generally obtained though investigating the installed equipment

configuration. Working distances are generally set at 18 inches for low voltage locations. Medium

voltage locations have working distances set based on procedures and equipment configurations. The

ATPV values for PPE and combinations available are also required to complete the assessment.

Site specific installation data is collected to take into account any installed conditions that may increasethe hazard/risk. This can include continuous process or chemical installations where an arc fault may

increase the risk of other hazards. It also must take into account the physical location with respect to

egress. Locations where the flash hazard boundary exceeds the limits of an electrical vault or room, or

are elevated, may increase the risk due to limited egress.


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Arc Flash Field Evaluation

FIGURE 4

The figure above is an example of a low voltage field evaluation spreadsheet used to determine incident

energy and required PPE using the simplified equations in IEEE 1584. This example uses minimal data

to determine PPE levels (and includes a table to indicate minimum distances for all PPE levels).

However, it is not as rigorous as a full analysis, and should not be used in place of a full study or to

develop labels for locations evaluated.

Complete data (either with field collected data or an existing short circuit and coordination study that isaccurate and complete), should be used to perform a full analysis using the IEEE calculation method.

Example: An infrared scan on 2000A 12.47kV switchgear lineup needs to be performed. The

switchgear is protected by ANSI very-inverse curve overcurrent protection (CT 2000:5, Pickup

2000A (5A secondary), Time Dial 3), with a 5-cycle circuit breaker. The available fault current is

14kA. The worst case working distance is 24 inches when the cover is being removed.

Determine the incident energy for this work.

Using the IEEE calculation method the arcing fault currents (100% and 85%) are determined to

be 13.51kA and 11.48kA. Looking up these currents in the manufacturers literature yields

clearing times of 0.55s and 0.65s. Adding these times to the clearing time for the breaker and

applying these values and the working distance yields a worst-case incident energy value of16.6cal/cm2 and a flash protection boundary of 358 inches. This is a Class 3 location that would

require a two layer arc-rated outfit (ATPV equal to or greater than the calculated incident energy)

with hooded head protection, Class II (15kV) gloves, electrical work boots, and hearing and eye

protection.

Figure 5 below uses the example data in a spreadsheet to determine incident energy and required PPE

using the IEEE 1584 equations.




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If this same equipment has buss differential protection the time can be reduced to the operating time of

the relay and the clearing time of the breaker (approx 7 cycles). The incident energy is reduced to 2.9

cal/cm2 and the flash boundary to 60 inches.



Full Arc Flash Field Analysis

FIGURE 5

When performing the assessment, it may be determined that some locations would require extreme

protective equipment (i.e. a flash suit), exceed the rating of available PPE, or be classified a prohibited

work area. Without modifying the equipment there are only two ways mitigation can be utilized to reduce

the incident energy to workable levels to allow for energized work.


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Increasing the Working Distance

The incident energy drops significantly over distance (proportional to the square of the distance in open

air), increasing the working distance will reduce the incident energy. Working distance can be increased

by using remote racking devices, remote operating devices, and extension tools (i.e. hotsticks).

Example: Using the above example determine the PPE required by the operator of the infraredcamera assuming a minimum 48 inch working distance.

Substituting 48 inches into the calculations reduces the incident energy to 11 cal/cm2.

Reducing the Clearing Time

Since the incident energy is directly proportional to the clearing time, reducing the clearing time can

significantly reduce the incident energy. Reducing the settings or adding instantaneous trips can reduce

the incident energy. The changes need to be weighed against the impact on selectivity.

An alternative to permanently lowering coordinated settings is to temporarily reduce settings for only the

time during which on-line work is performed. Locations with microprocessor-based relays can beprogrammed to implement lower settings (i.e. an instantaneous setting just above the peak demand level)

with a contact input, such as a front panel control and/or switch. The disadvantage of this technique is

that it results in nonselective operation for downstream faults during the maintenance window.

Lowering device settings is the least cost solution to lowering incident energy exposure, but is limited by

the range of available settings that will still achieve selective operation. In medium voltage relaying, this

can be achieved by changing the curve shape or lowering the time dial settings. Low voltage protection

changes are more limited due the device characteristics.

V. CASE STUDIES

The need for Arc Flash studies and training is not limited to commercial/industrial facilities. They areneeded in any location where energized work is necessary. This includes, but is not limited to hospitals,

data centers, utility substations and distribution systems, office buildings, factories, and municipal

facilities. The following case studies some typical applications and results from Arc Flash evaluations.

Naval Base: Relay testing and on-line partial discharge cable testing needed to be performed at a naval

station. The areas of work are the 69kV and 12kV substations and the 12kV distribution cables between

the substations out to the pad mounted unit substations. Many of the substations have single door access

with the protective relays in the same cubicle as the circuit breakers. Partial discharge cable testing will

require removal of switchgear covers to access the conductors. Additionally, access to the conductors

outside the substations will require entry into manholes and vaults. No Arc Flash analysis had been

performed for the base.

The 69kV and 12kV substations all have bus differential protection. All conductors between substations

have pilot wire protection. Conductors to feeder unit substations have no pilot wire or differential

protection.

A hazard analysis was performed based on an existing short-circuit and coordination study, which was

current and accurate. Without the differential and pilot wire protection incident energies in the

substations and conductor runs between substations were as high as 16 cal/cm2. With differential and


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pilot wire protection incident energies are less than 2 cal/ cm2. Analysis of the conductor runs to the unit

substations resulted in incident energies less than 4 cal/ cm2 for all runs within the scope of the project.

Review of all locations to be worked on determined that standard turnout gear consisting of 8.2 cal/ cm2

rated coveralls with appropriate hand, eye, head and foot protection was sufficient for all areas.

Waste Water Treatment Facility: Arc Flash hazard evaluations were performed for the distributionsystem which consists of 12kV fused switchgear and pad mounted switches, 5kV motor control centers,

and 480V switchgear and motor control centers. Energized work is performed at all levels for infrared

scans and meter readings are taken at all low voltage locations. Analysis indicated that all the medium

voltage locations had incident energies below 5 cal/cm2. Low voltage locations had incident energies

from below 1.2 cal/cm2 to above 40 cal/cm2.

Settings were recommended to reduce the incident energies with minimal impact on selectivity. As a

result only two locations (out of a total of 225) were labeled as prohibited for energized work.

Review of the incident energies calculated determined that a standard uniform consisting of a 8.2 cal/ cm2

rated shirt and 11 cal/cm2 rated pants with appropriate hand, eye, head and foot protection was sufficient

for approximately 170 locations where energized work was allowed. Around 30 locations required anadditional layer consisting of 11 cal/cm2 rated coveralls and a 25 cal/cm2 rated hood. The remaining

locations require a flash suit or PPE combination with a minimum 40 cal/cm2 rating.

Beverage Bottling Facility: Arc Flash hazard evaluations were performed for the distribution system

which consists of four 480V services that included 480V switchgear, panelboards and motor control

centers. Energized work including infrared scans and meter readings are performed on all equipment.

Fused main switches protect three of the four 480V switchgear locations. Analysis indicated these

locations have calculated incident energies above 100 cal/cm2 and cannot be worked on energized. The

fourth location is protected by a circuit breaker, and has calculated incident energies below 20 cal/cm2.

Recommendations were made to upgrade the fused switches mains to circuit breakers to reduce incident

energy to workable levels.

Review of the incident energies calculated determined that a standard uniform consisting of a 8.2 cal/ cm2

rated shirt and 11 cal/cm2 rated pants with appropriate hand, eye, head and foot protection was sufficient

for all but five of the locations where energized work was allowed. The remaining areas required an

additional layer consisting of 11 cal/cm2 rated coveralls and a 25 cal/cm2 rated hood.

CONCLUSIONS

The requirement to perform Arc Flash evaluations can no longer be ignored. The methods to quantify this

hazard exist, and as a result the standards require that action be taken to protect personnel who perform

energized work and assure they are properly trained to recognize the hazards present.

This paper has described the process of arc flash hazard analysis, including the calculation of incident

energy levels in arc flash faults and selection of appropriate Personal Protective Equipment (PPE) levels.

BIBLIOGRAPHY

1. NFPA 70E, Standard for Electrical Safety Requirement for Employee Workplaces, 2000 Edition2. IEEE Guide for Arc Flash Hazard Calculations, IEEE Standard 1584-2002.3. NFPA 70E, Standard for Electrical Safety in the Workplace, 2004 Edition.


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BIOGRAPHICAL SKETCHES

Christopher Inshaw is a Consulting Power Systems Engineer for Emerson Process Management,

Electrical Reliability Services (formerly Electro-Test) in Fresno, CA. He received his BSEE degree from

CSU Fresno. He is a Member of IEEE and a Registered Professional Engineer in California and Nevada.

He performs power systems studies including Arc-Flash on all levels and types of power systems.

[email protected]

Fred Toepfer is business development manager for Emerson Process Management Electrical

Reliability Services. He received his BSEE degree from Louisiana State University and MBA from

Vanderbilt University. He has over 25 years of sales and marketing experience in the power industry

related to service and new product/business development. He is currently responsible for the arc flash

solution business of Electrical Reliability Services in the US. [email protected]
mailto:[email protected]:[email protected]:[email protected]:[email protected]


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APPENDIX 1: IEEE 1584 Calculation Method

IEEE Std 1584-2002 contains calculation methods developed through testing by several sources to

determine boundary distances for unprotected personnel and the incident energy at the working distancefor qualified personnel working on energized equipment. The incident energy level can be used to

determine the proper PPE required for personnel.

The equations developed in the IEEE standard assess the arc flash hazard based on the available (bolted)fault current, voltage, clearing time, equipment type, grounding, and working distance. The working

voltage is also used to determine other variables. The equations account for grounding, equipment type,

and construction. This method can also determine the impact of some classes of current limiting low

voltage fuses as well as certain types of low voltage breakers. The calculations can be applied over a

large range of voltages.

The many variables of this method make it the preferred choice for Arc Flash evaluations, but at the same

time requires either a complex spreadsheet or computer program to be used efficiently. The calculations

are summarized as follows:

1. Determine the Arcing Current

For applications under 1000V

)(lg00304.0)(lg5588.0000526.00966.0lg662.0lg bfbfbfa IGIVGVIKI ++++= (1)

For applications 1000V and higher

bfa II lg983.000402.0lg += (2)

Convert from lg

aI

aIlg

10= (3)

where:

lg is the log10

Ia is the arcing fault current (kA)

K is 0.153 for open configurations

Is 0.097 for box configurations

Ibf is the bolted fault current for three-phase faults (symmetrical RMS)(kA)

V is the system voltage

G is the gap between conductors, (mm) (See Table 1)

Calculate a second arc current equal to 85% ofIa, so that a second arc duration can be determined.

2. Determine the Incident Energy

The following equations should be used for both values ofIa determined in the first step.

GIKKE an 0011.0lg081.1lg 21 +++= (4)nE

nElg

10= (5)

=

x

x

nfD

tECE

610

2.0(6)

for locations where the voltage is over 15kV the Lee method is used.


All Rights Reserved


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=

2

51012.5D

tVIxE bf (7)

where:

En is the incident energy (cal/cm2) normalized for time and distance

K1 is 0.792 for open configurations

Is 0.555 for box configurations

K2 is 0 for ungrounded or high resistance grounded systemis 0.113 for grounded systems

G is the gap between conductors, (mm) (See Table 1)

E is the incident energy (cal/cm2)

Cf is a calculation factor

1.0 for voltages above 1kV

1.5 for voltages at or below 1kV

t is the arcing time (seconds)

D is the distance from the possible arc point to the person (mm)

x is the distance exponent from Table 1


V is the system voltage

The arcing time tis the clearing time for the source-side protecting device that clears the fault first.

TABLE 1

Factors for equipment and voltage classes

System Voltage (kV) Equipment Type

Typical gapbetween

conductors

(mm)

Distancex Factor

Open Air 10-40 2.000

Switchgear 32 1.473

MCC and panels 25 1.641

0.208-1

Cable 13 2.000

Open Air 102 2.000Switchgear 13-102 0.973>1-5

Cable 13 2.000

Open Air 13-153 2.000

Switchgear 153 0.973>5-15

Cable 13 2.000

The incident energy is the worst case of the two values calculated (100 or 85% Ia).

3. Determine the Flash Boundary

The flash boundary is the distance from an arcing fault where the incident energy is equal to 1.2 cal/cm2.

For the IEEE Std 1584-2002 empirically derived model

x

B

x

nfBE

tECD

1

610

2.0

= (8)

For the Lee method




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=

B

bfBE

tVIxD 51012.5 (9)

where:

DB is the distance of the boundary from arcing point (mm)

En is the incident energy (cal/cm2) normalized for time and distance

Cf is a calculation factor1.0 for voltages above 1kV

1.5 for voltages at or below 1kV

t is the arcing time (seconds)

EB is the incident energy in cal/cm2 at the boundary distance

x is the distance exponent from Table 1




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