Practical Approach for Arc Flash Hazard Assessment · Practical Approach for Arc Flash Hazard Assessment International Journal on Advanced Electrical and Electronics Engineering,

International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278

Practical Approach for Arc Flash Hazard Assessment

Priyanka K Kolankar & U.

E-mail : [email protected]

Abstract - The exposure to hazards associated with electrical arcing phenomena, while working on energized equipment is a topic of significant interest to industrial plant personnel. This paper provides an overview of the arc flash hazard phenomena.discusses the method of carrying out arc flash assessment and reduction techniques.

Keywords - NFPA 70E, IEEE 1584, Arc flash hazard assessment, Incident energy calculations, curable burn distance

I. INTRODUCTION

Two types of electrical hazards now well recognized are electrical shock and electrical arc/flash burn. Electrical shock requires that a person have contact with the exposed energized electrical conductor or circuit part or be within the air flashover (bredistance of the conductor or circuit part. Electrical arc/flash burn can occur when the exposed body is yet several feet away from the exposed energized electrical conductor or circuit part.

This paper deals with the practical aspects of defining and establishing the arc/flash protection boundary around exposed energized electrical conductors or circuit parts.

II. CAUSES OF ARCS AND EFFECTS OF ARCING AND FLASHING

We all are aware of electricity’s hazards to life, but few of us are aware of the possibility of radiation burn from the fierce fire of electric arcs and flashovers. Electric arc burns account for a substantial portion of the injuries from electrical malfunctions and incidents. When electric current flows between two or more separated energized conducting surfaces, an arc occurs. Some arcs may be intentional such as arc welding or they may be accidental caused by a tool slipping or by touching a test probe to the wrong surface. A common cause of an arc is insulation failure. The fault curmagnetic effects cause conductors to separate producing an arc.

The results of such arcs are often devastating. Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit (F) or four times the

International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue

33


Priyanka K Kolankar & U. B. Sarode

PVG COET, Pune [email protected], [email protected]

exposure to hazards associated with electrical arcing phenomena, while working on energized equipment is a topic of significant interest to industrial plant personnel. This paper provides an overview of the arc flash hazard phenomena.

method of carrying out arc flash assessment and reduction techniques.

NFPA 70E, IEEE 1584, Arc flash hazard assessment, Incident energy calculations, curable burn distance

Two types of electrical hazards now well recognized are electrical shock and electrical arc/flash burn. Electrical shock requires that a person have contact with the exposed energized electrical conductor or circuit part or be within the air flashover (breakdown) distance of the conductor or circuit part. Electrical arc/flash burn can occur when the exposed body is yet several feet away from the exposed energized electrical

This paper deals with the practical aspects of and establishing the arc/flash protection

boundary around exposed energized electrical

CAUSES OF ARCS AND EFFECTS OF

We all are aware of electricity’s hazards to life, but ossibility of radiation burn

from the fierce fire of electric arcs and flashovers. Electric arc burns account for a substantial portion of the injuries from electrical malfunctions and incidents. When electric current flows between two or more

ergized conducting surfaces, an arc occurs. Some arcs may be intentional such as arc welding or they may be accidental caused by a tool slipping or by touching a test probe to the wrong surface. A common cause of an arc is insulation failure. The fault current’s magnetic effects cause conductors to separate producing

The results of such arcs are often devastating. Temperatures at the arc terminals can reach or exceed 35,000 degrees Fahrenheit (F) or four times the

temperature of the sun’s surface. The heat energy and intense light at the point of the arc is termed ARC FLASH. Air surrounding the arc is instantly heated and conductors are vaporized causing a pressure wave termed ARC BLAST. System voltage, arc resistance (impedance) and available shordetermine instantaneous arc energy. Total arc energy (incident energy) is the instantaneous arc energy times the arc duration. Conductive vapors help sustain the arc and the duration of the arc is primarily determined by the time it takes for over current protective devices to open the circuit. Current-limiting fuses for example may open the circuit in 8.3 ms (1/2 cycle) or less while other devices may take 100 ms (6 cycles) or more to open.

Fig. 1 : Effects of Arcing and Flashing

Therefore, the persons working on or near energized electrical equipment or circuit parts should wear personal protective clothing and other protective

1, Issue-1, 2012


exposure to hazards associated with electrical arcing phenomena, while working on energized equipment is a topic of significant interest to industrial plant personnel. This paper provides an overview of the arc flash hazard phenomena. It also

NFPA 70E, IEEE 1584, Arc flash hazard assessment, Incident energy calculations, curable burn distance.

The heat energy and intense light at the point of the arc is termed ARC FLASH. Air surrounding the arc is instantly heated and conductors are vaporized causing a pressure wave termed ARC BLAST. System voltage, arc resistance (impedance) and available short-circuit current determine instantaneous arc energy. Total arc energy (incident energy) is the instantaneous arc energy times the arc duration. Conductive vapors help sustain the arc and the duration of the arc is primarily determined by

s for over current protective devices to limiting fuses for example may

open the circuit in 8.3 ms (1/2 cycle) or less while other devices may take 100 ms (6 cycles) or more to open.

Effects of Arcing and Flashing

efore, the persons working on or near energized electrical equipment or circuit parts should wear personal protective clothing and other protective


International Journal on Advanced Electrical and Electronics Engineering, (IJAEEE), ISSN (Print): 2278-8948, Volume-1, Issue-1, 2012

34

equipment to prevent injuries to the eyes, head, face, body, limbs, and extremities from electrical arcs and flash due to equipment failure, malfunction, or human error.

A. Steps in calculating arc-flash energy

IEEE Std 1584-2002 provides a method of conducting an arc flash hazard analysis. This method uses equations that were developed using statistical analysis of data from arc-flash testing performed at laboratories. This standard includes a spreadsheet calculator that can be used to solve the equations.

A detailed arc-flash hazard analysis using this method requires the following steps.

Step 1) Collect the power system and installation equipment data.

Step 2) Review the system single line for each location and determine the power system’s modes of operation. If a single line does not exist, inspect the system and develop a single-line diagram.

Step 3) Perform an analysis of the maximum and minimum available short-circuit currents for each different mode of operation that applies. Determine the first cycle symmetrical root-mean-square bolted three phase fault current and X/R ratio at all locations where people could be working on energized electrical equipment. Equipment below 240 V need not be considered unless it involves at least one 125-kVA, or larger, low-impedance transformer in its immediate power supply.

Step 4) Determine the arc fault currents. The arc fault current for each location where an arc-flash hazard exists and the portion of that current that flows through the protective device that will clear this fault must be determined.

Log Ia = 0.00402+0.983*Log Ibf (1)

Ia = 10Log Ibf (2)

Where:

Ibf = Maximum bus fault current in kA

Ia = Maximum arcing current in kA

Step 5) From the protective device characteristics, find the arcing duration. The total clearing time of the fault will determine the “time” factor in the energy equation. If power in the arc remains constant, a longer time will produce more energy based on the following:

Energy = Power × Time. (3)

Step 6) Record the system voltages and the equipment classes for each bus. For example, record whether the equipment is switchgear or a motor control center.

Step 7) Determine working distances. Arc-flash protection is based on the incident energy level on the person’s face and chest area at the working distance. The degree of injury in a burn depends on the percentage of a person’s skin that is burned. Burns to the head and/or chest area are much more life threatening than burns on the extremities. The working distance based on the voltage level and equipment type using the table I.

Step 8) Determine incident energy for each work location in the study. This is best done using a software package. The equations in IEEE Std 1584-2002 are available in spreadsheet format. These equations have been incorporated into the packages of power system software vendors.

Step 9) Determine the flash-protection boundary for each work location in the study. Incident energy equations can be solved for the distance from the arc source at which the onset of a second-degree burn could occur. This is called the “flash-protection boundary.” The incident energy for the flash-protection boundary must be set at the minimum energy beyond which a second-degree burn could occur (1.2 cal/cm2).

TABLE I WORKING DISTANCE BASED ON VOLTAGE

LEVEL

Working Distance Equipment Type

KV

24 inches (610mm) Switchgear < 1

18 inches (455mm) Panel < 1

36 inches (910mm) Switchgear > 1 & < 35

72 inches (1829mm) Switchgear > 35

18 inches (455mm) all others

B. How arc-flash energy affects personal protective equipment (PPE)

NFPA 70E defines five levels of arc hazard. Table II shows the hazard/risk category levels and the calculated incident energy at the working distance. The table lists typical clothing and layer counts for the torso. In short, this is the level of clothing that should be worn to limit incident energy damage to a second-degree burn. Put another way, this guide is designed to protect the worker from heat to prevent a second-degree burn.



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TABLE I

PERSONAL PROTECTIVE EQUIPMENT (PPE) TO

LIMIT BURNS.

Hazard Risk

Clothing description (No. of clothing layers is given in parentheses)

Required Minimun Arc rating of PPE(cal/cm2)

0

Non melting,flammable materials(i.e untreated cotton,wool,rayon or silk or blends of these materials)

N/A

1

FR shirt and FR pants or FR coverall(1)

4

2

Cotton underware-conventional short sleeve and brief/shorts,plus FR shirt and FR pants(1 or 2)

8

3

Cotton underware plus FR shirt and FR pants plus FR overall or cotton underware plus 2 FR coveralls(2 or 3)

25

4

Cotton underware plus FR shirt and FR pants plus multilayer flash suit(3 or more)

40

III. EXAMPLE SYSTEM TO ANALYZE ARC FLASH

A. Steps to Calculate Arc Flash on an Example System

The system shown in Fig. 2 is used to help analyze these issues.

Fig. 2 Example system

1) Determine the Bolted Fault Currents

The first step in calculating an arc-flash number is to calculate the maximum available three-phase fault current. The utility may give a number based on fault MVA and an X/R ratio. As shown in (2), the utility has given the available source fault MVA as 583 and the X/R ratio as 15. To convert this to a percent impedance based on the transformer MVA and kV, we use (1):

2

12

*% 100 tan

*

kVu MVAt XZ

RkVt MVAu−

= (1)

Where ,

%Z= Utility impedence in percent based on the utility transformer base

kVu= utility voltage base

kV t= transformer voltage base

MVA u = Utility fault MVA

MVA t = Transformer MVA base.

X/R = Utility X/R ratio

The conversion gives the following result:

21

2

13.8 *10.5% 100 tan 15

13.8 *583Z −

= (2)

= 0.13+ j1.8%

The example shows switchgear and has no cable impedance to add to the total impedance to the bus. We must add the transformer impedance, which is listed as 4.1%. If we assume that the transformer impedance is all inductive, then the total impedance to the bus is:

%Ztotal = 0.13 + j1.8 +j4.1

= 0.13+ j5.9

To calculate the fault current we use Eq. (3)

* 57735%

* %fto ta l

M V A tI

kV t Z

= (3)

Where,

If = maximum bus fault current

kV t = transformer voltage base

MVA t = transformer MVA base

% Z total = total impedance on transformer base to bus in percent.

The fault current on this example is as follows:

If = 24.7kA



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2) Determine the Arc-Fault Currents

After calculating the maximum three-phase fault current, we calculate arcing current. The arc-fault current is typically lower than the bolted-fault current due to the arc impedance. In this example, the arcing fault current is 23.6 kA. Equation (4) is used to calculate the arcing current:

LogIa = 0.00402 + 0.983* LogIbf (4)

Ia = 10 log Ia

Where

Ibf = maximum bus fault current in kA

Ia = maximum arcing current in kA

The arcing current in this example is as follows:

Log Ia = 0.00402+0.983* Log (24.7) = 1.373

Ia = 101.373 = 23.6kA

We also want 85% of this value to see how the lower fault current impacts trip times (which may in fact increase energy). The 85% value is 20 kA.

3) Determine the Protective Relay/Device Operate Times

The breaker time of five cycles was added to obtain the total trip time. For the 23.6 kA current, the bus relay trip time is:

0.69 + 5/60 = 0.77 s

For the 20.0 kA current, the bus relay trip time is:

0.88 + 5/60 = 0.96 s

4) Document the system voltages, equipment class and working distances

IEEE 1584-2002 includes tables that provide typical bus gaps and working distances for 15 kV, 5 kV, and low-voltage switchgear, low-voltage motor control centres, panel boards, and cable. Also included are spreadsheets, which perform calculations based on selected parameters. For 5 kV switchgear, the gap between conductors is assumed to be 102 mm and the working distance is assumed to be 910 mm. Other factors, like the configuration of the switchgear, cable, or box, and the system grounding, are taken into account.

5) Determine the Incident Energy

The empirically-derived model presented in IEEE 1584 provides two equations to calculate the incident arc-flash energy. The first is the normalized incident energy. The second is the incident energy with specific parameters. The normalized incident energy assumes a

“typical working distance” of 610 mm and an arc duration of 0.2 s. The equation for this example is:

Log En = K1+K2+1.081* LogIa +0.0011* G (5)

En = 10En

Where,

En = normalized incident energy in J/Cm2

K1 = -0.555 for a box configuration

K2 = 0.0 for a resistance grounded system

G = gap between conductors = 102 mm

Thus the normalized incident energy for the 23.6kA arc current in this example is as follows:

Log En = -0.0555+1.081*Log (23.6) +0.0011*102 = 1.041

En = 101.0413 = 11 J/cm2

The normalized incident energy for the 20 kA arc current in this example is as follows:

Log En = -0.0555+1.081* Log (20) +0.0011*102 = 0.9636

En = 100.9636 = 19.2 J/cm2

Next we vary the parameters to calculate incident energy for our specific example system. For 5kV switchgear, we use a working distance of 910mm and then we calculate incident energy for the operating time of 0.77s.

610 4.183 * * * *

0.2

X

f X

tE C En

D

= (6)

Where,

E = Incident energy in J/cm2

En = Normalised incident energy in J/cm2

Cf = 1.0 for voltages above 1.0kV

t = Arcing time in seconds

D = Distance from the possible arc point = 910mm

X = Distance component = 0.973 for 5kV switchgear.

For this system, the incident energy is:

0.973

0.973

0.77 610 4.183 *1*11* *

0.2 910E

=

= 120J/cm2 @ 23.6kA

0 .9 7 3

0 .9 7 3

0 .9 6 6 1 0 4 .1 8 3 * 1 * 9 .2 * *

0 .2 9 1 0E

=



37

= 125J/cm2 @ 20kA

Note the 85% current actually has more incident energy due to the longer trip time delay from the bus relay.

Next we convert the arc energy into cal/cm2 using the conversion:

5.0J/cm2 = 1.2 cal/cm2

Thus the arc flash energy at the bus is 29 cal/cm2 @ 23.6kA and 30 cal/cm2 @ 20.0kA.

6) Determine the Flash-Protection Boundary

The flash boundary is calculated from (7):

1

6 1 0 4 .1 8 3 * * * *

0 .2

X X

b f nb

tD C E

E

= (7)

Where,

Eb = Incident energy at the boundary in J/cm2 = 5.0 for bare skin

Cf = 1.0 for voltages above 1.0kv

t = arcing time

Db = distance of the boundary from the arcing point in mm

X = distance exponent = 0.973 for 5.0kV switchgear

En = Normalized incident energy in J/cm2

10.973 0.9730.77 610

4.183 *1 *11* *0.2 5bD

=

= 23867mm = 24meters

This indicates that within 24 meters of the arc flash, any unprotected person could sustain second-degree burns from the fault incident energy. From this we also see that a worker must use Level 4 PPE to perform live work on this switchgear.

IV. REDUCTION STRATEGIES FOR ARC FLAS ENERGY

An arc-flash hazard evaluation for an industrial power system will generally uncover locations in the power system where the incident energy levels are high. This is considered to be levels above 40 cal/cm2.

1) Reduce Existing Pickup Settings

In some cases, the arcing fault current magnitude may not be large enough to operate the over current protective device in its short delay or instantaneous trip time. If possible, the short delay or instantaneous pickup should be reduced to allow for more sensitive sensing of the calculated arcing current that flows through the protective device. Changing the sensing element from

long delay to short delay or instantaneous can make a significant difference in incident energy levels. Care must be taken with the choice of the reduced pickup setting to ensure that over current coordination is maintained and nuisance tripping on load energization inrush will not occur.

2) Reduce Existing Time Delay Settings

If a protective device operates in its long time region, the time curve is normally an inverse type that has increased time delay as fault current decreases. Arcing fault magnitude is approximately 50% of the bolted fault current magnitude. Long delay trip time greater than 2 s may occur for arcing faults. It is generally recognized that the reaction time required to move away from an arc-flash hazard is 2 s. Based on the work task, if there is a good possibility that the worker can move away from the arc-flash area, then a maximum clearing time of 2 s may be used for incident energy calculations. When reviewing existing coordination studies, there are usually areas found where coordination between two devices can be eliminated which will allow the time delay setting for the upstream device to be reduced. Two areas where removal of the coordination interval can be considered are between the main and tie protective devices or between primary and secondary protective devices on transformers.

3) Reduce Fault Current

Incident energy can also be reduced by lowering the arcing fault current. The reduction of incident energy will depend on the trip time that results at the reduced current. A protective device with a definite time characteristic is best suited for this incident energy reduction technique. Note that for most inverse time current curve characteristic shapes, reduced arcing currents will result in increased trip time and increased incident energy. Fault levels can be reduced by one

of the following design changes, both of which require a significant amount of cost and engineering effort:

■ Installation of current limiting reactor

■ Use of multiple transformers instead of one large transformer.

4) Remote Racking Device

These devices are presently available for some equipment. Awareness of the arc-flash hazard is accelerating the development of these devices to work with an increasing number of equipment types from various manufacturers. By operating the breakers remotely, you have increased safety. However, this idea is cost prohibitive in low voltage switchboards. To make every breaker electrically operated would add



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approximately $1,000 per breaker to the switchboard plus the addition of a controller station. Remote racking is a great idea for draw out switchgear. However, most switchboards do not have draw out components and thus this must be ruled out.

5) Insulated Bus Bars

Insulating the bus bars is a positive addition to any design. It adds protection from inadvertent live bus bar contact. It protects the qualified electrician from the unqualified and startled rodent, snake, bird or other animal that makes phase to phase contact inside the gear. Also it can protect the electrician from a flash due to metal parts left loose inside the gear or from a falling metal tool.

6) Maintain Adequate Working Clearances

The safest thing to do is not to get near the gear. If you are not a qualified electrician, stay away from exposed live parts. Obviously this is not practical for the qualified electrician. In many applications it is often not practical. Testing, troubleshooting, diagnostics and a continuous process segment require that power remain on to complete the task and de-energizing would introduce an increased hazard or is infeasible. This is where many accidents occur and the risks and effects of an arc flash are the greatest. Finally, maintaining adequate work clearances does not minimize the hazard.

V. PROFICIENCY & RETRAINING

The employer shall provide training to each employee who is required by this section to use PPE. Each such employee shall be trained to know at least the following: When PPE is necessary; what PPE is necessary; How to properly don, doff, adjust, and wear PPE; The limitations of the PPE; and, The proper care, maintenance, useful life and disposal of the PPE. Distilling the requirements from the various standards yields the following requirements.

1. The arc flash hazard must be assessed

2. Appropriate PPE must be selected for non-prohibited work

3. The results must be documented

4. Personnel must be trained, understand the hazards, and take appropriate action.

5. Analysis should be revaluated if the standards, PPE types, or system configuration changes.

Documenting the results occurs in two forms. The first is the site safety manual. The Safety manual should include the results of the calculations, and required PPE classifications for each location, complete descriptions of the PPE classifications, and procedures associated

with performing energized work. The second is labelling at the locations where energized work is to be performed. In order to meet the requirements of the relevant standards, more than just a warning is necessary. Figures shown below are two examples of labels generated using the arc flash module in a power systems analysis software package.

Fig. 2.1 An arc flash label generated using arc flash module

Fig. 2.2 An arc flash label generated using arc flash module

The 2.1 label indicates a Class 1 protection (using the NPFA 70E example categories) and the 2.2 label indicates prohibited work (incident energy is far above the 40 cal/cm2 limit for energized work).

VI. IMPORTANCE OF ARC FLASH

ASSESSMENT

Without an arc flash study you will not know the actual level of danger or the appropriate personal



39

protective equipment (PPE) required for employees. Electrical systems are dynamic and change over time. Internal changes, such as adding new equipment can affect the level of arc flash energy. A study must be updated every time the system changes. External changes, such as utility changing transformers or changes at your utility’s closest substation can severely impact your level of arc flash energy. Performing this study will help you create a safer work environment.

Benefits of Performing an Arc-Flash Hazard Assessment

1) Provides workers with the best possible PPE.

2) Insurance premiums can be reduced.

3) Brings electrical system documentation up to date by providing current and accurate one-line diagrams.

4) System reliability can be enhanced with a proper protective device coordination study to insure device closest to the fault opens in the least amount of time.

5) Over-dutied equipment can be identified from an accurate system wide short circuit analysis.

6) Since the system is typically modelled on software, it will be easy to make future changes or upgrades with minimal expense or effort.

7) Most importantly, there will be fewer injured worker when an analysis is performed and recommended procedures are followed.

Costs of Not Performing an Arc-Flash Hazard Assessment

Costs due to not performing an arc flash hazard analysis can range from minor costs associated with fines, to millions of dollars for lifelong medical cost and can include any of the following:

1) Cost of non-compliance fines

2) Cost associated with lost productivity

3) Increased equipment repair costs

4) Medical expenses for injured workers

5) Legal costs

6) Most importantly, loss of life, there is no price for this.

VII. CONCLUSION

An arc-flash event occurs whenever a fault occurs. The attention given to the safety of personnel continues to increase. Conducting arc-flash studies allows engineers to determine personal protective equipment

required and flash boundaries. Implementing protection schemes to reduce trip increases safety and could reduce injuries or even save lives. On new and existing systems, consider adding controls to enable instantaneous tripping when personnel are in close proximity to energized equipment. The cost of implementation is small compared to the benefits of reduced trip times and reduced arc-flash hazards.

This paper has described the process of arc flash hazard assessment with an example system, including the calculation of incident energy levels in arc flash faults and selection of appropriate Personal Protective Equipment (PPE) levels as well various strategies for reducing this arc flash phenomena.

ACKNOWLEDGMENT

Special thanks are due Mr.Tushar Borole, Manager and Ms.Supriya Kulkarni ,Design Engineer at TridentTechlabs for their assistance in carrying out this study. Also many thanks to the reviewers of the draft copy of this paper for their very detailed and constructive critiques.

REFERENCES

[1] NFPA 70E-2004, Standard for Electrical Safety in the Workplace, National Fire Protection Association, Quincy,MA.

[2] IEEE 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE, New York, NY.

[3] Application of existing technologies to reduce Arc Flash Hazards, Jim Buff and Karl Zimmerman, Schweitzer Engineering Laboratories, Inc.

[4] Title 29 of the Code of Federal Regulations (29 CFR). Part 1910: General Industry; safety standards for electrical systems and safety-related work practices, Occupational Safety and Health Administration (OSHA) Standards, Washington DC

[5] R. Lee, "The Other Electrical Hazard: Electrical Arc Blast Burns,” IEEE Transactions on Industry Applications, vol IA-18, No. 3, May/June 1982, pp. 246-251..

[6] R. Lee, "Pressures Developed from Arcs,” IEEE Transactions on Industry Applications, vol IA-23, No. 4, July/Aug 1987, pp. 760-764.

[7] Arc flash hazard incident energy calculations A historical perspective and comparative study of the standards: IEEE 1584 and NFPA 70E, Copyright Material IEEE, Paper No. PCIC-2007

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Practical Approach for Arc Flash Hazard Assessment · Practical Approach for Arc Flash Hazard Assessment International Journal on Advanced Electrical and Electronics Engineering,