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A Practical Approach to Arc Flash Hazard Analysis and ReductionWhite Paper
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
By:
H. Wallace Tinsley III
Member, IEEEEaton Electrical130 Commonwealth DriveWarrendale, PA 15086
Michael Hodder
Member, IEEE
Eaton Electrical4120B Sladeview CresMississauga, ON L5L 5Z3
Presented at the 2004 IEEE IAS Pulp and Paper Industry Conference in Victoria, BC: IEEE 2004 - Personal use of this material is permitted.
TABLE OF CONTENTS
Abstract ....................................................................
I. Introduction ..........................................................A. Standards
B. Arc Flash Analysis
II. Generalization of Arc Flash ..............................A. Fault Magnitudes
B. Constant Energy
C. Overcurrent Device Responses
III. Time, Current, & Energy Relationship ...........A. Relationship Equations
B. Sotware Application
IV. System Models & Analysis ..............................A. Data Collection
B. Unbalanced Faults
C. Analysis Philosophy
V. Considerations & Solutions ...............................
VI. Conclusions ........................................................
Acknowledgement .................................................
References ...............................................................
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
ABSTRACT
Recent eorts to quantiy the dangersassociated with potential arc fash hazardsrely on overcurrent protection to remove agiven ault condition. The eectiveness ovarious devices is determined by a clearingtime related to the maximum availableault current or each system location. Asindustrial and commercial acilities begin to
embrace arc fash labeling procedures andbegin to recognize arc fash prevention asa part o a complete saety program, thecurrent method o calculation will allow themto quantiy the incident energy (cal/cm2)associated with a maximum, three-phaseault condition. Most aults producecurrent magnitudes less than the three-phase maximum. This paper will consider
ault current magnitudes less than thato the maximum, threephase condition
and discuss the resulting calculations orincident energy across the range o cur-rent magnitudes. Under these additionalscenarios, the perormance o variousovercurrent protection devices will bedemonstrated. Associated considerations
or design, modeling, and maintenancewill be presented.
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
Index Terms Constant Energy C-Line, Arc Flash Hazard,Unbalanced Faults, Worst-Case Scena
I. INTRODUCTION
Extensive research and experimentation have led to
the development o detailed calculation methods or
determining the magnitude and intensity o Arc Flash
Hazards. These methods have been presented in IEEEStandard 1584 2002 and the NFPA 70E 2000
Edition. Recommended practice now requires that
the incident energy due to an Arc Flash be quantifed
at each system location potentially accessed by
authorized personnel while the equipment is
energized.
This requirement suggests the need or a thoughtul
understanding o the power system and the meth-
ods o calculation. This paper provides a ramework
o considerations on which to base the methods o
calculation presented in the most current standards.
These considerations include worstcase scenarios,
data collection or analysis, design concerns, and
maintenance.
A. Standards
NFPA 70E-2000 Edition, Table 3-3.9.1 requires acil-
ity personnel to wear Personal Protective Equipment
(PPE) when perorming various tasks in locations
susceptible to potential Arc Flash Hazards.[1] These
requirements are mandated on the basis o feld
experience and are categorized by associated voltage
levels. The Hazard/Risk Category is determined by the
nature o the work to be completed, the operating
voltage, and the available short circuit current or thatgeneral location in the electrical distribution system.
The Hazard/Risk Category reers to the appropriate
protective clothing and personal protective equip-
ment (PPE) to be utilized.
In 2002, the IEEE reported the results o extensive
laboratory experiments and calculations. IEEE
Standard 1584 2002 describes the procedures
and provides direction or an accurate means o
determining a sae Arc Flash Boundary and
associated Hazard Level.
The basis or this method is experimental data
recorded rom simulated arcs corresponding to bolt-
ed, three-phase ault current magnitudes measured
at the terminals o an experimental enclosure.
B. Arc Flash Analysis
The Arc Flash analysis requires the completion o a
Short Circuit Study and a Coordination Study. Theresults o the Arc Flash calculations are based on the
calculated values o ault current magnitudes ound
in the short circuit study and the associated
clearing times o overcurrent protection devices
as determined by the coordination study.
The goal o this type o analysis is to determine the
incident energy potentially present during an arc
ash event. The magnitude o the incident energy is
calculated on the basis o the available ault current,
the clearing time o associated system protection,
and the physical parameters o the system location.
Associated with this calculation is the determination
o an approach distance within which the incidentenergy level is above 1.2 cal/cm2. Appropriate Per-
sonal Protection Equipment (PPE) shall be used when
working on or near energized equipment within the
ash protection boundary.[1]
The results o the approach boundary and incident
energy calculations may be displayed in labels
on equipment enclosures to inorm and direct
acility personnel with respect to the potential arc
ash hazard.
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
II. GENERALIZATION OF ARC FLASH
For proper evaluation o a power system with respect
to potential Arc Flash hazards, accurate generalization
o these hazards is imperative to describe the
worst-case scenario. To understand the worst-case
conditions, one must relate potential ault magni-
tudes to the clearing time associated with various
overcurrent devices.
Figures 1 & 2
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
A. Fault Magnitudes
IEEE Standard 1584 2002 cautions, it is important
to determine the available shortcircuit current or
modes o operation that provide both the maximum
and the minimum available short-circuit currents.[2]
The importance o this statement is demonstrated
when an o-peak maintenance scenario is compared
with the ull load operating condition. A hazardous
arc ash condition may arise rom various causes,
and oten occurs during maintenance. Maintenance
tasks are oten perormed at times when the acility
and/or its processes are not ully operational.
Although the power system is energized, some o
the contributing motor loads may be shut down.
Thereore, during maintenance operations, when
the propensity or arc ash conditions is high, the
available ault current may be signifcantly lowerthan the calculated maximum.
To demonstrate the eect o various scenarios, we
have modeled a sample system that represents three
acilities that are supplied by a single utility substa-
tion. A portion o this system is shown in Figures 1
and 2. In the Figure 1, all contributing motors are in
service and the utility contribution is at a maximum.
In Figure 2, the low voltage motors have been
reduced to 10% o ull-load and the medium voltage
motors have been removed. The same is true or the
adjacent neighbors and the total ault contribution at
a typical 480V substation is signifcantly decreased.The magnitude o the available bolted ault current
is decreased rom Figure 1 to Figure 2 by approxi-
mately 30%. The arcing current is also reduced by
approximately 30% between the two fgures. For the
calculation o incident energy, we should consider the
range defned by this minimum calculation and this
maximum calculation or any given location.
B. Constant Energy
By the method presented in IEEE Standard 1584
2002, incident energy (E) is calculated or specifc
system locations. This calculated value o energy is
determined by the physical environment at the given
location and the duration o a previously calculated
magnitude o arcing ault current. The duration o the
ault condition is dependent on the clearing time o
the upstream overcurrent protection. This clearing
time is determined by the actual magnitude o
arcing ault current or a given occurrence.
For a given location, there exists a series o potential
arcing ault current magnitudes and theoretical
clearing times or which incident energy remains
constant. Several o these series are shown in Figure
3. On a log-log plot, these combinations o constant
energy points with respect to time and current
appear as a linear line segments. For a typical low-
voltage, grounded, enclosed substation, these
selected lines correspond to the PPE classes outlinedin IEEE Standard 1584 2002. The lowest line shown
in Figure 3 represents a constant energy o 1.2cal/cm2.
This corresponds to the upper limit o PPE Class 0.
The uppermost line represents the maximum value
o 40 cal/cm2 or which PPE Class 4 provides sufcient
protection. Above this line, no PPE class has been
Figure 3
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
C. Overcurrent Device Responses
For the majority o system locations that are
protected by a use, the minimum available arcingault current is the basis o the worst-case calculation
or incident energy. (See Figure 4.)
For a system location protected by a circuit breaker,
the worstcase calculations vary with the regions o
the clearing characteristic. When the considered range
o ault current magnitudes alls completely within
any region o the timecurrent curve (TCC) across
which the time remains constant, the maximum
available ault current will result in the calculation o
the worst-case incident energy. Such regions include
defnite-time relays and defnite-time delay regions o
electronic trip unit characteristics. (See Figure 5.) For
regions o the TCC where the tripping characteristicis inverse or based on the I2t or I4t model, the
lower arcing ault values will correspond to longer
clearing times; resulting in the worst-case scenario.
(See Figure 5.)
Figure 4
Figure 5
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
III. TIME, CURRENT, & ENERGY RELATIONSHIP
A. Relationship Equations
In order to demonstrate the worst-case arc-ash
scenario across a given range o arcing ault currents,constant-energy lines can be plotted on the TCC plot
in conjunction with tripping characteristics o various
devices.
For voltage levels less than 15kV, the IEEE 1584
2002 presents the equation or incident energy as
shown in Equation (1) [2]. The values or the variables
shown in this equation are presented in Table 1. In
equation (1), the units o energy are Joules/cm2.
Equation (2) shows Equation (1) algebraically
rearranged in order to calculate values or time with
respect to a given set o parameters. The conversion
actor between Joules and calories has also been
included so that the units o Energy (E) in Equation (2)are cal/cm2.
Equation (1):
Where:
E is energy inJoules
/cm
2
.C is a calculation actor, equal to1.0 or voltages above 1kV and1.5 or voltages at or below 1kV.
K1 is -0.792 or open confgurations (no enclosure) andis -0.555 or closed confgurations (enclosed).
K2 is 0 or ungrounded and HRG systems andis -0.133 or grounded systems.
Ia the magnitude o the arcing ault current (kA)that may be determined according to IEEE1584 2002, equation (1).
G is the gap between conductors (mm).t is the duration o the arc (seconds).x is the distance exponent.D is the distance rom the arc to the worker (mm).
Equation (2):
Where:
t is the duration o the arc (seconds).E is energy in cal/cm
2.
C is a calculation actor, equal to1.0 or voltages above 1kV and1.5 or voltages at or below 1kV.
K1 is -0.792 or open confgurations (no enclosure) andis -0.555 or closed confgurations (enclosed).
K2 is 0 or ungrounded and HRG systems andis -0.133 or grounded systems.
Ia the magnitude o the arcing ault current (kA)that may be determined according to IEEE1584 2002, equation (1).
G is the gap between conductors (mm).x is the distance exponent.D is the distance rom the arc to the worker (mm).
Equation (3) shows the linear relationship between
time and arcing current with respect to a given
energy and specifc system parameters shown in
Table 1. With the aid o curveftting sotware [3], this
relationship was ound consistent or all systemconfgurations considered.
Equation (3):
Where:
t is time in seconds.k is a unique constant based on specifc system
parameters (See Table 1 or a summary and thediscussion below or details.)
Ia is the magnitude o arcing ault current.b is a constant value = -1.081.
E=(4.184)C10[K1 + K2 + 1.081 log [Ia]+ 0.0011G]
[t/0.20][610x
/Dx]
E (0.20) 4.1667 ____________________________________t =
(4.184)C10[K1 + K2 + 1.081 log [Ia]+ 0.0011G]
[610x
/Dx]
t =k (Ia)b
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
Constant k is determined or each system location
according to system parameters and a distance actor
related to the equipment type and the system location
voltage.
This determination was made according to the
ollowing steps:
1. For each system location considered, a fnite
series o time-current ordered pairs (Ia, t) was
ound, or which incident energy remains
constant. (See Equation (1) and Figure 3)
2. This series o ordered pairs (Ia, t) was
provided as input or the curve-ftting
sotware [3].
3. The resulting time versus current plot was
consistently ftted with a curve o the ormshown in Equation (3). The constant b (-1.081)
remained constant regardless o the system
parameters. The constant k was ound to be
unique or each new set o parameters.
The system parameters are shown in Table 1 and
include: system voltage, equipment type, bus gap
(mm), working distance (mm), enclosure confgura-
tion, and grounding. For some typical systemlocations, Table 1 shows the resulting values or
the unique constant k.
With a point defned on a TCC plot by the magnitude
or arcing ault current and the associated clearing
time or a specifc device; it is useul to defne a
corresponding line that represents all combinations
o time and arcing current or which energy remains
constant with respect to the given point. This line on
the TCC plot is called a C-line, and the points (Ia, t)
along this line o constant energy can be defned by
the constant C in Equation (4).
Table 1
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
Equation (4):
For a given system location (defned by k), C is a
unique constant describing the relationship o a fnite
series o time and current combinations or which
energy remains constant. For increasing energy, C
is also increasing. Using this relationship, any two
regions on a TCC can be compared to determine the
worst-case scenario.
Consider the clearing time or both the maximum and
minimum ault conditions and let the ordered pair,
(Ia1, t1), represent the maximum arcing ault current
and the associated clearing time. Let the ordered pair,
(Ia2, t2), represent the minimum arcing ault current
and the associated clearing time. Compare as ollows:
I C1 > C2, then E1 > E2 and vice versa. The larger value
or C will correspond to the energy (E) greater value.
Using the relationship one can quickly determine
the worstcase condition between any number o sce-
narios (time and arcing current) at a given location.
B. Sotware Application
On a standard time-current curve (TCC), sotware
packages could use a location-specifcC-lineto
provide a visual representation or the severity o
several incident energy calculations within the range
o possible arcing ault conditions at a given location.
Provided with the values shown or k in Table 1, a
C-linecan be generated or each device with respect
to the bus location immediately downstream or withrespect to a selected bus downstream o several
devices. To aid in overcurrent device coordination, the
unique C-linewill visually demonstrate which setting
regions might be adjusted to reduce the arc ash haz-
ard. Figure 6 shows the tripping characteristics o two
devices. The electronic-trip circuit breaker is shown as
the 480V main breaker o a typical unit substation. The
use characteristic is representative o the primary
device on the 13.8kV side o the source transormer.
Ater determining maximum and minimum
magnitudes o the available bolted ault currents
at the substation bus, the corresponding arcing ault
magnitudes can be calculated. These arcing ault
current magnitudes are calculated according to
IEEE Standard 1584 2002 using specifc system
parameters. Given these parameters, the appropriate
value or k may be selected rom Table 1 or alterna-tively, k can be calculated or system parameters not
ound in the table.
In Figure 6, the maximum arcing ault current magni-
tude (Ia1) o 15.7kA is shown. For an arc ash event at
the substation bus, the associated clearing time o the
main breaker will be used to determine the incident
energy or this ordered pair (Ia1, t1). A clearing time o
0.323 seconds is shown or t1.
t
________C =k (Ia)
-1.081
t1
Set C1
=
/k (Ia1)-1.081
t2
and C2
= /k (Ia2)-1.081
Figure 6
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
Similarly, or an arc ash event on the line side o
the main breaker or the secondary terminals o the
substation transormer, the incident energy is deter-
mined by the clearing time o the primary use. Thispoint on the plot or the maximum arcing current and
associated clearing time is shown as (15.7kA, 4.1s).
Figure 6 also shows the minimum magnitude o the
available arcing ault current at the substation bus.
This minimum value o available ault current
(Ia2 = 9.1kA) relates to the systemoperating scenario
when motor contributions are the lowest. The time
required to clear the potential arc ash event rom
the substation bus is 2.7 seconds, and is shown in
the long-delay region o the circuit breaker trip unit.
Likewise, or an event on the line side o the substa-
tion main breaker or the secondary terminals o thetransormer, the time required or the primary use
to clear the ault is ound to be 90 seconds.
Using the points that correspond to the maximum
value o arcing ault current, a uniqueC-lineis drawn
or each protective device characteristic in Figure 7.
From Equation (4), the C-lineor each device is deter-mined with the value o k selected rom Table 1 and
the time-current pairs associated with the maximum
available arcing ault current.
By visual inspection o Figure 7, it is shown that, or
both protective devices, the greatest incident energy
is present under the minimum ault condition. This is
evident because the point on the tripping characteris-
tic o each device that is associated with the minimum
arcing ault magnitude is shown above theC-line
that passes through the similar point associated with
the maximum available ault current. Each time-cur-
rent point on a TCC that is above a givenC-linehasa corresponding value or incident energy (E) that is
greater than the value o incident energy (E) associ-
ated with all points shown on or below thisC-line.
Figure 7 Figure 8
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A Practical Approach to Arc Flash Hazard Analysis and Reduction
The plotting o the C-Line line may be automated
within a sotware package, but can be quickly plotted,
by hand. For the main substation breaker in this par-
ticular scenario, the ollowing steps are demonstratedin Figure 8:
1. For a typical, solidly grounded, low voltage
switchgear location, select the value or k
rom Table 1: 0.6841.
The tripping characteristic o the main breaker is
plotted in Figure 8. The breakers maximum clearing
time at the maximum arcing ault current o 15.7kA is
shown to be 0.323 seconds.
2. Select (Ia1, t1) to correspond to the maximum
arcing ault current and the associated
clearing time: (15.7kA, 0.323).
3. Calculate C rom Equation (4):
Remember,Cisonlyanenergyspecifcconstant.
4. Select Ia2: 9.1kA
Thisvaluecanbeanyothercurrentvalueontheplot,but itisconvenienttousethecalculatedminimumvalue.
5. Calculate t2 using Equation (4):
6. Connect the two points with a line segment.
ThisistheC-Lineassociatedwiththesubstation mainbreaker.
Using the C-Line in Figure 8, one can be visually
observe that the minimum arcing ault condition has
a higher incident energy that the maximum arcing
condition. Following the determination o the worst-
case scenario, system changes may be recommended
or specifed to reduce the incident energy potentially
present at the substation bus.
Figure 9 shows a change in settings or the main
breaker and a change in use type or the primary
device. In both cases, the original C-lines are still
shown or comparison. For the electronic-trip circuit
breaker, the maximum magnitude or arcing ault
current now corresponds to with the worst-case
scenario. For the primary use, the minimum magni-
tude or arcing ault current remains the worst-case
t ________C=k (Ia)
-1.081
0.323 ________________C=0.6841 (15.7 103)
-1.081
16.213103
C=