Download A Time-Current Curve Approach to Arc

A Time-Current Curve
Approach to Arc-Flash Hazard Analysis
Jeanne M. Avendt
Distribution System Engineer
UNITED SERVICES GROUP
17845 East Highway 10
Elk River, MN 55330
763.241.5793
July 9, 2008
DEFINITIONS
Some of the following definitions were obtained from the IEEE Guide for Performing
Arc-Flash Hazard Calculations – Standard 1584 - 20021
Arc-flash hazard: A dangerous condition associated with the release of energy caused
by an electric arc.
Arcing fault current: The fault current flowing through an electrical arc plasma, also
called fault current and arc current.
Available fault current: The electrical current at a given location that can be provided
by the available source considering the impedance, or resistance, of the current path.
Bolted fault current: A short circuit or electrical contact between two conductors at
different potentials in which the impedance or resistance between the conductors is
essentially zero.
Circuit: A conductor or system of conductor through which an electric current is
intended to flow.
Electrical hazard: A dangerous condition in which inadvertent or unintentional contact
or equipment failure can result in shock, arc-flash burn, thermal burn, or blast.
Electrical shock:
through the body.
Physical stimulation that occurs when electrical current passes
Energized: Electrically connected to or having a source of voltage.
Exposed (live parts): Capable of being inadvertently touched or approached nearer
than a safe distance by a person. It is applied to parts that are not suitably guarded,
isolated, or insulated.
Fault current: A current that flows from one conductor to ground or to another
conductor due to an abnormal connection (including an arc) between the two.
Flash hazard analysis: A method to determine the risk of personal injury as a result of
exposure to incident energy from an electrical arc-flash.
Flash protection boundary2: An approach limit at a distance from live parts that are
uninsulated or exposed within which a person could receive a second degree burn.
Also called an arc-flash protection boundary.
1
Institute of Electrical and Electronics Engineers, Inc., IEEE Guide for Performing Arc-Flash Hazard
Calculations – IEEE Std. 1584™ – 2002, (New York: Institute of Electrical and Electronics Engineers,
Inc., 2002).
2
NFPA 70E-2000 additionally defines a “limited approach boundary”, a “prohibited approach boundary”,
and a “restricted approach boundary”.
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Incident energy: The amount of energy impressed on a surface at a certain distance
from the source generated during an electrical arc event. Incident energy is measured
in joules per centimeter squared (J/cm2) or calories per centimeter squared (cal/cm2).1
Working distance: The distance between the possible arc point and the head and
body (torso) of the worker positioned in a place to perform the assigned task.
1
To convert from cal/cm2 to J/cm2, multiply cal/cm2 by 4.184 (See IEEE 1584 – 2002 Annex E for further
information.
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ABSTRACT:
The 2007 Edition of the National Electric Safety Code, Section 41.410.A.3 – page 246,
has regulated that employers conduct an arc-flash hazard assessment prior to
January 1, 2009 to ensure adequate protection of employees from exposure to electrical
arcs when working near energized equipment. This paper details a method of arc-flash
hazard analysis using the IEEE Standard 1584-2002, IEEE Guide for Performing ArcFlash Hazard Calculations, to create a time-current curve (TCC) reflecting the arc-flash
hazard of a device or work location at various fault currents.
INTRODUCTION:
Arc-flash hazard analysis is important in determining the personal protective equipment
(PPE) required to keep personnel safe when working with energized equipment.
Contact with energized equipment is a commonly known risk; however, exposure to
incident energy from an electrical arc is sometimes overlooked. Whether the electrical
arc is a result of equipment failure, equipment operation or switching, or unintentional
contact, the resultant incident energy can result in serious and life-threatening thirddegree burns and/or exposure to molten metal and other projectiles. With adequate
PPE and knowledge of the hazard category at a particular work location, the likelihood
of a serious injury is reduced.
As stated in 2007 Edition of the National Electric Safety Code, Section 41.410.A.3 –
page 246:
“Effective as of January 1, 2009, the employer shall ensure that an
assessment is performed to determine potential exposure to an electric
arc for employees who work on or near energized parts or equipment. If
the assessment determines a potential employee exposure greater than
2 cal/cm2 exists…, the employer shall require employees to wear clothing
or a clothing system that has an effective arc rating not less than the
anticipated level of arc energy.”1
There is much discussion regarding how thorough an arc-flash hazard assessment
must be. A complete examination of the system would require assessment at each and
every possible work location, a task that is unrealistic to complete. Even if this task was
undertaken, some of the accepted analysis methods pose some concerns as to whether
the assessment considers the ‘most likely’ fault scenarios.
This paper will first discuss the fundamentals of arc-flash hazard analysis using the
IEEE Std. 1584-2002 methodology of arc-flash analysis. It will then go on to discuss the
results of the arc-flash analysis provided by IEEE Std. 1584-2002. Finally, it will discuss
a TCC-based methodology that provides further arc-flash hazard risk assessment for
varying scenarios. The TCC-based methodology provides the information to deduce
appropriate PPE at any work location given basic knowledge of available fault currents
for the given system.
1
Institute of Electrical and Electronics Engineers, Inc., Nation Electric Safety Code® C2-2007, (New York:
Institute of Electrical and Electronics Engineers, Inc., 2006), pg 246.
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ARC-FLASH FUNDAMENTALS:
This section will cover the fundamentals of arc-flash hazard analysis. The methodology
used in this section is that of IEEE Std. 1584-2002, IEEE Guide for Performing ArcFlash Hazard Calculations. Please reference the IEEE Standard 1584-2002, IEEE
Guide for Performing Arc-Flash Hazard Calculations for detailed equations and
calculation parameters.
First and foremost, when considering arc-flash hazard there are four (4) primary factors
that determine the hazard category:
1. System voltage.
2. Bolted fault current – calculated at the location/equipment to be assessed and
subsequently used to calculate the theoretical arcing fault current.
3. Working distance – as measured from the personnel’s head/torso to the location
of the arc source.
4. Fault clearing time.
The IEEE Std. 1584-2002, IEEE Guide for Performing Arc-Flash Hazard Calculations
provides a spreadsheet to ease the assessment of the arc-flash hazard. While detailed
equations for the various calculations are provided in Std. 1584, the spreadsheet
ensures higher confidence that, provided the correct values are entered, the results are
accurate.
It must be said that, aside from the four factors listed above, there are other factors
determining the arc-flash hazard. For example, the gap between the conductors at the
location in which the arc occurs is a factor in the both the arcing current calculation and
the resultant incident energy calculation. The spreadsheet provides a typical gap based
on the equipment type and system voltage. More specific conductor gap values could
be substituted, if desired; however, the conductor gap could vary based on location of
the arc-flash event. Due to the unpredictable nature of fault events, a typical / average
value is suitable for calculations. In the instance of a fault due to equipment failure or
operation, the conductor gap is somewhat predictable because of the design of the
physical equipment. In the case of inadvertent contact being the source of the fault, the
gap in which the arc occurs can vary greatly.
Of the four primary factors that determine the arc-flash hazard category, two (2) have a
larger impact than the others: working distance and fault clearing time. The working
distance is defined as the distance from the personnel’s head/torso to the location of the
arc source and does not consider hands, arms, or legs. While incident energy exposure
to one’s hands is something to be avoided, injury to appendages is not considered lifethreatening. Incident energy quickly decays as distance from the fault source
increases; therefore, the hazard is reduced with distance. An arc-flash boundary
distance should be provided in any arc-flash hazard analysis. An arc-flash boundary is
the distance from the arc source in which the incident energy is less than 1.2 cal/cm2,
the energy needed to obtain a second degree burn.
In the author’s opinion, fault clearing time plays the largest role in the arc-flash hazard
category. The longer one is exposed to fault incident energy, the greater the degree of
injury. Due to this, low fault currents are often more dangerous than high fault currents.
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A time-current curve (TCC) is frequently used to show the relationship between current
(amps) and response time (seconds). Most protective devices have an inverse
characteristic: as current increase, time decreases. Figure 1 illustrates the TCC for an
80T fuse element. Note that the TCC is displayed using a non-linear logarithmic (log)
scale.
Figure 1
80T Fuse Time-Current Curve
Clearing times can be extrapolated from the TCC by first selecting a current at which
the response time is desired. For this example, we will use 1000 A. The vertical line at
the 1000 A mark is followed up until it intersects the device response curve. Fuse
elements typically have a minimum melt and total clear time associated with the device
response. The minimum melt time is the time characteristic in which the fuse element
first begins to melt due to the fault energy. The total clear time is the time in which the
manufacturer anticipates the fuse element will completely melt thus removing the fault.
In the case of arc-flash analysis, the author uses the total clear time as it is the slower of
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the two characteristics. As previously stated, the longer the clearing time, the greater
the arc-flash hazard. At the point where the 1000 A vertical line intersects the 80T total
clear curve, a line is drawn horizontally to the time axis. The point at which the
horizontal line intersects the time axis is the time in which a 1000 A fault will be cleared,
in this case, approximately 0.7 seconds. It is important to note that the ‘current’ axis
(x-axis) can be displayed in amps or multiple of the device pickup. Additionally, the
‘time’ axis (y-axis) can be displayed in seconds or cycles. Some TCC's contain a line
between the first and second multiples of 10, such as the one in Figure 1. For example,
a line indicating 1500 A exists between the 1000 A and 2000 A lines.
In evaluating the arc-flash hazard category, the first step is to calculate the bolted fault
current at the evaluation point. This can be done by hand or with the aid of modeling
software. In the case of substation transformers, it is usually relevant to look at the
available source data. Contrary, when evaluating distribution transformers, an infinite
bus calculation can be used since, for most systems, a value very close to the infinite
bus fault is readily obtainable given the source. Once bolted fault currents are
calculated, the arcing fault current can be calculated either by hand or with the aid of the
IEEE 1584 spreadsheet. The arcing fault current value will be lower than the bolted
fault current value due to the impedance of the arc, and will vary according to the
system voltage. Refer to the IEEE Standard 1584-2002, IEEE Guide for Performing
Arc-Flash Hazard Calculations, section 5.2 for arcing current equations. For some
voltage classes, a reduced arcing fault current will be calculated in addition to the arcing
fault current.
In calculating the arcing fault current, equipment class, system voltage and grounding
type must be considered. In using the IEEE 1584 spreadsheet, the equipment class
and system voltage are considered when determining the conductor gap, which is a
variable in the calculation of the arcing fault current.
Once the arcing fault current has been determined, and the reduced arcing fault current
if required, the protective device trip time can be input. As previously stated, the fault
clearing time has the largest impact on what the hazard category will be. The protective
device that will clear a fault at the analysis location must be determined. In the example
of a distribution transformer, this is the transformer’s highside (primary) fuse. Using the
protective device’s time-current curve, the protective device’s clear time must be input
into the incident energy formula or IEEE 1584 spreadsheet. It is the author’s
recommendation to use the total clear time as a worse case (slowest) scenario when a
fuse is the upstream protective device. The trip time lookup should be done for the
reduced arcing fault current, if required. In addition to the trip time, an opening time is
required. For various equipment classes the opening time may vary therefore the
manufacturer's literature should be referenced. In the case of a fuse element in which
the total clear time (as opposed to the minimum melt time) is used, an opening time of a
cycle will suffice. Refer to the IEEE Standard 1584-2002, IEEE Guide for Performing
Arc-Flash Hazard Calculations, Table 1 for IEEE opening time recommendations.
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Working distance is another important factor in assessing the arc-flash hazard. As
previously stated, incident energy quickly decays as distance from the arc source
increases. Typical working distances include work done with one’s hands: 12 – 15
inches (304.8 – 381 mm), and work done with a hotstick or shotgun stick: 7 – 8 feet
(2133.6 – 2438.4 mm). It is recommended that arc-flash analysis be performed for all
possible working distances.
Table 1, below, illustrates the effect that each variable has on the arc-flash hazard
category, provided that all other variables remain the same.
Table 1
Arc-flash Hazard Variable Relationships
Provided all other parameters remain the same:
As x increases:
x = Conductor gap
x = Working distance
x = System voltage
x = Clearing time
Hazard category:
Increases
Decreases
Increases
Neither – results in an increase in
arcing fault current
Neither – typically results in an
increase in clearing time
Increases
Equipment class changes effect:
Grounding changes effect:
Arc-flash boundary
Arc-flash boundary
x = Bolted fault current
x = Arcing fault current
ARC-FLASH ASSESSMENT RESULTS:
Following the input of required parameters, the IEEE 1584 spreadsheet calculates
various values that are helpful in assessing the arc-flash hazard. Foremost is the
assigned hazard category for the working distance that was input into the spreadsheet.
(Note: the IEEE 1584 specifies the hazard ONLY at the input working distance. If the
actual working distance is less than the input value, the actual hazard category will be
greater than calculated). Hazard categories are defined by exposure to incident energy
in cal/cm2, which is available as a reference parameter within the spreadsheet. Table 2,
below, illustrates the various categories and incident energy ranges corresponding to
each category.
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Table 2
Hazard Category as a Function of Incident Energy
Arc-flash Hazard Category
Minimum cal/cm2
0
1.2001
4.001
8.001
25.001
40.001
Maximum cal/cm2
1.2
4
8
25
40
No maximum
Risk Category
0
1
2
3
4
X
In addition to a hazard category assessment, the IEEE 1584 spreadsheet calculates a
flash protection boundary, or the distance in which less than 1.2 cal/cm2 is impressed on
the head/torso of personnel in the event of an arc-flash. If outside of the flash protection
boundary, personnel should not receive injuries greater than a second degree burn.
The flash protection boundary is independent of the working distance that was input into
the spreadsheet.
While the IEEE 1584 spreadsheet provides assessment that satisfies the requirements
of the NESC, it is the author’s opinion that the arc-flash hazard can vary greatly at any
given work location due to the multitude of variables that exist. Possible fault values at
a single work location are only truly limited by the maximum available fault current at
that location. The magnitude of the fault can be influenced greatly by a number of
factors, including but no limited to, the nature/cause of the fault, the arcing resistance
and even the atmospheric humidity. As previously stated, it is the lower magnitude
faults that pose a greater risk, due to increased clearing time; however, in the author’s
experience, the majority of faults are within 70% of the maximum available fault current
at a given location. It is a tough decision to make, but while worst case scenario events
should be examined, it is the events of higher probability that should be planned for.
In addition to the challenges presented by the range of potential fault magnitudes, is the
issue that results from the millions of possible work locations. It would be time
consuming and labor intensive to assess the arc-flash hazard at all possible work
locations. The author’s preferred method is to perform assessments at some fixed
points, but then use a TCC based approach to deal with the unassessed areas. It is
advised that arc-flash hazard assessment be performed for the following locations at
multiple working distances:
• Faults within the substation cleared by the transformer highside, lowside and/or
other applicable protective device.
• Faults cleared by the feeder protective device upon leaving the substation.
• Faults downstream or within padmounted switchgear or other underground or
overhead switching points that are routinely operated.
• Lowside distribution transformer faults cleared by the distribution transformer
highside protective device.
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Figure 2 provides a simplified one-line diagram that highlights the points that are
typically assessed using the IEEE 1584 spreadsheet.
Figure 2
Simplified One-Line w/ Arc Flash
Analysis Points
Other points on the system, such as tap fuses, can be addressed using the TCC based
approach to arc-flash assessment which is later discussed.
Table 3 demonstrates typical results obtained using the IEEE 1584 spreadsheet. Some
cells have been removed from the table to simplify results. In the case of the
distribution transformers, voltage and current parameters are with respect to the
transformer lowside; however, the arc clearing time is determined using the
characteristics of the highside fuse, which would clear the lowside fault. Also note the
units in which parameters are expressed.
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Table 3
IEEE 1584 Spreadsheet Example
for Figure 2 One-Line
TIME-CURRENT CURVE ARC-FLASH HAZARD ASSESSMENT:
A time-current curve based approach to arc-flash assessment entails superimposing the
hazard category at various current magnitudes upon the TCC for a specific protective
device. The result is a color-coded TCC in which the hazard category can be
extrapolated for any available fault current. All other variables affecting the arc-flash
hazard category must remain the same, such as working distance, arc gap, system
voltage, etc.
This approach allows quick assessment for any work location given
knowledge of the protective device upstream of the work location. It is the protective
device upstream of the work location that will clear any fault in the immediate vicinity of
personnel performing work.
Figure 3 and 4 provide examples of an arc-flash hazard TCC. Note the specified
working distance and system voltage. While not documented, all other variables also
remained the same.
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Figure 3
80T Fuse Arc-Flash Hazard TCC at 15”
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Figure 4
80T Fuse Arc-Flash Hazard TCC at 8’
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INTERPRETING AN ARC-FLASH TCC:
One can extrapolate the arc-flash hazard category from an arc-flash TCC by simply
referencing the color coded hazard at the desired current. In an example of line work
being performed on a fused tap, the personnel must only know the protective device
upstream of the work location and approximate fault currents to determine the
approximate arc-flash hazard and appropriate PPE. In the case of the 80T fuse shown
in Figure 3 and 4, and assuming fault currents greater than 1000 A, it can be seen that
the arc-flash hazard at 15” is a category 1 and a category 0 at 8’.
Discussion often arises regarding what can be considered a reasonable minimum fault
value when using an arc-flash TCC. While low magnitude faults can occur, in most
cases, the fault current quickly escalates as arcing further reduces the fault impedance.
The worst case scenario is always a low magnitude fault near the device pickup, but this
is not the most likely scenario. Fault magnitudes can vary greatly, but a good place to
start when considering the most likely fault scenario would be approximately 6 – 10
times the device pickup. This range is determined using field experience and
assumptions regarding the power system protection.
CONCLUSION:
While the hazard created by arc-flash events is nothing new, NESC regulations have
prompted those with personnel exposed to electrical systems to further investigate and
plan for the associated hazard. The vast number of unpredictable variables makes arcflash hazard assessment some what of an imperfect science. With reasonable
assumptions and basic system knowledge, the use of arc-flash TCC's provides a
method of determining the hazard category at virtually any location without performing
time consuming calculations at seemingly infinite work locations.
As an added benefit, it is in the author’s experience that the reaction of personnel tends
to be more favorable and understanding when the arc-hazard potential is presented in a
visual medium, as opposed to a data-packed spreadsheet. In a short safety meeting
the basics of reading an arc-flash TCC can be explained. Personnel often reflect on
fault magnitudes that they have commonly seen and the associated hazard in the case
of an arc-flash event. Even when not required by company policy, linemen have opted
to wear a face-shield when performing a certain task only because they were not aware
of the potential arc-flash hazard should something go wrong.
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