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
July 9, 2008
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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: Physical stimulation that occurs when electrical current passes through the body. 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 Arc-Flash 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 third-degree 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 Arc-Flash 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 personnels 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 personnels head/torso to the location of the arc source and does not consider hands, arms, or legs. While incident energy exposure to ones hands is something to be avoided, injury to appendages is not considered life-threatening. 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 authors 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 transformers highside (primary) fuse. Using the protective devices time-current curve, the protective devices clear time must be input into the incident energy formula or IEEE 1584 spreadsheet. It is the authors 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 ones 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: Hazard category: x = Conductor gap Increases x = Working distance Decreases x = System voltage Increases
x = Bolted fault current Neither results in an increase in arcing fault current
x = Arcing fault current Neither typically results in an increase in clearing time x = Clearing time Increases Equipment class changes effect: Arc...