Integrating Arc Flash Analysis with Protective Device Coordination

Christopher Lee Brooks, PE Senior Member, IEEE ESC Engineering Inc.

3540 JFK Parkway Fort Collins, CO 80528, USA

CBrooks@ThinkESC.com��

�Abstract — This paper presents a novel method to integrate

arc flash assessment with the important task of protective device coordination. Since the National Electrical Safety Code (NESC) arc flash rules are now in force, there is a need for a consistent approach to continually assess the arc flash levels on a power system. By integrating arc flash analysis with Time-Current Coordination (TCC) analysis, the full range of fault currents at locations on the power system can be fully evaluated for their arc flash levels. Once the coordination of a system’s protective devices is established, the devices can be checked for their arc flash level and then, if required, adjusted to reduce the arc flash hazard level. This integration method introduces and defines a new concept called a Human Damage Curve (HDC) for Personal Protective Equipment (PPE) in discrete energy (calories/cm2) levels that hold to a set of assumptions based the NESC and/or IEEE 1584 standards.

Index Terms-- Arc flash hazard, human damage curve, equal energy line, protective device coordination, Time-Current Coordination analysis, fuses, reclosers, relays, power system safety, personal protective equipment, NFPA70E, IEEE 1584.

I. INTRODUCTION

This paper introduces a novel method for integrating arc flash analysis with protective device coordination on Time-Current Coordination (TCC) plots. It includes a new concept, called the Human Damage Curve (HDC) into the coordination process, which relates directly to calories/cm2 energy withstand levels of Personal Protective Equipment (PPE). Each curve in an HDC family holds to a set of consistent arc flash assumptions based on those found in the NESC and IEEE 1584 standards. That set of assumptions includes values for the working distance, voltage level, arc gap width, open air/cabinet conditions, and the full consideration of escape time.

This paper will describe the components of the HDC, followed by how they interact with three different protection device arrangements. Finally, the paper will conclude with some examples of mitigating arc flash situations with this method.

II. BACKGROUND

Adequacy for PPE protection from the arc flash hazard for workers in the electric power industry was defined in the

2007 National Electrical Safety Code (NESC) [1] and was also declared therein to start on January 1, 2009 by Rule 410A3. It predefined three clothing systems for voltages from 1kV to 800kV at the levels of 4, 8, and 12 calories/cm2 (or cals) in Tables 410-1 and 410-2 and later again in the 2012 NESC [2] in Tables 410-2 and 410-3. Table 410-1 was added in 2012 for voltages below 1kV.

These PPE protection levels are rated in cals and have energy equivalence representations on a TCC plot defined by fault currents and exposure times to an arc flash. This manner of representation now allows for the simultaneous evaluation of power equipment protection from overcurrents and faults, along with the evaluation of human protection from the potential hazard of an arc flash event. Just as a transformer damage curve defines the boundary where a transformer would, by design, begin to experience loss of life from overheating at currents that exceed the rated level or from sustain severe winding damage from very high currents. So it is with the PPE human damage curve (HDC) which represents the cal energy absorption level for a specific cal rating of PPE before the unabsorbed energy can damage the human skin underneath. In other words, the HDC defines the boundary beyond which PPE can no longer adequately protect the wearer from the energy released from an arc flash.

The objective is the same for the protective device for the power equipment and the human – clear the fault and/or overcurrent before it damages the respective object. The responsibility for that clearing in a radial system is usually the nearest protective device toward the source, which is routinely either a fuse, or circuit breaking device working with a relay or a built-in predefined overcurrent response curve. Combining an HDC for a given cal level with the respective protective device curve on a TCC plot provides an instant format for evaluating the adequacy of arc flash rated PPE.

With this method, adequate levels of protection can be quantitatively demonstrated to protect the public, the workers, and the equipment on a power system from an arc flash event.

This paper will now describe the derivation of the HDC and its components.

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III. HUMAN DAMAGE CURVE (HDC)

The concept of a human damage curve on a TCC plot has emerged as a premier tool to help provide a qualitative and quantitative solution to mitigate an arc flash hazard. Mitigating options include adjusting: the protection device parameters, the level of protection of the PPE, and/or the level of the fault current. These three options will be discussed later in this paper.

The next section defines the arc flash calculation basis for the values on the HDC and the HDC’s two main components: the Equal Energy Line (EEL) and the escape time line.

A. Arc Flash Calculation Basis

The recent demand on the electrical power industry to quantitatively estimate the levels of arc flash hazards on a power system requires the use the best calculation methods available to help engineers develop adequate protection and protection models for line workers. To date, there are several calculation methods that have emerged as “standard,” but there are two methods that, due to lab validation and industry use, stand out: the IEEE 1584 [3] empirical formulation up to 15kV and the ARCPRO™ [4] program. The IEEE 1584 empirical formulation was developed through multiple lab experiments that varied key variables and then found a regression fit to them in logarithmic forms. ARCPRO™ formulation has also found support through lab evaluations and from being the calculation method which is purported to have been used to create the values in the arc flash tables in the NESC. For the purposes of this paper, IEEE 1584 calculations will be used for voltage from 208V to 15kV and ARCPRO™ for all voltages over 15kV.

For electrical utility purposes, the default values for arc flash calculation variables will be those defined within the NESC arc flash tables for the respective voltages. Other default values not covered in the NESC tables will be derived from the tables in IEEE 1584. These variables include the arc gap width, the working distance from the arc source, and the working condition. The working condition can be open air or cabinet (a.k.a. arc-in-box).

ARCPRO™ normally produces arc flash estimates only for a single-phase fault. To use it for three-phase purposes, the arc flash results must be increased by a program-documented multiplier that ranges from 1.2 to 7. Where 7 is the most conservative and 1.2 is the least conservative. For the purposes of this paper a mid-range multiplier of 5.5 will be used.

B. Equal Energy Lines (EEL)

The first and most basic component of the HDC is the Equal Energy Line (EEL) on the TCC plot. Each EEL represents an estimated level of energy absorption of a PPE clothing item in the units of cals, which was previously defined as calories/cm2. The lines are constructed on a TCC log-log plot template for a consistent energy level from

multiple runs of an arc flash calculation method such as IEEE 1584 or ARCPRO™. These points roughly form a straight line on the log-log TCC plot for a given arc gap width, working distance, and working condition. Voltage level only affects the calculation directly for voltages less than 1kV. However, voltage level does indirectly affect all calculations through the arc gap width, which increases as the voltage increases. This gap width attempts to represent a spacing of energized conductors where, at that distance and voltage, an arc could start and maintain itself in our atmosphere.

The default arc gap widths for these calculations are those used in the NESC tables by voltage and are repeated here as: two inches for 1kV to 15kV, four inches for 15.1kV to 25kV, six inches for 25.1kV to 36kV, and nine inches for 36.1kV to 46kV. For over 46kV, the NESC calculates the arc gap width using the phase-to-ground voltage of the circuit and dividing it by 10 and the working distance is derived from a calculation using the NESC Minimum Approach Distances (MAD) distances. [2, p. 267] See also IEEE Std 4-1995. [5]

The NESC tables assume the open air condition, not cabinet conditions, and model the worker’s body (not hands or arms) to be 15 inches from the point of inception of an arc flash for voltages 46kV and below. For industrial and commercial sites covered by NFPA 70E [6], the benchmark worker distance for calculations is assumed to be 18 inches.

An example of an EEL appears in Fig. 1 on a TCC plot. The points forming the line were computed from IEEE 1584 using a voltage of 12.47kV, an arc gap width of two inches, a working distance of 15 inches, and a working condition of open air.

The next step is to create a family of EELs that represent the various PPE cal levels. A clothing rating essentially means that if an arc flash event exceeds the cal rating of the clothing item, then the underlying skin could receive a second degree or greater burn, which is unacceptable. Therefore, the energy from a fault must be stopped by the respective upstream protection device and not be allowed to exceed the let-through energy cal level of the PPE.

On the TCC plot, the area below and to the left of the EEL can be considered a zone where that rating of PPE can be considered adequate (the adequate zone) and should be able to withstand the energy created by all the various combinations of the arc durations and faulting currents without failing. The area above and to the right of the EEL represents the opposite. It is where the energy created by longer durations or higher currents will cause the PPE to fail and allow damage to the skin.

Note that the EEL is inversely proportional between time on the vertical axis and current on the horizontal axis. For the EEL, time represents the duration of an arc flash, and the current represents the fault current level that created an arc flash. Every point on the line represents the same amount of incident energy in cals. In other words, the same incident energy can occur for a short time at a high current, or for a

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longer time at a lower current. The objective for the protection device is to stop an arc flash within the adequate zone in order to have the cal rating of the PPE be considered adequate.

Fig. 1. Equal Energy Line (EEL) for 1.2 cal (12.47kV, 15-inch working distance, IEEE 1584).

C. Escape Time Line

The second component of the human damage curve is the line representing the escape time. This component suggests the worker will not remain in the same initial location where he was located at the start of an arc flash event but will make haste to escape. There is also the reality that the worker will be forced away involuntarily from the event location by an arc blast. In either case, the worker will be moving away rapidly from the site and will continue that movement even further to a safer location.

This behavior must be modeled because it is unreasonable to believe that a worker would stand static at the same distance from an arc flash for its entire duration. This escape time is modeled in the HDC by adding a vertical line to the EEL at a selected number of seconds.

The common maximum time used for a sustained arc flash is two seconds of exposure at a constant distance. The best known origin for the two-second time period is from IEEE 1584-2002 [3] standard on page 76. It states:

“If the time is longer than two seconds, consider how long a person is likely to remain in the location of the arc flash. It is likely that a person exposed to an arc flash will move away quickly if it is physically possible and two seconds is a reasonable maximum time for calculations. A person in a bucket truck or a person who has crawled into equipment will need more time to move away.”

Therefore within a two-second time period it is reasonable to assume that a worker would be, or has begun to be, well on his way to escaping to a safe location. Thus, the vertical line

represents the mark in time when a worker would be far enough away from the arc source where there would no longer be any harm. Also note from the previous quote that the two-second reference implies there is an open area around the worker where he could retreat in any direction.

To consider the situation when an arc flash occurs in a confined space, the escape time must be longer. It is understood that having adequate time to exit an arc flash event is subjective and is dependent on the person and the environment around a possible source of an arc flash. In reality, as a worker moves away from the thermal energy source, the energy exposure will be decreasing exponentially with that distance. This vertical time line component intentionally overstates the energy exposure by holding the arc flash energy level for the full two seconds, and then the vertical time line component assumes an instantaneous departure to a safe distance. Until models of escape times can be more clearly understood and defined, this conservative estimate provides a reasonable case to use for decision making.

For the situation where the worker is in a confined space, such as a vault, the bucket of a line truck, or a tight control room site where escape routes are minimal, a more conservative modeling for exposure time must be considered. This author uses a breakpoint on the EEL of 10 seconds for the time line to go vertical as a benchmark for confined space situations. Other consultants suggest a five-second extraction time. Whatever the escape time, the method allows the user to vary it as he desires. It is certainly hoped that any protective device that clears such a fault will happen well before 10 seconds. Therefore, the best way to interpret the use of a longer time, like 10 seconds, is to think of it as providing up to 10 seconds of thermal energy protection before the PPE fails. It is hoped that within that time frame the worker can also increase his distance as much as possible from an arc flash location to further reduce the thermal exposure or to possibly experience a complete extraction to a safe location. However, there are situations where there could be fault durations from lengthy intentional delays in clearing a fault where the 10-second benchmark could be exceeded. If a worker is in such a confined situation, then additional mitigation measures must be put in place.

D. An NESC Example

An example of the creation of an HDC family of curves can be taken directly from the NESC tables. This example will use Table 410-2 from the 2012 NESC [2]. Note that the NESC table is populated with the clearing times for a single phase line-to-ground protection device, such as a fuse, that is upstream from a worker. The columns on the horizontal of the table are grouped into three cal levels of rated protection for a PPE clothing system. They are 4, 8, and 12 cal. On the vertical axis there are two layers to the groups: voltage and fault current. The fault current levels are fixed at 5, 10, 15,

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and 20kA for four voltage ranges that are from 1kV to 15kV, 15.1kV to 25kV, 25.1kV to 36kV, and 36.1kV to 46kV.

Figs. 2 through 5 show how the HDCs for 12.47kV (using the 1kV to 15kV range) are developed from the NESC tables onto a TCC plot. Fig. 2 displays the four values from the 1kV to 15kV section for the fault currents 5, 10, 15, and 20kA for 4 cal. Fig. 3 adds the plotting of the points for 8 and 12 cal. Fig. 4 then displays the effect of connecting these points to create an EEL for each of the three cal levels of PPE clothing system. Fig. 5 finally shows the EEL extended to higher and lower fault currents and adds the vertical escape time line to create the final HDCs using an escape time of two seconds.

E. Family of Curves for a Clothing System

The HDC can be expanded beyond the three cal levels of the NESC to other voltage levels, gap sizes, worker distances, and working situations. This is done by: 1) selecting a set of values for arc flash variables that will define the PPE HDC family, 2) running either IEEE 1584 or ARCPRO™ calculations with those values to generate the respective EELs, and 3) then adding the vertical line for the maximum exposure (escape) time. This approach was developed to create a family of discreet HDCs for several cal levels that can be somewhat aligned with commercially available cal ratings of PPE. Additional cal levels were also developed to better evaluate other situations.

Fig. 6 displays a family of HDCs for 12.47kV, for a working distance of 15 inches, with an arc gap width of two inches in an open air environment. The HDC family starts at the far left, with a dashed line representing the naked human skin at 1.2 cal. It proceeds to the right for PPE levels of 4, 8, 12, 16, 20, 25, 40, 60, 80, and 100 cals. All escape times are two seconds.

IV. INTEGRATING ARC FLASH EVALUATION AND DEVICE

COORDINATION

A. Introduction

The goal of an adequate arc flash program for a utility is to address all known locations where an arc flash can occur in the system. This means having a quick enough protective device that is between the worker and the power source(s) to stop the energy of an arc flash before it exceeds the cal level of the worker’s PPE. With this objective met, utility management can be assured that their workers would be adequately protected should the unexpected occur.

However, it must be stated that an arc flash event like any hazard has a strong component of unpredictability and that risk can never be eliminated, only minimized with the best available knowledge that the industry has at that time.

B. Determination of an Adequate PPE

There are three components in an arc flash TCC evaluation for a site or situation being evaluated. They are: 1) the TCC curve of the respective upstream protective device or devices,

2) the maximum fault current at the site or in the situation under evaluation, and 3) the PPE HDC family of curves for the respective voltage and other given conditions including working distance, open air/cabinet, escape time, etc.

Fig. 2. NESC – 4 cal points (12.47 kV – 15-inch distance).

Fig. 3. NESC - 4,8,12 cal points (12.47 kV – 15-inch distance).

A simple example will be used to explain the interaction of

a PPE HDC and a protection curve of a fuse, which is shown in Fig. 7. Notice the fuse protection curve and the two PPE HDCs. Each HDC represents a level of PPE protection. If this fuse is the main protection device between the worker

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and the power source, then the arc flash level in cals of the worker’s PPE must be able to handle more energy than the respective protecting fuse at the worker’s site. In other words, the fuse must interrupt the circuit before the PPE on the worker fails.

Fig. 4. NESC - 4,8,12 cal EELs (12.47 kV – 15-inch distance).

Fig. 5. NESC 4,8,12 cal Human Damage Curves (HDC) (4, 8 & 12 cal HDC from the left to right)

(12.47 kV – 15-inch distance).

Fig. 6. PPE HDC Family for 12.47kV, 15 in, Open Air, 2 sec escape line, IEEE 1584.

1) Adequate Condition

Only the green PPE HDC in Fig. 7 that is above and to the right of the fuse curve, which has a higher cal rating, is adequate. It is adequate because as the faulting and arcing of the arc flash progresses vertically up through time on the TCC, the fuse can be seen as being the first to absorb all the energy and open, and stop the arcing event. Since the representative HDC is above the fuse on the TCC plot, it has a higher energy absorption level at all fault currents from the two-second escape point up to the maximum fault current level (the vertical line) – thus adequacy is obtained for the cal level represented by this PPE HDC.

2) Inadequate Condition

However, the reverse is true for the red PPE HDC in Fig. 7 which is below and to the left of the fuse curve. Here the represented PPE will experience the full blast of the arc flash energy and fail before the fuse can experience enough energy to open. With the PPE failing before the fuse can open, it means that the skin of the worker could experience harm in the form of a burn. The cal level represented by the lower HDC is inadequate.

3) Plot Usefulness

Notice that the plot provides much valuable information on the interaction between the clothing PPE HDC and the protective device. Mitigation options, for example, can be considered immediately on the TCC plot by: changing the fuse size, changing the level of PPE, or considering other mitigation options that affect the escape time and/or maximum fault current which could alter the adequacy of a

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level of PPE. All of these can be modeled and will be addressed.

Another benefit of this TCC plot method over other arc flash programs and methods is the display of the full range of currents that are covered from the escape time line to the maximum fault current. Most of these other programs only evaluate at one or two fault current levels and provide no perspective on the range of options that can be implemented to mitigate the situation.

Fig. 7. - An example of a fuse TCC response curve . and two PPE HDC curves.

C. Basic Types of Interactions with HDC

The effort to address adequate PPE protection starts with an analysis to find out the current levels of a potential arc flash hazard around the system. This begins by subdividing the power system into those groups that can be analyzed in the same manner. This author has organized a typical power system into eight analytical groups. Within each group there is a strong similarity in how to approach and evaluate an arc flash hazard with this method. Even though the evaluations are very similar, the groupings allow for the conclusions to be generalized and extrapolated to the entire system where applicable. This approach will be the topic of another paper.

The groups are: 1) Generation and generation plants 2) Transmission/Subtransmission 3) Substations and substation buses 4) Distribution feeders 5) Distribution transformers 6) Secondary drops 7) Secondary networks 8) Spot networks This paper will only address groups 2 through 6. These

five groups are the most common and involve the greatest number of utility workers that work in the field. The other three groups require some special treatment as follows. Group 1 (generation) has many similarities in the way industrial sites are modeled; thus, much of the current analytical approach has been to follow a similar, but not the same approach as that found in NFPA 70E [6]. As for Groups 7 and 8 (networks), these sites have multiple sources of power that can contribute to an arc flash event, thus requiring some additional considerations that will not be addressed here but are the subject of another paper.

One commonality among all eight groups from generation sites to secondaries and networks is that within each there are protection devices that are in place to open a circuit or circuits if a fault should occur within their zones of protection. For the protective devices on radial based portions of the power system, those protection zones have the simplicity of being directed to downstream portions of the system from their locations. In other portions of the system where the flow of power could be in any direction at any time, the protective devices need to be more robust and evaluated a little differently.

To evaluate arc flash levels for groups 2 through 6, there are three basic types of protections that will be modeled in this evaluation. They are:

1) Overcurrent Curves - These are used by both relays and fuses to protect medium voltage (MV) and low voltage (LV) circuits.

2) Differential Curves - These are used to protect buses, transformers, and HV line sections.

3) Standard Service Equipment Protection Curves - These are mainly fuses that protect the same size equipment or equipment arrangements throughout a power system, but the concern here is at secondary or low voltage (LV) level.

It is understood that protection schemes may be more complex at any one location than just the use of an overcurrent device. For example, additional features of a single relay, or the use of other relays, may be involved in monitoring for adverse changes in other parameters, such as voltage and frequency. However, since arc flash levels are directly related to faulting current levels, those respective protection devices that monitor the level of current and power flow are the ones important to this method.

Next, each of these three scenarios will be reviewed with the inclusion of the HDC. Behind each scenario is the important aspect of insuring that the data and modeling are accurate.

D. Overcurrent Curves and the HDC

This type of evaluation includes fuses, reclosers, and overcurrent relay/breakers on MV and LV circuits. Fig. 8 displays an example using a feeder relay/breaker protection curve and Fig. 9 displays an arrangement with a fuse curve.

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In Fig. 8 the accuracy and tolerances for the relay instrument itself and the CTs are accounted for with a minimum and maximum curve around the nominal protection relay/breaker curve. For this paper a maximum response curve will reflect a +4% adjustment in time from the nominal curve due to relay accuracy, a +8% adjustment in current due to CT tolerance and relay accuracy, plus a fixed time adder of +1.5 cycles for relay accuracy. The minimum response curve will reflect a -4% adjustment in time for relay accuracy, a -8% adjustment in current for CT tolerance and relay accuracy, plus a fixed time adder of -1.5 cycles for relay accuracy. A final fixed time adder of five cycles is added to the nominal relay curve to account for the vacuum breaker clearing time. This adder will also move up both the minimum and maximum curves on the TCC. For other breakers (i.e. oil, air, etc.), the adder could result in a different clearing time.

Note the vertical line on the right side of the plot represents the maximum current on the feeder. Using the 12.47kV family of PPE HDC, the closest curve to fit above the relay/breaker set is a 16 cal with a two-second escape time. Any curve representing a lesser cal level will cross below some or all portions of the relay curve and create an inadequate condition.

Fig. 8. A TCC plot example of an overcurrent relay response curve, the maximum fault current, and the resulting 2-second 16 cal PPE HDC. Note that this 16 cal level PPE is valid for all levels of

fault current on the feeder up to the maximum, which means that all locations on a radial feeder are adequately covered by a 16 cal rated PPE and no additional evaluation of downstream conditions is required. However, there are exceptions that should be easy to identify. They include distributed generation and large motor sites that could contribute large portions of fault current from a different direction. These types of locations will require a special

evaluation, resulting in a possible change to their protective devices and/or settings to mitigate their contributions to the fault current. Also note that the overcurrent curve stops at the vertical line for the maximum fault current, since this fault current level will not be exceeded therefore extended curves have no significance and that portion is typically removed.

The fuse example is in Fig. 9 for a fictitious 69kV/12.47kV substation with only a HV fuse. It parallels the process discussed above for a relay/breaker. Instead of the maximum and the minimum curves for a relay, fuses have their range defined with two other curves known as the minimum melt and the total clear curves. The plot also has the maximum fault current noted with a vertical line and the resulting 40 cal PPE HDC for the 12.47kV side of the substation which was fit just above the total clear curve.

Note that in cases where there is an intentional overlap of multiple overcurrent protection zones, adequacy of a PPE level is only valid for the first level of protection. If an arc flash evaluation were to consider cases where the first level of protection were to fail, then that assumption must be clearly stated and the evaluation will most likely suggest higher cal levels so the PPE can withstand the longer delays of the backup protection devices.

Fig. 9. A TCC plot example of a fuse response curve, the maximum fault current, and the resulting 2-second 40 cal PPE HDC for the 12.47kV.

For illustrative purposes a second HDC at 1.2 cal appears

in both Fig. 8 and Fig. 9 and represents the cal energy level where bare skin will receive a second degree burn. Note the locations of these curves on the TCC with respect to the protective device curves. Both figures show that the bare skin will fail and a severe burn will occur at all current levels. These 1.2 cal HDCs were placed in the plots to emphasize that without protective PPE over the worker’s bare skin, there will likely be severe burns because the skin will not be able to

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safety absorb enough energy before the circuit opens.

E. Differential Curves and the HDC

Evaluating arc flash levels with differential relays is only slightly different than evaluating with an overcurrent relay. The difference is that instead of the overcurrent curve, this type of evaluation uses a simpler “L” shape of the curve of the differential relay. Fig. 10 displays a single 69kV subtransmission line section with an ideal pilot wire differential L curve. The vertical line to the right represents the maximum bus fault level between the two ends of a line section. The PPE HDC for this voltage is then set above only one point on the TCC, that being the point where the horizontal clearing line of the relay crosses the highest of the two bus fault currents. Note that the PPE HDC is obviously above the differential curve at all current levels between the pickup level of the relay to the maximum possible fault current. This is an adequate condition and represents coordination between protection devices, the power system equipment, and the human working on it.

The evaluation of a differential relay for a bus and/or transformer site is the same process used for the differential pilot wire relay scheme. Instead of the bus faults from both ends of the line section, the maximum bus faults within the zone would be on the vertical lines. The PPE HDC would then be set above the same crossing point of the horizontal portion of the differential relay curve and the vertical line representing the highest fault level.

Fig. 10. A TCC plot example of a differential relay, maximum fault current between both directions, and the resulting 2-second 8 cal PPE HDC.

The vertical portion of the “L” curve of the differential represents a threshold or current pickup level above which a current difference will signal the breakers to open. Whereas,

the horizontal portion of the L curve is mainly defined by the clearing time of the slowest breaker in the differential zone, plus the time it takes to communicate and sum the current loads between all the lines in a zone and compute a decision as to whether or not a fault is in progress and then signal the breakers to open. This could be a one or two cycle adder.

F. Standard Service Equipment Protection and the HDC

Most equipment on power systems, like distribution transformers and other assemblies that service residential, commercial, and light industrial loads, standardized the type and size of fusing. With standardized fusing this type of TCC arc flash evaluation can be done in groups that can be set up with representative models, evaluated, and the results applied across the system. The most common use of this would be the evaluation of distribution transformers by kVA size and type (such as pole-mount or padmounted), where the main arc flash concern is not on the distribution voltage side, but mainly on the low voltage side, where fault currents are substantially higher. This method plots each standard transformer kVA size with its maximum through-fault current and standard fuse on a TCC plot. Then, an adequate PPE HDC is fitted above the fuse curve to determine the minimum PPE that would be required when it is approached.

Fig. 11 displays a 12.47kV/480V three-phase 1000 kVA padmounted transformer with: 1) the transformer’s damage curve, 2) the curve for its standard internal fusing, 3) a vertical line representing the maximum through-fault current, and 4) the resulting PPE HDC of 20 cal at the 480V level. This means that any place where there is a 1000kVA padmounted transformer using the standard fusing, a line worker should be prepared to wear at least 20 cal PPE.

This method can also be extended to larger assemblies that for example may include a riser, cablings, and a transformer and that would all be protected by an upstream standardized external fuse. In this situation, the upstream fuse would be the main fuse that is used to estimate the level of an arc flash at the LV level, and not the internal transformer fuse.

V. USING THE TCC PLOT TO EXAMINE THE MITIGATION OF

AN ARC-FLASH

A. Introduction

With the plots established for the existing conditions on the system for the various line sections and evaluation groups, the task of mitigation of unacceptable situations is now possible. There are many mitigation options that can be examined through the curves plotted on TCCs. They include:

1) Changing the escape time 2) Changing the maximum fault current level 3) Adjusting the protection curve with a short time

setting 4) Adjusting the protection curve with an instantaneous

setting

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5) Changing the relay curve or fuse curve choice 6) Changing the type of relay 7) Changing the PPE cal level 8) Consider varying the PPE level to the task 9) Changing the working distance 10) Changing the arc gap width assumption to something

closer to an actual gap spacing For this paper only three of these options will be discussed

with an example. However, the reader should be able to extrapolate from these three examples on how the other change options could be utilized. The three are:

1) A PPE HDC curve will have its escape time changed. 2) A relay curve will be modified. 3) The fault current will be lowered.

Fig. 11. A TCC example of a 12.47kV/480V three-phase 1000kVA transformer with its standard bay-o-net expulsion fuse, the maximum

through fault current, and the resulting 2-second 20 cal PPE HDC.

B. Changing the Escape Time on the Human Damage Curve

As examined earlier, the escape time line of the HDC intersects at a point with the EEL line that creates a knee in the curve. This point is important because its location must be kept above (occurs later in time) the respective protection curve to insure that the protective device will stop the flow of energy before it can fail the PPE.

Fig. 12 displays the same 1000kVA transformer condition found in Fig. 11 which had resulted in a 20 cal PPE, where a two-second escape time was considered adequate. However, if the area around this transformer were to be considered confined, then more time must be allowed for escape. The second PPE HDC in Fig. 12 displays the effect of protecting

the worker for a full 10 seconds. The result is that the PPE cal level must now be raised to 80 cal. This increase in cal level for the PPE can be understood from the point of view that the PPE is now being pushed to withstand 10 seconds of an arc flash before it fails versus only two seconds.

Fig. 12. A TCC example of a 12.47kV/480V three-phase 1000kVA transformer with its standard bay-o-net expulsion fuse, the maximum

through fault current, and the resulting 2-second and 10-second PPE HDCs.

C. Changing the Short Time or Instantaneous Settings

Changing the relay settings of a relay or changing the type of fuse will likely be the most common mitigation option on a modern power system. For modern microcomputer-based relays the flexibility of change is straightforward. However, to change a fuse will still require a manual effort.

Fig. 13 displays a very real situation that has been, and will continue be, occurring on many power systems. It is where a worker only has an 8 cal PPE to wear and he must now work where the arc flash level is higher than his PPE. This inadequacy is seen in the figure over a small range of high currents. To mitigate this, the curve of the protecting relay could be slightly modified to allow the relay to clear the fault a little quicker over this small range. This is done by using a short time relay setting that would “notch” the curve to allow for the earlier clearing. Fig. 14 shows such an adjustment to the curve that will now allow the worker to use his 8 cal PPE and have it be adequate. Note also that by using modern electronic relays, this arrangement could be implemented temporarily, instead of permanently, with a “second settings set” that would only be used during line work.

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D. Changing the Fault Current Level

This third mitigation option is the simplest graphically but the hardest in reality to accomplish. In this example, the same situation found in Fig. 13 will be used, where the worker only has an 8 cal rated PPE and he will be working where it will be inadequate over a range of high currents. Instead of changing the relay protection curve, the solution will be to reduce the maximum possible fault current. Such a situation could occur where the supplying source has two transformers in parallel feeding a common secondary bus with a tie-breaker that is normally closed. Opening the tie breaker will split the bus and typically cut the supplying fault current by about 50%. This result is shown in Fig. 15 as a movement in the vertical maximum fault current line to a current level of half as much. In this case, the change happens to be enough to allow the 8 cal PPE to be considered adequate.

Fig. 13. A TCC plot example of an overcurrent relay response curve with a fixed 8 cal PPE HDC and the indication that the PPE will likely fail at high

fault currents.

VI. FUTURE WORK

There is always room for improvement and this method offers the industry a platform for discussion of ideas on how best to address the hazard of an arc flash in the context of protective device coordination. As mentioned in the paper, there is a need for some additional work discussing an arc flash under multiple power source conditions, like distributed generation. Also mentioned was the need to better model the escape time portion of the HDC. However, the effect of changing the escape portion of the HDC will be to move it away from this conservative model and allow the relaxation in the cal level of the PPE.

Fig. 14. A TCC plot example from Fig. 13 of an overcurrent relay response curve with a short time setting changed to make the 8 cal PPE work.

Fig. 15. A TCC plot example from Fig. 13 of an overcurrent relay response curve with the fault reduced to one half that would then allow the 8 cal PPE

to work.

The applicability of this method to the industrial environment is also another avenue to explore in more detail. This has already occurred and indeed provides the same ease of use and mitigation considerations. Note that the four arc flash Risk/Hazard Categories in NFPA 70E of 4, 8, 12, and 40 cals can also become HDCs themselves on the TCC and interact in the same way with industrial protective devices. However, because of the strong use in industry of a handful of

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existing software packages that provide turnkey solutions that include one-line development, calculation method choices, and arc flash equipment label printing, it may not find as strong a following. However, this author has found the best use of this method in an industrial environment is where there is a medium voltage distribution system in a campus arrangement.

VII. CONCLUSION

The author is aware that the use of the term Human Damage Curve (HDC) may strike a strong emotional response in some readers. This anticipated reaction is intentional. Avoidance of harm is always a top priority in the design, construction, operation and maintenance of an electric power system; and, if the HDC term sharpened the need to make the intentional effort to avoid the harm, then it has met the intention of the seriousness of mitigating the hazard.

This paper does not propose to be an ideal solution that will insure safety for electrical workers, but is presented to the industry as a tool that can help provide a quantitative framework for evaluating an arc flash hazard and mitigating a solution to protect workers.

One of the greatest benefits of this method by using a TCC plot is that it now forces a consideration of protecting the human at the same time engineers are looking to protect the electrical equipment. However, this new objective may be a challenge for management to implement and a culture shock for coordination engineers, who will now have the added responsibility to consider a safety objective. To overcome this adjustment in objectives, utility management will likely have to be involved in a greater way.

To the author’s knowledge, this method is currently the best and most informative method available to model, evaluate, and mitigate an arc flash for meeting compliance requirements of both the NESC and NFPA 70E. However, it does not in any way replace the tables of the NESC or create a new calculation method, but enhances all the current knowledge of an arc flash by providing a method that uses the existing framework of TCC analysis to actually address two problems at once: safety of workers and protection of utility equipment.

ACKNOWLEDGMENTS

The author would like to acknowledge various individuals (Mark, Carol, Dev, Jason, and Terry) from three different major utilities, a cooperative electrical utility, and a major power equipment manufacturer for their comments and contributions. Because of the permission issues involved, only their first names are mentioned.

In all four utilities over which the method has been recently applied, it received broad acceptance, and in many cases generated immediate mitigation solutions to reduce the arc flash levels. In two cases the management of the utility requested that the results be presented to the full strata of

management, engineering, and line personnel. In all those presentations a clear understanding was accomplished for all attendees. The benefit came from using the graphical medium of TCC plot to visually present 1) the development and extinguishment of an arc flash event, 2) the benefit of cal rated PPE, and 3) the mitigation of inadequate conditions.

REFERENCES [1] National Electrical Safety Code (NESC) C2-2007, The Institute of

Electrical and Electronics Engineers, Inc., 3 Park Avenue, New York, NY, USA, 2006.

[2] National Electrical Safety Code (NESC) C2-2012, The Institute of Electrical and Electronics Engineers, Inc., 3 Park Avenue, New York, NY, USA, 2011.

[3] IEEE Guide for Performing Arc-Flash Hazard Calculations, IEEE Std 1584-2002, The Institute of Electrical and Electronics Engineers, Inc., 3 Park Avenue, New York, NY, USA, 2002.

[4] ARCPROTM ver. 2.01, From help documentation of program, Kinectrics Inc., 800 Kipling Avenue, Toronto, Ontario, Canada. Available at: http://www.hdelectriccompany.com/hd-electric-products/ utility-specialty-products/arc-analysis-software/ARCPRO-Version-2.01.htm.

[5] IEEE Std 4-1995 IEEE Standard Techniques for High-Voltage Testing, The Institute of Electrical and Electronics Engineers, Inc., 3 Park Avenue, New York, NY, USA, 1995.

[6] NFPA70E-2012, Standard for Electrical Safety in the Workplace, National Fire Protection Association, Quincy, MA, USA, 2011.

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