THE EFFECTS OF SYSTEM GROUNDING, BUS INSULATION AND PROBABILTY ON ARC FLASH HAZARD REDUCTION THE MISSING LINKS

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1 THE EFFECTS OF SYSTEM GROUNDING, BUS INSULATION AND PROBABILTY ON ARC FLASH HAZARD REDUCTION THE MISSING LINKS Copyright Material IEEE Paper No. PCIC John P. Nelson Joshua Billman James Bowen, P.E. Fellow, IEEE Member IEEE Fellow IEEE NEI Electric Power Engineering NEI Electric Power Engineering Aramco Services Company P.O. Box 1265 P.O. Box West Loop South Arvada, CO Arvada CO Houston, TX USA USA USA Abstract This paper provides a discussion on the theory behind reducing the risk and severity of an arc flash incident. In particular, the variables associated with the calculations of energies from an arcing fault are presented in an effort to show the futility of present methods for the determination of incident energy levels in the electrical industry. A number of commonly ignored design concepts that significantly reduces the risk of electrical hazards will be discussed two of which include 1) the system grounding and, 2) solid insulation. This paper will discuss risk and the management of risk as a means of reducing the probability of an incident. It will then show how risk-reduction should be used in the design, construction, operation and maintenance of electrical equipment as means for the safeguarding of employees in the workplace. Index Terms Arcing faults, bolted fault, electromechanical reset, ground clearance, impedance grounding, incident energy levels, insulated bus, latent heat of vaporization, latent heat of fusion, phase-spacing, risk, reactance, grounding, resistance grounding, solidly grounded system grounding, significant figures, significant digits, I. INTRODUCTION Models for the calculation of a bolted fault 1 and associated fault studies are essentially a proven technology. Fault simulations on power systems have been performed in order to adequately verify the validity and accuracy of the models. However, the modeling of an arcing fault has proved to be very difficult, if not totally elusive. The reason for this elusiveness is that each arcing fault is unique and cannot be repeated. Even tests on arcing faults in the laboratory are difficult to repeat. There are many variables that make the repeatability of any particular test difficult. In fact, the probability of two identical arcing faults in the real world is a physical impossibility because of the random nature of the arcing in a plasma cloud. Acknowledgement of this fact will allow a more reasonable approach to understanding the arcing fault which involves the consideration of numerous variables. Some of these variables are controllable, but many are not. Once there is a good understanding of the controllable versus non-controllable variables, it is of importance to develop a simple yet effective way of understanding the electrical hazards. Then, and only 1 Bolted fault is a fault with no fault arc impedance then, can reasonable progress be made in an effort to develop reasonable solutions to improve electrical arc flash safety in the workplace. At best, the present arc flash models can only provide an estimate of the incident energy levels that may be present from an arcing fault. This estimate is based on equations developed through extensive test data from the laboratory which may or may not represent the actual real world conditions. Furthermore, these equations involve a number of variables including the correct operation of protective devices. The correct operation of protective devices during an arcing fault is one of the more serious issues in arc flash hazard calculations. A large number of arc-flash incidents occurs on older and poorly maintained equipment in which the protective devices may be extremely slow to operate. While determining a value for incident energy levels during an arcing fault is important, the more important consideration is in minimizing the potential or risk of such an event taking place. Since it is impractical if not impossible to remove all risk from life, reducing the probability of an accident from taking place is most important to reducing fatal and nonfatal injuries. To this end, there is an extensive discussion on risks and how to minimize the probability of an incident. While 20/20 hindsight in every serious accident investigation can show how an accident could have been prevented, that knowledge is not totally available prior to the accident. If it were, there would be no accidents. Therefore, knowing the risks, minimizing the risks and taking proper precautions will minimize the severity and number of injuries in the workplace. II. VARIABLES AND FACTO Knowledge and understanding of the incident energy level variables and factors is important. These will be further discussed in more detail later after the model is presented. The following is a list of the three commonly identified variables: 1. Time Duration of the fault measured in seconds. 2. Current magnitude of the fault measured in Amperes or kilo-amperes 3. Distance away from the arcing fault typically measured in meters, centimeters or inches. Next, the somewhat neglected factors that may impact the incident energy levels and/or arc-flash hazard risks include the following: 1

2 1. System Grounding is the method by which the system is grounded (solidly, impedance or ungrounded). The probability of an arc-flash incident can be significantly reduced depending on the type of system grounding. 2. Solid insulated versus bare bus is an important factor on reducing the probability of an arcing fault from occurring, and if one were to occur, whether or not the fault will be limited to a single phase. 3. Electrode arrangement is the physical arrangement of the electrical buses and terminations. Many buses are arranged vertically or horizontally rather than in a delta formation. See Fig 1 as an example. 4. System impedances and modeling data for the study can vary significantly. For example, the accuracy of transformer impedances can vary by as much as +7.5%. Likewise, most impedance calculations for electrical lines and conductors are dependent on such factors as temperature, spacing and transposition assumptions which may not always be accurate. In addition, source impedances can vary significantly depending on the time of day and day of the year. Therefore, the impedance data of the model may have inaccuracies which may vary in excess of five to ten percent. 5. Electrode spacing which includes phase-to-phase and phase-to-ground distances can vary significantly from one system to another, and such spacing is typically ignored. 6. Electrode material will normally consist of copper or aluminum conductors with steel or aluminum enclosures. 7. Relative Air Density is dependent on temperature and atmospheric pressure with the relative density and insulating quality air decreasing with increased temperature and decreased barometric pressure. Relative air density decreases at a rate of approximately 1% per 100 meters above sea level. (3% per 1000 feet) 8. Latent heat of fusion and vaporization is a physical property of a material that is typically endothermic on changing state one direction and exothermic on changing the state another direction. For example, water absorbs heat in changing from liquid to gas and releases heat in changing from gas to liquid. Heat transfers also occur when materials sublimate (change directly from solid to vapor.) 2 9. Probability of an event - The probability of an event taking place is an important factor which needs to be considered. As will be discussed later, safety involves minimizing the probability of a serious event from occurring. At the present time, little appears to be written about the probability of a given incident taking place. The lack of complete knowledge of these variables and factors is sufficient to show that calculating incident energy levels is not an exact science. In addition, the industry should be made aware of the limited accuracy of the studies being performed. For example, consider the significant figures of the equations being used in comparison with the accuracy of the data being used for the studies. (A review of this topic is included in Appendix A) III. GENERAL ARC FLASH SYSTEM MODEL The Circuit Prior to further analysis of the electrical model, a common electrical circuit consisting of three buses is shown in Fig 1. In this arrangement, note that the bus is arranged horizontally. A B C Fig 1 Electrical Buses in an Enclosure It has been long assumed that an arcing fault model consists of two elements at the point of fault: Arc Voltage Arc resistance Papallo 3 recently presented an arc flash model that assumes these two elements for an arcing fault. He further utilized data from IEEE 1584 to refine his model and presented convincing evidence on the accuracy of his model. By utilizing the two elements for the arcing fault, the model consists of a linear element (arc voltage) and a non-linear, exponential element (arc current) for the power calculation which when integrated over time is equal to energy. The basic power equation is shown in equation 1. Where, P a = V di a + I a 2 R d (1) P a V d I a R d Power at the point of the arcing fault (Watts) Arc Voltage (Volts) Arc current (Amperes) Arc resistance (Ohms) Papallo used the subscript ( d) to represent the fact that the arcing voltage and arc resistance are both functions of the gap distance. It should be intuitively obvious that that both the arc voltage and arc resistance increase with the length of the gap. Therefore, a greater gap distance will increase the arc voltage and arc resistance most likely resulting in less arc current and lower arc energy. Assuming that arcing can take place from any bus to a second bus and from any bus to the ground plane, an equivalent circuit can be drawn showing the system using the arc voltages and arc resistances. See Fig 2. 2 Dunki-Jacobs Page Papallo Pages

3 VdAC RAC A B C VdAB RAB VdBC RBC predict where the arc will occur next. This is obvious when inspecting the remnants of severely damaged enclosures after an arcing fault event. Numerous arc marks to ground, burned buses and burned sheet metal show the evidence of the randomness or dancing of the arc that takes place during the fault. VdAG VdBG VdCG RAG G RBG Fig 2 Equivalent Circuit from Fig 1 Where phase-to-phase arc resistances are, R AB = Arc Resistance between bus A and bus B R BC = Arc Resistance between bus B and bus C R CA = Arc Resistance between bus C and bus A And where phase-to-ground arc resistances are, R AG = Arc resistance between bus A and ground R BG = Arc resistance between bus B and ground R CG = Arc resistance between bus C and ground RCG The circuit represented in Figs 1 and 2 is assumed to be for bare buses. Note that typical bus arrangements are in a flat horizontal or vertical configurations rather than a delta configuration. Based on the flat configuration and equal spacing between buses A, B and C, it stands that: R AB = R BC R AC R AB + R BC (Assuming that an arc can jump over B phase) Let R φφ = R AB = R BC = ½ R AC Then R AC 2 R φφ Also, let R φg = R AG = R BG = R CG The system supplying the fault can be represented by an impedance (Z s = R s + jx s) and a voltage source, and ground return impedance consisting of any system neutral impedance (Z N) and ground return impedance (Z G = R G + jx G) Since most fault studies utilize the per unit system, the voltage reference will assumed to be operating at a particular voltage level of 1.0 per unit. A higher or lower per unit voltage source may be used depending on the situation. For the purpose of this paper, Fig 3 represents the equivalent source to the arcing fault. Random arcing can take place across each of the six gaps shown by the arc voltage elements with the possible exception of the gap between A and C buses due to the fact that bus B is between A and C. The energy dissipated in an arcing fault is most likely not continuous since the arc most likely momentarily extinguishes each time that the current goes through a current zero. Dunki-Jacobs 4 states the arc of an arcing fault is momentarily extinguished and another arc is formed. The randomness of the arcing is such that there is no means to 4 Dunki-Jacobs Pg 182 ZN VSA VSC VSB RG XG A B C G SEE FIG. 2 Fig 3 Equivalent Arcing Fault Circuit with Source Where, V S are the three phase voltage sources, R S is the system equivalent phase resistance X S is the equivalent system phase reactance, R G is the ground return resistance, X G is the ground return reactance, and Z N is the system neutral grounding impedance From a practical point of view, the probability of a balanced, three-phase arcing fault taking place is zero for the standard horizontal or vertical configuration of the buses. With the exception of a bare bus with a delta configuration, an arcing three-phase fault cannot exist. However, an arcing fault involving three phases can exist which can be correctly identified as an arcing fault involving three phases. Therefore, it would appear that the arcing fault condition will include the following arcing conditions: 1. Phase-to-ground faults 2. Phase-to-phase faults 3. Phase-to-phase-ground faults. A conclusion that can be drawn here is that the use of three phase arcing fault currents may not be a reasonable representation of an arcing fault. Bolted phase-phase faults involve currents that are approximately 86.6% of a three phase bolted fault and an arcing phase to phase fault will have fault current that is approximately 74% of a three phase bolted fault. 5 Therefore, assuming that a three phase arcing fault does not exist, the incident energy level calculations of an arcing fault involving only phase to ground and phase to phase should be different that those for a three phase arcing fault. This conclusion does not necessarily mean that the incident energy level will be lower, just that it will most likely be different. Similar assumptions will be made that are commonly used in short circuit studies. First, the bus will be considered to be in a delta configuration so that phase-to-phase fault currents can exist equally between all three phases. Second, it will be assumed that the fault is balanced from a standpoint that fault current can and will exist between each phase and the other 5 Dunki-Jacobs Page 185 3

4 two phases, and between each phase and ground. By doing so, an equivalent wye connected fault will be substituted for the combination delta and wye fault represented in Figure 2 and 3. That representation is now shown in Figure 4. ZN ZN VSA VSC VSC VSB RG XG VdB RaB a) Three Phase Equivalent Circuit RG XG b) Single Phase Equivalent Circuit Figure 4 Equivalent Arcing Fault Circuit RaA RaC VdA VaC In reviewing Figure 4, it becomes quite obvious that an impedance Z N in the ground path of the circuit will have an impact on the energy of an arcing fault for arcing between a phase conductor and the ground plane. While this representation does not properly show the energy being generated due to the phase-to-phase arcing, it does show the major impact of energy that is being generated by the arcing between the conductors and ground. For example, with a solidly grounded system, Z N will be zero and the resulting ground fault current will be high. Therefore, there will be a significant propensity for large amounts of energy to be dissipated to the ground plane from each of the three phases. On the other hand, if Z N is quite large as in the case for high resistance grounded (HRG) systems, the energy dissipated to the ground plane will approach zero since the HRG system limits the current to just a few amps. This is consistent with the results of testing a three phase arcing fault created inside of switchgear from a solidly grounded source results in much greater energy levels than the same fault on a high resistance grounded system. 6 6 Result of preliminary testing which will lead to further testing. Vd Ra IV. INITIATION OF THE ARCING FAULT Anecdotally, it has been reported that over 95 percent of the faults in an industrial facility originate as a phase-to-ground fault, less than 3% originate as a phase-to-phase fault and less than 1% originate as a three-phase fault. Furthermore, one company reported that practically all faults originating as a three-phase fault, in that company s experience, were the result of maintenance errors where the equipment grounds were found to have been left on the system when the system was reenergized. 7 Little of this information has been noted in the current literature on arc flash hazards and in the arc flash testing. The importance of the initiating event is clearly evident when inspecting the equivalent circuit for an arcing fault and what will happen once the fault is initiated. If the system is solidly grounded, Z N will be zero. In addition, R G and X G in the switchgear or motor control center may be assumed negligible and equated to zero. As a result, the system impedance, arc voltage and arc resistance are the limiting factors for the ground fault current. With the minimal clearances in low and medium voltage bare bus equipment, the air surrounding the bare bus will quickly ionize allowing the fault to propagate into other phases and ground. Large air gaps or solid insulation will minimize the probability of the arc propagating into other phases. While a fault may originate phase-to-phase, the probability is quite low and the probability of a fault initiating as a three-phase fault is practically zero. Therefore, the testing procedures for arcing faults where the initiating mechanism is a three phase fault appears to be inappropriate and of only academic value. In conclusion, the model of the arcing fault circuit provides a significant amount of information on the make-up of an arcing fault. First, there is the initiating event which most likely will involve ground. Second, energy will be produced with arcs from phase-to-ground as well as phase-to-phase. However, the likelihood of energy being produced by an arcing three-phase fault appears to be remote at best. V. Understanding the Incident Energy Variables The three major variables that are most commonly used in the calculation of incident energy levels were previously listed as 1) fault current, 2) time duration of the fault, and 3) distance from the fault. In addition, there is a plethora of minor variables that are neglected. It is these additional variables that further lead to the difficulties in obtaining realistic and meaningful results from incident energy level calculations. The following is a discussion of major and minor variables in calculating incident energy levels: A. Fault Current The level of fault current is normally given in terms of a bolted, three-phase fault in ka. However, as can be seen in the system model, some of the incident energy is produced based on phase-to-ground and phaseto-phase faults. The power of a particular incident is dependent on current, arc voltage, arc resistance and time as shown in equation which is shown again below: 7 Unless the system grounding conductors have been improperly placed on the system, energizing a system with properly installed grounding jumpers should preclude an arcing fault from taking place. 4

5 P a = V di a + I a 2 R d (1) Time The duration of the arcing fault is a second key variable. It should be noted that the fault duration most likely consists of a large number, of intermittent phase-toground and phase-to-phase faults each re-igniting in a random manner. Therefore, the energy produced from the fault is the integral of the power produced over time: E a = P a(t) dt (2) Where, E a = energy produced from the incident P a(t) = Power as a function of time t = time in seconds Note: If the average power is considered a constant which for all practical purposes is correct, Equation 2 simplifies to the following equation: E a = P a t (3) It should be noted that many of the electronic protective devices have both an electronic reset and an electromechanical reset option. The electronic reset feature provides instantaneous reset which will reset on intermittent faults whereas the electromechanical reset feature emulates the reset feature of an electromechanical disk of an over current relay which has an inherent time delay for resetting. B. Distance The incident energy with which a person is exposed decreases with the square of the distance from the arcing fault. In open air, the dissipation of energy occurs in all three-dimensions. However, the energy is more directed from an enclosure where the energy dissipates more directly in two dimensions, much like the effect of a shotgun blast. The additional variable adds distance to the incident energy level at a given distance from the arcing fault: E i = K Ea/d n (4) Where, E i = Incident energy at a given location I = RMS current t = Time in seconds d = Distance from the arc in meters n = An exponent which equals 2 for a fault in open air and some lesser value for a fault in an an enclosure C. Arc Voltage The voltage produced at the point of arc which is a function of the arc length and may vary with time. On a 480 Volt system, the arc voltage E arc has been found to be in the range 140 Volts. 8 D. Arc Resistance The resistance produced by the arc which is a function of the arc length. The arc resistance has found to be time varying and may in fact be non-linear. 8 Dunki-Jacobs Page 182 E. Latent Heat of Vaporization for Aluminum and Copper Energy is absorbed by metals when vaporized and, therefore, has a cooling effect. However, energy is released when it solidifies, so the net effect of this energy variable should be minimal. The time and location of the heat absorption and release are variables which cannot be adequately defined. For example, the release of energy on the skin of worker may worsen the injury. Suffice to say, the latent heat of vaporization cannot be reasonably measured or anticipated in an electrical arc flash and will most likely be ignored as being of minor importance. However, for reference please see Table 1 for some constants of aluminum and copper. Table 1 Characteristics of Copper and Aluminum Aluminum Copper Atomic Weight Melting point 660 O C 1083 O C Boiling point 2519 O C 2562 O C Latent heat of fusion 397 kj/kg 209 kj/kg Latent heat of 10,900 kj/kg 4,730 kj/kg vaporization kj/mole 300 kj/mole F. Exothermic Reaction for Aluminum - Dunki-Jacobs 9 expressed the concern that under high temperatures which occur under arcing conditions that aluminum can sublimate and become combustible (exothermic) resulting in additional incident energy being released. The literature shows that aluminum can burn at approximately 4000 Kelvin or 4273 o C. 10 The following chemical reaction exists with the burning of aluminum: 4AL + 3O 2 2AL 2O 3 + heat Research into the burning of copper resulted in little information about the exothermic reaction of copper under combustion. However, according to Dunki-Jacobs, the burning of copper does not appear to be much of an issue. However, copper at some temperature should burn and the likelihood is that some heat may be produced from the combustion of copper. G. Bare versus insulated bus The probability of an arcing fault taking place on insulated bus is low. Furthermore, if a fault were to take place on a system that is insulated, the fault will be limited to an area where the insulation is missing or damaged. Similar to faults originating on insulated cables, faults occurring on insulated systems are extremely limited. As an observation on preliminary testing, an arcing three phase fault was initiated on arc resistant switchgear utilizing insulated bus. As expected the energy exposure was minimal. However, an equivalent fault was initiated on equivalent switchgear composed of bare bus. The resulting arcing fault and energy was severe. 9 Dunki-Jacobs Pg Beckstead Pg 19 5

6 An important area to consider in bare versus insulated bus is that of the cable terminations. The exposure of the terminations should be minimized using properly rated electrical tape or cable boots. Fig 5 shows a 15 kv termination which has not been insulated with tape or boots. As such, there is exposure for an arcing three phase fault in this compartment. It should also be noted that the terminations are much closer to the switchgear door than is normally expected. This particular compartment was for the termination of a 10 MW gas turbine. The circuit breaker compartment was placed in a bus transition cubicle which forced the cable terminations closer to the rear compartment door. See Fig 6 for a more typical arrangement. Fig 5 15 kv Terminations in Transition Cubicle Fig 6 15 kv Terminations in Normal Cubicle H. Grounded versus ungrounded Reference to Fig 2 reveals an interesting point concerning the energy released at the point of an arcing fault. For a solidly grounded system, an arcing fault will release energy between all three phases and between each of the three phases and ground. With an impedance grounded system, the energy released between the phases and ground will be limited due to the neutral impedance to such point that the energy released to ground from each of the phases should approach zero with a high resistance grounded or ungrounded system. VI. DEGREE OF RISK IN THE WORKPLACE Unfortunately, all activities in life have a certain amount of inherent risk. For example, the risk of average American dying in a commercial aircraft is approximately 1 in seven million 11 while that of dying in a motor vehicle is 1 in There are many other examples where the average American is subjected to similar risks. However, this paper limits the population of injuries to the workplace, not the entire American population. In reviewing workplace risks, consider the following examples. According to the US Department of Labor, the category of slips, trips and falls account for largest category of work-related injuries for a total 15 percent of all accidental work related deaths and 17 percent of all disabling work related injuries. In general, a worker is five times more likely to suffer serious injuries due to a slip, trip or fall over being seriously injured in a work related vehicular accident. 13 In 2003, there were 696 fatalities and 257,100 employees injured from the category of slips, trips and falls. 14 With regard to workplace fatal injuries, during the period of 1992 through 2002, transportation was listed as the leader in fatal occupational injuries with 23,272 fatalities over that period of time for a total of approximately 35% of all occupational fatalities. Therefore, while slips trips and falls was the statistical leader in workplace injuries, transportation was the statistical leader in occupational fatalities. 15 The average number of fatal occupational injuries from transportation is approximately 2,327 or a little over three times of that for slips, trips and falls. Therefore, it is important to analyze safety from two perspectives: 1) Fatal Injuries 2) Serious non-fatal injuries Electrical related occupational fatalities accounted for approximately 5% of occupational incidents Based on these statistics, an electrician is much more likely to be injured by a cause other than from electricity. In performing work tasks, the employer and employee must take into consideration the degree of risk involved in such a task and minimize that risk. In planning and executing a work plan, there is no way to eliminate all risk except for avoiding that work task. However, after an accident has taken place, it is only in rare cases that someone can say that the accident could not have been prevented. This is a paradox in our safety culture where people are prepared to lay blame post-accident. A good safety program involves controlling the degree of risk in performing each task, and so our objective in electric safety is to minimize the degree of risk to which an electrical technician 16 may be exposed. Therefore, the safety program RISK*TEX Page Page 1 15 Trends in Electrical Injury, Page Electrical technician in this context means a qualified person 6

7 should consider the degree of risk from injuries caused by all non-electrical risk hazards including but not limited to: 1) Slips, trips and falls 2) Vehicular accidents 3) Pinch points 4) Cuts and abrasions 5) Heat and cold Statistics have shown that electrical technicians are more likely to be injured by these hazards than from electrical shocks and burns. Once consideration has been taken into the higher degree of risk tasks, the electrical technician should assess the electrical specific risks that are present. This assessment should include such things as: 1) Condition of the Equipment: Most accidents occur on poorly designed, constructed and/or maintained equipment for which the electrical technician has little or no control. The equipment should have proper, up to date documentation including manuals and drawings. 2) Familiarity with Equipment: The electrical technician should be familiar with the equipment associated with the work task. If not, the degree of risk to the electrical technician in working around that equipment can increase significantly. 3) Experience of Worker: The experience of the worker is an important factor when working on or near electrical equipment. The degree of risk for a given task decreases with increased experience. Electrical technicians working for electrical testing and maintenance contractors are oftentimes exposed to equipment that has been in service for many years, has poor documentation, has been poorly maintained and may be in a poor environment. In addition to the higher degree of risk associated with all of these factors, that technician s degree of risk is higher due to his lack of familiarity on that particular equipment. 4) System Grounding: Within an industrial facility, experience has shown that the vast majority of faults originate as a phase-to-ground fault. High resistance grounded (HRG) and ungrounded systems typically have insufficient fault current to create an arc flash, and if cleared in a timely fashion, do not propagate into a multi-phase, high energy fault. On the other hand, a ground fault on a solidly grounded system begins as a high energy fault which can propagate into a multi-phase fault with high incident energy levels. Therefore, HRG and ungrounded systems significantly reduce the probability of a high energy incident. As a result, an electrical technician is exposed to a significantly higher degree of risk on solidly grounded system that on an ungrounded or HRG system. 5) Insulated bus and terminations: Insulated bus and terminations reduce the probability of a fault. Furthermore, the probability of a ground fault propagating into a multi-phase fault is significantly reduced. As a result, the degree of risk of injury or death to the electrical technician is reduced where the equipment utilizes insulated bus and terminations. 7) Task Being Performed: The risk to the electrical technician is dependent on the task being performed. For example, if the task in question is a normal or routine task for which the equipment was designed and constructed, the degree of risk is relatively low. However, if the task involves exposure to live parts or other exposures which are unusual to that equipment, the degree of risk will increase and can increase significantly. For example, normal opening and closed of a circuit breaker is a normal and ordinary task. For that matter, even racking a breaker in and out of service is a normal and ordinary task. However, closing the breaker after an incident or racking a breaker into service after an incident or maintenance increases the degree of risk to the technician. 8) Magnitude of bolted three phase fault current: The degree of risk and risk of injury increases with greater magnitudes of bolted single phase, phase to phase and three phase fault currents. This is one reason why low voltage systems, especially above 1000 kva, can expose a worker to greater arc-flash hazards than at higher voltages. 9) Distance from arcing fault: The degree of risk to the worker is greater near the arcing fault and decreases rapidly with distance from the point of arcing fault. 10) Type and speed of the electrical protection: The risk to the worker is dependent on the clearing time of the fault and, therefore, the degree of risk to the electrical technician increases with the amount of time for which the fault is present. Failure of a protective device to properly operate in a timely fashion is one of the higher degrees of risk that is taken by an electrical technician. VII. ELECTRICAL INJURY STATISTICS In order to put the subject matter degree of risk into perspective with electrical safety, there is a need to cover electrical injury statistics. In this section, electrical statistics for over two decades will be discussed and an effort will be made to further refine the degrees of risk for various types of electrical accidents. During the period of 1992 through 2002, there were 3,378 fatal electrical injuries listed in the Census of Fatal Occupational Injuries (CFOI) 17. Of those 3,378 electrical fatalities, all but 30 were attributed to electrocution. In other words, less than 1% of the electrical fatalities were attributed to injuries resulting from electrical burns. During the same period, 47,406 nonfatal electrical injuries were categorized by the type of injury which resulted in the following types of injuries: 18,360 or 38.7% were electrical burns 29,046 or 61.3% were electrical shocks The above statistics are based on ten years of data and provide the following average annual statistics: 338 electrical related fatalities per year o 335 fatalities due to electrocution o 3 fatalities due to electrical burns 4741 electrical injuries o 1,836 electrical burn injuries o 2,945 electrical shock injuries A breakdown of the electrical 18,360 electrical fatalities reveals that that 1,432 (42%) of those fatalities were the result of contact with overhead power lines and 175 (5%) were the result of being struck by lightning. Further substantiation of these statistics were presented at the 2011 annual technical conference of the Petroleum and Chemical Industry Committee where 23 years of OSHA burn working around electrical equipment such as electricians, electrical apprentices, engineers and operators. 17 Cawley pp

8 injury statistics were revealed for the period of April 1984 to June : 37 Low Voltage Fatalities 48 Medium to high Voltage Fatalities Based on 23 years of OSHA records, 85 fatalities in 23 years equates to 3.7 fatalities per year which is remarkably close to the CFOI statistics previously discussed. The significance of these statistics is to show that the degree of risk for an electrical related burn fatality is extremely low and is less than the probability of being struck by lightning. Furthermore, when comparing the number of annual electrical burn related injuries (1,836) to the number of annual serious injuries due to slips trips and falls (257,000), the relative degree of risk for the electrical technician is low. VIII. EQUIPMENT TESTING In an effort to better understand the effects of high resistance grounding and bus insulation on electrical equipment, testing was performed using 600 Volt equipment in an effort to better understand these two variables. 19 1) Switchgear using bare copper bus versus switchgear using insulated copper bus: The main reason for comparing insulated versus bare stems from anecdotal comments (Dunki- Jacobs and others) who contend that arcing faults do not normally occur on insulated buses and, if they do, they are not as severe as an arcing fault on bare bus. The testing was performed at 635 Volts using similar equipment that is used for arc resistant switchgear. [However, one test was performed applying a three phase arcing fault on a 600 Volt equipment with insulated bus while the other test was performed applying the same type of arcing fault on similar equipment bus with bare bus. The video results were remarkable in the incident energy on the fault utilizing insulated bus was contained in the arc resistant gear while the energy on the fault initiated on the bare bus was orders of magnitude more severe and exited the switchgear. 2) Switchgear using high resistance grounded source versus switchgear using a solidly grounded source: There were several reasons for comparing the solidly ground versus impedance grounded systems including but not limited to: IEEE 1584 results show greater incident energy levels for ungrounded/high resistance grounded than for solidly grounded systems. From basic physics, if a fault is truly a three phase fault and does not involve ground, then there should be absolutely no difference in incident energy levels between the solidly grounded and ungrounded systems. Based on the physical presence of an enclosure and physical evidence of arcing to the enclosure on practically all arcing faults, energy must be dissipated to the enclosure and not contained within the three phase faults. Finally, since practically all industrial system faults originate as a phase-to-ground fault, it is important to understand the impact of system grounding on arcing faults. 18 Wellman Sept 2011 Power Point Presentation 19 See Acknowledgements section An arcing, three phase fault was initiated in the 600 Volt equipment. One test utilized a solidly grounded source while the second system utilized a high resistance grounded system. The significance was that the energy dissipated from the fault with the high resistance grounded source was significantly lower than that with the solidly grounded system. IX. TESTING VEUS REAL LIFE In an effort to maximize incident energy measurements through a three phase fault, practically all testing is conducted with the fault originating as a three phase fault using bare electrodes. Some type of initiating event, most commonly, a thin conductor, is used to initiate the arcing three phase fault. The fault is allowed to continue as a three phase fault due to the bare electrodes and electrode spacing. In real life, practically all faults originate as a phase to ground fault which if not cleared can propagate into a multi-phase fault. The multi-phase fault is typically a phase-to-phase (most likely including ground) and possibly to a three phase fault. Therefore, the testing results which are used for system models do not reasonably represent real life conditions. X. BREAKDOWN VOLTAGES The breakdown voltage in air at one atmosphere is approximately 327 Volts according to Paschen s Law 20 (Paschen s law further states that the minimum distance is 7.5 μm or meters.) According to Dunki-Jacobs the restrike voltage of an arcing fault is approximately 325 Volts. 21 Also, the flat top arc voltage of 140 Volts (These are not RMS values. Please note that Dunki-Jacobs is referring to the restrike voltage of an arcing fault on 600 volt equipment that is surrounded by highly conductive plasma gas. Since the 325 Volt restrike voltage is a peak or dc voltage, a 60 Hz restrike voltage of 325/ 2 = 230 Volts. Therefore, the probability of a restrike on a system of 230 volts or less is extremely low. Furthermore, with an arc voltage of approximately 140 volts, the amount of arcing current will be severely limited as the literature has shown 22 and which is replicated in Table 2. Table 2 Arcing-Fault rms current multiplier in percent of bolted equivalents System Voltage Type of Arcing Fault 480Y/277 V 208Y/120 V Arcing-fault multiplier (min) Three phase 89% 12% Phase-phase 74% 2% Phase-ground 38% 0% The OSHA statistics for injuries confirm that the probability of maintaining an arcing low voltage fault is low and the probability of a burn related injury is equally low. This is shown in Table Dunki-Jacobs Page Dunki-Jacobs Page 185 8

9 3. 23 In fact, there are no burn related fatalities shown for voltages of 277 volts and below. One note should be made regarding the completeness of this table. It is obvious that not all of the nonfatal injuries have been reported. However, it is highly probable that due to OSHA requirements for reporting fatal injuries that the number of fatal injuries reported in Table 3 is close to the actual number. The significance of Table three lies in the relative percentages of injuries at the various voltage levels. Table 3 Number of Injuries at Low Voltage Voltage Burns Shocks Fatalities 120 V V V V V V 5 1 Unknown XI. CONCLUSIONS A considerable amount of time, money and effort has been and continues to be expended on arc flash hazard prevention. While there has definitely been some good due to this effort, there has also been a lot of misdirected time, money and effort spent in the name of electrical safety. While there may be a number of people that will take exception to that statement, the facts clearly show that the number of burn related deaths and injuries are low when compared to other hazards such as 1) occupational vehicular accidents: 2) falls; 3) exposure to heat and cold; 4) electrical shock and even being struck by lightning. In addition, some of the arc flash hazard mitigation measures such as certain personal protective equipment may in fact increase the worker exposure to other types of hazards such as inadvertent contact with electrical equipment, falls and heat related injuries. Many people will advocate that the efforts that are being made are improving electrical safety in the workplace, and this cannot be denied. Improvements in electrical safety awareness and the prevention of some electrical burn injuries are evident. However, our limited resources may be better spent in properly designing, constructing and maintaining the electrical systems including retrofitting older electrical systems to make them safer. The following is a list of conclusions that were addressed in this paper: 1) High resistance grounding, especially on low voltage systems with a line to neutral voltage 150 volts or greater reduces the probability of an arcing fault significantly. 2) High resistance grounding in all probability will reduce the incident energy level of a fault significantly. Additional testing of this hypothesis needs to be performed. 3) Insulating the bus and equipment will significantly reduce the probability and most likely the severity of an arcing fault. Additional testing of this hypothesis need to be performed. 4) Future modeling of the arcing fault should involve the variables of arc voltage and arc resistance as Papallo has recently shown. This appears to be a more basic and realistic model. It is also a model that is more easily understood by electrical technicians and engineers. 5) The industry needs to recognize the vast number of variables associated with the incident energy level produced by an arcing fault. 6) The industry needs to reassess the accuracy of the data and the significance of the variables in the accuracy of arc flash calculations. As such, the significant figures being used in all of the calculations shown in the industry need to be reassessed from a practical and scientific standpoint. 7) Increased spacing of the electrodes will increase the arcing voltage, V d, and arc resistance, R d. As such, the distance can be increased to such a value as to not allow an arcing fault to occur. 8) Electromechanical reset should be used on electronic protective devices to minimize tripping time for the intermittent arcing fault which is common on 480-1,000 Volt systems and can occur on medium voltage systems. 9) The risk of death from an arc flash incident is extremely low, less than 1% of all electrical fatalities each year. While the awareness of electric injury and death has been increased with the efforts in electric arc flash hazards, a greater emphasis should be placed on reducing electric injuries and fatalities from electrocution. 10) The statistics show that the greatest number of arc flash burn injuries and fatalities occur at voltages above 240 Volts. In fact, in the OSHA provided data, during a 23 year period, there appear to have been no electric burn fatalities at voltages below 240 Volts. XII. ACKNOWLEDGEMENTS The authors wish to acknowledge Powell Industries Inc. for the testing and test videos that were used for this paper. XIII. BIBLIOGRAPHY 1) Beckstead, MW, A summary of Aluminum Combustion, Brigham Young University, May 2002, Page 19 2) Blackburn, J.L. Applied Protective Relaying, Principles and Applications, New York, NY: Marcel Dekker, Inc ) Cawley, James C. and Gerald T. Homce, Trends in Electrical Injury, , IEEE-PCIC Paper , Petroleum and Chemical Industry Committee Conference, pp Wellman 2011 Presentation 9

10 4) Dunki-Jacobs and Shields, Industrial Power System Grounding Design Handbook, Thomson-Shore Printer, Copyright ) Kaufmann, RH and Page, JC, Arcing Fault Protection for Low-Voltage Power Distribution Sytems Nature of the Problem, pp ` June ) NFPA 70, 2012 National Electrical Code, Quincy, MA:NFPA. 7) Land, H. Bruce, The Behavior of Arcing Faults in Low- Voltage Switchboards, IEEE-Transactions of Industry Applications, Vol. 44, No. 2, March/April 2008, pp ) Papallo, Thomas, Arc Flash Calculations Using Physics Based Circuit Model, IEEE PCIC Conference Record, 2011, pp ) RISK*TEX, Slips, trips and falls common at work, interagency communications from the State Office of Risk Management, Texas, April 2011, Page 1 James E. Bowen earned a BSEE degree from Texas A&M University in After working for SIP Engineering as a power engineer for three years, he joined Exxon Chemicals in His duties included maintenance, project design, construction follow-up and commissioning for the petrochemical and cogeneration processes. In 1997, Mr. Bowen joined Powell Electrical Manufacturing Company as the Technical Director where he provided leadship in the design development of MV switchgear and circuit breakers. In 2009, Mr. Bowen accepted the position of vice president of Advanced Technical Services at Dashiell Corporation where he advanced the concepts of safety by design into the high voltage substation. In 2010, Mr. Bowen accepted a position with Aramco Services Company as Power System Technologist. His current role includes investigating technologies that can be applied by Aramco to improve safety, reliability and profitability. Mr. Bowen has presented at numerous technical seminars for the IEEE Houston Section s Continuing Education on Demand. He is a professional engineer in the state of Texas and an IEEE Fellow. 10) Wellman, Craig, OSHA Arc Flash Injury Analysis, Power Point Presentation presented at the Electric Safety Subcommittee at the IEEE-IAS-PCIC 2011 Technical Conference, September 20, 2011, Toronto, Canada. 11) Fast Facts: Slips, Trips & Falls, July 2005, Page 1 XIV. VITAE John P. Nelson graduated from the University of Illinois Champaign-Urbana in 1979 with a BSEE and from the University of Colorado Boulder in 1975 with an MSEE. Mr. Nelson was employed by Public Service Company of Colorado from , Power Line Models from and with NEI Electric Power Engineering from where he is presently a principle engineer and CEO. Mr. Nelson has been active in the IAS Petroleum and Chemical Industry Committee since 1980 and is presently the Chair of that Committee. Mr. Nelson was elevated to IEEE Fellow in 1999 and is the recipient of the 2012 Harold Kaufman award. Mr. Nelson is a registered professional engineer in the state of Colorado and nine other states. Joshua D. Billman earned a BSEET from Metropolitan State College in 2006 and is located in Denver Colorado. He is presently an electrical engineer with NEI Electric Power Engineering located in Arvada, Colorado. He is a registered professional engineer in the State of California. 10

11 APPENDIX A SIGNIFICANT FIGURES AND CALCULATION ACCURACY OF MODELS Appendix A has been included to provide the reader with a refresher for the term significant figures or significant digits. The reason for this discussion is based on the apparent disregard of significant figures in both NFPA 70E and IEEE 1584 as with as other work in the area of incident energy levels. There is an odd but true saying that engineering is an exact science based on assumptions and approximations. With the use of scientific calculators and high speed computers, engineers need to recall this phrase and pay attention to it. In basic science and mathematics, most if not all engineers learned the concept of significant figures. The term significant figures which may also be referenced as significant digits refers to the concept of carrying those numerical digits that are meaningful to its precision. In this day of modern computers, all too often calculations are performed without consideration to the accuracy of the data and the meaningfulness of the answer. For example, consider a simple short circuit calculation for an industrial plant in which the following information is known: X system = 0.04 Per Unit Z trans = Per Unit (5.75% Z) Base values of per unit: 13.8 kv, 480 V and 1000 kva A calculation is performed to determine the short circuit current for a three phase, 480 Volt fault: I 3φ = 1.0 PU V/(Z sys + Z trans) Per Unit = 1.0 PU V/(j0.04+j.0575) Per Unit = 1.0/j.0975 Per Unit = Per Unit temperature and operating conditions such as transformer tap positions. Even the per unit system voltage may be above or below 1.0 per unit by + 5%. Therefore, an overall error of + 5% is not a bit unusual for such a calculation. Five percent of 12, is approximately 616 Amps resulting in an answer that could be in the range of 11,719 to 12,952 Amps. It is obvious that there is no significance with the original number of 12, Amps. Unfortunately, the basic concept of significant figures has been abandoned by many engineers in recent times, especially when it comes to utilizing software to perform complex calculations. The results from the computer software are no more accurate than the data (based on assumptions and approximations) entered into the software. In fact, the results could even be worse based on the assumptions utilized within the computer software. Therefore, some discussion is warranted on the significance of the accuracy of arc flash hazard calculations. APPENDIX B FAILURE TO TRIP The amount of incident energy in a fault is directly proportional to time. The following five photographs are of incidents that took place in a line-up of 15 kv switchgear operating at 12.5 kv. During a six month period, a similar incident took place in two separate cubicles of that switchgear. The first failure was properly cleared by high speed differential relays that provided a combination of transformer and bus differential protection. Figure A-1 one shows the cubicle where the first failure took place. I Base = (1000 kva)/ kv = Amps I 3φ = I Base I 3φ Amps = 12, Amps In reviewing the data given, the first assumption was to assume that the transformer impedance was accurate. However, ANSI/IEEE Standards allow a tolerance of +7.5% on the nameplate impedance. Next, the calculation above assumes that the impedances are purely reactive. However, with an X/R ratio of 10, the true impedance of the transformer would actually be: Z Trans = /84.3 o Or approximately, j.0572 Per Unit The term approximately is used because the angle of 84.3 o was rounded from and the number shown does not have an X/R ratio of 10, but approximately Next, the system impedance of X sys = 0.04 Per Unit may have significant errors based on numerous system conditions that may vary the system impedance. An error of + 10% or more is not a bit uncommon for system impedances. Likewise, the transformer impedance may have similar types of errors based on manufacturing tolerances, ambient conditions such as Figure A-1 Failure No. 1 11

12 The apparent failure was due to condensation on the barrier board shown in Figure A-2. Condensation formed on the barrier board associated with the breaker. Figure A-2 shows the tracking that took place between phases B and C in the breaker. Notice the minimal amount of damage on the barrier board from the fault. It is estimated that the fault was sensed and cleared in approximately 6 cycles or 0.1 seconds. and most likely there was tracking on the barrier board due to tracking. However, the substation had been out of service for approximately seven days and the 125 V dc battery discharged to approximately 50 volts. There was insufficient voltage on the battery to trip the substation main circuit breaker which ultimately was mechanically opened. Figure A-4 shows the resulting damage to the breaker cubicle. Note that the circuit breaker barrier board has not been removed in Figure A-4. Figure A-2 Tracking between Phases on Barrier Board The damage to the circuit breaker is shown in Figure A-3 which shows damage at the base of the breaker. It should be noted that this breaker was open at the time of the fault, the bus connection to the breaker was to the bottom of the breaker and the load cables/by-pass switch were connected to the top of the breaker. Figure A-4 Cubicle Damage from 2 nd Incident Figure A-3 Breaker Damage from First Incident Unfortunately, the results from the second incident were considerably different from the first. The apparent cause of the second failure was identical to the first in that the breaker was open, the live bus was connected to the lower breaker stabs Figure A-5 Breaker Damage from 2 nd Incident. 12

13 The breaker from the first incident was returned to the factory for inspection and minor repairs due to the minimal damage. However, the breaker from the second incident, Figure A-5, was a total loss as well the cubicle. APPENDIX C ELECTRICAL INJURY STATISTICS The occupational electrical fatal and nonfatal statistics were presented in the paper. In order to put these numbers into perspective, the numbers have been included in the charts and tables are included in this appendix. Figure C-1 shows the top ten causes of occupational fatal injuries with electricity listed as the sixth leading event. Table C-1 provides the tabulated number of fatalities per event along with the percentage. It should be noted that fatal electrical injuries account for approximately 5% of the occupational fatal injuries and electrical burn fatalities less than 1% or those or less than 0.05% of the occupational fatal injuries Occupational Fatal Injuries Transporation Violent Acts Falls Struck by Caught in Electricity Aircraft Exposure Watercraft Explosions All others Electrical Burns(est) Figure C Occupational Injuries by Event - Chart Table C Occupational Fatal Injuries by Event - Table DESCRIPTION NUMBER OF INCIDENTS PCT OF INCIDENTS Transportation 23, Violent Acts 12, Falls 7, Struck by 6, Caught in 4, Electricity 3, Aircraft 3, Exposure to 2, Watercraft 1, Explosion 1, Other Causes 2, Electrical Burns 34 (est) 0.05 Now contrast the number of occupational nonfatal electrical injuries to the number of occupational fatal electrical injuries. During the same reporting period of , there were a total of 47,406 occupational nonfatal injuries of which 29,046 were reported to be shock related injuries while 13,360 were reported to be burn related injuries. See Figure C-2 which compares occupational electric Fatalities to Figure C-3 which compare occupational nonfatal injuries. Shock injuries by contact are by far the greatest threat to the worker for a fatal injury over a burn related fatality. With regard to nonfatal injuries, burn related injuries are significant but still account for less than one third of the nonfatal electrical injuries. Shock by contact is still the greatest threat to the electrical worker. See Figure C ,000 45,000 40,000 35,000 30,000 25,000 20,000 15,000 10,000 5,000 0 Occupational Electrical Fatal Injuries Total Electric Fatalities Figure C Occupational Electric Fatalities Fatality by Electrocution Fatality by Electrical Burns Occupational Nonfatal Electrical Injuries Nonfatal Electrical Injuries Electrical Burns Electrical Shock Figure C Occupational Nonfatal Electrical Injuries The injury statistics have been misrepresented in the literature where it has been reported that one to two workers a day are killed by arc flash and that people a day require professional medical treatment. 24 In actuality, the fatal electrical arc-flash injuries have been exaggerated by over 18,000% and the number of arch-flash injuries has been exaggerated by approximately 250%. 24 H. Bruce Land, III The behavior of Arcing Faults in Low-Voltage Switchboards. 13