Presentation on theme: "1 Electrical Awareness Arc Flash Overview & Qualifications By: Joseph F. Maida, PE August 18, 2009 Fort Washington, PA P 215.542.8700 F 215.542.5652 Orlando,"— Presentation transcript:
1 Electrical Awareness Arc Flash Overview & Qualifications By: Joseph F. Maida, PE August 18, 2009 Fort Washington, PA P F Orlando, FL P F Visit us online at
2 Joseph F. Maida, P.E. President Maida Engineering, Inc. Qualifications: BSEE – Drexel University – 1971 MSEE – Drexel University (Power) – 1976 Licensed Electrical Contractor Officer - US Army Reserve 1971 – 1979 Delmarva Power & Light Co.1972 – 1974 Day & Zimmermann, Inc Maida Engineering, Inc. – 1978 – Present PE License – PA (NJ, NY & DE 1976) PE License – ID, MA, RI, NC, GA, FL, TX, IA, WV, AK and Alberta, Canada LEED Accredited Professional PA UCC Review and Advisory Council
3 Why the NEC ® ? The National Electrical Code NFPA 70 - NEC ® is the least amended model code in the world and no court in the USA has faulted anyone for using the latest version of the NEC ®, even when the local code was not updated International Building Code has incorporated the 2008 NEC ®. 1.
4 NEC ® Article Flash Protection. Equipment such as switchboards, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are in other than dwelling occupancies and are likely to require examination, adjustment, servicing, or maintenance while energized shall be field marked to warn qualified persons of potential electric arc flash hazards. The marking shall be located so as to be clearly visible to qualified persons before examination, adjustment, servicing, or maintenance of the equipment.
5 NEC ® Article Flash Protection. FPN No. 1: NFPA 70E-2004, Standard for Electrical Safety in the Workplace, provides assistance in determining severity of potential exposure, planning safe work practices, and selecting personal protective equipment. FPN No. 2: ANSI Z , Product Safety Signs and Labels, provides guidelines for the design of safety signs and labels for application to products.
6 NEC ® Definition Panelboard. A single panel or group of panel units designed for assembly in the form of a single panel, including buses and automatic overcurrent devices, and equipped with or without switches for the control of light, heat, or power circuits; designed to be placed in a cabinet or cutout box placed in or against a wall, partition, or other support; and accessible only from the front.
7 NEC ® Definition Switchboard. A large single panel, frame, or assembly of panels on which are mounted on the face, back, or both, switches, overcurrent and other protective devices, buses, and usually instruments. Switchboards are generally accessible from the rear as well as from the front and are not intended to be installed in cabinets.
8 NEC ® Definition Motor Control Center. An assembly of one or more enclosed sections having a common power bus and principally containing motor control units. ARTICLE 312 Cabinets, Cutout Boxes, and Meter Socket Enclosures Scope. This article covers the installation and construction specifications of cabinets, cutout boxes, and meter socket enclosures.
9 NEC ® Article Industrial Control Panel. An assembly of two or more components consisting of one of the following: (1) Power circuit components only, such as motor controllers, overload relays, fused disconnect switches, and circuit breakers (2) Control circuit components only, such as pushbuttons, pilot lights, selector switches, timers, switches, control relays (3) A combination of power and control circuit components These components, with associated wiring and terminals, are mounted on or contained within an enclosure or mounted on a sub-panel. The industrial control panel does not include the controlled equipment.
10 NEC ® Definition NFPA 70 National Electrical Code ®, 2008 Edition Qualified Person. One who has skills and knowledge related to the construction and operation of the electrical equipment and installations and has received safety training to recognize and avoid the hazards involved. FPN: Refer to NFPA 70E®-2004, Standard for Electrical Safety in the Workplace, for electrical safety training requirements.
11 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition Qualified Person. A qualified person shall be: Trained and knowledgeable of the construction and operation of equipment or a specific work method Trained to recognize and avoid the electrical hazards that might be present with respect to that equipment or work method.
12 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition Qualified persons shall be familiar with the proper use of: The special precautionary techniques, Personal Protective Equipment, including Arc-flash, Insulating and Shielding materials, Insulated tools and test equipment.
13 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition A person can be considered qualified with respect to certain equipment and methods but still be unqualified for others.
14 What is an Arc Flash? Question What is an arc flash?
15 Arc Flash NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition – Appendix K Arc-Flash - When an electric current passes through air between ungrounded conductors or between ungrounded conductors and grounded conductors. Exposure to these extreme temperatures both burns the skin directly and causes ignition of clothing, which adds to the burn injury. Arc-flashes can and do kill at distances of 3 m (10 ft).
16 What is an Arc Flash? Arc-Flashes occurs when electricity flows through air between two (2) parts of a power circuit which are not at the same voltage. The parts could be two (2) conductors of different phases or a phase conductor and ground when the system is grounded. Arc flashes are more likely to occur where dust or humidity are present or after an initial electrical short because the vaporized particles provide low impedance media for current to flow between the phases or to ground. Circuit breakers can explode if subjected to short circuit currents higher than their rating.
17 What is an Arc Flash? Electricity can flow through any medium but cannot flow through a vacuum. Insulated materials present a significantly high impedance but will conduct more electricity as the voltage increases. The majority of hospital admissions due to electrical accidents are from arc-flash burns. Each year more than 2,000 people are admitted to burn centers with severe arc-flash burns. Almost 5 people every day. Hazards are shown on the following slide.
18 What is an Arc Flash? Flame Retardant Clothing Provides added protection from arc flash burns only! Electrical hazards are shown on the following slide.
19 What is an Arc Flash?
20 Electric Hazards Electric Shock – Electricity, resulting from electromagnetism, passing through the body can cause shock, cardiac arrest and internal burns. Arc-Flash Burns – Caused by air that can reach 35,000° F. Hotter than the sun! Hot enough to vaporize metal. Arc-Flash Blasts – Pressure waves that will throw a person away from the blast into walls and other equipment or off ladders and platforms.
21 Electric Hazards Intense Light – Electric arcs can create light that will damage eyes and may cause cataracts years later. Sound Waves – Noise levels that can cause temporary or permanent loss of hearing Projectiles – Molten pieces of metal, vapors and shrapnel that can penetrate flame retardant clothing.
22 Why now and not before? Question Why now and not before?
23 Why now and not before? Electrical power distribution system are being designed to generate and distribute more electrical energy. Personnel are more often exposed to recognized electrical hazards that could cause death or serious physical harm when examining, adjusting, servicing, or maintaining electrical equipment. Understanding of the arc flash hazards has increased Personal Protective Equipment (PPE) to prevent death or severe injuries due to arc flash burns are available.
24 Why now and not before? In the early 1980s, Dupont developed flame retardant material (Nomex). Ralph Lee, a Dupont Consultant, looking for uses for the new flame retardant material, performed tests to determine the effect heat from an electric arc flash would have on human skin. Ralph Lee developed the first set empirical equations \ that could be used to calculate incident energy associated with an arc flash. The equations are still used for electrical power systems rated above 15,000 Volts.
25 Why now and not before? Ralph Lees work showed that skin temperatures above 96°C for.1 seconds or 6 cycles would result in incurable 3rd degree burns and that at a temperature of 80°C the skin would be just curable or sustain a 2nd degree burn. In 1998, Dougherty, Neill and Floyd developed the first Equations which considered the effect of an arc in a box versus an arc in open air. They developed the Arc Hazard Classifications for flame retardant clothing. Their equations are only for systems rated below 1,000 volts with short circuit currents between 16,000 and 50,000 amps. Equations are no longer utilized.
26 Why now and not before? An IEEE industry group sponsored by petroleum and chemical industries developed the latest formulas that are published in the IEEE Standard Guide for Performing Arc Flash Calculations. It contains: Empirical formulas, derived from tests that can be used for 3Ø power systems up to 15,000 volts and for short circuit currents between 700 and 106,000 amps. Generally, no need for calculation on systems rated at 50 volts or less and on systems rated 240 volt and less that are derived from transformers rated less than 125 KVA. The guide does not state if this rating is for 1Ø and 3Ø transformers.
27 Why now and not before? IEEE Standard Guide for Performing Arc Flash Calculations contains equations to calculate Arc Flash Incident Energy based on: The short circuit current and duration of an arc that consider if the arc occurs within a box or in open air and if the power system is grounded or ungrounded. The short circuit current and the type of fuse or circuit breaker, if the arc flash current will trip the circuit breaker or blow the fuse. Incident Energy is the amount of energy impressed on a surface a certain distance from the source.
28 Why now and not before? Occupational Safety and Health Administration (OSHA) was created in OSHA has worked with employers and employees to provide a better working environment and since its creation, has helped to cut workplace fatalities by more than 60 percent and occupational injury and illness rates by 40 percent while employment in the United States has doubled. 1 OSHA is part of the US Department of Labor. OSHA regulations are written under Title 29 of the Code of Federal Regulations 1.
29 Why now and not before? On January 16, 1981, OSHA by reference incorporated the relevant requirements from Part 1 of then new NFPA 70E as its electrical standard for general industry. The first substantial changes to NFPA 70E were introduced in 1995 and included a consensus standard on work practices and PPE application based on theoretical modeling developed by Ralph Lee. Subpart S of 29 CFR Part 1910 was updated on August 13, 2007 to reference NFPA 70E – 2000 and NFPA 70 – 2002.
30 Why now and not before? Calculations: - Methods and Empirical Equations Developed by - Ralph Lee IEEE Group (1584) – 2002 Charts: - Published within NFPA 70E – 2004 Calculators -Provided with IEEE 1584 – 2002 Computer Software -Developed by SKM Systems Inc. and others
31 Why now and not before? Calculation - IEEE Group (1584) – 2002 Empirical Formulas that consider for many variables including short circuit current and time of exposure. Formulas specific to types of fuses. General Formulas that are based on fuse or breaker sizes and available short circuit current. Defines when calculations are not required. Refers to Lees equations for systems greater than 15,000 Volts.
32 Why adhere to NFPA 70 E Question Why adhere to NFPA 70E Article Working on or near live parts ?
33 Why adhere to NFPA 70 E OSHA Non-mandatory Compliance Guidelines for Hazard Assessment and Personal Protective Equipment Selection Subpart I App B Electrical Codes NFPA 70 National Electrical Code ®, 2008 Edition NFPA 70E Standard for Electrical Safety in the Workplace, 2009 Edition
34 Why adhere to NFPA 70 E? With the addition of Article in the 2002 National Electrical Code (NEC®), employers and employees have become aware of electric arc flash hazard. With the publication of NFPA 70E Standard for Electrical Safety in the Workplace, 2009 Edition, employers and employees have methods to safeguard employees from at least one of the electric arc flash hazards. 1: NFPA 70E Standard for Electrical Safety in the Workplace, 2004 Edition
35 Why adhere to NFPA 70 E?
36 January 9, 2002 Conclusion: Though OSHA does not, per se, enforce NFPA 70E Standard, 2000 Edition, OSHA considers the NFPA Standard a recognized industry practice. The employer is required to conduct hazard assessment in accordance with 29 CR (d)(1). If a arc flash hazard is present, or likely to be present, then the employer must select and require the employees to use the selected apparel.
37 January 9, 2002 Employers who conduct the hazard/risk assessment, and select and require their employees to use protective clothing and other PPE appropriate for the task, as stated in NFPA 70E 2000 edition, are deemed in compliance with the Hazard Assessment and Equipment Selection OSHA Standard U. S. Department of Labor – Jan. 9, 2002
38 OSHA OSHA commonly uses the General Duty Clause, which is paraphrased below or as an alternate uses NFPA 70E or the NEC for citations related to arc flash incidents. General Duty Clause: Section 5(a)(1) of the Occupational Safety and Health Act requires an employer to furnish to its employees employment and a place of employment which are free from recognized hazards that are causing or are likely to cause death or serious physical harm to his employees.
39 Why adhere to NFPA 70 E Answer It is the right thing to do!
41 Arc Flash Labels Question What is and where are Arc Flash Label Required?
42 Arc Flash Labels Arc Flash labels are required on equipment such as panelboards, switchboards, motor control centers, industrial control panels and meter sockets, Arc Flash labels are required on motor starters, variable frequency drives, plug-in bus duct, equipment control panels and building management panels if they contain equipment that is 50 volts or higher and are likely to require examination, adjustment, servicing or maintenance while energized.
43 Arc Flash Hazard Labels The following arc flash label would suffice in meeting the requirements of NEC Article but may not suffice in meeting NFPA 70E – Equipment Labeling. Equipment shall be field marked with a label containing the available incident energy or required level of PPE. 1. NFPA 70E 2009
44 Arc Flash Hazard Labels Arc flash labels can contain other information that can be provided by the owner or employer to the employee when needed based on the system configuration at the time
45 Arc Flash Hazard Labels Limited Approach: The closest distance an unqualified person can approach unless made aware of the danger and accompanied by a qualified person. Restricted Approach: The closet distance a qualified person can approach with proper PPE and tools. Prohibited Approach: The minimum distance to prevent flashover and arcing. Flash Protection Boundary: The distance where the energy from the arc will not cause a 3 rd degree burn to unprotected skin.
46 Arc Flash Hazard Labels
47 Shock Hazard Analysis Electric Shock Hazard Analysis The following chart is the basis for determining electric shock boundaries. It is contained within NFPA 70 E The actual chart covers voltage as high as 800,000 volts.
48 Shock Hazard Analysis Table 130.2(C) Approach Boundaries to Energized Electrical Conductors or Circuit Parts for Shock Protection (All dimensions are distance from energized electrical conductor or circuit part to employee Limited Approach Boundary Restricted Approach Nominal System Exposed Exposed Boundary; Includes Prohibited Voltage Range, Movable Fixed Inadvertent Movement Approach Phase to Phase Conductor Circuit Part Adder Boundary Less than 50 Not specified Not specified Not specified Not specified 50 to ft 0 in. 3 ft 6 in Avoid contact Avoid contact 301 to ft 0 in. 3 ft 6 in. 1 ft 0 in. 1 in. 751 to 15 kV 10 ft 0 in. 5 ft 0 in. 2 ft 2 in. 7 in kV to 36 kV 10 ft 0 in. 6 ft 0 in. 2 ft 7 in. 10 in.
49 Arc Flash Hazard Analysis Arc Flash Hazard Analysis are performed to determine the Arc Flash Hazard Category And Arc Flash Protection Boundary
50 Arc Flash Labels Arc Flash Hazard Analysis. An arc flash hazard analysis shall determine the Arc Flash Protection Boundary and the Personal Protective Equipment that people, within the Arc Flash Protection Boundary, shall use. 1 The Arc Flash Protection Boundary is the distance at which a person is likely to receive a second-degree burn. All parts of the body inside the Arc Flash Protection Boundary shall be protected NFPA 70E -2009
51 Arc Flash Hazard Analysis Arc Flash Hazard Category – Is used to define the appropriate level of Personnel Protective Equipment based on a working distance in front of exposed electrical parts and circuits. Exposed - Capable of being inadvertently touched or approached nearer than a safe distance by a person. It is applied to electrical conductors or circuit parts that are not suitably guarded, isolated, or insulated NFPA 70E -2009
52 Arc Flash Hazard Analysis Typical working distances used for incident energy calculations are as follows: Low voltage (600 V and below) MCC and panelboards 455 mm (18 in.) Low voltage (600 V and below) switchgear 610 mm (24 in.) Medium voltage (above 600 V) switchgear 910 mm (36 in.) Reference – NFPA 70E – 2009 Appendix D
53 Arc Flash Hazard Analysis Almost all arc flash analysis require knowing the available short circuit current. Table 130.7(C)(9) within NFPA 70E can be used in lieu of arc flash calculations if one know the maximum available 3 phase bolted fault current and the overcurrent device clearing time at and for the equipment. Part of the Table 130.7(C)(9) Hazard/Risk Category Classifications and Use of Rubber Insulating Gloves and Insulated and Insulating Hand Tools from NFPA 70E – 2009 and selected footnotes for the Table are shown on the following slides.
54 Arc Flash Hazard Analysis Table (C)(9) from NFPA 70E Task Performed on Energized Equipment Hazard/Risk V-rated V-rated Category Gloves Tools Panelboards and Other Equipment Rated 240 V and Below Notes 1 Perform infrared thermography and other non-contact inspections outside the restricted approach boundary 0 N N Circuit breaker (CB) or fused switch operation with covers on 0 N N CB or fused switch operation with covers off 0 N N Work on energized electrical conductors and circuit parts, including voltage testing 1 Y Y Remove/install CBs or fused switches 1 Y Y Removal of bolted covers (to expose bare, energized electrical conductors and circuit parts) 1 N N Opening hinged covers (to expose bare, energized electrical conductors and circuit parts) 0 N N Work on energized electrical conductors and circuit parts of utilization equipment fed directly by a branch circuit of the Panelboard 1 Y Y
55 Arc Flash Hazard Analysis Footnotes from Table (C)(9) NFPA 70E kA short circuit current available, 0.03 second (2 cycle) fault clearing time kA short circuit current available, 0.03 second (2 cycle) fault clearing time. Generally, the short circuit current must be within the instantaneous operating time of the circuit breaker or the current limiting range of a current limiting fuse.
56 Arc Flash Hazard Analysis Table (C)(9) from NFPA 70E Both larger and smaller available short-circuit currents could result in higher available arc flash energies. * If the available short-circuit current increases without a decrease in the opening time of the overcurrent protective device, the arc flash energy will increase. * If the available short-circuit current decreases, resulting in a longer opening time for the overcurrent protective device, arc flash energies could also increase NFPA 70 E 2009
57 Source of Power
58 Arc Flash Hazard Analysis
59 Arc Flash Hazard Analysis Who is Qualified?
60 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition Qualified persons permitted to work within the Limited Approach Boundary of exposed energized electrical conductors and circuit parts operating at 50 volts or more shall, at a minimum, be additionally trained in all of the following: (1) The skills and techniques necessary to distinguish exposed energized electrical conductors and circuit parts from other parts of electrical equipment.
61 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition (2) The skills and techniques necessary to determine the nominal voltage of exposed energized electrical conductors and circuit parts (3) The approach distances specified in Table 130.2(C) and the corresponding voltages to which the qualified person will be exposed. (4) The decision-making process necessary to determine the degree and extent of the hazard and the personal protective equipment and job planning necessary to perform the task safely
62 Task Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition An employee who is undergoing on-the-job training and who, in the course of such training, has demonstrated an ability to perform duties safely at his or her level of training and who is under the direct supervision of a qualified person shall be considered to be a qualified person for the performance of those duties. Tasks that are performed less often than once per year shall require retraining before the performance of the work practices involved.
63 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition Employees shall be trained to select an appropriate voltage detector and shall demonstrate how to use a device to verify the absence of voltage, including interpreting indications provided by the device. The training shall include information that enables the employee to understand all limitations of each specific voltage detector that may be used.
64 Qualified Person NFPA 70 E Standard for Electrical Safety in the Workplace, 2009 Edition The employer shall document that each employee has received the training required. This documentation shall be made when the employee demonstrates proficiency in the work practices involved and shall be maintained for the duration of the employees employment. The documentation shall contain each employees name and dates of training.
65 NFPA 70E The arc flash hazard analysis shall take into consideration the design of the overcurrent protective device and its opening time, including its condition of maintenance. Exception No. 1: An arc flash hazard analysis shall not be required where all of the following conditions exist: (1) The circuit is rated 240 volts or less. (2) The circuit is supplied by one transformer. (3) The transformer supplying the circuit is rated less than 125 kVA.
66 Arc Flash Hazard Analysis Therefore, there is NO requirement to determine the following for AC power system rated below 240 Volts and above 50 volts originating from one transformer rated below 125 KVA. Arc Flash Protection Boundary Personal Protective Equipment Arc Flash Hazard Category
67 Arc Flash Hazard Labels The following arc flash label would suffice in meeting the requirements of NEC Article and NFPA 70E – 2009.
68 Arc Flash Hazard Analysis NFPA 70 E defines Arc Flash Protection Boundary as follows When an arc flash hazard exists, an approach limit at a distance from a prospective arc source within which a person could receive a second degree burn if an electrical arc flash were to occur
69 Arc Flash Hazard Analysis NFPA 70E states the following: When an employee is working within the Arc Flash Protection Boundary he or she shall wear protective clothing and other personal protective equipment in accordance with All parts of the body inside the Arc Flash Protection Boundary shall be protected.
70 Arc Flash Hazard Analysis IEEE 1584 – 2002 states the following relative to the use of PPE: Where used, PPE for the arc-flash hazard is the last line of defense. The protection is not intended to prevent all injuries but to mitigate the impact of an arc flash upon the individual, should one occur. In many cases, the use of PPE has saved lives or prevented injury. The calculations in this guide will lead to selection of a level of PPE that is a balance between the calculated estimated incident energy exposure and the work activity being performed while meeting the following concerns:
71 Arc Flash Hazard Analysis a)The desire to provide enough protection to prevent a second degree burn in all cases. b) The desire to avoid providing more protection than is needed. Hazards may be introduced by the garments such as heat stress, poor visibility, and limited body movement.
72 Arc Flash Hazard Analysis Without arc flash hazard analysis, which at a minimum requires the determination of the potential short circuit current and knowledge of the overcurrent protective device, short of testing, it is impossible to determine the Incident Energy. Therefore, without arc flash hazard analysis, the only information that can be included on label is the Required Level of PPE.
73 Arc Flash Hazard Analysis NFPA 70 E does not define what is meant by the Required Level of PPE and it only defines methods that should be used for the selection of protective clothing and other personnel protective equipment for work performed within the Arc Flash Protection Boundary.
74 Arc Flash Hazard Analysis Arc Flash Labels used to mark equipment for which arc flash hazard analysis has not been performed should indicate that the Level of PPE shown on the label are DEFAULT And NO ARC FLASH HAZARD ANALYSIS WAS PERFORMED An Arc Flash Protection Boundary need not be listed on the label.
75 Arc Flash Hazard Analysis Considering that the hazard within the equipment could be equivalent to the hazard associated with changing a 120 volt. 100 watt light bulb, the requirement for PPE should depend on the type of equipment. Examples of Level of PEE that could be defined on a label, as determined by the employer, could include :
76 Arc Flash Hazard Analysis Panelboards – Arc Hazard Category 0 Industrial Control Panels - Arc Hazard Category 0 Motor Controllers – Arc Flash Hazard Category 0 Building Management Panels – No Arc Flash Hazard Category Instrumentation Panels - No Arc Flash Hazard Category
77 Arc Flash Hazard Analysis Even if no Arc Flash Hazard Category is defined, Safety glasses or goggles and hearing protection should be required for any electrical work near exposed energized parts except for circuits originating from Class II power sources as defined by Section 725 of the NEC.
78 Arc Flash Hazard Analysis Based on the selection of standard personal protective equipment (PPE) levels (1.2, 8, 25, and 40 cal/cm2), it is estimated that the PPE is adequate or more than adequate to protect employees from second-degree burns in 95 percent of the cases.
80 Arc Flash Hazard Analysis When incident energy exceeds 40 cal/cm2 at the working distance, greater emphasis than normal should be placed on de-energizing before working on or near the exposed electrical conductors or circuit parts.
81 Arc Flash Hazard Analysis What are the formulas?
82 Arc Flash Hazard Analysis SKM Software Incident Energy Calculations: To determine the incident energy based on the arcing fault currents, first use empirically derived equation shown on the next slide to determine the log10 of the normalized incident energy. The equation is based on data normalized for an arc time of 0.2 second and a distance from the possible arc point to the person of 610 mm. Afterwards convert from the normalized value using the equations on the subsequent slides.
83 Arc Flash Hazard Analysis For Applications with system voltages below 1,000 Volts where: l g = the log10 Ia = arcing current in kA K = for open air arcs; for arcs-in-a-box I bf = bolted three-phase available short-circuit current (symmetrical rms) (kA) between 700 and 106,000 amps V = system voltage in kV G = conductor gap (mm) (See Table on following slide)
84 Arc Flash Hazard Analysis For applications with a voltage of 1,000 Volts or higher: where: lg = the log10 I a = arcing current in kA I bf = bolted three-phase available short-circuit current (symmetrical rms) (kA) V = system voltage in kV G = conductor gap (mm) (See Table on previous slide)
85 Arc Flash Hazard Analysis Table D.8.2 Factors for Equipment and Voltage Classes System Typical Voltage ConductorDistance (kV) Type of Equipment Gap (mm) X-Factor Open-air 10– –1 Switchgear MCCs and panels Cables Open-air >1–5 Switchgear 13– Cables Open-air 13– >5–15 Switchgear Cables
86 Arc Flash Hazard Analysis For all applications two (2) time durations are used to calculate the incident energy from the TCC Curve: Use 0.85I a to find a second arcing time. This second arc current accounts for variations in the arcing current and the time for the overcurrent device to open. Calculate the incident energy using both values (I a and 0.85 I a ), and use the higher value.
87 Source of Power
88 Arc Flash Hazard Analysis Normalized Incident Energy Calculations: where: E n = incident energy (J/cm2) normalized for time and distance k 1 = for open air arcs; for arcs-in-a-box k 2 = 0 for ungrounded and high-resistance grounded systems = for grounded systems G = the conductor gap (mm) (See Table on previous slide)
89 Arc Flash Hazard Analysis Converting from Normalized Incident Energy: where: E = incident energy in J/cm2 C f = calculation factor = 1.0 for voltages above 1 kV = 1.5 for voltages at or below 1 kV E n = incident energy normalized t = arcing time (seconds) from TCC Curve D = distance (mm) from the arc to the person (working distance) X = the distance exponent from Table on previous slide
90 Arc Flash Hazard Analysis Flash Boundary Calculation: The Flash Protection Boundary is the distance at which a person is likely to receive a second-degree burn. The onset of a second-degree burn is assumed to be when the skin receives 5.0 J/cm 2 or 1.2 cal/cm 2 of incident energy. The formula on the following slides is then used to calculate the Flash Boundary Distance.
91 Arc Flash Hazard Analysis Flash Boundary Calculation: where: D B = the distance (mm) of the Flash Protection Boundary from the arcing point C f = a calculation factor = 1.0 for voltages above 1 kV = 1.5 for voltages at or below 1 kV E n = incident energy normalized E B = incident energy in J/cm2 at the distance of the Flash Protection Boundary t = time (seconds) X = the distance exponent from Table I bf = bolted three phase available short-circuit current V = system voltage in kV
92 Arc Flash Hazard Analysis Equations for estimating incident energy and Flash Protection Boundaries based on statistical analysis and curve fitting of available test data was produced by an IEEE working group that produced the data from tests it performed to produce models of incident energy. Based on the selection of standard personal protective equipment (PPE) levels (1.2, 8, 25, and 40 cal/cm2), it is estimated that the PPE is adequate or more than adequate to protect employees from second-degree burns in 95 percent of the cases.
93 Arc Flash Hazard Analysis Question What can be done to reduce arc flash exposure?
94 Arc Flash Hazard Analysis Design with Arc Flash in mind Install smaller transformers KVA or less for 240 Volt and 120/208 Volt Power Systems. Compartmentalize Use individual secondary main circuit breakers that are in separate compartments or enclosures Require designs that have a maximum arc flash hazard incident energy of 8 cals/cm 2.
95 Arc Flash Hazard Analysis Design with Arc Flash in mind Do not oversize transformers KVA SC = KVA BASE / Z PU I SC = (KVA SC X 1000) / (V LL X 1.732) where I SC is in amperes, V LL is in volts, and Z PU is based on the transformer rated KVA Install new light sensing relays in medium voltage switchgear Apply overcurrent protective devices properly Use circuit breakers with a maintenance instantaneous trip switch.
96 Arc Flash Hazard Analysis NFPA 70 National Electrical Code ®, Many Editions Interrupting Rating. Equipment intended to interrupt current at fault levels shall have an interrupting rating sufficient for the nominal circuit voltage and the current that is available at the line terminals of the equipment. Equipment intended to interrupt current at other than fault levels shall have an interrupting rating at nominal circuit voltage sufficient for the current that must be interrupted.
97 Arc Flash Hazard Analysis Arc Flash Hazards can be reduced by clearing the arcing fault faster by doing one of the following: Reduce Existing Pickup and Delay Settings wherever possible. Enable Instantaneous Functions or Retro-fit with Instantaneous Functions Reduce Fuse Sizes wherever possible. Use Current-limiting breakers or fuses for high arcing fault currents Add Differential Protection Use Temporary Instantaneous Trip Settings when work is being performed Add optical sensors to trip when flash occurs
98 Flash Hazard Analysis
99 Arc Flash Hazard Analysis
100 Arc Flash Hazard Analysis
101 Arc Flash Hazard Analysis
102 Arc Flash Hazard Analysis Must consider all of the sources of power including generators run in parallel and motors. Cannot account for services that only have overload protection and hence must be consider DANGEROUS. Must account for current limiting fuses operating within the current limiting range. Panelboards on the secondary of a transformer may present a very high hazard because the trip time of the transformers primary protection is very long for a fault on the secondary of the transformer.
103 Arc Flash Hazard Analysis
104 Arc Flash Hazard Analysis Arc Flash Hazards can be reduced by current limiting fuses. The clearing time for a current limiting fuse is approximately ¼ cycle or second. The clearing time of a 5 kV and 15 kV circuit breaker is approximately 0.1 second or 6 cycles. This can be broken down as follows: actual breaker time (approximately 2.0 cycles), plus relay operating time of approximately 1.74 cycles, plus an additional safety margin of 2 cycles, giving a total time of approximately 6 cycles.
105 Arc Flash Hazard Analysis Arc Flash Hazards can be reduced by: Current Limiting Fuses and Cable Limiters
106 Arc Flash Hazard Analysis Arc Flash Hazards can be reduced by: Current Limiting Fuses and Cable Limiters
107 Arc Flash Analysis Question What is required to do an arc flash analysis?
108 Arc Flash Analysis All analyses require the following: Accurate One Line Diagrams Overcurrent Device Equipment Data Realistic Short Circuit Current Calculations Arc Flash Calculations or Fault Clearing Time. A copy of NFPA 70E Standard for Electrical Safety in the Workplace, 2004 Edition
109 Arc Flash Analysis General arc flash study procedure 1 1.Collect field data sufficient to perform a short-circuit and coordination study. 2.Identify the possible system operating modes including tie-breaker positions, parallel generation, etc. 3.Calculate the bolted fault current at each fault location. 4.Calculate the arcing fault current flowing through each branch for each fault location. 1. Arc Flash Users Guide – SKM Power Systems, Inc.
110 Arc Flash Analysis General arc flash study procedure 1 5. Determine the time required to clear the arcing fault current using the protective device settings and associated trip curves. 6. Select the working distances based on system voltage and equipment class. 7. Calculate the incident energy at each fault location. 8. Calculate the flash protection boundary at each fault location. 1. Arc Flash Users Guide – SKM Power Systems, Inc.
111 Overall Benefits Question What are the overall benefits of an arc flash analysis?
112 Overall Benefits Why Perform Arc Flash Studies? Possibly prevent worker injury or death Avoid or reduce litigation expense associated with an electrical injury. Comply with codes and safety regulations (OSHA, NFPA, NEC) thereby avoiding citations and fines. Insurance requirements Because you can and you want to!
113 Overall Benefits Potential Benefits from an Arc Flash Studies? Lesson injuries caused by an arc flash thus increasing worker safety and productivity. Minimize equipment damage and system down time Increase selectivity and hence the reliability of the Power Distribution System. Reduce the cost of future electrical projects. Reduce maintenance cost.
114 Arc Flash Summary Proper Maintenance Define and follow necessary PM procedures Knowledge Understand the true dangers of electricity Planning Develop and follow a work plan that includes identifying the arc flash hazard STAY ALERT – STAY WELL – STAY ALIVE
Questions? Additional Presentations ARC FLASH OVERVIEW ARC FLASH LABELS AND REPORTING REQUIREMENTS FR CLOTHING AND PPE PERFORMING ARC FLASH ANALYSIS VS USING CHARTS & TABLES UNDERSTANDING SHORT CIRCUIT AND ARC FLASH CALCULATIONS AN OVERVIEW IEEE 1584 Joseph Maida President x141