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Safety Engin Methods

by: Loy King

Safety Engin Methods IOE 539

Loy King
GPA 3.76

W. Keyserling

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W. Keyserling
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This 54 page Class Notes was uploaded by Loy King on Thursday October 29, 2015. The Class Notes belongs to IOE 539 at University of Michigan taught by W. Keyserling in Fall. Since its upload, it has received 37 views. For similar materials see /class/231585/ioe-539-university-of-michigan in Industrial Engineering at University of Michigan.

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Date Created: 10/29/15
Halon Notes IOE 539 HALONS AND THE ENVIRONMENT W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved MONTREAL AGREEMENT 1987 United Nations Protocol on Substances that Deplete the Ozone Layer A Halon is an ozone depleting agent B In the US there is a tax of 2650 per pound for new Halon Therefore production has ceased Fixed extinguishing systems typically hold 6001200 lbs The cost of replenishing with new taxed Halon is more expensive than installing a new system with a different extinguishing agent C Recycled Halon is available and not subject to tax However the supply is finite and diminishing DESIRED FEATURES OF REPLACEMENT AGENTS TO HALON A Excellent extinguishing capability B No residue particularly critical around computers and other electronic equipment C Nontoxic in concentration levels that extinguish fire D Littleno decrease in visibility following discharge E Acceptable cost D Can be used in existing storage and distribution systems HISTORY AND CURRENT USES A Naming scheme Halon abcde where a is the number of carbon atoms b is the number of fluorine atoms c is the number of chlorine atoms d is the number of bromine atoms e is the number of iodine atoms if there are no iodines the fifth digit is dropped Note Many halons are toxic or carcinogenic eg carbon tetrachloride CCI4 072000 Page 1 In the US Halon 1301 used in fixed systems while 1211 is used in portable systems 0 Halons do not ionize on decomposition therefore they are nonconductive Effective extinguishing occurs at volume concentrations of 5 to 10 percent These levels are low enough to allow occupants to escape without excessive exposure Halons interfere with the fire chain reaction extinguishing certain types of fires eg flammable liquids very quickly It is difficult to find a nonozonereducing substitute for Halon that has equivalent firefighting effectiveness IV REPLACEMENT AGENTS See NFPA 2001 Standard on a clean agent fire extinguishing system A Heptafluoropropane CF30HFCF3 Trade Name FM200 Halon Notes 1 Effective extinguishing agent at 7 volume concentration no asphyxiation or overexposure at this level 2 To achieve weightvolume equivalent of Halon 1301 storage capability must be increased by 70 percent 3 No effects on ozone layer Rated by EPA as an acceptable Halon substitute Perfluorobutane C4F10 Trade Name CEA410 1 Effective extinguishing agent at 7 volume concentration no adverse effects at up to 40 concentration based on toxicity tests on dogs This is better than Halon 1301 2 To achieve weightvolume equivalent of Halon 1301 storage capability must be increased by 100 percent ie doubled Piping and nozzles may need to be modified in order to handle increased volume when converting from Halon 1301 3 No effects on ozone layer However high potential for global warming since it acts as a greenhouse gas Rated by EPA as acceptable Halon substitute only where other alternatives are not technically feasible or where human exposure to high concentrations is a likely scenario lnergen trade name Mixture of 52 nitrogen 40 argon 8 carbon dioxide 1 Acts as a suffocating agent by reducing available oxygen to 125 percent below the 15 level needed for flaming combustion 2 125 oxygen is normally not sufficient for maintaining consciousness However ambient carbon dioxide increases to more than 4 times normal levels Since this increases respiration there may be sufficient oxygen exchange to maintain consciousness allowing healthy persons without obstructive pulmonary disease to escape 3 To achieve weightvolume equivalent of Halon 1301 storage capability must be increased by a factor of 10 4 No effects on ozone layer and approved for normally occupied areas 072000 Page 2 JOB SAFETY ANALYSIS JSA W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved Job Safety Analysis JSA is a hazard identification technique that involves breaking a 39ob down into basic work elements A Each element is then scrutinized to identify all conditions or actions that could possibly lead to an accident injury or illness B A complete JSA involves the following steps A template for performing JSA is provided at the end of this module It is also available a downloadable PDF file if you desire additional copies for your term project 1 Document the quotbasic factsquot a Job Identification Job title job number department etc b Work Objectives Use a few short phrases to describe major functions the worker is expected to accomplish c Locations where the job is performed d Operator Identification Depending on the level of detail in your analysis you can document one or more of the following items i Names ii Height iii Hand Dominance iv Experience on thisjob years months v Experience on similarjobs years months vi How many other workers perform this job e Production standards i Use information such as line speed work standards quality standards historical production information etc to estimate the number of units produced per minutehourday or to estimate the number of work cycles completed per minutehourday This information is generally available from industrial engineering records andor interviews with the worker or supervisor For operations that use incentive pay plans this information may be available through the payroll office The type of pacing and the compensation plan can greatly affect the actual work rate so it is important to document these factors ii Keep in mind that the official production rate ie the rate required to meet work standards may be slower than the actual production rate in situations where workers must meet a daily quota on selfpaced jobs Job Safety Analysis Notes 07l21l00 Page 1 f Shift schedule shift length i When does the shift begin and end ii How many breaks are provided during the shift including lunch iii Are special rest breaks provided for an unusually strenuous job iv How long are breaks for lunch and rest v Compute the total duration of work and rest in minutes during a typical shift g Job rotation schedule if applicable h Date and time of JSA i Names of JSA team members N Sketch the work station showing the location of all equipment conveyors and material handling devices fixtures tools and work benches Also showthe locations where raw materials and parts are stored Use overhead plan and side elevation views as needed a Record all important dimensions Excessive reach distances can require the use of awkward postures that may lead to musculoskeletal injuries and disorders b Nearby operations including materials handling aisles can expose workers to hazards lf significant nearby hazards are present these should be included on the sketch 00 Describe all equipment and tools ldentify all inherent hazards An inherent hazard is one that is not related to specific applications of the equipment or tool as used on thisjob See Table l of Introductory Systems Safety notes for examples of generic hazards found in many types of equipment and tools In this section you should also describe personal protective equipment PPE available to the worker 4 Describe all parts that are handled Identify inherent hazards Use Table I from the introductory notes as needed 5 Describe all process chemicals including byproducts or wastes produced at the work station Identify all hazards 6 Break the job down into basic steps or work elements On complex jobs it is useful to first identify major tasks and then break each task down into basic steps or work elements For each element identify the following a Hazards associated with normal operations eg exposures to chemicals and or physical stresses that are expected to occur on a continuing basis Identify all exposures and conditions that could cause an accident injury or disease including overexertion and cumulative trauma disorders b Hazards associated with human error accidental or deliberate c Hazards associated with process or hardware failures d Hazards associated with problems involving nearby operations equipment andor employees You should use this entry to cover environmental Job Safety Analysis Notes 07l21l00 Page 2 concerns not addressed elsewhere such as ambient noise from nearby equipment Note The discipline in Job Safety Analysis requires that you identify hazards at each sta e of our anal sis ste s 36 above lf ou are thorou h ou ma identif the same hazard more than once Do not be discouraged or frustrated by this While redundancy is a nuisance overlooking a hazard can result in a catastrophe 7 Summarize yourfindings For each identified hazard a Identify the work tasks that is are most likely to be associated with exposure to the hazard b Determine the types of accidents or injuries that are likely to occur c Estimate the probability of occurrence for each accident or injury You may wish to use qualitative rankings since quantitative data is not always available d Estimate the severity of each accident or injury You may wish to use qualitative rankings 8 Develop a prioritized plan for hazard control Priority is usually established based on a combination of probability and severity In addition the benefits and costs of each intervention must be considered It is sometimes useful to develop a menu of interventions for each identified hazard Interventions can range from band aids inexpensive temporary fixes that can quickly installed to substantial investments in engineering controls It is also important to consider how many persons would benefit from a specific intervention a Where feasible a single intervention should address multiple hazards b In general try to use proven interventions in your menu Offtheshelf technology is generally more reliable than new designs C A unique feature of JSA is that it utilizes a team approach to hazard identification and control Typical team members include the plant safety professional the supervisor responsible for the job and the workers who perform the job In certain instances where special expertise is needed the team should be expanded to include physicians engineers skilled trades maintenance personnel andor outside consultants The team approach is good for the following reasons 1 Each team member has different insights and experience this helps in the identification of hazards 2 Collectively the team has considerable knowledge of the job being studied When problems are found this collective knowledge is used to develop practical and effective solutions D Other hazard identification methods eg procedure analysis interface analysis follow similar procedures to those outlined for JSA In general however these other methods are performed by safety professionals and may not formally utilize the team approach Job Safety Analysis Notes 07l21l00 Page 3 JSA FORMS Job Safety Analysis Notes 07l21l00 Page 4 BASIC FACTS Job Identification Work Objectives Work Locations Operator Identification Production Standards ShiftShift Length Job Rotation if any JSA DateTime JSA Team Members Job Safety Analysis Notes 07l21l00 Page 5 WORK STATION LAYOUT Job Safety Analysis Notes 07l21l00 Page 6 EQUIPMENT AND TOOLS 5 3 Hazard Job Safety Analysis Notes 07l21l00 Page 7 PARTS HANDLED 5 3 Hazard Job Safety Analysis Notes 07l21l00 Page 8 PROCESS CHEMICALS 5 3 Hazard Job Safety Analysis Notes 07l21l00 Page 9 MAJOR TASKS AND WORK ELEMENTS list tasks and elements Job Safety Analysis Notes 07l21l00 Page 10 ELEMENTAL ANALYSIS one page per element Element Description Hazards from quotnormalquot operations Hazards from human error Hazards from processhardware failure Hazards from nearby operations Job Safety Analysis Notes 07l21l00 Page 11 ELEMENTAL ANALYSIS one page per element Element Description Hazards from quotnormalquot operations Hazards from human error Hazards from processhardware failure Hazards from nearby operations Job Safety Analysis Notes 07l21l00 Page 12 ELEMENTAL ANALYSIS one page per element Element Description Hazards from quotnormalquot operations Hazards from human error Hazards from processhardware failure Hazards from nearby operations Job Safety Analysis Notes 07l21l00 Page 13 ELEMENTAL ANALYSIS one page per element Element Description Hazards from quotnormalquot operations Hazards from human error Hazards from processhardware failure Hazards from nearby operations Job Safety Analysis Notes 07l21l00 Page 14 HAZARD DESCRIPTION one page per hazard Hazard Associated Tasks Likely accidentsinjuriesdisorders Probability SeveritySeriousness Job Safety Analysis Notes 07l21l00 Page 15 HAZARD DESCRIPTION one page per hazard Hazard Associated Tasks Likely accidentsinjuriesdisorders Probability SeveritySeriousness Job Safety Analysis Notes 07l21l00 Page 16 HAZARD DESCRIPTION one page per hazard Hazard Associated Tasks Likely accidentsinjuriesdisorders Probability SeveritySeriousness Job Safety Analysis Notes 07l21l00 Page 17 HAZARD DESCRIPTION one page per hazard Hazard Associated Tasks Likely accidentsinjuriesdisorders Probability SeveritySeriousness Job Safety Analysis Notes 07l21l00 Page 18 IOE 539 Notes PRESSURIZED CONTAINERS AND EXPLOSIONS W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved OVERVIEW A Generic Causes of Explosions Most explosions result from the following events 1 Deliberate detonation of extremely reactive chemicals rapid chemical reactions produce a large volume of gas in a short period of time The gas expands rapidly producing high temperatures and a shock wave with substantial destructive potential Applications include a Blasting common in mining and road construction activities b Demolition controlled implosions of buildings c Military d Terrorism N Accidental detonation of mixture of air and flammable vapors or combustible dusts Chemistry is similar to deliberate detonations however reaction typically occurs in relatively large container or building where reactive mixture has accumulated 3 Rupture of a pressurized container due to structure failure of the container In general this type of event can be attributed to one of the following causes a Internal pressure of the container exceeds structural design limits b Structural weakening of the container causes failure at pressures below the design limit A pressurized container is any device or system that is designed to hold a liquid gas or vapor at an internal pressure that exceeds the pressure of the surrounding environment Examples include boilers water heaters hydraulic and pneumatic systems compressed gas cylinders aerosol cans and tires Pressurized containers are usually classified into two basic categories 1 Fired Pressure Vessels usually Boilers Fuels are burned to produce heat which in turn boils water to produce highpressure steam 2 Unfired Pressure Vessels Containers used to store gases or vapors with an internal pressure greater than atmospheric pressure a Atmospheric pressure at sea level is approximately 15 pounds per square inch 147 PSI if you prefer three significant digits FT Absolute pressure abbreviated PSla is measured relative to a pure vacuum Therefore an unpressurized tank one where the internal pressure is the same as external pressure at sea level would have an absolute pressure of 147 PSla c Gauge pressure abbreviated PSlg is measured relative to the ambient atmosphere The unpressurized tank in the previous example would have a Pressure Vessel Notes 072600 Page 1 gauge pressure of 0 PSlg At sea level the gauge pressure is 147 PSI less than the absolute pressure Piping systems hoses and gauges that are connected to pressurized containers are also considered to be pressurized containers and present potential safety hazards Applicable Codes Design testing and inspection standards have been developed by the following organizations 1 ASME Boiler and Pressure Vessel Code Considers issues related to the design construction installing and testing of pressurized systems lncludes specifications for thickness of walls welds etc 2 Compressed Gas Association Design transportation and use of highpressure gas cylinders 3 Department of Transportation Establishes federal guidelines for the transportation of compressed gases over highways and railways 4 NFPA Life Safety Code Fire resistance of walls ceilings and doors in boiler rooms 5 American Petroleum Institute APl Standard No 620 Pressure Vessel Code Certification Testing and certification of inspectors is performed by the National Board of Boiler and Pressure Vessel Inspectors This is a specialized skill that is relatively rare among safety professionals and OSHA compliance officers PRESSURE VESSEL HAZARDS A Pressure Vessel Notes Slow Rupture A small crack occurs in the container allowing fluid to escape The vessel typically remains intact so there are no fragments Leak hazards are usually determined by the contents on the container and include toxic gasesvapors fire and explosion of flammable gasesvapors and thermal or chemical burns Highpressure systems may emit high velocity gases In addition to the basic hazards listed above high velocity gases may be difficult to see and can generate tremendous cutting or puncturing forces Amputations due to escaping gases have occurred Prevention measures include 1 Policies and procedures to assure pressure release in order to achieve Zero Mechanical State ZMS prior to maintenance 2 Sometimes it is necessary to keep the system pressurized in order to detect leaks In this case probing should never be done with the fingers Instead use one of the following o Cloth streamers o Radioactive gases 0 Odorants 072600 Page 2 B Rapid Rupture Sudden and total structural failure of container due to internal pressures exceeding structural limits The container is rapidly destroyed producing fragments and sometimes a shock wave If the container has been used to store toxic or flammable materials these are released and may increase the likelihood of injury death or property damage Rapid ruptures occur when the internal pressure exceeds design limits andor when structural damage due to normal wear and tear corrosion galvanic action or an acute accident reduces the strength of the container x Boilers Overpressure can occur if the rate of heat energy ie steam flow leaving the vessel is less than the rate of heat energy generated When this occurs energy is added to the steam inside the boiler resulting in higher pressures This risk of boiler explosion due to this cause can be controlled by a number of mechanical devices including Safety valves Frangible discs Fusible plugs Sight glasses to gauge water level Safety valves frangible disks and fusible plugs should be located so that vented highpressure gasesvapors will not cause injury or property damage N BoyleCharles Law can be used to predict pressure changes caused by changes in temperature PlvlTl PszTz Because the volume of the container is typically fixed we can reduce the expression to P2P1 T2T1 Note When using the above expressions it is necessary to convert the temperature to an absolutezero scale Kelvin or Rankin F Steam boilers should be built to comply with the ASME Boiler Code and should carry the ASME medallion Regular inspections of overpressure safety devices as well and structural integrity of boiler walls and seams greatly enhances safety C Compressed Gas Cylinders Compressed gas cylinders vary in size the smallest are 2 in OD x 15 in long The most common cylinder for industrial applications is 9 in OD x 51 in long The US Dept of Transportation DOT has developed extensive standards for compressed gas cylinders that are transported under pressure via road or rail The Compressed Gas Association CGA is a trade group that has developed extensive standards for the safe design and use of cylinders 1 Major Hazard Break off of control valve producing an unguided missile Effective thrust can be 200 times the weight of the cylinder Contrast to space launch vehicles where thrust to weigh ratio does not exceed 25 at launch 2 Hazard control achieved by 0 Consistent use of valve caps on cylinders during transportation storage and any other time when the cylinder is not in use Pressure Vessel Notes 072600 Page 3 o Securing of cylinders when in use 0 Appropriate equipment and procedures for transporting cylinders 3 Compressed gas cylinders must also not be subjected to excessive heat Storage facilities should not exceed 130 F Je For cylinders containing flammable gases rooms must be ventilated and equipped with explosion proof electrical equipment Cylinders that contain flammable gases must be separated by at least 20 feet from cylinders that contain oxygen or other oxidizing gases I TESTING METHODS A Hydrostatic Testing nondestructive 1 Vessel is filled with water or alternate liquid and pressurized to 15 times the maximum expected operating pressure Strain measurements are taken before during and after the test to determine if deformation has occurred 2 Advantages over using a pressurized gas 0 Leaks can frequently be detected visually seepage o If the vessel should rupture there is no shock wave due to the rapidly expanding gas 3 OSHA requires periodic hydrostatic testing of pressured fire extinguishers eg 002 the testing interval is either five or ten years depending on type B Nondestructive tests for surface imperfections 1 Visual Inspection Look for obvious problems such as corrosion blistering and evidence of mechanical damage Requires good lighting and a clean surface 2 Penetrant Inspection Special liquids are used that seep into small cracks and discontinuities The penetrant is subsequently detected 2 Magnetic Particle Inspection Magnetic fields are disturbed by discontinuities in a surface These can be detected by applying fine particles of a ferromagnetic material to the surface and then inducing a magnetic field C Nondestructive tests for wall thickness and hidden imperfections 1 Industrial Radiography Measures thickness of wall andjoints Can detect holes voids and other internal discontinuities 2 Ultrasound Uses reflection of mechanical waves vibration to detect internal discontinuities such as cracks Can also be used to measure thickness Pressure Vessel Notes 072600 Page 4 IOE 539 ELECTRICAL SAFETY NOTES W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved I OVERVIEW A Major Hazards Associated with Electricity 1 Electric Shock Caused by the flow of electrical current through body tissues Occurs when parts of the body become part of an electrical circuit This may occur as the result of direct contact with an energized object or as a result of arcing the flow of electrons through a gas such as air Shockrelated injuries range from burns to the currentcarrying tissue to death from electrocution 2 Flash burns from heat generated by an electrical arc 3 Falls caused by involuntary muscle contractions 4 Heat fires and explosion caused by an electrical ignition source B Major Topics 1 Review basic electricity and simple circuits 2 Causes and prevention of shock and electrocution 3 Causes and prevention of electrical fires II STATISTICS NIOSH A The number of work related electrocutions has been dropping from 600700 per year in the 1970s to 300400 per year in the 1990s NIOSH 1998 Construction is the most hazardous industry for electrocution fatalities 1 5th leading cause of workrelated fatalities after motor vehicles homicide falls mechanical trauma 2 An OSHA analysis of three years of occupational electrocution cases from the mid 1980s revealed 0 60 percent of cases involve high voltage gt480 volts 0 32 percent of cases involve low voltage 120480 volts 0 8 percent of cases not specified Electrical Safety 07l19l00 Page 1 B Course will emphasize safety issues associated with low voltage nonelectrical work exposures FUNDAMENTALS OF ELECTRICITY A Ohm39s Law I VR where l current measured in amperes or amps V electrical potential between two points measured in volts R electrical resistance between two points measured in ohms 1 Ohms Law is important to electrical safety because a Health effects are most strongly related to the amount of current flowing through the body FT Overheating is related to the amount of current flowing through a conductor wire relative to its capacity to carry current 2 Joules Law Joule heating P i2R or P N where P is power measured in Watts B Current Density Current Density is the amount of current flowing through a conductor per unit of cross sectional area If the area is large the current density is low Large diameter conductors have a greater currentcarrying capacity than small diameter wires made of the same material The temperature of a conductor rises as the current flow through the conductor increases C Resistance All materials exhibit some resistance to the flow of electricity Materials with low resistance are called conductors copper aluminum electrolytic fluids Materials with high resistance are called insulators rubber glass wood air and many plastics Human tissues and body fluids are relatively good conductors of electricity This allow current flow to reach levels that may cause burns or electrocution D Direct Current Current flow is in a single direction E Alternating Current Current flow oscillates following a sinusoidal curve Electrical Safety 07l19l00 Page 2 IV A 1 2 3 4 B H x Electrical Safety an HEALTH EFFECTS Electric Shock For electric shock to occur a person must become part of an electric circuit ie the person must become a conductor between two points that differ in electrical potential Electric shock effects are mainly a function of the amount of current that flows through the body particularly the amount of current that flows through the chest cavity See Table 2 in Course Pack and Figure 10A on page 231 of your textbook Normally the brain sends electrical impulses to muscles to control contraction An externally applied current can produce the same effect If the magnitude of the current is high enough to dominate impulses from the brain involuntary contractions occur and people report that they quotcan39t let goquot If sufficient current passes through the chest cavity to cause involuntary contractions the contractionrelaxation cycle associated with breathing cannot occur and asphyxiation may result Electrical current can also cause fibrillation of cardiac muscle effectively stopping the pumping action of the heart While there appear to be differences among people in the ability to resist fibrillation the amount of current required to produce fibrillation can be quite low Statistical studies have been performed yielding the following equations for armtoleg or legtoleg connections 05 165 SQRT t 995 495 SQRT t where lp is the current in milliamps at which p percent of the population experiences fibrillation t is the time of exposure in seconds tlt5 dtofoot electrocution The resistance of the human body depends on environmental conditions a Dry skin 100000 and 600000 ohms b Wet skin 500 ohms c The internal resistance of the body is also low The internal resistance from the hand to the foot can be as low as 400 ohms d When the skin is wet the total handtofoot resistance of the body can be quite low Skin resistance hand 500 ohms Internal resistance handfoot 400 ohms Skin resistance foot 500 ohms TOTAL BODY RESISTANCE sum 1400 ohms 07I19l00 Page 3 c The Gmund Faun Aeemem 1 A gvuund auh acmdem uncuvs when a pawnquot muehes m wasps ah 2 25mc3Hy Enevnged amen WW2 the ee ave h camade the gmund m a gmundad swaee he auh acmdem eah uccm Hhe hands ave h E ndud Wuh the gmund m a gmundad swaee mhe yesmahee unhe may 5 uw the yesumhg ewem can he mgh enuugh m cause exeeuueuheh See ame be uw a e shun chmxhs ee the M w zheveue the C25 Sevwze Enuznze cum AND numeuwzerzn Euuxmzm wanuT A ERDUNDINE uNDucTnR Skemh cuunesy u OSHA Examp e Awuvkev wasps a memeasee hm hm e g a nun Due m msu a mn a uve mSME the uh the casmg 5 Va SEd m semce vuhage 121 vuhs m a typma edncaHyrqueved hahe mm Cumpme the amuum u EUHEHHHE heme aw thvuugh the may Nme Undenhe hm aha hum cundmuns ypma u many cunshucuumubs aha uthekuvk ehwuhmeme the 5km can he mmsL thus DWEYmg the exeemeax yesmahee Semen Use ohm s Law VR Enevgy Sauce 5 12m vuhs an Hz exeemeax servme The vesmahee s MUD ohms We mhe hum ease snua un descnbed ahuve Electrical Szl y n719nn P2926 Substituting into Ohm39s Law Current 120 Volts1400 Ohms 0086 Amperes 86 milliamperes Note that the resulting current is suf ciently high i e in excess of 75 millamps to ca use ventricular brillation and death unless the victim can be quickly removed from the circuit and resuscitated 2 If we ignore electrical accidents experienced by electricians most occupational and residential electrocutions are the result of ground fault accidents Many ground fault accidents are associated with hand tools andor appliances that are defective or used incorrectly V REVIEW OF BASIC CIRCUITS A Series Circuits 1 Current follows a single path and loads ie resistances are encountered sequentially 2 Req R1R2R3Rn 3 As additional loads are added to the path resistance increases and current decreases Electrical PPE such as insulated gloves and shoes enhance safety by increasing the resistance between high voltage surfaces and grounded surfaces B Parallel Circuits 1 Current may follow alternate parallel paths 2 1Req 1lR11lR2 1lR3 1an 3 As additional paths are added to the circuit resistance decreases and total current increases 4 The voltage drop across all parallel branches of a circuit is constant In North America household circuits are wired as parallel circuits with a voltage drop of 120 or 240 volts For industrial applications parallel circuits with a voltage drop of 120 240 or 480 volts are common C Complex Circuits 1 Many circuits include both series and parallel sections 2 Steps in solving current flow through branches of complex circuits a Compute Req for each set of simple series loads quotSimplequot implies two or more adjacent loads with no intervening parallel branches ReqR1R2R3Rn Electrical Safety 07l19l00 Page 5 VI Electrical Safety FT 0 51 FD o Compute Req for each section of the circuit that contains parallel branches In other words simplify the parallel branch into a single Req 1Req 1R11R2 1R3 1Rn Repeat steps 1 and 2 as necessary until you are left with a simple series circuit with no parallel branches Find Req for the complete circuit If you have done everything right up to this point you should be solving a simple series circuit as described in step a Use Ohm39s Law to compute the current flow through the entire circuit To do this use Req from step I VReq Going back to your original ie unsimplified circuit remember that the voltage drop across all parallel branches in a given section of the circuit is the same Because the total current entering a parallel section of a circuit is equal to the total current leaving the section we use the total current known and Req of the parallel section also known with Ohm39s Law to compute the voltage drop V across all branches of the circuit section vquotIxReq where Req is the equivalent resistance for the parallel circuit section computed in step Use Ohm39s Law to compute the current flow through each branch of the parallel circuit section I vR where V is the voltage drop across the entire parallel circuit section from step f and R is the simple resistance of the branch To check the sum of currents across all branches must equal the current entering and leaving the parallel current section GROUNDING AND GROUND FAULT ACCIDENTS A System Grounding Primary purpose is to equalize neutral conductors that are inside and outside of a structure It is desirable that all neutral conductors have zero voltage relative to ground System grounding is accomplished by establishing two lowresistance paths to ground one inside the structure and the second outside of the structure See figure on next page 07l19l00 Page 6 Mas mum vzzewivs mm mm m mm mm m m mum m uuhxe Pvavev mn 5M gem 251M mmquot Evaunqu mam A gem mum mam 5 needed when vhsuz zandum mrmmum mma zzb e m athev wmnq mums m sea m m quota zvvvwed 5 WWW mums mnsmmu Sezandzw emma Canduzmvav Wm E ezmzz SVmba mama SYSTEM AND EQLIIPMENT GROUNDING Ske ch cuunesy u OSHA casmg um exemmax dewce bemmes gnawed Anumbev uvsymms have been dwe uped mc udmg 1 ThwdWwe m2mudsmtemaandextemal Seahguvebe uw umuma emma CW Canuzt o w am Wwe WMur3939 swam Wwe Canuzt msz mw camame 557mm r y M Gveen aszve ewqu Canduzmv Gveen mum Hud Tevmmz SIM Wex mama Emmy wm on new mun unesy m a Ske ch cu OSHA Electrical Samy n719nn Page 2 Ground Fault Circuit lnterrupters GFCI 3 Double Insulation 4 lntrinsic Safety VII OVERCURRENT AND FIRE PREVENTION A Electrical Safety Joules Law W2R 0 Heat generated by electrical current is proportional to the square of the current flow Relatively small increases in current flow can result in substantial heat generation Common Causes of Electrical Fires 1 Short Circuits 2 Overloaded Circuits 3 Locallyhigh resistance poor mechanical coupling or damage to conductors Resistance of a Conductor R p X lA where R total resistance of conductor ohms p resistivity of material ohmcm l length of path cm A cross sectional area cm2 Resistivity p of common materials all values in ohmcm Silver 159 x106 Copper 171 X 10396 Aluminum 286 x106 Bakelite 5 x1017 Rubber 1020 quot1022 Selection of Conductors to Accommodate Current see Tables 1 and 3 in Coursepack Copper vs Aluminum Wiring 07l19l00 Page 8 IOE 539 FIRE SAFETY NOTES W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved OVERVIEW A Statistics 1 National Fire Protection Association fires reported to US fire departments NFPA 1997 million s a 205 million US fires in 1994 reported rate has been approximately fires per year since 1990 down from over 3 million per year in the 197 b 43000 workplace fires attended by municipal fire departments c 4275 civilian fatalities does not include fire fighters d 85 Billion in direct property losses e Smoke and gasses cause approximately 75 of deaths 2 0 2 OSHA 1993 1995 a 75000 workplace fires b 200 fatalities 5000 injuries firefighters most at risk approximately 50 of work related fire fatalities c 23 billion in losses 3 Famous workplace fires a 1911 Triangle Shirtwaist Factory New York City 150 women and young girls died This incident played a significant role in the development of child labor protection laws and the development of Workers Compensation legislation b September 1991 Hamlet NC 25 deaths in poultry processing plant Je Following the North Carolina fire OSHA stepped up inspection and enforcement activities related to fire safety in the workplace Key aspects of compliance inspections include a Number and condition of fire exits b Adequacy of portable fire extinguishers and employee training in extinguisher e us c Emergency evacuation plan must be written and employee training d Adequate fire alarm system e Written fire prevention plan including control of flammable and combustible materials and ignition sources f Employee training in specific fire hazards associated with theirjobs g Appropriate fire suppression systems standard is somewhat vague Fire Safety Notes 07l19l00 Page 1 Definitions 1 Fire Prevention refers to actions taken to prevent the inception of fire It includes both engineering and administrative controls to reduce or eliminate the presence of combustible materials andor the presence of ignition sources 2 Fire Protection refers to actions taken to minimize losses should a fire occur Again this involves both engineering and administrative controls 3 Life Safety involves assuring that people can rapidly escape from a burning building or find an area of refuge somewhere in the building where they can await rescue FIRE CHEMISTRY A Fire chemistry is poorly understood Fire is generally described as a rapid exothermic oxidation process that yields hot gases This reaction involves a chain reaction of short lived free radicals 1 Methane example CH4 202 gt 002 H20 net input and outputs of reaction 2 Free radicals created during combustion include H OH O These radicals are constantly being liberated and captured there lifetime may be on the order of microseco nds Components of the quotFire Pyramidquot 1 Heat 2 Fuel 3 Oxidizer 4 Chain Reaction 5 Fire fighting strategy involves breaking the pyramid by either removing a core component or by disrupting the chain reaction Iquot FLAMMABLE LIQUIDS Gen Ref NFPA 30 Flammable and Combustible Liquids Code A B Fire Safety Notes Principal hazards are explosion and fire Definitions 1 Flash Point 2 Lower Flammable Limit lower explosive limit LFL LEL 3 Upper Flammable Limit upper explosive limit UFL UEL 4 Flammable Range 5 Vapor Pressure propensity to evaporate 07l19l00 Page 2 C Classes 1 Flammable Liquids Flash point below 100 F a Class IA FP lt 73 F boiling point lt 100 F b Class IB FP lt 73 F boiling point gt 100 F c Class IC 2 Combustible Liquids a Class II FP lt 140 F b Class IIIA FP lt 200 F c Class IIIB FP gt 200 F Classes of Flammable and Combustible Liquids as Defined in 29 CFR 1910106 200 x 2quot a LA EDMBUSTIBLE 14a Flash Point gt 100 F n 5 II r 6 100 Ll IC 73 FLAMMnBLE Flash Point lt mum IA IB Boiling Point F Figure courtesy of OSHA D Engineering Controls 1 General Strategies Keep concentration of vapors below the flammable range isolate liquidsvapors from sources of ignition eliminate sources of ignition 2 Local Exhaust Ventilation remove vapors at their source in order to keep ambient concentration below the flammable range 3 General Dilution Ventilation provide adequate makeup air to building or subsection of building in order to keep concentration of vapors below the flammable range Can be very expensive due to energy costs associated with moving and tempering large volumes of air Used where it is dif cult to contain liquids or control the release of vapors eg drying ovens Fire Safety Notes 07l19l00 Page 3 01 Fire Safety Notes a Formula for general dilution ventilation assumes standard conditions of room temperature and sealevel pressure 21 Celsius 70 F and 760 mm Hg This formula also assumes that the user has accurate information on the rate at which the flammable liquid is being lost from a process Make up air cu ft per pintof liquid evaporated 403x104xSGxC MWxLFLxB where SC is the specific gravity of the flammable liquid MW is the molecular weight of the flammable liquid LFL is the lower flammable limit expressed as a percentage and C is a safety factor 4 for continuous operations and 10 for batch operations B is a constant based on temperature 1 for temperatures up to 250 degrees F and 07 for temperatures above 250 degrees F b Note Because warmer air or air at elevation is less dense than the standard conditions assumed volume must increase if nonstandard conditions are present in order to achieve equivalent dilution Recall from freshman chemistry that the volume of a perfect gas is proportional to its absolute temperature Because the Fahrenheit and Celsius scales use an arbitrary zero point it is necessary to convert to an absolute zero temperature scale The absolute zero scale for Fahrenheit degree intervals is the Rankin scale Rankin degrees are calculated by adding 460 to the Fahrenheit equivalent In the metric system the absolute zero scale for Celsius degree intervals is the Kelvin scale computed by added 273 to the Celsius equivalent 0 Note In general additional dilution is required to keep concentrations below PELs Storage of Flammable Liquids a Drums portable bulk storage 3 60 gallons Portable Safety Cans small quantity 3 5 gals transfer and storage requires approved container with spring closing lid to minimize spillage and to relieve internal pressure under fire conditions c Flammable Liquid Storage Rooms d Safety Cabinets typically found in laboratories and other occasional use areas e Explosionproof refrigerators FT Breakout exercise Develop concept designs for drums safety cans and flammable liquid storage rooms Substitution general strategy is to go for the highest flash point available however it is necessary to consider the toxicity and other characteristics of the alternative 07l19l00 Page 4 6 Open tanks vs Closed Systems 7 Exclusion of Ignition a b c Explosion Proof electrical systems and components control of arcing Intrinsicallysafe systems Control of static electricity sparks i Bonding and grounding ii Antistatic shoes and flooring Maintenance of bearings belts and other components where excessive friction can generate sufficient heat to ignite vapors 8 Hydraulic Fluids Due to high pressures in hydraulic systems leaks can atomize the hydraulic fluids into fine mists The flash point of a mist can be substantially lower than the flash point of the base hydraulic fluid a Approved hydraulic fluids will not ignite or combust under either of the following conditions atomized into open flame 1000 F ii sprayed or splashed onto a metal surface at 1300quot F Shutoff valves provide good secondary protection in the event that a leak is present and there is either a fire or the danger of a fire D Administrative Controls 1 2 No Smoking Policies Inventory Control 3 Hot Work Permits IV FIRE DETECTION A Four Stages of Fire 1 2 Fire Safety Notes Incipient no visible smoke flame or heat Smoldering concentration of combustion products is high enough to be seen by the human eye Flame emission of radiation of light electromagnetic energy Frequency may or may not be in the visible range of the electromagnetic spectrum Heat exothermic release of energy produces rapid build up of heat Progression depends on the composition of the burning material the availability of oxidizing agents and ventilation of combustion products 07l19l00 Page 5 V Fire Safety Notes Detector Systems 1 Smoke a Ionization b Light Interference i obstruction detectors scattering detectors c Combination 2 Flame 3 Heat a Bimetallic b Fusible element similar to metals in sprinkler heads c Confined Fluid d Fusible Insulated Wire FIRE EXTINGUISHING SYSTEMS A Fire Classes 1 A Ordinary Combustibles 2 B Flammable Liquids 3 C Electrical 4 D Combustible Metals Common Extinguishing Agents 1 Water 2 Carbon Dioxide 3 Halons manufacturing has been discontinued due to environmental impact on ozone While very effective and still widely used they are being phased out See supplemental notes on Halon alternatives 4 Foams 5 Solids Fixed Extinguishing Systems 1 Water Sprinklers Wet pipe Dry pipe Preaction Deluge 515quot FT 07l19l00 Page 6 2 Carbon Dioxide 3 Halon 4 Foam VI BUILDING DESIGN A General Principles 1 Minimize Spread of Flame and Smoke 2 Maintain Structural Integrity 3 Promote Escape 4 Facilitate Firefighting Activities B SmokeGas Venting 1 Flatroof buildings a b c Basic concepts Advantages and disadvantages Design concepts 2 Multistory buildings VII HEALTH EFFECTS OF FIRE AND SMOKE A Hazards 1 Heat 2 Impaired Vision 3 Narcosis 4 Respiratorylrritation 5 Toxicity B Effects 1 Immediate 90 F75 Burns Incapacitation Faulty judgment and panic Delayed escape no escape 2 Delayed Fire Safety Notes 07l19l00 Page 7 C Fire Products 1 Smoke 2 Carbon Monoxide C02 chemical asphyxiant produced as a result of incomplete combustion C02 is also a fuel that can act explosively if large quantities of air are introduced to a closed system 3 Hydrogen Cyanide HCN chemical asphyxiant produced from combustion of nitrogen containing compounds including natural and synthetic fabrics and fibers 4 Hydrogen Sulfide H2S toxic gas produced during incomplete combustion of materials containing sulfur includes rubber leather wool 5 Phosgene COClz toxic gas produced by combustion of halogenated hydrocarbons 6 Heat VIII BUILDING EGRESS AND LIFE SAFETY A NFPA Code Section 101 quotLife Safety Codequot B Human Responses to Fire C Goal of Life Safety Code is to make escape as simple as possible 1 Keep people moving to safe area in order to avoid panic The potential for panic is greatly controlled if people can their exit and are moving toward the exit 2 Direct people toward exits a Signs b Illuminated paths D Building Design 1 Two independent escape paths from occupied areas 2 Protection of vertical exits from smoke 3 Elevators are not considered safe fire exits 4 Areas of refuge in high rise buildings and in cases where occupants are non ambulatory 5 Design of Exit Doors E Life Safety Code Classifications classification determines specific requirements for building design 1 Occupancy a Assembly b Educational c Health Care d DetentionCorrection Fire Safety Notes 07l19l00 Page 8 IOE 539 POWER LOCK OUT NOTES W Monroe Keyserling PhD or Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved INTRODUCTION Definitions 1 LOCKOUT The mechanical interruption and neutralization to a ZERO ENERGY STATE of all energy sources to and within a piece of equipment or machinery Common energy sources include electrical mechanical hydraulic pneumatic steam and stored potential energy springs gravity etc 2 ZERO ENERGY STATE Occurs when every energy source that may create a hazard has been neutralized andor locked out Power lock out is not a new issue OSHA began discussing the need for a federal standard in the mid1970s Companies with progressive safety program have been using lockout procedures for over 50 years ANSI adopted a Consensus Standard Z2411 in 1981 during the development of the OSHA lock out standard The federal Power Lock Out Standard became effective October 31 1989 REF CFR 1910147 quotControl of Hazardous Energy LockoutTagoutquot Following promulgation lockout violations were one of the most common causes of OSHA citations During fiscal year 1993 7131 citations were written for violations of the lockout standard REF Rougton JE quotLockoutTagout Standard Revistedquot Professional Safety pp 3337 April 1995 The OSHA standard applies to the servicing andor maintenance of machines and equipment in which the unexpected energization or startup of the machines or the release of stored energy could cause injury to employees Note that the standard does not cover activities 39 39with normal production operations STATISTICS Part of OSHA s justification REFS USDOLBLS quotInjuries Related to Servicing Equipment Bulletin 2115 October 1981 NIOSH ALERT Preventing Worker Deaths from Uncontrolled Release of Electrical Mechanical and other Types of Hazardous Energy NIOSH Pub No No 99110 80 percent of all major injuries during maintenance operations associated with machines that were not turned off 82 percent of fatalities during maintenance were due to failure to completely deenergize isolate block andor dissipate energy Seven percent of injured workers reported that machine movement was caused by accidental contact with a machine operating control during the performance of maintenance work while the machine was in a powered state Lockout Notes 072000 Page 1 C 11 percent of fatal injuries during maintenance attributed to failure to lockout and tag out equipment following deenergization D Seven percent of fatal injuries during maintenance attributed to failure to verify that all energize sources had been disabled prior to beginning work The machine was never tested to assure that all power sources had been disabled E Interviews with workers revealed the most common reasons for not turning off equipment 1 Thought it was unnecessary and that maintenance could be safely performed with the machine on 2 De energizing machine either impossible timeconsuming or not convenient F In cases where power was turned off prior to beginning maintenance 1 50 percent of injuries occurred when another worker turned the power on 2 20 percent of injuries due to residual power coasting and or release of stored energy quotI HIGHLIGHTS OF THE OSHA LOCKOUT STANDARD A The lockout standard along with the confined spaces standard was one of OSHAs earliest attempts to issue a performanceoriented programmatic standard B The standard requires employers to establish procedures and training programs to a sure that machines are in a quotsafe statequot prior to maintenance The procedures must be written and include the following 1 Policy and scope 2 Authorization for personnel to utilize lock out devices 3 Procedural steps for shutting down isolating and blocking machines Because different machines have different requirements for shutdown a written protocol must be developed to provide machinespecific instructions These include but are not limited to a Procedures for identifying and removing all sources of power or energy to or in the machine including potential energy that is quotstoredquot in the machine after external sources have been quotremovedquot b Procedures for the placement and removal of lockout and tagout devices c Procedures for testing a machine to assure that a safe state has been achieved C The standard is not absolute There is a grandfather clause Existing equipment which is not capable of being locked out may utilize tagout procedures as long as the tagout procedure provides full employee protection D All equipment installed after October 31 1989 must be designed to accept a lockout device Any equipment which undergoes major repairs or renovation after this date must be modified to accept lockout devices Lockout Notes 072000 Page 2 E General Requirements 1 Durability of locks and tags a Withstand environmental exposures for the maximum period of time that the dev39ce will be u b Tagout devices must be legible and survive exposure to all chemicals expected to be encountered 2 Standardized Devices must be standardized within an organization wrt color shape print andor format 3 Substantial a Lockout devices must require the use of excessive force andor special tools for removal b Tagout devices must survive a 50 lb pull 4 Identification Each device must identify the employee who attached it 5 Audit Annual reviews of procedures and equipment are required IV BASIC HARDWARE A Locks B Scissors typically can accommodate up to six individual padlocks Scissor device with three personal locks on electric shutoff switch gure courtesy of SHA Lockout Notes 072000 Page 3 Key lock switches Key can only be removed when the switch is in the quotOFFquot position Worker controls possession of key during all maintenance activities Lockable valves in pneumatic and hydraulic systems Movtng l mu to quoterr cull on an m apply In use mum Al um um llme uhluu par un ovum mmquot m an 9mm n m mum lo umupnm Autumn nleeaev um inched m orr women m women oi low employ Scissor device with three personal locks on pneumatic valve shutoff switch figure courtesy of OSHA Lockout Notes 072000 Page 4 COMM DOW Lockout Notes Chains with padlocks Safety Blocks SAFEW max Safety blocks on power press figure courtesy of OSHA ON SOURCES OF ENERGY Electrical Hydraulic Pneumatic Mechanical Stored Energy 1 Gravity Fork Trucks Conveyors Presses 2 Momentum Flywheels Grinding Wheels Drill Bits Abrasive Wheels Saw Blades 3 Springs Special Systems 1 Gas toxic ammable 2 Water can be used for heat transfer and can be hot 3 Steam pressure to drive turbines cleaning agent high temperature and high pressure 4 Chemical Piping Systems Corrosive toxic thermal 072000 Page 5 IOE 539 Notes FAULT TREES AND CUT SETS W Monroe Keyserling PhD Professor Industrial and Operations Engineering The University of Michigan Ann Arbor Michigan 2000 All rights reserved Fault Tree Analysis FTA is perhaps the most widely used systems safety technique in the catastrophic event prevention category A Fault tree analysis involves identification of basic equipment malfunctions andor human error that may lead to serious undesired events By effectively controlling these root causes the probability of the catastrophic event can be reduced substantially B A complete FTA involves the following steps 1 Identify the catastrophic event to be prevented This is called the TOP LEVEL event See Figure 1 EXAMPLE A construction company is concerned that electric hand tools may cause ground fault electrocutions The company wants to minimize the likelihood of this event Ground Fault Accident Energized Case Low resistance path through No alternate path to No GFCI rker No third wire Nongrounded outlet Grounding Pin cutoff Ground wire not continuous Figure 1 Fault tree of ground fault accident Dashed lines separate first second and thirdlevel events Fault Tree and Cut Set Notes 072400 Page 1 N 00 Fault Tree and Cut Set Notes Construct a logic tree the Fault Tree showing the necessary conditions or events that must occur in order for the top event to occur First the SECOND LEVEL events are identified and their logical relationships to the toplevel event are determined In the construction example above four events must occur simultaneously for the ground fault accident to proceed a The casing of the tool becomes energized to service voltage b The worker and hisher clothing provides a path of low electrical resistance between the tool and ground c There is no alternate path of low electrical resistance eg an equipment grounding wire between the tool and ground and d The circuit that powers the tool is not protected by a functioning ground fault circuit interrupter GFCI Note that the accident could not happen if any one of these events does not occur This is known as an quotANDquot logical relationship in the Fault Tree terminology In an quotANDquot relationship all input events must be satisfied concurrently for the higher level event to occur Once the second level of the Fault Tree is completed the analysis continues To proceed THIRD LEVEL events and their logical relationships to each second level event are identified In the construction example above consider second level event quotCquot no alternate path of low electrical resistance Any of the following events would cause this condition a The tool is not equipped with a quotthird wirequot b The tool is plugged into a nongrounded outlet c The grounding pin on the tool39s plug has been cut off d The electrical continuity of the tool39s grounding wire has been broken This is an example of an quotORquot relationship In an OR relationship the higherlevel event will occur if m subevent is satisfied Construction of the Fault Tree continues by identifying the appropriate thirdlevel events and their logical relationship to the secondlevel events When all thirdlevel events have been established the analyst identifies the fourthlevel events and their logical relationshipto the thirdlevel This process is continued until basic failure events are identified Basic failures are those which are not analyzed further The basic failures or root causes are placed at the bottom of the tree In quantitative FTA the objective is to compute the probability that the top event will occur To do this it is first necessary to determine the probability of each basic failure These values can be determined through direct experimentation historical experience or guesstimation Once the basic failure probabilities are known the fault tree is solved from the bottom up using Boolean logic and mathematics Different procedures are used to 072400 Page 2 solve AND relationships and OR relationships In all cases however the basic failures are considered to be independent events a The probability associated with an AND connector is easy to solve To compute the probability of any event simply multiply the probability of all subevents EXAMPLE Two events B and C serve as inputs to event A with an AND gate The probability of Event A is computed as Pr A Pr B X Pr C EXAMPLE Three events G H and I serve as input to Event F with an AND gate The probability of Event F is computed as Pr F Pr G X Pr H X Pr b Computing the probability of an OR connector is slightly more complicated The probability of the output event is determined by adding the probabilities of all subevents The sum must be adjusted however to avoid quotdouble countingquot EXAMPLE Two events N and P serve as inputs to event M with an OR gate Event M will occur if either Event N or Event P occurs The probability of Event M is computed as Pr M Pr N Pr P Pr N AND P Pr N Pr P Pr N x Pr P Fault Tree and Cut Set Notes 072400 Page 3 EXAMPLE Event W will occur if any of Events X Y or Z occurs The probability of Event W is computed as Pr W Pr X Pr v Pr Z Pr x AND v Pr v AND Z Pr x AND Z Pr x AND v AND Z For a more complete explanation and additional examples refer to Roland and Moriarty 1983 7 A general weakness in PTA and other catastrophic event prevention methods is that the top event must be recognized prior to performing the analysis In general FTA cannot be used to discover previously unidentified catastrophic events 8 A second problem with fault tree analysis is that Boolean logic assumes that basic failure events occur independently of each other In real systems this is not true Significant environmental events eg earthquake lightning strikes terrorist attacks etc may simultaneously cause failures of multiple components or subsystems FAULT TREE SYMBOLOGY Fault event The rectangle is used to represent a fault event either at the top level or at an intermediate level You should never have a rectangle at the lowest level of a fault tree A B Basic event The circle represents a primary failure or fault It is not necessary to determine a specific cause In order to perform quantitative analysis all basic event failures must have a known probability Although there may be one or more basic faults that contribute to this event the analysis is not continued below this level often to save time Operationally a Basic event and an undeveloped event are similar In order to perform quantitative analysis all undeveloped events must have a known probability Normal event The house represents a normal event that must be satisfied in order for a higherlevel event to occur For example air oxygen is required to support combustion Because air is present in most work environments it can usually be modeled as a normal event C ltgt Undeveloped event The diamond represents an undeveloped failure or fault D D Fault Tree and Cut Set Notes 072400 Page 4 OR gate AND gate INHIBIT gate Allows output event to occur only if a specific prior condition has been satisfied Transfer out and transfer in Used when the fault tree is too large to fit on a single page A small triangle is used and a letter code designates where to branch Dobm CUT SETS A Fault Tree Analysis frequently produces large and complex diagrams of the conditions or events required for a catastrophic accident or other undesired event These diagrams can be so complex that it becomes difficult to extract essential information B The concept of cut sets was developed to simplify the results of fault tree analysis so that safety professionals can focus their attention on preventing the basic events that must be satisfied for the accident to occur Fault Tree and Cut Set Notes 072400 Page 5 IV COMPLETE CUT SET A Definition Any set of basic events whose occurrence will cause the top event to occur A given fault tree will usually have multiple cut sets B Example 1 1 The following are all cut sets of the above tree AB CD ABC ABCD ABD ACD BCD 2 If we know the probabilities of basic events A B C and D we can compute the probability of the quottopquot event Fault Tree and Cut Set Notes 072400 Page 6 B Example 2 In this case we modify Example 1 so that one of the AND gates becomes andORgate 1 The cut sets are now AB C D AC ABC ABCD BD BC A CD ACD AD BCD BD 2 Again if we know the probabilities of basic events A B C and D we can compute the probability of the quottopquot event C Note that in Examples 1 and 2 we have developed exhaustive lists of all cut sets Some of these sets include basic events that are not necessary for the quottopquot event to occur Fault Tree and Cut Set Notes 072400 Page 7 V MINIMUM CUT SETS A Definition A minimum cut set is a cut set in which the presence of ALL events is necessary for the realization of the top event B Example 3 In this case we modify Example 1 so that ALL gates are now OR gates 1 The cut sets are now A AB ABC ABCD B AC ABD O AD BCD D BC ACD BD CD 2 The MINIMUM cut sets are A B C and D since the other cut sets require unnecessary conditions 3 We can compute the probability of the top event based on probabilities of basic events Fault Tree and Cut Set Notes 072400 Page 8 C Example 4 In this case we modify Example 1 so that ALL gates are now AND gates 1 Here there is only one out set ABCD 2 The out set is also the minimum out set 3 We can compute probabilities of the top event based on the probabilities of basic events Fault Tree and Cut Set Notes 072400 Page 9


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