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AMAZON multi-meters discounts AMAZON oscilloscope discounts INTRODUCTION Modern society has produced several generations that have grown accustomed to electricity. This acclimatization has been made easier by the fact that electricity is silent, invisible, odorless, and has an "automatic" aspect to it. In the late 1800s, hotels had to place signs assuring their guests that electricity is harmless. By the late 1900s, signs had to be hung to remind us that electricity is a hazard. In fact, the transition of electricity from a silent coworker to a deadly hazard is a change that many cannot understand until it happens to them. Because of these facts, the total acceptance of an electrical safety procedure is a requirement for the health and welfare of workers. Understanding the steps and procedures employed in a good electrical safety program requires an understanding of the nature of electrical hazards. Although they may have trouble writing a concise definition, most people are familiar with electric shock.
Understanding the nature of the hazards is useless unless protective strategies are developed to protect the worker. This section also includes a synopsis of the types of protective strategies that should be used to safeguard the worker. HAZARD ANALYSIS The division of the electrical power hazard into three components is a classic approach used to simplify the selection of protective strategies. The worker should always be aware that electricity is the single root cause of all of the injuries described in this and subsequent sections. That is, the worker should treat electricity as the hazard and select protection accordingly. SHOCK Description Electric shock is the physical stimulation that occurs when electric current flows through the human body. The distribution of current flow through the body is a function of the resistance of the various paths through which the current flows. The final trauma associated with the electric shock is usually determined by the most critical path called the shock circuit. The symptoms may include a mild tingling sensation, violent muscle contractions, heart arrhythmia, or tissue damage. Detailed descriptions of electric current trauma are included in Section 9. For the purposes of this section, tissue damage may be attributed to at least two major causes. Burning. Burns caused by electric current are almost always third degree because the burning occurs from the inside of the body. This means that the growth centers are destroyed. Electric-current burns can be especially severe when they involve vital internal organs. Cell Wall Damage. research funded by the Electric power research Institute (EprI) has shown that cell death can result from the enlargement of cellular pores due to high- intensity electric fields. This research has been performed primarily by Dr. Raphael C. Lee and his colleagues at the University of Chicago. This trauma, called electroporation, allows ions to flow freely through the cell membranes, causing cell death. === TBL. 1 Important Frequency ranges of Electrical Injury Frequency regimen Applications Harmful effects DC-10 kHz Low Commercial electrical power, Joule heating; destructive cell frequency soft tissue healing; trans- membrane potentials cutaneous electrical stimulation 100 kHz- radio Diathermy; electrocautery Joule heating; 100 MHz frequency dielectric heating of proteins 100 MHz- Microwave Microwave ovens Dielectric heating of water 100 GHz 1013-1014 Hz Infrared Heating; CO2 lasers Dielectric heating of water 1014-1015 Hz Visible light Optical lasers retinal injury; photochemical reactions 1015 Hz and Ionizing radiotherapy; x- ray imaging; Generation of free radicals higher radiation UV therapy ==== Influencing Factors Several factors influence the severity of electrical shock. These factors include the physical condition and responses of the victim, the path of the current flow, the duration of the cur rent flow, the magnitude of the current, the frequency of the current, and the voltage magnitude causing the shock. Physical Condition and Physical Response. The physical condition of the individual greatly influences the effects of current flow. A given amount of current flow will often cause less trauma to a person in good physical condition. Moreover, if the victim of the shock has any specific medical problems such as heart or lung ailments, these parts of the body will be severely affected by relatively low currents. A diseased heart, For example, is more likely to suffer ventricular fibrillation than a healthy heart. Current Duration. The amount of energy delivered to the body is directly proportional to the length of time that the current flows; consequently, the degree of trauma is also directly proportional to the duration of the current. Three examples illustrate this concept: 1. Current flow through body tissues delivers energy in the form of heat. The magnitude of energy may be approximated by J = I 2 Rt where J = energy, joules I = current, amperes R = resistance of the current path through the body, ohms t = time of current flow, seconds If sufficient heat is delivered, tissue burning and/or organ shutdown can occur. Note that the amount of heat that is delivered is directly proportional to the duration of the current (t). 2. Some portion of the externally caused current flow will tend to follow the current paths used by the body's central nervous system. Since the external current is much larger than the normal nervous system current flow, damage can occur to the nervous system. Note that nervous system damage can be fatal even with relatively short durations of current; however, increased duration heightens the chance that damage will occur. 3. Generally, a longer duration of current through the heart is more likely to cause ventricular fibrillation. Fibrillation seems to occur when the externally applied electric field overlaps with the body's cardiac cycle. The likelihood of this event increases with time. Frequency. Tbl. 1 lists the broad relationships between frequency and the harmful effects of current flow through the body. Note that at higher frequencies, the effects of Joule (I 2 t) heating become less significant. This decrease is related to the increased capacitive current flow at higher frequencies. It should be noted that some differences are apparent even between DC (zero Hz) and standard power line frequencies (50 to 60 Hz). When equal current magnitudes are compared (DC to AC rms), anecdotally, DC seems to exhibit two significant behavioral differences: 1. Victims of DC shock have indicated that they feel greater heating from DC than from AC. The reason for this phenomenon is not totally understood; however, it has been reported on many occasions. 2. The DC current "let- go" threshold seems to be higher than the AC "let- go" threshold. Despite the slight differences, personnel should work on or near DC power supplies with the same level of respect that they use when working on or near AC power supplies. This includes the use of appropriate protective equipment and procedures. Note: Unless otherwise specifically noted, the equipment and procedures suggested in this guide should be used for all power frequencies up to and including 400 Hz. Voltage Magnitude. The magnitude of the voltage affects electric shock in one or more of the following three ways: 1. At voltages above 400 volts (V), the electrical pressure (voltage) may be sufficient to puncture the epidermis. Since the epidermis provides the major part of the resistance of the human body, the current magnitude will increase dramatically and lethally when this puncture occurs. 2. The degree of electroporation is higher for greater voltage gradients. That is, the higher voltages cause more intense fields, which in turn increase the severity of the electroporation. 3. Higher voltages are more likely to cause electrical arcing. While this is not a shock hazard per se, it’s related to the shock hazard since arcing may occur at the point of contact with the electrical conductor. Although current regulatory and consensus standards use 50 V as the lower limit for the shock hazard, recent research has shown that harmful or even fatal shocks can result from contact with circuits as low as 30 V. Current Magnitude. The magnitude of the current that flows through the body obeys Ohm's law, that is, I E R = (1.2) where I = current magnitude, amperes (A) E = applied voltage, volts (V) R = resistance of path through which current flows, ohms (Omega) In Fig. 1 the worker has contacted a 120-V circuit when an electric drill short-circuits internally. The internal short circuit impresses 120 V across the body of the worker from the hand to the feet. This creates a current flow through the worker to the ground and back to the source. The total current flow in this case is given by the formula: I E R R = + 1 2 Variable R2 is the resistance of the earth and for the purposes of this analysis may be ignored. Variable R1 is the resistance of the worker's body and includes the skin resistance, the internal body resistance, and the resistance of the shoes where they con tact the earth. Typical values for the various components can be found in Tables 1.2 and 1.3. Assume, For example, that the worker shown in Fig. 1 is wearing leather shoes and is standing in wet soil. This person is perspiring heavily and has an internal resistance of 200 ohm. From Tables 1.2 and 1.3 the total resistance can be calculated as: 500 ohm (drill handle) + 200 ohm (internal) + 5000 ohm (wet shoes) = 5700 ohm From this information the total current flow through the body for a 120-V circuit is calculated as I = = 120 5700 21 1 . milliamperes (mA) Tbl. 4 lists the approximate effects that various currents will have on a 68-kilogram (kg) human being. The current flow of 21.1 mA is sufficient to cause the worker to go into an "electrical hold." This is a condition wherein the muscles are contracted and held by the passage of the electric current-the worker cannot let go. Under these circumstances, the electric shock would continue until the current was interrupted or until someone intervened and freed the worker from the contact. Unless the worker is freed quickly, tissue and material heating will cause the resistances to drop, resulting in an increase in the current. Such cases are frequently fatal. FIG. 1 Electric shock current path. ==== TBL. 2 Nominal resistance Values for Various parts of the Human Body resistance Condition (area to suit) Dry Wet Finger touch 40 k?-1 M? 4-15 k? Hand holding wire 10-50 k? 3-6 k? Finger-thumb grasp* 10-30 k? 2-5 k? Hand holding pliers 5-10 k? 1-3 k? palm touch 3-8 k? 1-2 k? Hand around 1 1 2 -inch (in) pipe (or drill handle) 1-3 k? 0.5-1.5 k? Two hands around 11 2 -in pipe 0.5-1.5 k? 250-750 ? Hand immersed - 200-500 ? Foot immersed - 100-300 ? Human body, internal, excluding skin - 200-1000 ? *Data interpolated. ==== TBL. 3 Nominal resistance Values for Various Materials Material resistance* rubber gloves or soles >20 M? Dry concrete above grade 1-5 M? Dry concrete on grade 0.2-1 M-ohm Leather sole, dry, including foot 0.1-0.5 M? Leather sole, damp, including foot 5-20 k? Wet concrete on grade 1-5 k? *resistances shown are for 130-cm^2 areas. === The reader should note that the values given in this example are for illustration only. Much lower values can and do occur, and many workers have been electrocuted in exactly this same scenario. Parts of the Body. Current flow affects the various bodily organs in different manners. For example, the heart can be caused to fibrillate with as little as 75 mA. The diaphragm and the breathing system can be paralyzed, which possibly may be fatal without outside intervention, with less than 30 mA of current flow. The specific responses of the various body parts to current flow are covered in later sections. ==== TBL. 4 Nominal Human response to Current Magnitudes Current (60 Hz) physiological phenomena Feeling or lethal incidence <1 mA None Imperceptible 1 mA perception threshold Mild sensation 1-3 mA painful sensation 3-10 mA 10 mA paralysis threshold of arms Cannot release hand grip; if no grip, victim may be thrown clear (may progress to higher current and be fatal) 30 mA respiratory paralysis Stoppage of breathing (frequently fatal) 75 mA Fibrillation threshold 0.5% Heart action discoordinated (probably fatal) 250 mA Fibrillation threshold 99.5% (=5-s exposure) 4 A Heart paralysis threshold Heart stops for duration of current (no fibrillation) passage; for short shocks, may restart on interruption of current (usually not fatal from heart dysfunction) =5 A Tissue burning Not fatal unless vital organs are burned ==== ARC Caution: The calculations and formulas in this section are shown to illustrate the basic concepts involved in the calculation of arc parameters including current, voltage, and energy. The calculation of actual values for specific field conditions is a complex, safety related procedure and should be done only under the direction of experienced engineers. Definition and Description ANSI/IEEE Std 100-1988 defines arc as: "A discharge of electricity through a gas, normally characterized by a voltage drop in the immediate vicinity of the cathode approximately equal to the ionization potential of the gas. A similar definition, perhaps more useful in the discussion of electrical safety, is given in the glossary of this hand guide as: "The heat and light energy release that is caused by the electrical breakdown of and subsequent electrical discharge through an electrical insulator such as air." Electric arcing occurs when a substantial amount of electric current flows through what previously had been air. Since air is a poor conductor, most of the current flow is actually occur ring through the vapor of the arc terminal material and the ionized particles of air. This mixture of superheated, ionized materials, through which the arc current flows, is called a plasma. Arcs can be initiated in several ways: • When the voltage between two points exceeds the dielectric strength of the air. This can happen when overvoltages occur due to lightning strikes or switching surges. •When the air becomes superheated with the passage of current through some conductor. For example, if a very fine wire is subjected to excessive current, the wire will melt, superheating the air and causing an arc to start. •When two contacts part while carrying a very high current. In this case, the last point of contact is superheated and an arc is created because of the inductive flywheel effect. Electric arcs are extremely hot. Temperatures at the terminal points of the arcs can reach as high as 50,000 kelvin (K). Temperatures away from the terminal points are somewhat cooler but can still reach 20,000 K. Although the specific results of such temperatures will vary depending on factors such as distance from the arc, ambient environmental conditions, and arc energy, anecdotal evidence supported by experimental results developed by the Institute of Electrical and Electronics Engineers (IEEE) clearly shows the following: • The heat energy of an electrical arc can kill and injure personnel at surprisingly large distances. For example, second-degree burns have been caused on exposed skin at distances of up to 12 feet (ft) or (3.6 meters [m]) and more. • Virtually all types of clothing fibers can be ignited by the temperatures of electrical arcs. Clothing made of non-flame-resistant fibers will continue to burn after the arc source has been removed and will continue to cause serious physical trauma. Tbl. 5 shows the ignition temperature of various fabrics and identifies those that will support combustion after the arc energy is gone. The amount of energy, and therefore heat, in an arc is proportional to the maximum avail able short circuit volt-amperes in the system at the point of the arc. Calculations by Ralph Lee indicate that maximum arc energy is equal to one- half the available fault volt- amperes at any given point. Later research by Neal, Bingham, and Doughty shows that while the maximum may be 50 percent, the actual value will usually be somewhat different depending on the degree of distortion of the waveform, the available system voltage, and the actual arc power factor. The same research also shows that enclosing the arc to create a so- called "arc in the box" focuses the incident arc energy and increases its effect by as much as threefold. The arc energy determines the amount of radiated energy and, therefore, the possible degree of thermal injury from radiation effects, including convective, infrared, and ultra violet radiation. The arc energy will be determined by the arc voltage drop and the arcing current. After the arc is established, the arc voltage tends to be a function of arc length; consequently, the arc energy is less dependent on the system voltage and more dependent on the magnitude of the fault current. This means that even low-voltage systems have significant arc hazard and appropriate precautions must be taken. Figs. 2 and 3 show the results of two experiments that were conducted with manikins exposed to electric arcs. As can be seen, both high and low voltages can create significant burns. ==== TBL. 5 Ignition Temperatures and Characteristics of Clothing Fibers Melt Decomposition Ignition Burning Fiber temperature, temperature, temperature, temperature Cotton/rayon* N/A 554/581 752/788 1562 polyester 482 734 1040 1337 Wool N/A 446 1112 Unknown Nylon 6,6 490 653 990 1607 pBI N/A 860 N/A N/A NOMEX N/A 900 N/A N/A ==== Arc Energy Release Arc energy is released in multiple forms, including electrical, thermal, mechanical, and photonic or light energy. Tbl. 6 describes the nature of these energy releases and the injuries that they cause. Note that light and heat tend to cause similar injuries and will, there fore, be treated as one injury source in later calculations. Also note that mechanical injuries are usually categorized as blast injuries, even though the ultimate cause is the electric arc. To be conservative in arc energy release calculations, two assumptions must be made: 1. All arc energy is released in the form of heat measured in cal/cm2 or J/cm2. The reader should remember that this assumption is made solely for the purpose of analyzing electric arc thermal injury. Other hazards such as shock and blast are considered separately. 2. Every arc is fed by a sinusoidal source, thereby creating the maximum amount of energy release. FIG. 2 Electric arc damage caused by a medium-voltage arc. (Brosz and Associates.) FIG. 3 Electric arc damage caused by 240-volt arc. (Brosz and Associates.) Arc Energy Several major factors determine the amount of energy created and/or delivered by an electric arc. Tbl. 7 lists the major factors and their qualitative effect. The quantitative effects of electric arc are the subject of many ongoing studies. An individual's exposure to arc energy is a function of the total arc energy, the distance of the subject from the arc, and the cross- sectional area of the individual exposed to the arc. ==== TBL. 6 electrical arc injury, energy sources energy Nature of injuries light Principally eye injuries, although severe burns can also be caused if the ultraviolet component is strong enough and lasts long enough heat severe burns caused by radiation and/or impact of hot objects such as molten metal Mechanical shrapnel injuries from flying objects or blast debris; concussive injuries to brain/central nervous system; air pressure wave injuries to ears, lungs, and gastrointestinal organs === Arc Energy Input The energy supplied to an electric arc by the electrical system, called the arc input energy, may be calculated using the formula J V I dt arc arc arc t = × × × ? cos? 0 where Jarc = arc energy, joules Varc = arc voltage, volts Iarc = arc current, amperes t = time, seconds research has shown that electric arcs are rarely perfect sinusoids; however, the perfect sinusoid creates the greatest arc power. therefore, eq. 1.5 can be solved as Jarc = Varc × Iarc × t × cos ? (1.6) where ? = the angle between current and voltage ==== TBL. 7 factors that affect the amount of trauma caused by an electric arc distance the amount of damage done to the recipient diminishes by approximately the square of the distance from the arc. Twice as far means one- fourth the damage. (empirical evidence suggests that the actual value may be somewhat different because of the focusing effect of the surroundings.) temperature the amount of energy received is proportional to the difference between the fourth power of the arc temperature and the body temperature (T a 4 - T b 4 ). Absorption coefficient the ratio of energy received to the energy absorbed by the body. Time energy received is proportional to the amount of time that the arc is present. Arc length the amount of energy transmitted is a function of the arc length. For example, a zero-length arc will transmit zero energy. Note that for any given system, there will be an optimum arc length for energy transfer. Cross- sectional area of the greater the area exposed, the greater the amount of energy received. Body exposed to the arc angle of incidence of energy is proportional to the sine of the angle of incidence. thus, energy the arc energy impinging at 90° is maximum. ==== Arcing Current. The actual arcing current varies as a function of several variables and has been calculated or estimated in different ways. IEEE Standard (Std) 1584-2002, For example, gives two equations that may be used to calculate the arcing current. Equation 7 is the formula used for electrical arcs in systems with voltages less than 1000 V, and Eq. 8 is used for systems with voltages equal to or greater than 1000 V. log10(Ia) = K + 0.662 log10(Ibf) + 0.0966V + 0.000526G + 0.5588V log10(Ibf) - 0.00304G log10(Ibf) log10(Ia) = 0.00402 + 0.983 log10(Ibf) (1.8) where Ia = arcing current (kA) K = a constant (-0.153 for open configurations or -0.097 for box configurations) Ibf = the bolted, rMS symmetrical, three-phase fault current (kA) V = system phase-to-phase voltage (kV) G = the gap between the arcing conductors (mm) Note that these equations are based on a specific model as developed for Std 1584-2002. The model includes the following: • Three-phase voltages in the range of 208 to 15,000 V phase-to-phase • Frequencies of 50 or 60 Hz • Bolted fault current in the range of 700 to 106,000 A • Grounding of all types and ungrounded • Equipment enclosures of commonly available sizes • Gaps between conductors of 13 to 152 mm • Faults involving all three phases Arcing Voltage Arc voltage is somewhat more difficult to determine. Values used in power system protection calculations vary from highs of 700 V/ft (22.72 V/cm) to as low as 300 V/ft (9.84 V/cm). Two things are well understood: 1. Arc voltages start low and tend to rise. periodically, the arc voltage will drop if the arc lasts long enough. 2. Arc voltage is proportional to arc length. Therefore, from Eq. 6, arc power and energy are proportional to arc length. Modern software programs used to calculate incident arc energy take different approaches to determine arc voltage. It should be noted that, at best, arc voltage calculation is only approximate for any given scenario. Arc Surface Area While the actual shape of an electrical arc may vary, all classic, realistic solutions start by assuming that an arc causes an approximately cylindrical plasma cloud with length L and radius r. This cylindrical structure will have a lateral surface area equal to 2prL. The areas of the ends of the cylinder are ignored in this calculation since they are so small relative to the side of the arc. To simplify the calculation of energy density, the arc is assumed to form a sphere with a surface area equal to the cylinder ( Fig. 4). Thus, the arc sphere will have a radius of where rs = radius of equivalent sphere r = radius of arc cylinder L = length of arc FIG. 4 Arc cylinder and equivalent arc sphere. a. Arc cylinder with area A; b. Equivalent arc sphere with area A Incident Energy The most commonly used arc calculation for personnel protection is the one that determines the radiant energy transfer from the electrical arc source to a body or equipment surface at an assumed distance away from the electrical arc. This is called the incident energy. This information can be used to determine the necessary level of protective clothing required and can also be used in the performance of a risk analysis. The estimation for tissue injury from electric arc depends on the temperature change and duration of the tissue exposure. However, the estimation starts with the calculation of the amount of energy, or heat flux (measured in calories per square centimeter), delivered to the skin. Many methods have been developed to calculate the incident energy-some more conservative than others. The following sections describe some of the methods that have been determined either empirically or theoretically. The reader should be aware that research into these areas is continuing at a frantic pace. Always refer to the most recent industry literature for the most up-to-date information. The Lee Method. Ralph Lee has predicted that the heat energy received by an object (or worker) can be calculated using Eq. 10. where Qo = heat flux received by the object (cal/cm^2 ) Qs = heat flux generated by source (cal/s/cm^2 ) As = surface area of arc sphere r = distance from center of source to object (cm) t = length of arc exposure Using Eq. 8 as a starting point, Lee determined that the energy received by the worker is calculated using Eq. 11. where E = incident energy in J/cm^2 V = system voltage (phase-to-phase) t = arcing time (seconds) D = distance from arc point to person or object (mm) Ibf = bolted fault current (kA) Other Research. Research by Bingham and others has yielded a slightly different result based primarily on empirical results. Using an experimental setup, the researchers measured energy received from an electric arc at various distances. The arc was created using a 600- V source, and different configurations were used to simulate a completely open- air arc versus the so- called "arc-in-a- box." Using these experiments, they developed two equations to model the amount of energy received. EMA = 5271DA where EMA = maximum open-arc incident energy (cal/cm^2) EMB = maximum arc-in-a-box incident energy (cal/cm^2) D = distance from the arc electrodes in inches (D = 45.7 cm [18 in]) F = bolted fault current available in kA (16 to 50 kA range) tA = duration of the open-air arc (seconds) tB = duration of the arc-in-a-box (seconds) Note that these equations were developed with three constraints: 1. System voltage = 600 V 2. System available fault current F - 16,000 A < F < 50,000 A 3. Electrode distance D = 45.72 cm (18 in) Using the model previously described (see "Arcing Current"), the IEEE Std 1584-2002 calculates incident energy by first calculating the normalized incident energy. The normalized energy is calculated for an arc time of 0.2 second and a distance of 610 mm. The empirically developed formula for this calculation is shown in Eq. 14. log10(En) = K1 + K2 + 1.081[log10(Ia)] + 0.0011G where Ia = arc current calculated from Eq. 7 En = incident energy (J/cm^2) normalized for time and distance K1 = constant and is equal to -0.792 for open configurations and -0.555 for enclosed configurations K2 = constant and is equal to 0 for ungrounded and high-resistance systems and -0.113 for rounded systems G = gap between conductors (mm) After the log10 En is calculated from eq. 14, eq. 15 is used to calculate En, and eq. 16 is used to calculate the actual incident energy. where E = incident energy (J/cm^2) Cf = calculation factor equal to 1.0 for voltages above 1 kV and 1.5 for voltage equal to or below 1 kV En = normalized incident energy calculated from eqs. 14 and 15 t = arcing time in seconds D = distance from the arc point to the exposed worker (mm) x = distance exponent whose value is dependent on voltage (see ieee std 1584 2002, table 4 for values of x) extensive research continues to be performed on the subject of incident arc energy. Practical applications used in the selection of protective equipment are covered in detail in section 3. Arc Burns: Arc burns are thermal in nature and, therefore, fall into one of four categories: 1. First-degree burns. First-degree burning causes painful trauma to the outer layer (epidermis) of the skin. Little permanent damage results from a first-degree burn because all the growth areas survive. Healing is usually prompt and leaves no scarring. 2. Second-degree burns. Second-degree burns are sometimes called partial thickness burns and cause damage to the epidermis and dermis layers. The second-degree burn is usually very painful and typically causes blistering of the skin; however, this type of burn will heal without skin grafts. Second-degree burns are often further classified into two subcategories: a. Superficial: this type of second-degree burn affects the epidermis and the upper layer of the dermis called the papillary dermis. b. Deep: the deep secondary burn involves the epidermis and extends through the papillary dermis into the reticular dermis. 3. Third-degree burns. Third-degree burns (sometimes called full thickness burns) destroy the epidermis and the dermis and usually cause damage to the subcutaneous layer. These types of burns result in complete destruction of the growth centers. If the burn is small, healing may occur from the edges of the damaged area; however, extensive third-degree burns require skin grafting. 4. Fourth-degree burns. Fourth-degree burns cause severe damage to all three skin layers and extend into the muscle, nerve, tendon, ligament, vascular, organ, and bone tissues below the skin. Most severe electrical burns are fourth degree. Refer to section 9 for more detailed coverage of electrical arc trauma. Blast: When an electrical arc occurs, the vaporization of solid metal conductors into a gas is an exothermic or heat-releasing event that leads to rapid superheating of the surrounding air. The metallic vapor can be toxic exposure to respiratory or lung tissue because of its chemical composition and high heat. The superheating of the surrounding air can create a blast effect leading to acoustic trauma or tissue destruction from explosion. This rapid expansion of the air creates a wavefront that can reach pressures of 100 to 200 lb per square foot (lb/ft^2) (4.79 to 9.58 kpa). Such pressure is sufficient to explode switchgear, turn sheet metal into shrapnel, turn hardware into bullets, push over concrete walls, and propel molten metal and superheated plasma at extremely high velocities. Blasts don’t always occur. Sometimes an arc is not accompanied by a blast, but when it is, it can be lethal. Fig. 5a to c shows physical evidence of the pressure exerted by an electric blast. In Fig. 5a the interior of a medium-voltage cubicle can be seen. The severe scorching on the right-hand side of the interior and exterior of the cubicle is clear evidence of a significant arc-flash event. By looking closely at the cable terminations, evidence of the con tact points for the electric arc as it occurred can be seen. FIG. 5 (a) Interior of a medium-voltage cubicle showing the results of an electrical arc and accompanying electrical blast; (b) external view of an aisle and adjacent switchgear for arc-flash event shown in Fig. 5a; (c) close-up view of adjacent switchgear showing metal covers damaged by impact of panel blown across aisle by arc-flash event. Fig. 5b shows the aisle between the switchgear where the arc-flash occurred (right side) and the adjacent gear across the aisle. The metal panel in the aisle is the one that covered the cubicle where the arc-flash event occurred. Note that the fully secured panel was blown completely off the faulted switchgear across the aisle and smashed into the adjacent switchgear. Fig. 5c is a close-up view of the adjacent switchgear showing where the metal panel, seen in the lower right side of the photo, is dented and crumpled in the cubicle cover. Taken together, these three photos clearly illustrate the following two key points: •Workers may not assume that they are safe from electrical arc-flash events even though the access doors and panels are fully secured. • Unless it’s specially designed arc-resistant switchgear, metal-clad equipment will probably not withstand the explosive force of an electrical blast. AFFECTED BODY PARTS General: Detailed information on the medical aspects of electrical trauma is provided in Section 9. The following sections are for overview only. Skin: Definition and Description. Skin is the outer layer that completely encloses and envelops the body. Each person's skin weighs about 4 lb, protects against bacterial invasion and physical injury of underlying cells, and prevents water loss. It also provides the body with sensation, heat regulation, and excretion (sweat) and absorbs a few substances. There are about 20 million bacteria per square inch on the skin's surface as well as a forest of hairs, 50 sweat glands, 20 blood vessels, and more than 1000 nerve endings. Fig. 6 is a cross section of the upper layers of skin tissue. The main regions of importance for electrical purposes are the epidermis, the dermis, and the subcutaneous layers of the skin. For severe electrical burns, the underlying muscles and bone tissues may be involved as well. The epidermis, the topmost layer of skin, is 0.1 to 1.5 millimeters thick; however, it’s made up of five layers including the basal cell layer, the squamous cell layer, the stratum granulosum, the stratum lucidum, and the outermost layer called the stratum corneum or "horny layer." The stratum corneum comprises 10 to 30 thin layers of dead cells that have been "pushed" up from the lower layers in the process of the normal growth process. It’s called the horny layer because its cells are toughened like an animal's horn. The stratum corneum is composed primarily of a protein material called keratin. Of all the skin layers, keratin exhibits the highest resistance to the passage of electricity. When areas of the epidermis such as the hands or feet are subjected to friction, the horny layer becomes thickened and toughened. Areas that are toughened in this manner are called corns or calluses. The sweat glands and the blood vessels have relatively low resistance to the passage of electricity and provide a means of electrical access to the wet and fatty inner tissues. Most of the electrical resistance exhibited by the human body (see Tbl. 2) comes from the stratum corneum. Internal resistance is typically in the area of 200 ohm. Effects on Current Flow. Since the body is a conductor of electricity, Ohm's law applies as it does to any other physical substance. The thicker the horny layer, the greater the skin's electrical resistance. Workers who have developed a thick stratum corneum have a much higher resistance to electricity than a child with an extremely thin layer. However, as Tbl. 2 shows, even high skin resistance is not sufficient to protect workers from electric shock. Skin resistance is also a function of how much skin area is in the circuit. Therefore, grasping a tool with the entire hand gives a much lower resistance than touching the tool with a finger. Also, any cut or abrasion penetrates the horny layer and significantly reduces the total resistance of the shock circuit. Moisture, especially sweat, greatly reduces the skin's resistance. A remarkable thing occurs to the skin insulation when voltages above 400 V are applied. At these voltages the epidermis is punctured like any film insulation and only the low resistance inner layers are left. This is a major consideration for the many 480-volt distribution systems commonly used today. Note that the epidermis may not puncture, but if it does, the current flow increases and shock injury is worse. === FIG. 6 Typical skin cross section. Duct of sweat gland Hair Epidermis Malpighian layer Arrector muscle Sebaceous gland Hair follicle Sweet gland Pacinian corpuscle Subcutaneous fatty tissue Blood vessels Papilla of hair Subcutaneous Dermis === FIG. 7 Thermal burns caused by high-voltage electric arc. Burns. Electrically caused burns can come from at least four different sources: • Physical contact with conductors, tools, or other equipment that have been heated by the passage of electrical current flow. These types of burns are no different than burns received from any hot object. • Current flow through the skin can cause burns from the I^2 R energy. Since such burns occur from the inside-out, they are frequently third degree. • Thermal or radiation burns are caused by the radiant energy of the electrical arc. • Contact with superheated plasma caused by the vaporization of solid materials in the vicinity of an electrical arc. Fig. 7 shows a victim of burns caused by both the radiant arc energy and contact with the superheated plasma. The Nervous System Definition and Description. The nervous system comprises the electrical pathways that are used to communicate information from one part of the human body to another. To communicate, electric impulses are passed from one nerve to another. For example, the heart beats when an electric impulse is applied to the muscles that control it. If some other electric impulse is applied, the nervous system can become confused. If the current is high enough, the damage can be permanent. Shock. As far as the nervous system is concerned, at least three major effects can occur when current flows through the body: 1. Pain. Pain is the nervous system's method of signaling injury. When current flows through the nerves, the familiar painful, tingling sensation can result. 2. Loss of control. An externally applied current can literally "swamp" the normal nervous system electric impulses. This condition is similar to electrical noise covering an information signal in a telemetering or other communications system. When this hap pens, the brain loses its ability to control the various parts of the body. This condition is most obvious during the electrical paralysis, or electrical hold, that is described later in this section. 3. Permanent damage. If allowed to persist, electric current can damage the nervous sys tem permanently. This damage takes the form of destroyed neurons and/or synapses. Since the nervous system is the communications pathway used to control the muscles, such damage can result in loss of sensation and/or function depending on the type of injury. Muscular System Definition and Description. The muscular system provides motor action for the human body. When the nervous system stimulates the muscles with electric impulses, the muscles contract to move the body and perform physical activity. Skeletal muscles are either flexor or extensor muscles. When a flexor muscle, such as the biceps, contracts, it bends or closes the joint. When an extensor muscle, such as the triceps, contracts, it unbends or opens the joint. The heart and pulmonary system are also muscle related, and they will be covered in a later section. Shock. Electrical shock can affect muscles in at least three significant ways: 1. Reflex action. Muscular contractions are caused by electric impulses. Normally these impulses come from the nervous system. When an externally induced current flows through a muscle, it can cause the muscle to contract, perhaps violently. This contraction can cause workers to fall off ladders or smash into steel doors or other structures. 2. Electrical paralysis. Current magnitudes in excess of 10 mA may be sufficient to block the nervous system signals to the muscular system. Thus, when such an external current is flowing through the body, the victim may be unable to control his or her muscles. This means that the victim cannot let go-he or she is caught in an electrical hold. As the current continues, the heating and burning action can lower the path resistance and cause an increase in the current. If the current is not cut off or if the victim is not freed from the circuit, death will occur. 3. Permanent damage. If the current is high enough, the muscle tissue can be destroyed by burning. Even very low currents will cause tissue destruction if they last long enough. Because such burning destroys the growth areas in tissue, the damage can be extremely slow to heal. Physical therapy and other extraordinary methods may be required to restore muscular function. The Heart Definition and Description. The heart is a fist-size pump that beats more the 2.5 billion times in a 75-year lifetime. Fig. 8 shows the structural layout of the heart. The atria and ventricles work together, alternately contracting and relaxing to pump blood through your heart. The electrical system of your heart is the power source that makes this possible. Normally a heartbeat starts in the sinus node, travels at approximately 2.1 m/s (7 ft/s) through the AV node, "HIS bundle," and right and left bundle branches. The resulting con traction sends blood flowing from the heart. This sequence occurs with every beat (usually 60 to 100 times per minute). If the path is interrupted for any reason, even for a few minutes, changes in the heart rate and rhythm occur that can be fatal. FIG. 8 The heart. Shock. When the heart's electrical system is disturbed for any reason, such as an outside current from an electric shock, changes in the heart's rate and rhythm occur. Such disruptions result in a large percentage of heart deaths. The electric impulses in the heart must be coordinated to give a smooth, rhythmic beat. An outside current of as little as 60 to 75 mA can disturb the nerve impulses so that there is no longer a smooth, timed heartbeat. Instead, the heart fibrillates-that is, it beats in a rapid, uncoordinated manner. When a heart is fibrillating, it flutters uselessly. Prolonged exposure to an outside current exceeding 75 mA is likely to result in death. Like any muscle, the heart will become paralyzed if the current flowing through it’s of sufficient magnitude. Oddly, paralysis of the heart is not often fatal if the current is removed quickly enough. In fact, such paralysis is used to an advantage in defibrillators. A defibrillator intention ally applies heart-paralyzing current. When the current is removed, the heart is in a relaxed state ready for the next signal. Frequently the heart restarts. Burns. Any internal organ, such as the heart, can be burned by current flows in excess of 5 A. Such burns are often fatal. The Pulmonary System Definition and Description. With the exception of the heart, the pulmonary system is the most critical to human life. If breathing stops, which can happen with as little as 30 mA, brain damage can occur in as little as four minutes. When the lower diaphragm moves down, it creates a partial vacuum in the chest chamber. This in turn draws air into the sacs in the lungs. The oxygen is then passed to the bloodstream through the tiny capillaries. At the same time, carbon dioxide is returned to the air in the lungs. When the lower diaphragm moves up, the air is forced out of the lungs, thus completing the breathing cycle. Shock. Current flow through the midsection of the body can disrupt the nervous system impulses that regulate the breathing function. This disruption can take the form of irregular, sporadic breathing, or-if the current flow is sufficient-the pulmonary system may be paralyzed altogether. When such stoppage occurs, first aid is often required. SUMMARY OF CAUSES-INJURY AND DEATH Shock Effect Tbl. 4 summarizes the effects that electric shocks of varying amounts of current will have on a 68-kg (150-lb) person. Note that these effects are only approximate and vary among individuals. Arc-Flash Effect Tables 6 and 8 summarize the effects that electric arc-flash energy have on personnel. Note that these effects are still being studied and the full range of injury potential is not yet fully evaluated and documented. === TBL. 8 equipment and Procedural strategies for Protection from the three electrical hazards (see sections 3 and 4 for detailed information.) Hazard | Equipment strategy | Procedural strategy for all three hazards shock arc Blast • de-energize all circuits and conductors in the immediate work area. • develop and follow a lockout/tagout procedure. • Maintain a safe working distance from all energized equipment and conductors. • Use all specified safety equipment. • follow all safety procedures and requirements. • carefully inspect all equipment before placing it into service. This includes tools, test equipment, electrical distribution equipment, and safety equipment. • Make certain that all non-energized equipment is properly grounded. This applies to both normal system grounding and temporary safety grounds. • design or redesign systems to be intrinsically safe. • rubber insulating equipment including gloves with leather protectors, sleeves, mats, blankets, line hose, and covers. • insulated tools when working near energized conductors. • Approved flash/flame-resistant work clothing. • Approved flash suits when performing work with a high risk of arcing. • Use hot sticks to keep as much distance as possible. • Wear eye protection. • Wear rubber gloves with leather protectors and/or other flashproof gloves. • Approved flash/flame-resistant work clothing. This may protect from splashed molten material. • Approved flash suits when performing work with a high risk of arcing. This may protect from splashed molten material. • Wear face shields. === Causes of Injury: Injury from electrical hazard can come from both direct and indirect sources: • The reflex action caused by the passage of current flow can cause falls resulting in cuts, abrasions, or broken limbs. • Nerve damage from shock or burns can cause loss of motor function, tingling, and/or paralysis. • Burns, both thermal- and current-induced, can cause extremely long duration and intensely painful suffering. Third- and fourth-degree burns may require skin grafting to heal. • The light intensity, molten metal, and/or burns to the eyes can cause blindness. • The concussion of a blast can cause partial or complete loss of hearing. • Current-induced burns to internal organs can cause organ dysfunction. • The superheated plasma may be inhaled, causing severe internal burns. • Metal vapors may be inhaled, filling the lungs with toxic residues. Causes of Death: If the electrical injury is severe enough, death can result. • An electric shock-induced fall can cause fatal physical injuries. • When the skin is severely burnt, large quantities of liquid are brought to the burnt areas to aid in the healing process. This creates a stress on the renal system and could result in kidney failure. • Severe trauma from massive burns can cause a general systemic failure. • Burnt internal organs can shut down-causing death. Thus, the more critical the organ that is burnt, the higher the possibility of death. • The pressure front from the blast can cause severe injury to the lungs, called blast- lung, resulting in death. • Heart failure can result from fibrillation and/or paralysis. PROTECTIVE STRATEGIES The types of strategies that may be employed to protect from each of the three electrical hazards are remarkably similar. Tbl. 8 summarizes the types of protective strategies that may be used. Note that the information given in Tbl. 8 is general. Specific equipment and procedures are covered in Sections 3 and 4. Be aware that any given strategy may not be applicable in a given situation. For example, •When troubleshooting equipment, de-energization may not be possible. • De-energization may create an additional, unacceptable hazard. For example, if de energization shuts down ventilation equipment in a hazardous area, workers may opt for working with energized equipment. • Shutdown of an entire continuous process plan to work on or around one small auxiliary circuit may not be economically feasible. If electrical equipment cannot be de-energized, a hierarchy of electrical safe work practices and procedures must be implemented as follows: • Document the reason(s) for having to work on or near energized electrical conductors or circuit parts. Identify that de-energizing is infeasible. • Obtain responsible manager's (employer's designated person) signature, verifying knowledge and recognizing the need to work on or near exposed energized electrical conductors or circuit parts. • Perform a job briefing that includes a hazard-risk analysis. • Prepare and follow a documented plan of work processes and tasks. • if worker(s) must enter closer to the potential arc source(s) than the flash protection boundary, then ensure that the worker is a "qualified person" and is wearing appropriate personal protective clothing (PPc) and using other personal protective equipment (PPe). Also see: Electrical safety systems
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