Every time you work on energized high-power circuits, you run the risk of electric shock, arc/blast, or suffocation/poisoning. Here's how to protect yourself from thse three killers.
Hazards are everywhere when you're working on live high-power electrical equipment and circuits. However, the threat of electric shock, arc/blast, or suffocation/poisoning doesn't automatically spell disaster. If you can clearly identify and understand the power of these three major killers, you can take the necessary precautions to avoid serious, or even fatal, electrical accidents. It's not that difficult: All it takes is a little time and a lot of attention. To understand just how lethal these hazards can be, let's look at some graphic details.
Electric shock. Fleshy tissue is destroyed at 122DegrF. Vascular tissue serving the nerves suffers damage at considerably less. Temperatures generated by short-duration contact (milliseconds) with a conventional 120V, 15A, 60 Hz electrical circuit can exceed 10 times these dangerous temperatures. This is the carnage left by electric shock.
One important factor that determines the severity of body damage is the length of contact time with an energized circuit. Contact time also affects the point at which fibrillation (erratic heartbeat) occurs. Suppose you go into fibrillation after an electric shock. If someone doesn't stop the fibrillation quickly, death is inevitable. Fig. 1 (original article) shows fibrillation threshold (in mA) versus shock duration (in sec). We've included a comparison of effects between AC and DC to make the causation differences clear.
Sure, we'd all like to think we're aware of the danger from electricity, but few of us realize just how little electrical energy is required for electrocution. For example, the same amount of current passed from hand-to-hand, as that drawn by a 75W, 120V lamp, can cause fibrillation. Fibrillation incidence is also a function of frequency. At 60 Hz, the pulsing sensed by the body from the positive and negative values of the sine wave is 120 cycles, which is a close multiplier of the heart rate.
Most industrial electrocutions are due to contact with sufficiently high voltage. Duration of contact and current path greatly influence the effects of shock. Secondary factors include age, physical condition, and size of the victim. The lethal element of electric shock is current. (Voltage, resistance, and frequency are also contributing parameters.)
Here's where human skin comes into play. Your skin resistance is changeable: It varies with the amount of moisture present in the skin's external and internal layers. This resistance can vary from as little as 500 ohms (when skin is wet) to 600,000 ohms (when skin is dry). The skin's moisture content changes with ambient temperature, humidity, fright, anxiety, overwork, and other factors. As electric current breaches your skin's resistance, body tissues, bones, vital organs, and nerves become current-carrying conductors. As a result, your skin resistance drops, and the offending current level increases. At this point, you'll probably suffer internal injury. But unlike thermal burns (which are external), electrocution burns are internal and external, since they take place from the inside out.
Differences between AC and DC shock. At 60 Hz, AC produces a slight-to-violent tingling sensation, while DC produces a warm-to-hot sensation. Remember: With DC, only establishing and interrupting the current will cause muscle reaction.
Let-go thresholds. During electrocution, your arm muscles paralyze, making you unable to release your grip. The minimum amount of current causing this inability is the let-go current, which varies with frequency. Some electrical equipment, such as UPS systems and drives, operate at higher frequencies than 60 Hz, so it's important to recognize this relationship.
Effect of body pathways. Another important aspect of electrical shock is the pathway a current takes through your body. This determines which of your tissues and organs suffer damage or destruction. The pathways are touch potential, step potential, and touch/step potential. If the current path doesn't pass through a vital organ, you may survive. For instance, a right hand/foot path is less likely to cause fibrillation than a left hand/foot path.
Preventing electric shock. There are two effective methods of preventing electric shock. The first is obvious: Work only on de-energized circuits. Unfortunately, too many "dead" circuits have electrocuted workers. This scenario emphasizes the importance of testing for voltage, following lockout/tag-out procedures, using safety grounds, etc. The second method is using proper safety equipment and procedures. Careful and consistent use of personal protective equipment (PPE) always results in safety improvements.
Electric arc and blast. Most of us are aware of the dangers of electrocution, but we often forget the hazards of electric arc and blast resulting from short circuits or electrical faults.
Electric arc. You can suffer extremely bad burns or even death from the heat generated by an electric arc. It can reach temperatures from 15,000DegrF to four times the temperature of the sun's surface. Its radiated thermal energy not only produces this extremely high heat, but it also creates other damaging energy. During a fault, at least 80% of the thermal radiation is available to cause terrible burns.
A curable burn, categorized as first or second degree, is one where the temperature rise of the skin was limited to 46DegrC in 0.1 sec. (Noncurable burns are third degree.) The calculated power to raise the skin temperature to this level in this time interval is 0.09kW per sq in. This is important when you're determining the minimum safe approach distance for curable burns. In addition, heating the surrounding air and melting (vaporizing) the arc electrodes consume significant energy. For example, a 277V, single- phase-to-ground arcing fault with an available bolted fault of 30,000A consumes approximately 20% of the maximum arc power.
Pressure wave aspects of a blast. How can the pressure wave generated by a high-energy arcing fault save your life? It can rapidly hurl you away from the heat source. However, in most cases, it causes other injuries, including physical injury from impact with objects, hearing damage, and possible concussion. You may also suffer injuries from propelled mechanical parts of an enclosure.
A pressure wave can even generate a force strong enough to propel relatively large objects (such as switchgear parts and electrical cabinets) many yards. Suppose you're standing 2 ft away from a 25,000A arc. The pressure wave may hurl you many yards because you would experience about 160 lb per sq ft (about 480 lb of force) on the front of your body. Some pressures are high enough to blow out the windows of a building. In several documented cases, these pressures have even blown over construction walls.
What causes arc and blast? When a fault occurs, the available electrical energy at the fault location changes into another form of energy: an electrical arc. It's usually very powerful and typically results in high thermal radiation, damaging noise levels, explosiveexpansion of surrounding air, and vaporization/splattering of conductors and metal components of the electrical equipment.
Arc blast pressure comes from two sources: expansion of metal in a boiling, vaporizing state, and the heating of ambient air by passage of the arc. Copper expands significantly when it vaporizes; water expands dramatically when it becomes steam. This mixture of vaporized water and metal in air near the arc generates a rapidly expanding plasma of ionized vapor, which can cause extensive injury. Further expansion occurs so rapidly that the vaporized metal droplets expel at speeds sometimes exceeding the speed of sound. These molten pieces of metal have an initial temperature close to that of the arc, at times up to 35,000DegrF. When such an arc occurs, this molten metal travels for 10 ft or more, causing noncurable burns and igniting most non-flame-retardant clothing. The shrapnel effect of small molten metal droplets hitting bare skin also compounds this hazard.
Flash protection boundary. NFPA 70E, Standard for Electrical Safety Requirements for Employee Workplaces, 1995 edition, Part II, Safety-Related Work Practices, recommends the safe distance a person can be from a flash/arc blast. Chapters 2 and 3 cover working on or near electrical conductors or circuits as well as personal and other protective equipment. A significant portion of this section covers the effect of temperature on human tissue and clothing. (See sidebar "Flash injury calculations").
Personal protective equipment. Part II, Chapter 3 of NFPA 70E addresses the standards on PPE, which are based on American National Standards Institute (ANSI) and American Society for Testing and Materials (ASTM) standards. PPE must protect specific body parts in the following categories: head, face, neck, and chin; eyes; body; hand and arm; and foot and leg. In addition to the above, you're also required to use other equipment, such as insulated hand tools, safety grounds, nonconductive ladders, barricades, safety signs, tags, etc.
To provide protection from flash, you must wear protective gear and limit the amount of exposed skin. Electrical workers must wear clothing resistant to flash flame wherever exposure to an electric arc flash is possible. Depending on the type and thickness of material used, clothing ignites from 400DegrC to 800DegrC. Because it takes several seconds to remove clothing or snuff out fire, you may be subjected to direct contact of the flame temperature of the burning cloth. Serious deep thermal burns, which are frequently fatal, can result. Electric arcs also expel droplets of molten metal, which shower the immediate area and anyone in it. At temperatures of about 1000DegrC or more, these droplets ignite clothing instantly and cause burns. If you're working close to the flash, suitably designed protective apparel, such as flash suits made of aramid fiber, will protect you.
Suffocation and poisoning. Sometimes you're required to enter hazardous enclosures or confined areas, such as manholes, underground equipment rooms, or power-cable tunnels. You may encounter these typical hazards:
Combustible gas: process, sewer, natural, and other combustible gases that may accumulate in an enclosed space. These are potentially explosive in various concentrations.
Inadequate oxygen supply: Many confined areas have accumulations of gases heavier than air (such as nitrogen or carbon dioxide) that displace oxygen. Excess oxygen can also be hazardous.
Carbon dioxide: This gas can accumulate from a cable fault, combustion engine exhaust, and other sources.
Hydrogen chloride: This gas can be a byproduct of a fault or fire involving polyvinyl chloride (PVC) conduits or polychlorinated biphenyl (PCB) insulating oils.
PCBs: Hazards can result from exposure to this insulating fluid, which you can find in old capacitors, ballasts, oil-filled transformers, and hydraulic lines.
Asbestos: You can find this heat-resistant insulating material in old electrical equipment, such as fireproof taping of cables, arc chutes on air circuit breakers, duct banks, and panelboards.
You can avoid suffocation and poisoning by testing the atmosphere, using an external oxygen supply, and applying hazard recognition training procedures. For detailed procedures, refer to NFPA 70E, 1995 Edition and the latest OSHA standards.
Papers: R.L. Doughty, R.A. Epperly, R.A. Jones, "Maintaining Safe Electrical Work Practices in a Competitive Environment," IEEE Transaction on Industry Application, Vol. 28. No. 1., Jan./Feb., 1992; R. H. Lee, "The Other Electrical Hazard: Electric Arc Blast Burns," IEEE Transaction on Industry Applications, Vol. 1A-18, No. 3, May/June, 1982; R. H. Lee, "Physical Mechanism of Tissue Injury in Electrical Trauma," IEEE Transactions on Education, Vol. 34, No. 3, August 1991; T. Bernstein, "Electric Shock Hazards and Safety Standards," IEEE Transaction on Education, Vol. 34, No. 3, August 1991; L. B. Gordon, " Electrical Hazards in the High Energy Laboratory," IEEE Transaction on Education, Vol. 3, No. 3, August 1991. For copies, call (800) 678-IEEE.
Books: J. Cadick, Electrical Safety Handbook, McGraw-Hill, Inc., 1994.
Standards: NFPA 70E-1995, Standard for Electrical Safety Requirements for Employee Workplaces. For copies, call (800) 344-3555; Fax (617) 984-7057.