|
An electric shock can occur upon contact of a human or animal body with any source of voltage high enough to cause sufficient current flow through the muscles or nerves. The minimum detectable current in humans is thought to be about 1 milliampere (mA). The current may cause tissue damage or heart fibrillation if it is sufficiently high. When (and only when) an electric shock is fatal, it is called electrocution. Psychological The perception of electric shock can be different depending on the voltage, duration, current, path taken, frequency, etc. Current entering the hand has a threshold of perception of about 5 to 10 milliamperes (mA) for DC and about 1 to 10 mA for AC at 60 Hz. Shock perception declines with increasing frequency, ultimately disappearing at frequencies above 15-20 kHz. Physiological Burns - Tissue heating due to resistance can cause extensive and deep burns. High-voltage (> 500 to 1000 V) shocks tend to cause internal burns due to the large energy (which is proportional to the square of the voltage) available from the source. Damage due to current is through tissue heating. Ventricular fibrillation - A low-voltage (110 to 220 V), 50 or 60-Hz AC current travelling through the chest for a fraction of a second may induce ventricular fibrillation at currents as low as 60mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheter or other electrodes), a much lower current of less than 1 mA, (AC or DC) can cause fibrillation. Fibrillations are usually lethal because all the heart muscle cells move independently. Above 200mA, muscle contractions are so strong that the heart muscles cannot move at all. Neurological effects - Current can cause interference with nervous control, especially over the heart and lungs. When the current path is through the head, it appears that, with sufficient current, loss of consciousness almost always occurs swiftly. (This is borne out by some limited self-experimentation by early designers of the electric chair and by research from the field of animal husbandry, where electric stunning has been extensively studied) *. Arc-flash hazards - Over 80% of all injuries and fatalities caused by electrical incidents are not caused by electric shock, but by the intense heat, light, and pressure wave (blast) caused by electrical faults. The arc-flash in an electrical fault produces the same type of light radiation from which electric welders protect themselves using face shields with dark glass, heavy leather gloves, and full-coverage clothing. The heat produced may cause severe burns, especially on unprotected flesh. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs. The degree of hazard present at a particular location can be determined by a detailed analysis of the electrical system, and appropriate protection worn if the electrical work must be performed with the electricity on. Worker safety standards in the USA require, though, that the electricity be turned off before work is performed unless a greater hazard will result from turning the power off. Issues affecting lethality Other issues affecting lethality are frequency, which is an issue in causing cardiac arrest or muscular spasms, and pathway - if the current passes through the chest or head there is an increased chance of death. From a mains circuit the damage is more likely to be internal, leading to cardiac arrest. The comparison between the dangers of alternating current and direct current has been a subject of debate ever since the War of Currents in the 1880s. DC tends to cause continuous muscular contractions that make the victim hold on to a live conductor, thereby increasing the risk of deep tissue burns. On the other hand, mains-frequency AC tends to interfere more with the heart's electrical pacemaker, leading to an increased risk of fibrillation. AC at higher frequencies holds a different mixture of hazards, such as RF burns and the possibility of tissue damage with no immediate sensation of pain. Generally, higher frequency AC current tends to run along the skin rather than penetrating and touching vital organs such as the heart. While there will be severe burn damage at higher voltages, it is normally not fatal. It is sometimes suggested that human lethality is most common with alternating current at 100-250 volts, however death has occurred from supplies as low as 32 volts and supplies at over 250 volts frequently cause fatalities. Electrical discharge from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest. Point of Entry Avoiding danger of shock It is strongly recommended that people should not work on exposed live conductors if at all possible. If this is not possible then insulated gloves and tools should be used. If both hands make contact with surfaces or objects at different voltages, current can flow through the body from one hand to the other. This can lead the current to pass through the heart. Similarly, if the current passes from one hand (especially the left hand) to the feet, significant current will probably pass through the heart. Also, remember there can be a voltage potential between neutral wires and ground in the event of an improperly wired (disconnected) neutral, or if it is part of certain obsolete (and now illegal) switch circuits. The electrical appliance or lighting may provide some voltage drop, but not nearly enough to avoid a shock. "Live" neutral wires should be treated with the same respect as hot wires. Also, the neutral wire must be insulated to the same degree as the hot wire to avoid a short circuit. Electrical codes in many parts of the world call for installing a residual-current device (RCD or GFCI, ground fault circuit interrupter) on electrical circuits thought to pose a particular hazard to reduce the risk of electrocution. In the USA, for example, a new or remodeled residential dwelling must have them installed in all kitchens, bathrooms, laundry rooms, garages, and any other room with a concrete floor such as a workshop. These devices work by detecting an imbalance between the live and neutral wires. In other words, if more current is passing though the live wire than is returning though its neutral wire, it assumes something is wrong and breaks the circuit in a fraction of a second. There is some concern that it might not be fast enough for infants and small children in rare instances. The plumbing system in a home or other building is connected to ground through its metal pipes. Because of the extensive use of plastic (PVC)piping in recent years, though, this is still often true but cannot be relied upon for safety purposes. Contrary to popular belief, pure water is not a good conductor of electricity. However, most water is not pure and contains enough dissolved particles (salts) to greatly enhance its conductivity. When the human skin becomes wet, it allows much more current to flow than the dry human body would. Thus, being in the bath or shower will not only ground oneself to return path of the power mains, but lower the body's resistance as well. Under these circumstances, touching any metal switch or appliance that is connected to the power mains could result in electrocution. While such an appliance is not supposed to be hot on its outer metal switch or frame, it may have become so if a hot bare wire is accidentally touching it (either directly or indirectly via internal metal parts). It is for this reason that mains electrical sockets are prohibited in bathrooms in the UK. However, widespread use of plastic cases (which won't conduct electricity), grounding of appliances, and mandatory installation of ground fault circuit interrupters have greatly reduced this type of electrocution over the past few decades. A properly grounded appliance eliminates the electric shock potential by causing a short circuit if any portion of the metal frame (chassis) is accidentally touching the hot wire. This will cause the circuit breaker to turn off or the fuse to blow resulting in a power outage in that area of the home or building. Often there will be a large "bang" and possibly smoke which could easily scare anyone nearby. However, this still much safer than risking electric shock as the chance of an out-of-control fire is remote. Many people in this situation have nevertheless called the fire department as a precaution. Where live circuits must be frequently worked on (e.g. television repair), an isolation transformer is used. Unlike ordinary transformers which raise or lower voltage, the coil windings of an isolation transformer are at a 1:1 ratio which keeps the voltage unchanged. The purpose is to isolate the neutral wire so that it has no connection to ground. Thus, if a technician accidentally touched the hot chassis and ground at the same time, nothing would happen. Neither ground fault circuit interrupters (RCD/GFCI) nor isolation transformers can prevent electrocution between the hot and neutral wires. This is the same path used by functional electrical appliances, so protection is not possible. However, most accidental electrocutions, especially those not involving electrical work and repair, are via ground -- not the neutral wire. First Aid In helping a victim of an electric shock, the first thing you must do is disconnect the power supply. Don’t even touch the victim until you are sure the power supply is turned off. Be especially careful in wet areas, such as bathrooms, since the salts in water conduct electricity. First aid instructions Handling downed power lines Sometimes, power lines are downed in car accidents, causing energized lines to drape over the vehicles. The metal structure of the car surrounds the occupants, behaving as a Faraday cage which protects them from electrical shock. Unless there is an immediate fire danger, urge the victims to remain inside the car. Do not approach the scene until it has been declared safe by the proper authorities. Stand well back and try to encourage any other bystanders to keep a distance of at least six metres. If occupants must exit the vehicle (say, because of fire), they should flee individually by jumping well clear of the energized car and wires. It is essential that no simultaneous contact be made with the car, wires, or ground since this may result in immediate electrocution. Electrocution statistics There were 550 electrocutions in the US in 1993, which translates to 2.1 deaths per million inhabitants. At that time, the incidence of electrocutions was decreasing. • Electric shock as medical treatment Electric shock can also be used as a medical therapy, under carefully engineered conditions: Torture Electric shocks have been used as a method of torture, since the received voltage and amperage can be controlled with precision and used to cause pain while avoiding obvious evidence on the victim's body. Such torture usually uses electrodes attached to parts of the victim's body. The genitalia are amongst the most painful, and at the same time humiliating. Nipples and the tongue are also frequent sites. Another frequent method of electrical torture is stunning with an electroshock gun such as a cattle prod or a taser, (provided a sufficiently high voltage and non-lethal current is used in the former case). The Nazis are known to have used electrical torture during World War II. An extensive fictional depiction of such torture is included in the 1966 book The Secret of Santa Vittoria by Robert Crichton. During the Vietnam War, electric shock torture is said to have been used against American prisoners of war. A scene of electrical torture in the American Deep South is included in the 1980 Robert Redford film Brubaker. Advocates for the mentally ill have often asserted that extensive use of electro-convulsive therapy is torture used without bona fide medical benefit against recalcitrant or non-reponsive patients; for more on ECT as medical therapy, see above. Capital punishment Electric shock delivered by an electric chair is sometimes used as a means of capital punishment, although its use has become rare in recent times. Although the chair was at one time considered a more humane and modern execution method than hanging, shooting, or decapitation, it is now being replaced for the same reasons by lethal injection. In addition, modern reportage has revealed that it sometimes takes several shocks to be effective, and that the condemned man may actually catch fire before the process is complete. Throughout the world, execution via electric shock has always been regarded as inhumane. Except for the United States, only the Phillipines used this method for a few years. It remains a legal means of execution in some states of the USA.http://www.deathpenaltyinfo.org/state/ It is reportedly one of the most grisly forms of modern execution to witness, with smoke or actual flame visible, coming from the prisoner's garments or cap. Games Electric shock is sometimes used as a punishment in novelty games such as Lightning Reaction, Shocking Roulette, Shocking Liar, Laser Shock Guns, and Shocking Tanks. In addition to these games, there are some prank toys like a fake pen or a chocolate candy which give out a mild shock. Notes See also | |||||||
|
| ||||||||
 |
|
| |