- Electric shock
:"Electrocute" redirects here. For the band, see
An electric shock can occur upon contact of a human's body with any source of
voltagehigh enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is thought to be about 1 milliampere (mA). The current may cause tissue damage or fibrillationif it is sufficiently high. Death caused by an electric shock is referred to as electrocution.
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 mA (milliampere) 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.
Heating due to resistance can cause extensive and deep
burns. Voltage levels of (> 500 to 1000 V) shocks tend to cause internal burns due to the large energy (which is proportional to the duration multiplied by the square of the voltage) available from the source. Damage due to current is through tissue heating. In some cases 16 volts might be fatal to a human being when the electricity passes through organs such as the heart.Fact|date=September 2008
A low-voltage (110 to 220 V), 50 or 60-Hz AC current through the chest for a fraction of a second may induce
ventricular fibrillationat currents as low as 60 mA. With DC, 300 to 500 mA is required. If the current has a direct pathway to the heart (e.g., via a cardiac catheteror other kind of electrode), 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 200 mA, muscle contractions are so strong that the heart muscles cannot move at all.
Current can cause interference with nervous control, especially over the heart and lungs. Repeated or severe electric shock which does not lead to death has been shown to cause
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 chairand by research from the field of animal husbandry, where electric stunning has been extensively studied) [http://www.grandin.com/humane/elec.stun.html] ."'
Approximately 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. [cite web|url=http://standards.ieee.org/announcements/pr_FINArc.html|title=Industry Backs IEEE-NFPA Arc Flash Testing Program With Initial Donations Of $1.25 Million|publisher=IEEE|date= 14 July, 2006|accessdate=2008-01-01] The arc flashin an electrical fault produces the same type of light radiationfrom 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.
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 main circuit or power distribution panel the damage is more likely to be internal, leading to cardiac arrest.
The comparison between the dangers of
alternating currentand direct currenthas been a subject of debate ever since the War of Currentsin 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 currentat 100–250 volts, however death has occurred outside this range, with supplies as low as 32 volts and supplies at over 250 volts frequently causing fatalities.
Electrical discharge from lightning tends to travel over the surface of the bodyFact|date=September 2008 causing burns and may cause
Lethality of a shock
The voltage necessary for electrocution depends on the current through the body and the duration of the current. Using
Ohm's law, Voltage = Current × Resistance, we see that the current drawn depends on the resistance of the body. The resistance of our skin varies from person to person and fluctuates between different times of day. In general, dry skin is a poor conductor that may have a resistance of around 100,000 Ω, while broken or wet skin may have a resistance of around 1,000 Ω. [cite web|url=http://www.cdc.gov/niosh/docs/98-131/overview.html|accessdate=2008-08-16|title=Publication No. 98-131: Worker Deaths by Electrocution|publisher= National Institute for Occupational Safety and Health]
The capability of a conducting material to carry a current depends on its cross section, which is why males typically have a higher lethal current than females (10 amperes vs 9 amperes) due to a larger amount of tissue Fact|date=August 2008. However, death can reportedly occur from currents as low as 30 mili-amperes.Fact|date=October 2008
Using Ohm's law, we may derive the voltages lethal to the human body. This is given in the following table: [cite web |url=http://acept.asu.edu/courses/phs110/ds/appendixC.html |accessdate=2007-06-14 |title=Dangers of electricity |publisher=
Arizona State University]
Point of entry
Macroshock: Current across intact skin and through the body. Current from arm to arm, or between an arm and a foot, is likely to traverse the heart, therefore it is much more dangerous than current between a leg and the ground.
Microshock: Direct current path to the heart tissue.
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 exist through the body from one hand to the other. This can lead the current through the heart. Similarly, if the current is from one hand to the feet, significant current will probably exist through the heart. An alternative to using insulated tools is to isolate the operator from ground, so that there is no conductive path from the live conductor, through the operator's body, to ground. This method is used for working on live high-voltage overhead power lines. [ [http://tdworld.com/mag/power_line_maintenance_reaches/ Philippe Morel, "Line Maintenance Reaches New Heights", "Transmission & Distribution World", Aug 1, 1999, accessed 2007-06-22] ]
It is possible to have a voltage potential between neutral wires and the ground in the event of an improperly wired (disconnected) neutral, or if it is part of certain obsolete (and now illegalFact|date=January 2008) switch circuits. The electrical appliance or lighting equipment might provide some voltage drop, but not nearly enough to avoid a shock. "Live" neutral wires should be treated with the same respect as live wires. Also, the neutral wire must be insulated to the same degree as the live wire to avoid a
It should be mentioned that much care needs to be taken with electrical systems on ships and boats, especially steel or aluminum ones. Anyone standing on a metal deck or leaning against a bulkhead is automatically grounded, so great care must be taken that all live electrical wires are well insulated. As an example of the danger, during WW II, the battleship "USS Washington" had not one casualty due to enemy action. However, there were some sailors killed by electrocution while doing such things as using electric drills that had defects in them. For the details, see the official histories of this distinguished USN warship.
Electrical codes in many parts of the world call for installing a
residual-current device(RCD or GFI, ground fault interrupter) in 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 also any other room with an unfinished 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 exits through the live wire than is returning though its neutral wire (presumably via ground), it assumes something is wrong and breaks the circuit in a tiny fraction of a second. There is some concern that these devices might not be fast enough to protect infants and small children in rare instances.
*Concrete contains a residual amount of water that makes it somewhat electrically conductive. Also, concrete in contact with any source of water or moisture will absorb some, and the water in concrete always contains dissolved minerals that makes the water significantly conductive.
The plumbing system in a home or other small building has historically used metal pipes and thus been connected to ground through the pipes*. This is no longer always true because of the extensive use of plastic piping in recent years, but a plastic system 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 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 severe electric shock or electrocution. While such an appliance is not "supposed" to be live on its outer metal switch or frame, it may have become so if a defective live 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 United Kingdom. However, the widespread use of plastic cases for everyday appliances, grounding of these appliances, and mandatory installation of Residual Current Devices (R.C.D.s) have greatly reduced this type of electrocution over the recent past decades.
*Connecting electrical neutrals to plumbing is against the electrical codes, at least in the United States of America. This is for several reasons. One of these is that connecting any electrical lines to plumbing presents a danger to plumbers or anyone else working on or around plumbing. Also, with metallic plumbing, even small amounts of electric current through them over a significant length of time can cause corrosion to the pipes, the removal of their zinc linings - if they have any, and the breakdown of the solder in their joints.
*The ground wire (grounding conductor) of the system is allowed to be connected to plumbing. However as previously stated, the neutral (grounded conductor) is not allowed to be connected. NEC 250.52 Grounding Electrodes (A)Electrodes Permitted for Grounding (1)Metal Underground Water Pipe. This requires: a metal underground water pipe in direct contact with the earth for 3.0m (10ft) or more and electrically continuous to the points of connection of the grounding electrode conductor and the bonding conductor.
A properly-grounded appliance greatly reduces the electric shock potential by causing a
short circuitif any portion of the metal frame (chassis) is accidentally touching the live wire. This will cause the circuit breakerto turn off or the fuseto 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 is still much safer than risking electric shock, since the chance of an out-of-control fire is remote.
Where live circuits must be frequently worked on (e.g.
televisionrepair), an isolation transformeris sometimes 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 touches the live chassis and ground at the same time, nothing would happen.
Neither ground fault interrupters (RCD/GFI) nor isolation transformers can prevent electrocution between the live 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.
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. [cite web | url = http://www.ilo.org/encyclopedia/?doc&nd=857100207&nh=0 | title = Electricity: Physiological Effects | accessdate = 2006-09-04 | author = Folliot, Dominigue | year = 1998 | work = Encyclopaedia of Occupational Health and Safety, Fourth Edition ] Electrocutions in the workplace make up the majority of these fatalities. From 1980–1992, an average of 411 workers were killed each year by electrocution. [ NIOSH (1998) "Worker Death by Electrocution" [http://www.cdc.gov/niosh/elecphsm.html] Cincinnati: National Institute for Occupational Safety and Health, NIOSH Pub. No. 98-131.]
Electric shock is also used as a medical therapy, under carefully controlled conditions:
Electroconvulsive therapyor ECT is a psychiatric therapy for mental illness. The objective of the therapy is to induce a seizure for theraputic effect. There is no sensation of shock because the patient is anesthetized. The therapy was originally conceived of after it was observed that depressed patients who also suffered from epilepsy experienced some remission after a spontaneous seizure.Fact|date=June 2007 The first attempts at deliberately inducing seizure as therapy used not electricity but chemicals; however electricity provided finer control for delivering the minimum stimulus needed. Ideally some other method of inducing seizure would be used, as the electricity may be associated with some of the negative side effects of ECT including amnesia. ECT is generally administered three times a week for about 8-12 treatments.
* As a treatment for fibrillation or irregular heart rhythms: see
* As a method of
pain relief: see Transcutaneous Electrical Nerve Stimulator(more commonly referred to as a TENS unit).
* As an aversive punishment for conditioning of mentally handicapped patients with severe behavioral issues. This method is highly controversial and is employed at only one institution in the United States, the
Judge Rotenberg Educational Center. The institute also uses electric shock punishments on non-handicapped children with behavioral problems. Whether this constitutes legitimate medical treatment versus abusive discipline is the subject of ongoing litigation.
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. Another method of electrical torture is stunning with an electroshock gunsuch as a cattle prodor a taser(provided a sufficiently high voltage and non-lethal current is used in the former case).
National Socialists are known to have used electrical torture during World War II. [cite web|url=http://www.boston.com/bostonglobe/ideas/articles/2007/12/16/torture_american_style/|title=Torture, American style: The surprising force behind torture: democracies|publisher= Boston Globe|date= 2007-12-16|accessdate=2008-01-01] 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 by both the Americans and Vietnamese.Fact|date=January 2008 A scene of electrical torture in the American Deep Southis included in the 1980 Robert Redfordfilm " Brubaker." Amnesty Internationalpublished an official statement that Russian military forces in Chechnyatortured local women with electric shocks by connecting electric wires to their bra straps. [ [http://web.amnesty.org/library/Index/ENGEUR460142006?open&of=ENG-385 Russian Federation Preliminary briefing to the UN Committee against Torture] 1 April 2006, statement by Amnesty International] Examples in popular modern culture are the electric torture of Martin Riggsin " Lethal Weapon" and John Ramboin "". Japanese serial killer Futoshi Matsunagaused electric shocks for controlling his victims. [cite news|url=http://www.asahi.com/english/Herald-asahi/TKY200709270058.html|title=Serial killer's death sentence upheld|publisher=Asahi Shimbun|date=2007-09-27|accessdate=2008-03-21]
Advocates for the mentally ill and some
psychiatrists such as Thomas Szaszhave asserted that electroconvulsive therapy is torture when used without a "bona fide" medical benefit against recalcitrant or non-responsive patients. See above for ECT as medical therapy. These same arguments and oppositions apply to the use of extremely painful shocks as punishment for behavior modification, a practice that is openly used only at the Judge Rotenberg InstituteFact|date=December 2007.
Low- to moderately high-voltage electric shocks do not result in the type of pain felt at death or organ failure, [ [http://www.tomjoad.org/bybeememo.htm Bybee Torture Memo- The Full Text ] ] nor have been proven to result in "significant psychological harm of significant duration, e.g., lasting for months or even years,".
Electric shock delivered by an
electric chairis sometimes used as an official means of capital punishmentin the United States, although its use has become rare in recent times. Although the electric chair was at one timeLopsided|date=July 2008 considered a more humane and modern execution method than hanging, shooting, poison gassing, the guillotine, etc., it has now been replaced in countries which practice capital punishment by lethal injections. Modern reporting has claimed that it sometimes takes several shocks to be lethal, and that the condemned person may actually catch fire before the process is complete. The brain is always severely damaged and inactivated.
Other than in parts of the
United States, only the Philippines reportedly has used this method, and only for a few years. It remains a legal means of execution in a few states of the USA. [ [http://www.deathpenaltyinfo.org/state/ Death Penalty Information Center ] ] .
Transcutaneous Electrical Nerve Stimulator
* [http://www.cdc.gov/niosh/docs/98-131/ National Institute for Occupation Safety & Health: Worker Deaths by Electrocution] a CDC study.
* [http://www.allaboutcircuits.com/vol_1/chpt_3/2.html Physiological effects of electricity]
* [http://www.allaboutcircuits.com/vol_1/chpt_3/3.html Shock current path]
* [http://merck.com/mmpe/sec21/ch316/ch316b.html Electrical injury (Merck Manual)]
* [http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html Electric Shock Hazards] (Hyperphysics)
* [http://vias.org/feee/safety_04.html Electric Shock] : a more technical perspective
* [http://csao.org/uploadfiles/magazine/vol11no1/shock.htm Construction Safety Association of Ontario: Electrocution] ... article with case studies
* [http://www.electrical-installation.schneider-electric.com/ei-guide/pdf_files/EIG-F-protection-electrical-shocks.pdf Protection against electric shocks] : physiological effects and protection rules
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