Electric shock

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Sign warning of possible electric shock hazard
Sign warning of possible electric shock hazard
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An electric shock can occur upon contact of a human's body with any source of voltage high enough to cause sufficient current flow 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 fibrillation if it is sufficiently high. Death caused by an electric shock is referred to as electrocution.

Contents

[edit] Shock effects

[edit] 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 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.

[edit] Burns

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.

[edit] 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 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 200mA, muscle contractions are so strong that the heart muscles cannot move at all.

[edit] Neurological effects

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 neuropathy.

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) [2].

[edit] Arc-flash hazards

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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.[1] 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.

[edit] 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 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.

[edit] Lethality of a shock

The voltage necessary for electrocution depends on the current flowing through the body and the duration of the current flow. 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 poor conductor having a resistance of around 10,000 Ω, while skin dampened by tap water or sweat has a resistance of around 1,000 Ω.

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. However, death can reportedly occur from currents as low as 0.1 amperes.

Using Ohm's law, we may derive the voltages lethal to the human body. This is given in the following table: [2]

Electric current (amperes) Voltage at 10,000 ohms Voltage at 1,000 ohms Maximum power (watts) Physiological effect
0.001 A 10 V 1 V 0.01 W Threshold of feeling an electric shock, pain
0.005 A 50 V 5 V 0.25 W Maximum current which would be harmless
0.01-0.02 A 100-200 V 10-20 V 1-4 W Sustained muscular contraction. "Cannot let go" current.
0.05 A 500 V 50 V 25 W Ventricular interference, respiratory difficulty
0.1-0.3 A 1000-3000 V 100-300 V 100-900 W Ventricular fibrillation. Can be fatal.
6 A 60,000 V 6,000 V 400,000 W Sustained ventricular contraction followed by normal heart rhythm.

These are the operation parameters for a defibrillator. Temporary respiratory paralysis and possibly burns.

[edit] Point of entry

  • Macroshock: Current flowing across intact skin and through the body. Current traveling 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 traveling between a leg and the ground.
  • Microshock: Direct current path to the heart tissue.

[edit] 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 to the feet, significant current will probably pass 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. [3]

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 illegal[citation needed]) 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 short circuit.

It should be mentioned that lots of 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 is passing through the live wire than is returning though its neutral wire, 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 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 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 passing 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. Don't do it. DAW - Master of Science in Electrical Engineering, Georgia Institute of Technology.


*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. 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 circuit if any portion of the metal frame (chassis) is accidentally touching the live 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 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. television repair), an isolation transformer is 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 the ground -- not the neutral wire. This is an odd statement, because the neutral wires are all connected to the ground as often as is practical - at least in North America.

[edit] 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. [4] 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. [5]

[edit] Deliberate uses

[edit] Electroconvulsive Therapy

Electric shock is also used as a medical therapy, under carefully controlled conditions:

  • Electroconvulsive therapy or 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.[citation needed] 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 time a week for about 8-12 treatments.
  • 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.

[edit] Torture

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Main article: 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. Another 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.[6] An extensive fictional depiction of such torture is included in the 1966 book The Secret of Santa Vittoria by Robert Crichton, this method of torture used by the Nazi's is also apparent in the computer game Return to Castle Wolfenstein, where you see a prisoner of war being given an electric shock in an attempt to get information out of him. During the Vietnam War, electric shock torture is said to have been used by both sides.[citation needed] A scene of electrical torture in the American Deep South is included in the 1980 Robert Redford film Brubaker. Amnesty International published an official statement that Russian military forces in Chechnya tortured local women with electric shocks by connecting electric wires to their bra straps.[7] Examples in popular modern culture are the electric torture of Martin Riggs in Lethal Weapon and John Rambo in Rambo: First Blood Part II. Japanese serial killer Futoshi Matsunaga used electric shocks for controlling his victims.[8]

Advocates for the mentally ill and some psychiatrists such as Thomas Szasz have 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 Institute[citation needed].

Since low- to moderately high-voltage electric shocks do not result in the type of pain felt at death or organ failure, [9] nor have been proven to result in "significant psychological harm of significant duration, e.g., lasting for months or even years," it is doubtful if whether these techniques would be considered torture by the current US administration in the context of the assertive interrogation of unlawful enemy combatants. Other gray-area methods such as waterboarding, for example, are used and referred to as "enhanced interrogation techniques." [10]

[edit] Capital punishment

Main article: Electric chair
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Electric shock delivered by an electric chair is sometimes used as an official means of capital punishment in the United States, although its use has become rare in recent times. Although the electric chair was at one time considered a more humane and modern execution method than hanging, shooting, poison gassing, the guillotine, etc., it has now been replaced in some countries which still practice execution 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.[11].

[edit] See also

[edit] References

  1. ^ Industry Backs IEEE-NFPA Arc Flash Testing Program With Initial Donations Of $1.25 Million. IEEE (14 July 2006). Retrieved on 2008-01-01.
  2. ^ Dangers of electricity. Arizona State University. Retrieved on 2007-06-14.
  3. ^ Philippe Morel, "Line Maintenance Reaches New Heights", Transmission & Distribution World, Aug 1, 1999, accessed 2007-06-22
  4. ^ Folliot, Dominigue (1998). Electricity: Physiological Effects (English). Encyclopaedia of Occupational Health and Safety, Fourth Edition. Retrieved on 2006-09-04.
  5. ^ NIOSH (1998) Worker Death by Electrocution [1] Cincinnati: National Institute for Occupational Safety and Health, NIOSH Pub. No. 98-131.
  6. ^ Torture, American style: The surprising force behind torture: democracies. Boston Globe (2007-12-16). Retrieved on 2008-01-01.
  7. ^ Russian Federation Preliminary briefing to the UN Committee against Torture 1 April 2006, statement by Amnesty International
  8. ^ "Serial killer's death sentence upheld", Asahi Shimbun, 2007-09-27. Retrieved on 2008-03-21. 
  9. ^ Bybee Torture Memo- The Full Text
  10. ^ Waterboarding would be torture to me: U.S. spy chief | Reuters
  11. ^ Death Penalty Information Center

[edit] External links

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Consequences of external causes (T15-T35, T66-T98, 930-959, 990-995)
General external causes
Other external causes
Certain early complications of trauma
Complications of surgical and medical care