Encyclopedia
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 milli
ampere .
The current may cause tissue damage or heart fibrillation if it is sufficiently high. When an electric shock is fatal, it is called
electrocution.
Shock effects
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 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 shocks tend to cause internal burns due to the large energy available from the source. Damage due to current is through tissue heating.
Ventricular fibrillation
A low-voltage , 50 or 60-Hz AC current traveling 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 , a much lower current of less than 1 mA, 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. .
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 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
- 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, and so is much more dangerous than current traveling 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 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.
Also, remember there can be a voltage potential between neutral wires and ground in the event of an improperly wired neutral, or if it is part of certain obsolete 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 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 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 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 . It is for this reason that mains electrical sockets are prohibited in bathrooms in the UK. However, widespread use of
plastic cases , 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 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 , 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 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 conducts electricity.
First aid instructions
- Check for a response and breathing. If necessary, start resuscitating the victim.
- Call emergency services for an ambulance. If you are unsure on resuscitation techniques, the ambulance call-taker will give you easy-to-follow instructions over the telephone, so you can increase the patient’s chances of survival until the ambulance arrives.
- If the breathing and pulse are steady, attend to injuries. Cool the burns and cover with dressings that won’t stick. Never put ointments or oils onto burns. If the victim has fallen from a height, only move them if there is chance of further danger . Try not to move them unnecessarily in case of spinal injuries.
- Talk calmly and reassuringly to the conscious victim.
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 , 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.
Deliberate uses
Electric shock as medical treatment
Electric shock can also be used as a medical therapy, under carefully engineered conditions:
- in a psychiatric therapy for mental illness, called in modern usage Electroconvulsive therapy or ECT; previously referred to as electroshock therapy or EST .
- as a treatment for fibrillation or irregular heart rhythms: see defibrillator and cardioversion.
- as a method of pain relief: see Transcutaneous Electrical Nerve Stimulator .
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, .
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 is a 1980 [i] film [i] about a prison [i]. ...
.
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 most of the world, execution via electric shock is now regarded as inhumane, but it remains a legal means of execution in some states of the
USA. 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
- Static electricity
- Electromagnetism
- Transcutaneous Electrical Nerve Stimulator
External links
- a CDC study.
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- - a more technical perspective
- ... article with case studies
- - Physiological effects and protection rules
