Electric shock
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- Electrocution redirects here, for deliberate execution by electric shock, see electric chair.
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 1mA. The current may cause tissue damage or heart fibrillation if it is sufficiently high.
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Description
An electric shock is usually painful and can be lethal. The level of voltage is not a direct guide to the level of injury or danger of death, despite the common misconception that it is. Physiological effects and damage are generally determined by current and duration. Even a low voltage causing a current of extended duration can be fatal.
'Let go' current
With sufficiently high current there can be a muscular spasm which causes the affected person to grip and be unable to release from the current source. The maximum current that can cause the flexors of the arm to contract but that allows a person to release his hand from the current's source is termed the let-go current. For DC, the let-go current is about 75 mA for a 70-kg man. For alternating current, the let go current is about 15 mA, dependent on muscle mass.
Shock effects
Psychological
The perception of electric shock can be different depending on the voltage, duration, current, path taken, 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.
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), 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 of DC is required. If the current has a direct pathway to the heart (eg, 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.
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. It is believed that human lethality is most common with AC current at 100-250 volts, as lower voltages can fail to overcome body resistance while with higher voltages the victim's muscular contractions are often severe enough to cause them to recoil (although there will be considerable burn damage).
Electrical discharge from lightning tends to travel over the surface of the body and may cause respiratory arrest.
Avoiding danger of shock
Current 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. 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 flows through the body can occur 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.
Shock due to differing ground potentials
In the case of lightning strikes to ground or other causes of large ground currents, large animals may be killed by the potential difference occuring on the ground between their front and rear limbs. Depending on the current, this could also affect humans if legs are spaced apart.
Deliberate uses
Electric shock as medical treatment
Electric shock can also be used as a medical therapy, under carefully engineered conditions:
- as a (disputed) psychiatric therapy for mental illness, where it's called Electroconvulsive therapy.
- as a treatment for fibrillation or irregular heart rhythms: see defibrillator and cardioversion.
Torture
Electric shocks are a convenient method of torture, since they can be controlled with scientific precision and used to cause internal injury, avoiding obvious evidence on the victim's body. Torture can use electrodes fixed to parts of the victim's anatomy. The genitalia are amongst the most painful, and at the same time humiliating. The nipples of a woman's breasts are also another very frequent target. Stunning with electroshock gun such as a cattle prod or taser, provided a sufficiently high but non-lethal voltage is used.
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. Throughout most of the world this practice is regarded as inhumane, but it remains a legal means of execution in some states of the USA.
See also
External links
- Electrical injury [1]
- Electric Shock Hazards (Hyperphysics)
- Question: Why Does Mixing Water and Electricity Cause Electric Shock?
- Electric Shock - a more technical perspective
