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.
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. A small shock from static electricity may contain thousands of volts but has very little current behind it due to high internal resistance. Physiological effects and damage are generally determined by current and duration. Even a low voltage causing a current of extended duration can be fatal. Ohm's Law directly correlates voltage and current for a given resistance; thus, for a particular path through the body under a particular set of conditions, a higher voltage will produce a higher current flow.
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.
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.
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 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.
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)
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 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). However, death has occurred from supplies as low as 32 volts.
Electrical discharge from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest.
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 in the event of an uncorrected high current condition. "Live" neutral wires should be treated with the same respect as hot wires (though a short circuit must involve the hot wire, not just neutral and ground).
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 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. 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.
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.
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 whilst 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. Another frequent method of torture is by 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).
In a case in Vietnam, a man was electroshocked by a guerilla warrior for not giving out information. The warrior discharged 5000 volts to the man's chest using a improvised electric shocking device made out of 2 electrodes connected to the man's chest (around the nipples) and 1 on the man's groin and set off by a computer. The man survived the extreme torture which he described as 'like going to hell and back'. The man had severe injuries to his heart and lungs and was rendered impotent by the local doctor. He suffered for 10 years with numerous treatments and finally died from cardiac arrest.
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