Sign warning of possible electric shock hazard

An electric shock may occur upon contact with electricity. An electric shock can be lethal, but the level of voltage is not a direct guide to lethality, despite the popularity of such a measure. Physiological effects are determined by voltage, current and duration. A high voltage and a high current together are lethal, but so are a lower voltage and current of extended duration. An example of the first would be a lightning strike and of the second would be contact with a live mains cable, but even a mains cable is carrying many times a minimum lethal shock.

Electrical discharge from lightning tends to travel over the surface of the body and causes respiratory arrest. From a mains circuit the damage is more likely to be internal, leading to cardiac arrest. With line currents above 2 milliamperes there can be a muscular spasm which causes the affected person to grip and be unable to release from the current source. It is believed that human lethality is most common with AC current at 100-250 volts, as lower voltages can fail to overcome 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). Damage due to current is through tissue heating and interference with nervous control, especially over the heart. Fibrillation can be induced (and removed) by 10 mA, although, oddly, with higher currents (20 mA and above) contractions in muscles around the heart can actually prevent the heart from fibrillating. Tissue heating due to resistance can cause extensive and deep burns. 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.

It is strongly recommended that people working with exposed parts of electrical machines should work with only one of their hands. The best way to do this is to keep your left hand in your pocket. This is because if both hands make contact with the wrong surfaces, the current flows through the body from one hand to the other. This can lead the current to pass through the heart. Conversely, if the current passes from one hand to the feet, little current will probably pass through the heart.

So depending on the circumstances, a human can survive 35 kV without great harm while 10 V can kill. The above information would appear to suggest that the requirements to distribute electrical current to domestic users have resulted in a combination that is quite deadly. In parts of the Americas and a few countries like Japan, the power is distributed at 110-120 V AC to the end users. But in Europe and most other countries, it is distributed at 220-240 volts.

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, however, it remains a legal means of execution in some states of the USA.

Electric shock as medical treatment

Electric shock can also be used as a medical therapy, under carefully engineered conditions.

Electric shock can be used as a psychiatric therapy for mental illness, where it's called Electroconvulsive therapy.

Electric shock is also used as a treatment for fibrillation or irregular heart rhythms: see defibrillator and cardioversion.

See also

• static electricity