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 mA. The current may cause tissue damage or heart fibrillation if it is sufficiently high.
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. It should be noted, however, that 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.
'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.
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.
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 (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.
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.
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.
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
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. Also, remember there can be a voltage potential between "neutral" wires and ground. The neutral wire from a high-wattage appliance will have nearly as much voltage potential to ground as its hot wire. However, even a low-wattage appliance isn't safe against electrocution from its neutral wire.
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.
The recommended first aid for someone who had received a severe electrical shock has three major components
- Call for help
- Make sure the victim is no longer in contact with the electrical current source. Turn off all power if this can be done quickly.
- Check for breathing and heart beat and apply cardiopulmonary resuscitation, if necessary