Electric shock:
An Electrical shock can occur upon contact of human's body with any source of voltage high enough to cause sufficient current through the muscles or hair. The minimum current a human can feel is through to be about 1 mili ampere(mA).
The current may cause tissue damage or fibrillation if it is sufficiently high. Death caused by an electric shock is referred to as electrocution.
Shock Effects:
1. 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 mA(miliampere) for DC and about 1 to 10 mA for AC at 60Hz.
Shock perception declines with increasing frequency, ultimately disappearing at frequencies above 15-20 khz.
2. Burns:
Heating due to resistance can cause extensive and deep burns. voltage levels of (>500 to 1000V) shocks tend to cause internal burns due to the large energy (which is proportional to the duration multiplied by the square of the voltage ) available from the source.
Damage due to current is through tissue heating. In some cases, 16 volts might be fatal to a human being when the electricity passes through organs such as the heart.
3. Ventricular fibrillation :
A low voltage (110 to 220V) , 50 or 60 Hz Ac current through the chest for fraction of a second may induce ventricular fibrillation at current as low as 60 mA.
With DC, 300 to 500mA is required. If the current has a direct pathway to the heart(e.g. via a cardiac catheter or other kind of electrode), 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.
4. Neurological effects:
Current can cause interference with nervous control, especially over the heart and lungs. The repeated or severe electric shock which does not lead to death has been shown to cause neuropathy.
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)
5. Arc Flash Hazards:
Approximately 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(blast) 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 flash. The blast produced by vaporizing metallic components can break bones and irreparably damage internal organs.
The degree of hazards present at a particular location can be determined by a detailed analysis of the electrical system, and appropriate protection is worn if the electrical work must be performed with the electricity on.
6. Issue affecting lethality:
Another issue 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 the main circuit or power distribution panel 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 leads 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, a 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 occured from supplies as low as 32 volts and supplies at over 250 Volts frequently cause fatalities.
Electrical dicharges from lightning tends to travel over the surface of the body causing burns and may cause respiratory arrest.
7. Lethality of shock:
The voltage necessary for electrocution depends on the current through the body and the duration of the current. Using ohm's law, voltage = current * resistance, we see that the current drawn depends on the resistance of the body.
The resistance of our skin varies from person to person and fluctuates between different times of day.
In general, Dry skin is poor conductor having a resistance of around 10000 ohm, while skin dampened by tap water or sweat has a resistance of around 1000 ohm.
The capability of a conducting material to carry a current depends on its cross-section, which is why males typically have a higher lethal current than females (10 amperes vs 9 amperes) due to a larger amout of tissue. However, death can reportedly occure from current as low as 0.1 amperes.
Effect of Various Voltages and currents on Human Body:
