Electric current is dangerous because it is invisible. Its effects on humans are sufficiently well-known nowadays for us to be able to protect ourselves effectively. In any electrical installation, the risk of electrocution (nonfatal accident), electrocution (leading to death), and fire is real. When a person is in contact with two electrodes, it passes through the body an electrical current.
Even the briefest contact with electricity at 50 volts for alternating current (V a.c.) or 120 volts for direct current (V d.c.) can have serious consequences for a person’s health and safety. High voltage shocks involving more than 1000 V a.c. or 1500 V d.c. can cause contact burns and damage to internal organs. Workers using electricity may not be the only ones at risk. That’s why everyone should know the effects of electricity on the human body.
Electric current has a different effect on every person. The effects of electric current on the human body depend on four factors:
Current and voltage
Although high voltage often produces massive destruction of tissue at contact locations, it is generally believed that the detrimental effects of electric shock are due to the current flowing through the body. Even though Ohm’s law (I=E/R) applies, it is often difficult to correlate voltage with damage to the body because of the large variations in contact resistance usually present in accidents. Any electrical device used on a house wiring circuit can, under certain conditions, transmit a fatal current. Although currents greater than 10 mA are capable of producing painful to severe shock, currents between 100 and 200 mA can be lethal.
With increasing alternating current, the sensations of tingling give way to contractions of the muscles. The muscular contractions and accompanying sensations of heat increase as the current is increased. Sensations of pain develop, and voluntary control of the muscles that lie in the current pathway becomes increasingly difficult. As the current approaches 15 mA, the victim cannot let go of the conductive surface being grasped. At this point, the individual is said to “freeze” the circuit. This is frequently referred to as the “let-go” threshold.
As the current approaches 100 mA, ventricular fibrillation of the heart occurs. Ventricular fibrillation is defined as “very rapid uncoordinated contractions of the ventricles of the heart resulting in loss of synchronization between heartbeat and pulse beat.” Once ventricular fibrillation occurs, it will continue and death will ensue within a few minutes. The use of a special device called a defibrillator is required to save the victim.
Heavy current flow can result in severe burns and heart paralysis. If shock is of short duration, the heart stops during current passage and restarts normally on current interruption improving the victim’s chances for survival.
Studies have shown that the electrical resistance of the human body varies with the amount of moisture on the skin, the pressure applied to the contact point and the contact area.
The outer layer of skin, the epidermis, has very high resistance when dry. Wet conditions, a cut or other breaks in the skin will drastically reduce resistance.
Shock severity increases with an increase in the pressure of contact. Also, the larger the contact area, the lower the resistance. Whatever protection is offered by skin resistance decreases rapidly with an increase in voltage. Higher voltages have the capability of “breaking down” the outer layers of the skin, thereby reducing the resistance.
Path through body
The path the current takes through the body affects the degree of injury. A small current that passes from one extremity through the heart to the other extremity is capable of causing severe injury or electrocution. There have been many cases where an arm or leg was almost burned off when the extremity came in contact with an electrical current and the current only flowed through a portion of the limb before it went out into the other conductor without going through the trunk of the body. Had the current gone through the trunk of the body, the person would almost surely have been electrocuted.
A large number of serious electrical accidents in the industry involve current flow from hands to feet. Since such a path involves both the heart and the lungs, the results can be fatal.
Duration of shock
The duration of the shock has a great bearing on the outcome. If the shock is of short duration, it may only be a painful experience for the person.
If the level of current flow reaches the approximate ventricular fibrillation threshold of 100 mA, a shock duration of a few seconds could be fatal. This is not much current when you consider that a small light-duty portable electric drill draws about 30 times as much. At relatively high currents, death is inevitable if the shock is of appreciable duration; however, if the shock is of short duration and if the heart has not been damaged, interruption of the current may be followed by a spontaneous resumption of its normal rhythmic contractions.
Effects of electric current on the human body
When subjected to voltage, the human body reacts like a conventional receiver that has a given internal resistance. Electric current flows through it with three serious risks:
The current keeps the muscles through which it passes contracted and if this involves the rib cage, this may lead to respiratory arrest.
This is a total disruption of the cardiac rhythm.
Thermal effects cause varying degrees of tissue injury, even deep burns in the case of high currents the table below shows that, for a touch voltage of 220 V, a current of 147 mA will pass through the human body. This current must therefore be broken in less than 0.18 seconds to avoid any risk.
Given the two parameters to be taken into account for evaluating the risk, the standards define the time/ current limit curves. these curves, taken from IEC 60479-1, give the various limits of the effects of 50 Hz alternating current on humans and define 4 main risk zones. In some installations or under particular conditions (damp areas, moist skin, low resistance flooring, etc.) a lower impedance value can be taken into account.
Time/current curves showing the effects of 15 to 100 Hz alternating current on humans
AC-1: Generally no reaction
AC-2: Generally no dangerous physiological effect
AC-3: Generally no organic damage. Probability of muscle contractions and breathing difficulties if current flows for longer than 2 s. Reversible interference in the formation of cardiac impulse propagation without ventricular fibrillation increases with the intensity of the current and the flow period.
AC-4: Increasing with the current intensity and duration, pathophysiological effects such as cardiac arrest, respiratory arrest, and severe burns can occur in addition to the effects of zone AC-3.
AC-4.1: Probability of ventricular fibrillation up to approximately 5%
AC-4.2: Probability of ventricular fibrillation up to approximately 50%
AC-4.3: Probability of ventricular fibrillation greater than 50%
Conventional time/current zones of effects of d.c. currents on persons for a longitudinal upward current path (hand to feet)
DC-1: Slight pricking sensation is possible when making, breaking or rapidly altering the current flow.
DC-2: Involuntary muscular contractions are likely, especially when making, breaking or rapidly altering current flow but usually have no harmful electrical physiological effects.
DC-3: Strong involuntary muscular reactions and reversible disturbances of formation and conduction of impulses in the heart may occur, increasing with current magnitude and time. Usually, no organic damage is expected
DC-4: Pathophysiological effects may occur such as cardiac arrest, breathing arrest, burns or other cellular damage. The probability of ventricular fibrillation increases with the current magnitude and time.
DC-4.1: Probability of ventricular fibrillation increasing up to about 5 %
DC-4.2: Probability of ventricular fibrillation up to about 50 %
DC-4.3: Probability of ventricular fibrillation above 50 %
The current and human body
0,045 mA: Sensory level language
0,5 to 1 mA: The perception threshold depends on the state of the skin and light muscle contractions.
6 to 8 mA: Perception of skin shock to the touch.
10 mA: Electrical shock, muscle contraction.
15,5 mA: Impossibility of self-liberation of the current.
20 mA: Electric shock, tetanization of the thorax, possibility. On ventilatory asphyxiation, if t is greater than 3 minutes if the current path of interest to the diaphragm (contact by hand)
30 mA: Electric shock, tetanization of the thorax, possibility of ventricular fibrillation if t is greater than one minute.
50 mA: Possibility of ventricular fibrillation with a probability greater than 50% and if t is greater than 1.5 of the cardiac cycle.
70 to 100mA: Ventricular fibrillation with a probability greater than 50% if t is less than 0.75 of the cardiac cycle; burns.
>500 mA: Nerve centers destroyed; internal chemical composition burns very important; almost immediate death.