Electrical injury
An electrical injury or electrical shock is damage sustained to the skin or internal organs on direct contact with an electric current.
The injury depends on the density of the current, tissue resistance and duration of contact. Very small currents may be imperceptible or only produce a light tingling sensation. However, a shock caused by low and otherwise harmless current could startle an individual and cause injury due to jerking away or falling. A strong electric shock can often cause painful muscle spasms severe enough to dislocate joints or even to break bones. The loss of muscle control is the reason that a person may be unable to release themselves from the electrical source; if this happens at a height as on a power line they can be thrown off. Larger currents can result in tissue damage and may trigger ventricular fibrillation or cardiac arrest. If death results from an electric shock the cause of death is generally referred to as electrocution.
Electric injury occurs upon contact of a body part with electricity that causes a sufficient current to pass through the person's tissues. Contact with energized wiring or devices is the most common cause. In cases of exposure to high voltages, such as on a power transmission tower, direct contact may not be necessary as the voltage may "jump" the air gap to the electrical device.
Following an electrical injury from household current, if a person has no symptoms, no underlying heart problems, and is not pregnant, further testing is not required. Otherwise an electrocardiogram, blood work to check the heart, and urine testing for signs of muscle breakdown may be performed.
Management may involve resuscitation, pain medications, wound management, and heart rhythm monitoring. Electrical injuries affect more than 30,000 people a year in the United States and result in about 1,000 deaths.
Signs and symptoms
Burns
Heating due to resistance can cause extensive and deep burns. When applied to the hand, electricity can cause involuntary muscle contraction, preventing the victim from untensing their hand muscles and releasing the wire, increasing the risk for serious burns. Voltage levels of 500 to 1000 volts tend to cause internal burns due to the large energy available from the source. Damage due to current is through tissue heating and/or electroporation injury. For most cases of high-energy electrical trauma, the Joule heating in the deeper tissues along the extremity will reach damaging temperatures in a few seconds.Ventricular fibrillation
A domestic power supply voltage, 50 or 60 Hz alternating current through the chest for a duration longer than one second may induce ventricular fibrillation at currents as low as 30 milliamperes. With direct current, 90 to 130 mA are required at the same duration. If the current has a direct pathway to the heart, a much lower current of less than 1 mA can cause fibrillation. If not immediately treated by defibrillation, ventricular fibrillation is usually lethal, causing cardiac arrest, because all of the heart muscle fibres move independently instead of in the coordinated action needed for successful cardiac cycle to pump blood and maintain circulation.Short single DC pulses induce VF dependent on the amount of charge transferred to the body, which makes the amplitude of the electrical stimulus independent of the exact amount of current flowing through the body for very short pulse durations. DC shocks of short duration are usually better tolerated by the heart even at high currents and rarely induce ventricular fibrillation compared to lower currents with longer duration with both DC or AC. The amount of current can easily reach very high values as amperage is only of second order importance to fibrillation risk in the case of ultra short contact times with direct currents. But even if the charge itself is harmless, the amount of energy being discharged still can lead to thermal and chemical hazards if its value is high enough.
One example of high current electric shock which may be usually harmless is an electrostatic discharge as experienced in everyday life on door handles, car doors etc. These currents can reach values up to 60 A without harmful effects on the heart as the duration is in the order of only several ns. Another example for dangerous electrostatic discharges even without flowing directly through the body are lightning strikes and high voltage arcs.
Mechanism
Mechanism of cardiac arrhythmias induced by electricity is not fully understood, but various biopsies have shown arrhythmogenic foci in patchy myocardial fibrosis which contained increased amount of Na+ and K+ pumps, possibly associated with transient and localized changes in sodium-potassium transport as well as their concentrations, resulting in changes in membrane potential.Neurological effects
Electric shock which does not lead to death has been shown to cause neuropathy in some cases at the site where the current entered the body. The neurologic symptoms of electrical injury may occur immediately, which traditionally have a higher likelihood for healing, though they may also be delayed by days to years. The delayed neurologic consequences of electrical injury have a worse prognosis.When the path of electric current proceeds through the head, it appears that, with sufficient current applied, loss of consciousness almost always occurs swiftly. This is borne out by research from the field of animal husbandry, where electric stunning has been extensively studied.
If ventricular fibrillation occurs, the blood supply to the brain is diminished, which may cause cerebral hypoxia.
Mental health
There are a variety of psychiatric effects that may occur as a result of electrical injuries. Behavioral changes can occur as well, even if the path of electric current did not proceed through the head. Symptoms may include:- Depression, including feelings of low self-esteem and guilt
- Anxiety spectrum disorders, including posttraumatic stress disorder and fear of electricity
- Moodiness, including a lower threshold for frustration and "losing one's temper"
- Memory loss, decreased attention span, and difficulty learning
Arc-flash hazards
Pathophysiology
The minimum current a human can feel depends on the current type as well as frequency for AC. A person can sense electric current as low as 1mA for 60Hz AC and as low as 5mA for DC. At around 10mA, AC current passing through the arm of a human can cause powerful muscle contractions; the victim is unable to voluntarily control muscles and cannot release an electrified object. This is known as the "let go threshold" and is a criterion for shock hazard in electrical regulations.The current may, if it is high enough, cause tissue damage or fibrillation which can cause cardiac arrest; of AC or of DC at high voltage can cause fibrillation. A sustained electric shock from AC at 120 V, 60 Hz is an especially dangerous source of ventricular fibrillation because it usually exceeds the let-go threshold, while not delivering enough initial energy to propel the person away from the source. However, the potential seriousness of the shock depends on paths through the body that the currents take. If the voltage is less than 200 V, then the human skin, more precisely the stratum corneum, is the main contributor to the impedance of the body in the case of a macroshock—the passing of current between two contact points on the skin. The characteristics of the skin are non-linear however. If the voltage is above 450–600 V, then dielectric breakdown of the skin occurs. The protection offered by the skin is lowered by perspiration, and this is accelerated if electricity causes muscles to contract above the let-go threshold for a sustained period of time.
If an electrical circuit is established by electrodes introduced in the body, bypassing the skin, then the potential for lethality is much higher if a circuit through the heart is established. This is known as a microshock. Currents of only 10 μA can be sufficient to cause fibrillation in this case with a probability of 0.2%.
Body resistance
The voltage necessary for electrocution depends on the current through the body and the duration of the current. Ohm's law states that the current drawn depends on the resistance of the body. The resistance of human skin varies from person to person and fluctuates between different times of day. The NIOSH states "Under dry conditions, the resistance offered by the human body may be as high as 100,000 ohms. Wet or broken skin may drop the body's resistance to 1,000 ohms," adding that "high-voltage electrical energy quickly breaks down human skin, reducing the human body's resistance to 500 ohms".The International Electrotechnical Commission gives the following values for the total body impedance of a hand to hand circuit for dry skin, large contact areas, 50 Hz AC currents :
Skin
The voltage-current characteristic of human skin is non-linear and depends on many factors such as intensity, duration, history, and frequency of the electrical stimulus. Sweat gland activity, temperature, and individual variation also influence the voltage-current characteristic of skin. In addition to non-linearity, skin impedance exhibits asymmetric and time varying properties. These properties can be modeled with reasonable accuracy. Resistance measurements made at low voltage using a standard ohmmeter do not accurately represent the impedance of human skin over a significant range of conditions.For sinusoidal electrical stimulation less than 10 volts, the skin voltage-current characteristic is quasilinear. Over time, electrical characteristics can become non-linear. The time required varies from seconds to minutes, depending on stimulus, electrode placement, and individual characteristics.
Between 10 volts and about 30 volts, skin exhibits non-linear but symmetric electrical characteristics. Above 20 volts, electrical characteristics are both non-linear and symmetric. Skin conductance can increase by several orders of magnitude in milliseconds. This should not be confused with dielectric breakdown, which occurs at hundreds of volts. For these reasons, current flow cannot be accurately calculated by simply applying Ohm's law using a fixed resistance model.